Biology Module Cytology

Biology Module Cytology

Lesson 1

Table of Contents

 01 Question No 1: Microscope device

Answer : The has the main parts of your microscope Three Part

  1. Mechanical
  • includes:
  • base,
  • arm,
  • stage,
  • a draw tube,
  • а coarse focus knob
  • and fine focus
  • On the stage is placed studied the slide.
  • Coarse focus knob  changes the distance from the objectives  to the slide and is used for Finding object.
  • Fine focus knob   use for observing most prepared slides.
  1. Optical The optical part includes: eyepiece and objectives.
  2. Lighting-
  • includes a lamp or mirror and condenser.
  • Using a mirror we catch the light beam.
  • Condenser regulates the intensity of the lighting.

02 Question 2: The rules for  the use of the microscope

Answer : Using Microscope 1 st all

Put the Microscope Plane Place and Connect the Power Connection

And Switch on the Microscope Light .

And Clean the Lenses with tissue paper

And Put the Clean dry Slide and Do not Touch the Center of Slide .

Put the Slide on the Stage and Force Arrestment 10 x and look in Eyepece  and find the Cell if  not Show Clear than Change the Lense Size 40x and using Find Know and Search the Cell.

03 Question 3: Procedure of the preparation of temporary slide.

Answer :

  1. Place a drop of fluid in the center of the slide.
  2. Position sample on liquid, using tweezers.
  3. At an angle, place one side of the cover slip against the slide makingcontact with outer edge of the liquid drop.
  4. Lower the cover slowly, avoiding air bubbles.
  5. Remove excess water with the paper towel.

04 Question 4: Forms of life: cellular  and non-cellular

1.Non Cellular





Archae  and Bacteria



Plantae of Vegtabillia

Fungi or Mycota

05 Question 5:Differences between prokaryotes and eukaryotes

Answer: Eukaryotes(True Cell):  Example -Animal ,Plant and Fungi and Prokaryotes : Example- Bacteria and Archaea

1.Nucleus Present in Eukaryotes and Nucleus Absent in Prokaryotes Cell.

  1. Eukaryotes Cell is Unicellular and Multicellular and Prokaryotes Cell Usually Unicellular
  2. Eukaryotes Cell Size 10-100 um and Prokaryotes Cell 1-10 um
  3. Eukaryotes Cell More than 1 Chromosomes and Prokaryotes Cell 1 Circular Chromosome Called Nucleoid.
  4. Eukaryotes Cell 80 S Ribosome and Prokaryotes Cell 70 S Ribosome
  5. Eukaryotes Cell Cytoskleton Present and Prokaryotes Cell Cytoskleton Absent .
  6. Eukaryotes Cell Membranous organelles Present and Prokaryotes Cell Membranous organelles Absent .

 06 Question 6 :Structure of Eukaryotic Cell

Answer: Structures in All Eukaryotic Cells

  • Nucleus
  • Cytoplasm
  • Plasma membrane

 07 Question 7 : Differences between Animal and Plant cells.

Answer : Plant Cell: Size in Large and Animal Cell Size in Smaller

 Plant Cell: Cell wall (Cellulose ) Present  and in Animal Cell Absent

Plant Cell :Vacuole is Present large Central one or More and Animal Cell Smaller and  temporary Present .

Plant Cell : Plastid Present And in Animal Cell Absent

Plant Cell Centrioles is Absent and Animal Cell Centrioles Present .

Plant Cell Store food in Starch and Animal Cell Store Food in glycogen.

Plant Cell Cytoplasm is Pushed to the periphery bya Vacuole and form a thin layer and Animal Cell Denser, occupies most of the Space in the Cell.

08.Question 8 : Composition of Cytoplasm:

Answer: Cytosol, Organelles and Cytoplasmic inclusions

Cytosol:  is the internal environment of the cell. Is a colloidal solution containing a variety of organic and inorganic substances (water-about 90%, -0.9% salt ions, proteins, carbohydrates, amino acids, nucleotides, vitamins and others).

The composition of the cytosol of the cell determines the osmotic properties.

In the cytosol important metabolic processes occur: glycolysis, synthesis amino acids, nucleotides and other monomers .

Organelles : Organelles are permanent structure of the cytoplasm. Each organelles has a certain structure and function .

cytoplasmic inclusions -Inclusion is a non-permanent components of the cytoplasm. They are products of cell activity

These include:

  • Reserve nutrients cells (grain starch, fat drops)
  • Secretory vesicles
  • The end products of metabolism, toxins, derived from the metabolism
  • Pigments

09 Question 9: Cytosol: Composition and Function.

Answer:Cytosol organic and inorganic substances (water-about 90%, -0.9% salt ions, proteins, carbohydrates, amino acids, nucleotides, vitamins and others).

The composition of the cytosol of the cell determines the osmotic properties.

In the cytosol important metabolic processes occur: glycolysis, synthesis amino acids, nucleotides and other monomers.

10 Question 10: Cytoplasmic inclusions: definition and differences from organelles, types of cytoplasmic inclusions.

 Answer: Cytoplasmic inclusions : is a non-permanent components of the cytoplasm. They are products of cell activity.

Differences between organelles and Cytoplasmic inclusions: The main difference between cell organelles and cell inclusions is that the cell organelles are membrane-bound compartments that perform a particular function in the cell whereas the cell inclusions are non-living materials in the cytoplasm.

 Types of cytoplasmic inclusions

It is Two Type: Excretory inclusion – Calcium oxalate crystals. The final product metabolism in plants.

Food storage inclusions- Include glycogen in the liver cells and  Starch Grains

in Potato cells

 11 Question 11: Classification of Organelles. Structure and Function of

Answer: Structure and Function of

12.Mitochondria Structure

  • Mitochondria have two membranes: Outer and Outer membrane is smooth.
  • The inner membrane of mitochondria forms numerous folds. They are called
  • the Cristae membranes contains many enzymes
  • The internal contents of the mitochondria is called matrix. He is a colloidal solution. Matrix contains many enzymes, DNA in the form of a ring, RNA and ribosomes
  • Scientists believe that mitochondria occurred by symbiosis of the two cells

13.Mitochondria Function

  • Mitochondria are rod-shaped organelles that can be considered the power generators of the cell, converting oxygen and nutrients into adenosine triphosphate (ATP).
  • ATP is the chemical energy “currency” of the cell that powers the cell’s metabolic activities
  • This process is called aerobic respiration and is the reason animals breathe oxygen.

 14.Plastids Structure

Plastid is a double membrane-bound organelle.

15.Plastids Function

  • The plastid is a major organelle found in the cells of plants and algae.
  • Plastids are the site of manufacture and storage of important chemical compounds used by the cell. They often contain pigments used in photosynthesis, and the types of pigments present can change or determine the cell’s color.
  • They possess a DNA molecule, which is circular, like that of prokaryotes.
  • occurred by symbiosis of the two cells
  • There are 3 types Plastids: chloroplasts, chromoplasts and leucoplasts

 16.Endoplasmic reticulum Structure

The endoplasmic reticulum Single Membrane The general structure of the endoplasmic reticulum is a network of membranes called cisternae.

17.Endoplasmic reticulum Function

  • The endoplasmic reticulum is a network of membrane sacs that manufactures, processes, and transports chemical compounds for use inside and outside of the cell.
  • It is connected to the double-layered nuclear envelope, providing a pipeline between the nucleus and the cytoplasm.
  • If the endoplasmic reticulum contains on the surface  the ribosomes, it is called a Rough (granular).
  • Endoplasmic reticulum without ribosomes is called

18.Rough  ER participates  in protein synthesis.

  • Smooth ER participates in the synthesis of lipids and
  • ER – synthesizes components of membranes. It is cell membranes factory.  

 19.Golgi bodies Structure

  • Golgi Bodies Single Membrane
  • Golgi complex are composed of numerous sets of smooth cisternae.
  • Each disc-shaped cisternae forms a structure that resembles a stack of plates.
  • The Golgi complex contains a great number of vesicles. These vesicles are used to send molecules to the cellular membrane, where they are excreted.

20.Golgi bodies Function

  • accumulation and maturation of substances synthesized in the ER
  • packaging and expelling of substances from the cells (secretion)
  • synthesis of polysaccharides (components of plants cell wall )
  • production of lysosomes

21.Lysosomes Structure

Lysosomes is a Single Membrane .

  • Lysosomes are small spherical membrane vesicles
  • They contains  about 40 different types of hydrolytic enzymes, all of which are manufactured in the endoplasmic reticulum and modified in the Golgi body.

22.Lysosomes Function

  • intracellular digestion
  • the destruction of defective organelles
  • destruction of bacteria and viruses (immune response ) Leukocytes contain a lot of lysosomes
  • the destruction of larval bodies.
  • It contains digestive enzymes.
  • It Called Sucide Bag

23.Peroxisomes Structure

It is a Single Membrane

Peroxisomes – a small organelle that contains  enzymes of the class oxidase.

 24.Peroxisomes Function

  • which primarily function together to rid the cell of toxic substances, and in particular, hydrogen peroxide (a common by product of cellular metabolism).
  • Peroxidase (catalase) enzyme cleaves hydrogen peroxide.
  • Especially a lot of peroxisomes in the cells of the liver and kidneys.

25.Vacuoles Structure

  • Vacuole is a membrane-bound organelle which is present in all plant and fungal cells.
  • Vacuoles are essentially enclosed compartments which are filled with water containing inorganic and organic molecules.
  • The organelle has no basic shape or size, its structure varies according to the needs of the cell.

26.Vacuoles Function

  • Plant cell growth
  • Isolating materials that might be harmful
  • Containing waste products
  • containing water in plant cells
  • Maintaining turgor within the cell
  • Maintaining an acidic internal pH
  • Containing small molecules

27.Ribosomes Structure

It is non Membrane Organelle

  • Ribosome is tiny organelles composed of approximately 60 percent RNA and 40 percent protein.
  • The ribosome consists of two subunits : large and small
  • Some cell types may hold a few million ribosomes.
  • The organelles require the use of an electron microscope to be visually detected.
  • Ribosomes are mainly found bound to the endoplasmic reticulum and the nuclear envelope, as well as freely scattered throughout the cytoplasm.
  • Eukaryote ribosomes are produced and assembled in the nucleolus.

28.Ribosomes Function

29.Centrosome Structure  and centrioles

  • Centrosome consists of two centrioles
  • Centrioles are self-replicating organelles made up of nine triplets of microtubules and are found only in animal cells.
  • They appear to help in organizing cell division
  • Centrioles participate in the formation of poles division and spindle division
  • Within the centrosome, the centrioles are positioned so that they are at right angles to each other

30.Centrosome Function 

A centrosome is a cellular structure involved in the process of cell division. Before cell division, the centrosome duplicates and then, as division begins, the two centrosomes move to opposite ends of the cell.

31.Centrioles Stucture

  • A centriole is made of nine sets of microtubules, each in groups of three known as triplet microtubules

32.Centrioles Function

  • The main function of centrioleis to help with cell division in animal cells.
  • The centriolesalso help in the formation of the spindle fibers that separate the chromosomes during cell division (mitosis).

Cytoskeleton: Microtubules and  Microfilaments

33.Cytoskeleton  Structure –The cytoskeleton is a structure that helps cells maintain their shape and internal organization.

34.Cytoskeleton Functioncytoskeleton that gives structure and shape to a cell, and also serve as conveyor belts moving other organelles throughout the cytoplasm.

35.Microtubules Structure

  • Microtubules – a component of the cytoskeleton
  • Microtubules are able to self-assemble
  • In addition, microtubules are the major components of cilia and flagella.
  • It is hollow like Structure.
  • And it is Thickest Part

36.Microtubules Function

  • microtubules are composed of globular protein tubulin subunits, about 25 nanometers in diameter,
  • In addition, microtubules are the major components of cilia and flagella, and participate in the formation of spindle fibers during cell division (mitosis). 

37.Microfilaments Structure

It is Thin ( Patla) Part of the Cytoskeleton

It is long Thread like Filament .

It is made up Actins.

38.Microfilaments Function

Microfilaments assist with cell movement.

39.Flagella and Cilia Structure

Cilia and flagella are motile cellular appendages found in most microorganisms and animals, but not in higher plants.

40.Flagella and Cilia Function

  • Cilia and flagella are essential for the locomotion of individual organisms.
  • In multicellular organisms, cilia function to move a cell or group of cells or to help transport fluid or materials past them. The respiratory tract in humans is lined with cilia that keep inhaled dust, smog, and potentially harmful microorganisms from entering the lungs.

41 Question No 41:Structure, property and Function Plasma membrane.

Answer: Plasma membrane Structure

The plasma membrane is the outer layer that covers all cells and certain cell organelles. It is a thin layer and delicate ,and Selectively Permeable Membrane.

The principal components of a plasma membrane are:

  • Lipids (phospholipids and cholesterol)
  • Proteins
  • Carbohydrates attached to some of the lipids and some of the proteins.

42.Plasma membrane Function

  • Protection of the Cell against the bacteria.
  • It allow the entry and exit of some Selective materials in and out the cell
  • It help to maintain the Shape of the cell

43.Plasma membrane Properties

  • Impermeable to water-soluble molecules (but not to water)
  • Soft and flexible
  • Spontaneously prone to forming self-repairing pores

Question No:44 Membrane Transport

45.Passive transport:  Three Type : filtration , Diffusion , Osmosis

  • Move Particles in the direction of the gradient, in the direction of decreasing concentration
  • Requires no energy.
  • Example of Passive Transport  
  • Diffusion:
  • Simple diffusion
  • Facilitated diffusion
  • Osmosis

* Filtration :moving of some ions through the membrane channels.


  • They are transmembrane proteins.
  • Channels are specific for the ions such as sodium, potassium, calcium, and chloride.
  • Aquaporins are channel proteins that allow water to pass through the membrane at a very high rate

46.Simple diffusion:

  • Molecules of oxygen and carbon dioxide have no charge and so pass through membranes by simple diffusion. 
  • Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues.
  • Fat-soluble drugs and hormones also gain easy entry into cells and are readily transported into the body’s tissues and organs.

47.facilitated diffusion:

Process by which  substances moves down a concentration gradient (from high to low concentration) using integral membrane proteins.

  • Polar substances present problems for the membrane.
  • they cannot readily pass through the lipid core of the plasma membrane.
  • These substances diffuse across the plasma membrane with the help of membrane proteins (carrier protein).
  • These substances are ions (РО4, СО3), polar molecules (Simple sugars and amino acids) that are repelled by the hydrophobic parts of the cell membrane


Transport of water through a semipermeable membrane according to the concentration gradient of water across the membrane.

  • Osmosis is a special case of diffusion.
  • Water, like other substances, moves from an area of high concentration of water to an area of its low concentration.
  1. Active Transport:

  • Moves particles against a concentration gradient require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP).
  • Some active transport mechanisms move small-molecular weight substances, such as ions, through the membrane (Proton Pump,Sodium-potassium pump) .
  • Other mechanisms transport much larger molecules (Vesicular transport: Endocytosis, Exocytosis)
  • Moves particles against a gradient
  • Requires

Example Active Transport:

  • Proton Pump
  • Sodium-potassium pump or Na +/K+-ATPase
  • Vesicular transport
  • Phagocytosis
  • Pinocytosis
  1. Sodium-potassium pump or Na +/K+-ATPase :

  • One of the most important pumps in animals cells is the sodium-potassium pump (Na+-K+ ATPase), which maintains the electrochemical gradient (and the correct concentrations of Na+ and K+) in living cells. The sodium-potassium pump moves K+ into the cell while moving Na+ out at the same time, at a ratio of three Na+ for every two K+ ions moved in.
  1. Phagocytosis :

    Fhagocytosis, or “cell eating”, is the process by which a cell engulfs a particle and digests it.

  • Cells in the immune systems of multicellular organisms use phagocytosis to devour bodily intruders such as bacteria, and they also engulf and get rid of cell debris. Some single-celled organisms like amoebas use phagocytosis in order to eat and acquire nutrients.
  1. Pinocytosis: Pinosytosis, also called “cell drinking”. It is one mechanism by which cells will take in fluids. When the cell takes in the fluid, it is stored in a tiny vesicle.

53 Question No: 53. Types of solutions: isotonic, hypertonic and hypotonic.

Answer: Types of solutions: Three Type – isotonic, hypertonic and hypotonic.

54.Isotonic : (Physiological solution, 0,85% NaCl)

[substances in solution ] = [ substances in cell ]

  • In an isotonic condition, the relative concentrations of solute and solvent are equal on both sides of the membrane.
  • There is no net water movement; therefore, there is no change in the size of the cell
  1.  Hypertonic : (4% NaCl, 10% NaCl )

[substances in solution ] > [ substances in cell ]

  • In a hypertonic solution, water leaves a cell and the cell shrinks.
  • an animal cell crenates (shrinks) and a plant cell will become plasmolysed
  1. Hypotonic : ( Distilled water )

[substances in solution ] < [ substances in cell ]

  • In a hypotonic environment, water enters a cell, and the cell swells.
  • an animal cell may burst (lysis) however, a plant cell which has a cell wall to protect it will become turgid.
  • Cytolysis occurs when a cell bursts due to an osmotic imbalance that has caused excess water to move into the cell.

57.Question No : 57. Behavior of cells in different solutions.


Hypertonic solution :so that the plasma membrane has pulled away completely from the cell wall, and the central vacuole has shrunk.

Isotonic solution :the plasma membrane has pulled away from the cell wall a bit, and the central vacuole has shrunk.

Hypotonic solution : The central vacuole is large, and the plasma membrane is pressed against the cell wall.

Image for only Self Understand


58.Question No: 58. Value of solutions in medicine.



Lesson 2

59.Question No :59 Structural components of the nucleus. The role of the nucleus in the cell activity.

59.The overall Plan of the nuclear Structure

Answer: Nucleus is the main part of Cell and control in Cell function. Nucleus contains most of gene and RNA and Protein Store etc. Nucleus is double membrane organelle.

Structural components of the nucleus:

  • Nuclear membrane (nuclear envelope)
  • Nucleoplasm
  • Nucleolus
  • Chromatin

Nucleoplasm, also known as karyoplasm, is the matrix present inside the nucleus.

The role of the nucleus in the cell activity. it controls the hereditary characteristics of an organism.  (Function of Nucleus)

  • Storage of hereditary material, the genes in the form of long and thin DNA (deoxyribonucleic acid) strands, referred to as chromatin.
  • Storage of proteins and RNA (ribonucleic acid) in the nucleolus.
  • Nucleus is a site for transcription in which messenger RNA (mRNA) are produced for protein synthesis.
  • Exchange of hereditary molecules (DNA and RNA) between the nucleus and the rest of the cell.
  • During the cell division, chromatins are arranged into chromosomes in the nucleus.
  • Production of ribosomes (protein factories) in the
  • Selective transportation of regulatory factors and energy molecules through nuclear pores.
  • Control the Cell

60.Question No :60. Structure and functions of the nuclear envelope. The structure of the nuclear pore.

Answer:  Nuclear Envelope Structure:

Nuclear envelope or Nuclear Membrane : Nuclear Membrane separate to Cytoplasm and Nucleus.It is double layered Membrane.Outer layer endoplasmic reticulum . The inner nuclear membrane is enclose to Nucleopasm .It is Covered is known as Nuclear lamina.

  • The nuclear envelope is a double-layered membrane structure that encloses the contents of the nucleus.
  • The outer layer of the membrane is connected to the endoplasmic reticulum.
  • A fluid-filled space or perinuclear space is present between the two layers of a nuclear membrane.

Nuclear envelope function

Nuclear Membrane separate to Cytoplasm and Nucleus.

It is connected to the outer membrane by nuclear pores which penetrate the membranes.Nuclear pore by Exchange Cytoplasm to Nucleus (Proteins and RNA ).

The structure of the nuclear Pore:

The nuclear pore is a protein-lined channel in the nuclear envelope that regulates the transportation of molecules between the nucleus and the cytoplasm.

61.Question No :61. The chemical composition and structure of chromatin. The concept of euchromatin and heterochromatin.

Answer: The chemical composition and structure of chromatin:

  • Chromatin – the form and appearance of DNA during interphase in the cell nucleus
  • it consists of a complex of nucleic acids – DNA and RNA (40%) and proteins (60%), primarily histones, which provide for the first level of DNA supercoiling.

Euchromatin: It is inside the Nucleus and Thread like Structure and Light Packed .It is active transcription.

Heterochromatin : It is in side the Nucleus envelop or Nucleus Membrane. It is tightly Packed From DNA.

62 Question No :62. Nucleosome – the structural unit of chromatin. Stages of chromatin packaging in chromosomes.

Answer: Nucleosome: The nucleosome is the fundamental subunit of chromatin. Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as a histone octamer. Each histone octamer is composed of two copies each of the histone proteins H2A, H2B, H3, and H4.

Stages of chromatin packaging in chromosomes : During cell division (prophase) take place all the dense packing of chromatin and chromosome formation.

The first level of packing of the chromosome – in Nucleosome

Metaphase Chromatin in Chromosome  

63.Question No :63. Structure of chromosomes.  Rules of chromosomes.

63. Modern ideas about the structure of chromosomes. Terms of chromosomes

Answer: Structure of chromosomes : Each cell has a pair of each kind of chromosome known as a homologous chromosome. Chromosomes are made up of chromatin, which contains a single molecule of DNA and associated proteins. Each chromosome contains hundreds and thousands of genes that can precisely code for several proteins in the cell. Structure of a chromosome can be best seen during cell division.

Centromere divides the chromosome into two parts, the shorter arm is known as ‘p’ arm and the longer arm is known as ‘q’ arm.

64 Rules of chromosomes:

1.Number is constant: each species of living organism has a certain fixed number of chromosomes

  • Human has 46 chromosomes.

2.Rule of Pairs -The number of chromosomes in somatic cells is always even, because the chromosomes are         paired. The chromosomes of one pair have the same structure, a set of genes and are called homologous

      3.Rule  personality there are no two identical pairs of hromosomes

  1. Rule of continuity He basis of doubling of chromosomes during cell division, DNA replication is semi-conservative mechanism for. Thus, each newly synthesized DNA molecule comprises a nucleotide chain from the parent molecule

65 Question No : 65. Karyotype. Methods of study of the karyotype. International classification of chromosomes (Denver and Paris)

Answer : Karyotype : Karyotype is show on Number of chromosome and Size and Shape of Chromosomes in Nucleus in Eukaryotic Chromosome .

  • The normal human karyotype has a total of 46 chromosomes.
  • Homologous Pairs 1-22 are called the
  • X and Y are the sex chromosomes.
  • Autosomes chromosomes:  They are exists in all the organisms’ cells. They manage the somatic features of an organism. In the human being diploid cell, 44 chromosomes are autosomes while the rest two are sex chromosomes.
  • somatic features: It is a Body organ Making like hand ,heart,brain etc . It is Diploid Chromosome (2n). Its like Autosomes Chromosomes.
  • Germ Cells : It is Participate only Sex Chromosome and it is formation Testis (Male)and Ovary (Feamle). It is Haploid Chromosome  (n).

66.Methods of study of the karyotype :

Karyotyping – a laboratory test used to study an individual’s chromosome make-up. Chromosomes are separated from cells, stained, and arranged in order from largest to smallest so that their number and structure can be studied under a microscope.

67.International classification of chromosomes (Denver and Paris)

68 .According to Denver :

The classification of human chromosomes on the basis of size and centromere position; the 23 pairs of chromosomes are classified in seven groups (A to G), in order of decreasing length. Used before it was possible to distinguish among the chromosomes of the groups.

Denver Group No of Chromosomes
Group A1-3
Group B4-5
Group C6-12,X
Group D13-15
Group E16-18
Group F19-20
Group G21-22, Y

69.According to Paris :

70 Question No :70. The structure of the DNA molecule. The functions of DNA. Properties of DNA: replication and repair. The mechanism of DNA replication.

Answer : The structure of the DNA molecule.

DNA is made up of molecules called nucleotides. Each nucleotide contains a phosphate group, a sugar group and a nitrogen base. The four types of nitrogen bases are adenine (A), thymine (T), guanine (G) and cytosine (C). The order of these bases is what determines DNA’s instructions, or genetic code. Human DNA has around 3 billion bases, and more than 99 percent of those bases are the same in all people.

It is Single Standard and Double Standard DNA

DNA is a polymer with a very high molecular weight.

DNA molecule consists of two long polynucleotide chains. Each of these chains is known as  a DNA chain, or a DNA strand. Each single strand of DNA is a chain  composed of four types of nucleotides

71 .The functions of DNA :  Storage,Transfer, and realization of genetic information.

72.Properties of DNA : Replication ability to self-doubling. And Repair -ability to restore the structure after damage .

73.The mechanism of DNA replication: DNA Replicate in 4 Stage

Initiation, Elongation  ,Termination , Modification

Initiation: Replication Forks: hundreds of Y-shaped regions of replicating DNA molecules and new strands are growing.

Replication Bubbles: Bubbles Like Structure Form

  1. Hundreds of replicating bubbles (Eukaryotes).
  2. Single replication fork (Prokaryotes – bacteria).

Strand Separation:

Helicase:  enzyme which catalyze the unwinding and separation (breaking H-Bonds) of the parental double helix.

Single-Strand Binding Proteins: proteins which attach and help keep the separated strands apart.

Topoisomerase:  enzyme which relieves stress on the DNA molecule by allowing free rotation around a single strand.

RNA primers:  before new DNA strands can form, there must be small pre-existing primers (RNA) present to start the addition of new nucleotides (DNA Polymerase).

Primase:  enzyme that polymerizes (synthesizes) the RNA Primer.

 Elongation :  Synthesis of the new DNA Strands:

DNA Polymerase: with a RNA primer in place, DNA Polymerase (enzyme) catalyze the synthesis of a new DNA strand in the 5’ to 3’ direction.

Leading Strand:  synthesized as a single polymer in the 5’ to 3’ direction.

Lagging Strand:  also synthesized in the 5’ to 3’ direction, but discontinuously against overall direction of replication.

Okazaki Fragments:  series of short segments on the lagging strand.

Termination : The RNA primers are degraded by Ribonuclease H and replaced with DNA nucleotides by DNA polymerase

DNA ligase:  a linking enzyme that catalyzes the formation of a covalent bond from the 3’ to 5’ end of joining stands.

Termination sequences “Ter” direct termination

  • Modification: Check daughter DNA strand and replication error correction. This process is called repair.
  • Post-Replication modification involves covalent alteration of DNA after replication, e.g. (methylation at either A or C bases within a particular DNA sequence.
  1. Question No 73.Differences of RNA from DNA. The functions of RNA.

74 Answer: 74.Differences of RNA from DNA

  • There are two differencesthat distinguish DNA from RNA:
  • RNAcontains the sugar ribose,  DNA contains the slightly different sugar deoxyribose
  • RNAhas the nucleobase uracil while DNA contains thymine.
  • RNA are Protein Synthesis and DNA Store Genetic Information

75.The functions of RNA : Transfer the genetic code needed for the creation of proteins from the nucleus to the ribosome.

76.Question No :76: Cytoplasmic inheritance. Plasmids and their role in prokaryotes and eukaryotes.

77.Answer  :Cytoplasmic inheritance :

The genes governing cytoplasmic inheritance are called “plasma genes”, “cytoplasmic genes”, “cytogenes” or “extranuclear genes”. While the sum total of the extrachromosomal genetic material is known as plasmon and this type inheritance is known as extrachromosomal inheritance.

78.Plasmids role in prokaryotes:

A plasmid is a small DNA molecule within a cell that is physically separated from a chromosomal DNA and can replicate independently. plasmids often carry genes that may benefit the survival of the organism, for example antibiotic resistance. (Only Prokaryotes Cell Found).

79.Plasmids role in eukaryotes:

Plasmids naturally exist in bacterial cells, and they also occur in some eukaryotes. Often, the genes carried in plasmids provide bacteria with genetic advantages.

Lesson 3

80 .Question: The concept of the gene.

Answer: Genes the functional segments of DNA which code for the transfer of genetic information.

gene is the basic physical and functional unit of heredity. Genes are made up of DNA. Some genes act as instructions to make molecules called proteins.

81.Question : The genetic code. Properties of genetic code: triplets, degenerate, non overlapping, comma-free, ordered, universal.

  1. Answer : The genetic code: The genetic code by which DNA stores the genetic information consists of “codons” of three nucleotides.
  2. 82. Properties of genetic code:
  • The Genetic code is composed of nucleotide triplets.
  • The genetic code is degenerate ()
  • The genetic code is non-overlapping.
  • The genetic code is ordered (non-ambiguous). (No Double Meaning )
  • The genetic code is comma-free.
  • The genetic code contains start and stop codons. (AUG – methionine or start codon

UAA, UAG, or UGA – stop codons

  • The genetic code is nearly universal

The genetic code is triplet.

  • The Genetic code is composed of nucleotide triplets.
  • Three nucleotides in mRNA specify one amino acid in the polypeptide product;
  • thus, each codon contains three nucleotides.

Degenerate : It may one Specific codon code an other Amino Acid.

All but two (met, trp) of the amino acids are specified by more than one codon (from 2 to 6 ).

Non overlapping: Each nucleotide in mRNA belongs to just one codon  .It means that the same letter does not take part in the formation of more than one codon.

The genetic code is ordered (non-ambiguous).

A particular codon will always code for the same amino acid.

It may also be that the same amino acid may be coded by two different codons.

However, when one codon codes for two amino acids, it is called ambiguous.

The genetic code is nearly universal

With minor exceptions, the codons have the same meaning in all living organisms, from viruses to humans.

For instance, mitochondria have an alternative genetic  code with small variations.

83.Question No.83: Central dogma molecular biology:   DNA  RNA  protein. Stages of gene expression: transcription and translation

83. Stages of synthesis of Protein: transcription and translation

84.Answer: Central dogma molecular biology : This is a Process for DNA information  to RNA or  Protein Making

Gene expression is described by central dogma – two-step process denoted DNA → RNA → protein.

Gene expression is the process by which cells convert DNA sequence information to RNA and then decode the RNA information to the amino-acid sequence of a polypeptide.

85.Stages of gene expression: transcription and translation

Transcription: DNA to RNA Making Process is Known as Transcription

  • All types of RNA are synthesized from the DNA template on the principle of complementarity.
  • This process is called transcription
  • The enzymes that perform transcription are called RNA polymerases

86.Stage of Transcription :


  • The first step in transcription is initiation, when the RNA pol binds to the DNA upstream (5′) of the gene at a specialized sequence called a
  • RNA polymerase unwinds and unzips DNA double strand, attaches to promoter region of gene, which marks the beginning point for transcription


RNA polymerase synthesizes a complementary RNA strand from nucleoside triphosphate taken from the surrounding solution in 5’ to 3’ direction

  • It adds nucleoside triphosphates using base pairing rules:

         A = U               T = A                    G ≡ C            C ≡ G


RNA polymerase reaches termination region of the gene, which marks the end of the coding sequence

Terminates transcription by releasing both DNA and RNA

Translation: mRNA to Protein making process is known as Translation.

  • Translation is the process of decoding the mRNA into a polypeptide chain
  • In the cytoplasm, a ribosome attaches to the mRNA (three bases or 1 codon) and translates its message into a polypeptide
  1. 87. Stages of translation:


Translation is initiated when an mRNA molecule, which has a ribosome-binding site in its 5′ UTR, binds the complementary sequence in the rRNA molecule that is part of the small ribosomal subunit, with the help of initiation factor proteins.

Another initiation factor assists in the binding of a charged tRNA molecule to the start codon.

This tRNA is formylmethionine in bacteria (the formyl group is later removed) and methionine in eukaryotes.

The large ribosomal subunit binds, with the initiatior tRNA in the P site.


Elongation begins when the next aminoacyl tRNA occupies the aminoacyl (A) site.

The amino acid shown here is glutamic acid, but any amino acid can be the second amino acid specified by a particular mRNA.

The peptidyltransferase center in the large subunit catalyzes the formation of a peptide bond bewteen the amino acid in the P site and the amino acid in the A site.

This reaction is catalyzed by ribosomal RNA (a ribozyme) rather than by a protein enzyme.

Once the peptide bond is formed, the mRNA is moved through the ribosome to place the tRNA with the growing peptide chain in the peptidyl (P) site.

A new aminoacyl tRNA is then free to occupy the aminoacyl site.

The next peptide bond is synthesized.

The mRNA is moved again, to place the tRNA with the growing peptide chain in the peptidyl (P) site.


When the stop codon is reached, a release factor protein whose structure resembles that of a tRNA enters the A site.

Hydrolysis of the bond between the aminoacyl tRNA and the carboxy terminus of the last amino acid releases the peptide.

The tRNAs are also released.

The ribosome separates from the mRNA and the two subunits of the ribosome dissociate.

Protein modifications

Most proteins undergo post-translational modifications.

Protein modification is the process by which some proteins from the rough ER are altered within the Golgi apparatus in order to be targeted to their final destinations.

Protein modifications Stage

  • N- and C-terminal modifications
  • Proteolytic Cleavage
  • Folding of proteins (Secondary and Tertiary structure)
  • Crosslinking (Quaternary structure)

88 Question No 88 :The role of RNA in gene expression. Types of  RNAs. Characteristics of tRNAs, mRNA, rRNA, snRNA.

88.Answer: The role of RNA in gene expression : RNA that play a role in translation, including transfer RNA (tRNA) and ribosomal RNA (rRNA).

89: Types of  RNAs

It is a three type : mRNA, tRNA and rRNA


  • Code for proteins
  • Long Straight chain of Nucleotides
  • Made up of 500 to 1000 nucleotides long
  • Made in the Nucleus
  • Copies DNA & leaves through nuclear pores
  • Contains the Nitrogen Bases A, G, C, U ( no T )
  • Carries the information for a specific protein
  • Sequence of 3 bases called codon
  • AUG – methionine or start codon
  • UAA, UAG, or UGA – stop codons


  • Clover-leaf shape
  • Single stranded molecule with attachment site at one end for an amino acid
  • Opposite end has three nucleotide bases called the anticodon
  • The tRNA carries a specific amino acid from the amino acid pool to the mRNA on the ribosomes to form a polypeptide, hence its name. The tRNAs form about 15% of the total RNA of a cell. Its molecule is the smallest and has the form of a cloverleaf. It has four regions:


  • rRNA is a single strand 100 to 3000 nucleotides long
  • Globular in shape
  • Made inside the nucleus of a cell
  • Associates with proteins to form ribosomes
  • Form the basic structure of the ribosome and catalyze protein synthesis
  • Site of protein Synthesis


  • snRNAs – small nuclear RNAs, function in a variety of nuclear processes, including the splicing of pre-mRNA
  • It is Connect to Exon to Exon ,Removing to Intron. Removing Process is known as splicing

RNA Processing (maturation of pre-mRNA to mRNA): pre-mRNA → mRNA

  • All the primary transcripts produced in the nucleus must undergo processing steps to produce functional RNA molecules
  • Most human genes are divided into exons and
  • Primary transcripts (pre-mRNA) contains exons and
  • The exons are the sections that are found in the mature transcript (messenger RNA), while the introns are removed from the primary transcript by a process called splicing

90.Question No 90: Basic structure of a protein-coding gene at Prokaryotes and Eukaryotes (operon, transcripton).

Answer: Basic structure of a protein-coding gene

Basic structure of a protein-coding gene at









polycistronic mRNA

the mRNA codes for two or more polypeptides;

only found in prokaryotes


monocistronic mRNA

each mRNA codes for a single polypeptide;

therefor each gene codes for a single polypeptide



Transcription unit – promoter , RNA-coding sequence and terminator regions of a gene.

The promoter  is upstream of the coding sequence, the terminator downstream . The coding sequence at nucleotide +1

Basic structure of a protein-coding gene at Eukaryotes

  • A promoter is a region of DNA that initiates transcription of a particular gene. Promoters are located near the transcription start sites of genes, on the same strand and upstream on the DNA (towards the 5’ region of the sense strand). Promoters can be about 100–1000 base pairs long.
  • Core promoter – Includes the transcription start site (TSS) and elements directly upstream:

1) A binding site for RNA polymerase

2) General transcription factor binding sites, e.g. TATA boxes – – direct RNA polymerase  to the correct initiation site for transcription.

  • Proximal promoter – the proximal sequence upstream of the gene that tends to contain primary regulatory elements 

91 As Same Basic structure of a protein-coding gene at Prokaryotes (Direct use in Making  Protein )

  1. Operon :
  • Prokaryotic cells have linear sequences of DNA called operons. An operon contains one or more structural genes which are generally transcribed into one polycistronic mRNA (a single mRNA molecule that codes for more than one protein).
  • The operon is composed of a promoter sequence, followed by an operator gene, followed by one or more structural genes.

An operon is made up of 3 basic DNA components:

  • Promotor – a nucleotide sequence that enables a gene to be transcribed. The promoter is recognized by RNA polymerase, which then initiates transcription.
  • Operator – a segment of DNA that a repressor binds to. In the case of a repressor, the repressor protein physically obstructs the RNA polymerase from transcribing the genes.
  • Structural genes– the genes that codes for
  • Regulator – These genes control the operator gene in cooperation with certain compounds called inducers and corepressors present in the cytoplasm. The regulator gene codes for and produces a protein substance called repressor. The repressor substance combines with the operator gene to repress its action. A regulator gene controls an operon, but is not the part of the operon.


  1. Transcription Show on 86 .

92 Question No 92 : Processing. Steps of Processing: capping, the “poly-A tail”, splicing. The Alternative splicing.

93.Answer : Processing is the New RNA( it means hnRNA- with impure RNA or Intron and Exon both are Present ) making and make a Caping and tailing this is Process.

Steps of Processing of capping:

When the Processing start Both ends of eucaryotic mRNAs are modified: by capping on the 5 end and by polyadenylation of the 3 end.

Capping : It is Starting point  attached 7 Methyl Guanonsine of hnRNA .It Point is Secure the New RNA. Its Provide Binding Site of Ribosome .

94.The “poly-A tail : It is end point of hnRNA . Polyadenylation attached in the 3 end and 3 end Protect it. By Degradation exonnuclous enzyme.

95.The Alternative splicing :

Alternative splicing of introns to form different mRNAs from the same gene.

Alternative Splicing of exons allows one gene to make several different mRNAs, depending on which exons are included in the final message. Hence, one “gene” may code for a large number of different products. Some genes are known to make close to 100 different transcripts based on splicing patterns.

  1. Question :Control of an expression of genes. The lac-operon Escherichia coli

97.Answer: Control of an expression of genes: DNA to RAN or Protein Making Processing Control is known as Control of Expression of gene. It Mean Regulate the Process when will be on and When ill be of . Some Gene to Increase and Some Gene to Decrease . It known as gene expression.

98.The lac-operon Escherichia coli :

 Function – to produce enzymes which break down lactose (milk sugar)

  • Lactose is not a common sugar, so there is not a great need for these enzymes
  • When lactose is present, they turn on and produce enzymes.

If Present Lactose in E. Coli (Inducer) It working Making Enzyme .

If not Present Lactose in E.Coli (Inducer) It now Making Enzyme .

Processing to Lac operon

When not Present Lactose in E.Coli (Inducer) . Than Regulatory Gene Making mRNA its name is Repressor. And Repressor attached the Operator and Stop the Structural function or Making a Enzyme .

When Present Lactose in E. Coli (Inducer) ,Than Inducer go to Operator and Active and RNA polymerase Bind the Promoter and Making Enzyme .

98. Gene Regulation in Eukaryotes

  • The latest estimates are that a human cell, a eukaryotic cell, contains approximately 35,000 genes.
  • Some of these are expressed in all cells all the time. These so-called housekeeping genes are responsible for the routine metabolic functions (e.g. respiration) common to all cells.
  • Some are expressed as a cell enters a particular pathway of differentiation.
  • Some are expressed all the time in only those cells that have differentiated in a particular way. For example, a plasma cell expresses continuously the gene for the antibody it synthesizes.
  • Some are expressed only as conditions around and in the cell change. For example, the arrival of a hormone may turn on (or off) certain genes in that cell.

There are several methods used by eukaryotes.

Transcription Control 

The most common type of genetic regulation

Turning on and off of mRNA formation

Post-Transcriptional Control

Regulation of the processing of a pre-mRNA into a mature mRNA

Translational Control

Regulation of the rate of Initiation

Post-Tranlational Control

Regulation of the modification of an immature or inactive protein to form an active protein

Regulatory sequences:

  • Enhancers,
  • Silencers,
  • Insulators


Some transcription factors (“Enhancer-binding protein”) bind to regions of DNA that are thousands of base pairs away from the gene they control. Binding increases the rate of transcription of the gene.


Regulatory sequences with similar characteristics, but the opposite effect, exist.  These are called silencers.

Silencers are control regions of DNA that, like enhancers, may be located thousands of base pairs away from the gene they control. However, when transcription factors bind to them, expression of the gene they control is repressed.


As you can see above, enhancers can turn on promoters of genes located thousands of base pairs away.

What is to prevent an enhancer from inappropriately binding to and activating the promoter of some other gene in the same region of the chromosome?

One answer: an insulator.

Insulators are stretches of DNA (as few as 42 base pairs may do the trick) located between the

  • Enhancer(s) and promoter or
  • Silencer(s) and promoter

of adjacent genes or clusters of adjacent genes.

Their function is to prevent a gene from being influenced by the activation (or repression) of its neighbors.


Major rearrangements of at least one set of genes occur during immune system differentiation.
B lymphocytes produce immunoglobins, or antibodies, that specifically recognize and combat viruses, bacteria, and other invaders.
Each differentiated cell produces one specific type of antibody that attacks a specific invader.
Functional antibody genes are pieced together from physically separated DNA regions.
Each immunoglobin consists of four polypeptide chains, each with a constant region and a variable region, giving each antibody its unique function.
As a B lymphocyte differentiates, one of several hundred possible variable segments is connected to the constant section by deleting the intervening DNA.
The random combinations of different variable and constant regions create an enormous variety of different polypeptides, which combine with others to form complete antibody molecules.
As a result, the mature immune system can make millions of different kinds of antibodies from millions of subpopulations of B lymphocytes.

99. Reproduction. Levels of reproduction. The evolution of reproduction

Reproduction :

  • Reproduction is the process by which living organisms produce new individuals of the same species.
  • Reproduction is necessary for the survival and increase in the population of a species

Levels of reproduction

1.molecular – DNA replication

2.cell – cell division

3.organismal – sexual and asexual reproduction of organisms

The evolution of reproduction

There are two main types of reproduction in living organisms: asexual reproduction and sexual reproduction.

  • Asexual reproduction – is reproduction in which new individuals are produced from a single parent.
  • Sexual reproduction – is reproduction in which two individuals are involved to produce a new individual.

Asexual reproduction is of different types. They are: fission, budding, regeneration, fragmentation, spore formation, vegetative propagation etc.

Asexual Reproduction

Asexual Reproduction

Asexual reproduction (Fission)

  • Fission is an asexual reproduction by which a unicellular organism divides and forms two or more new individuals. Fission is of two types. They are binary fission and multiple fission.

Binary fission:

  • In this method an organism divides and forms two individuals. First the nucleus divides and forms two nuclei. Then the cytoplasm divides and forms two daughter cells.
  • Eg: Amoeba, Paramaecium etc.


  • Multiple fission (schizogony):
  • In this method one organism divides into many daughter cells.
  • Eg. Malarial Plasmodium


  1. In this method a bud like projection is formed on the body of the organism. The bud then develops into a new individual. It then separates from the parent and forms an independent individual.
  2. Eg: Hydra, Yeast etc.


  • Parthenogenesis is a form of asexual reproduction in which growth and development of embryos occur without fertilization. In animals, parthenogenesis means development of an embryo from an unfertilized egg cell.
  • Parthenogenesis occurs naturally in many plants, some invertebrate animal species (including nematodes, water fleas, some scorpions, aphids, some bees, some wasps) and a few vertebrates (such as some fish, amphibians, reptiles and very rarely birds). This type of reproduction has been induced artificially in a few species including fish and amphibians.


100 .Reproduction of organisms: asexual and sexual reproduction; in single-celled and in multicellular organisms

Reproduction of organisms:

  • Reproduction is the process by which living organisms produce new individuals of the same species.
  • Reproduction is necessary for the survival and increase in the population of a species.

Asexual and Sexual Reproduction

  • Asexual reproduction – is reproduction in which new individuals are produced from a single parent.
  • Sexual reproduction – is reproduction in which two individuals are involved to produce a new individual.

in single-celled and in multicellular organisms

Asexual Reproduction

101. Cell reproduction. Life-cycle. Interphase. Mitosis. Cytokinesis. Characteristic of stages

Cell Reproduction :

Cell division is the process by which a parent cell divides into two or more daughter cells. Cell division usually occurs as part of a larger cell cycle.

Cell division

There are three methods of cell division:

  • Amitosis direct division, the exact distribution of genetic information does not occur
  • Mitosis – indirect division, take place the exact distribution of genetic information. Formed cells that are genetically identical to the mother Cell
  • Meiosis -the division in which a four haploid cells are formed from one diploid cell


in general, cell cycle consists of 2 main phases with their subdivisions

1. interphase, the phase between mitotic events ,

Interphase has

  • three main phase – G1,   S ,   G2 and also – G0, GH

2. Mitotic phase – Mitosis has two stages


  • prophase
  • metaphase
  • anaphase
  • telophase

2. Cytokinesis

Characteristic of stages

Interphase :

Interphase comprised of three periods:

1.G1 Phase Gap Phase

2.S Phase Synthesis Phase

3.G2 Phase Gap Phase

1.G1 Phase : 

  • This is the longest phase
  • Time of major growth.
  • Intensive cellular synthesis
  • Nucleus produces rRNA, mRNA and tRNA and ribosomes are synthesised.
  • Cell produces structural and functional proteins
  • Creating of new organelles
  • Cell metabolic rate high
  • Substances are produced to inhibit or stimulate onset of next phase.
  • production of proteins for DNA synthesis

2.S Phase Synthesis Phase :

  • S Phase Synthesis Phase
  • DNA is replicated 2n2c2n4c

      n – quantity of chromosomes

      c – quantity of DNA (or chromatids)

  • Protein molecules called histones are synthesised and cover each DNA strand
  • Each chromosome becomes double (contains two DNA molecule) – consists of two sister chromatids
  • Centrioles are doubled and become visible

G2 Phase Gap Phase :

  • G2 Phase Gap Phase
  • The shortest phase
  • cells continue to grow and prepare for mitosis
  • Produces many proteins and molecules needed for mitosis.
  • Energy stores are increased
  • A cell that is ready to divide is called: A mother Cell

the mitotic phase, where the mother cell divides into two genetically identical daughter cells

Note : if Need for mam asking or According to you 

G0 phaseresting phase

At this stage, the cell has not started to divide.

This step can last a few hours, a few days, a few years, or a lifetime.

The stem cells are in the G0 phase

These cells are in red bone marrow When the cell is signaled to reproduce, it moves to the next phase, G1 phase

Mitosis has two stages

Karyokinesis: 4 Part  or phage 


In the nucleus 

  • Chromosomes coil
  • Chromosomes pair
  • Chromosomes move towards

the center of the cell

  • Nuclear envelope disappears
  • Nucleolus disappears

In the cytoplasm

  • Centrioles migrate to opposite sides of the cell via microtubules.
  • Begin to form mitotic spindles


  • Chromosomes line up along the equatorial plane of the cell.



  • Enzymes dissolve bonds the centromere and allows the chromatids to separate – once separated are called daughter chromosomes
  • Chromotids travel towards the poles of the cell. The spindle fibers pull/push the chromatid towards the opposite ends of the cell
  • When chromosomes reach ends of cells anaphase has ended



  • A new nucleus begins to re-form
  • Chromosomes begin to uncoil.
  • Nucleoli re-appear
  • Spindle fibers disappear
  • When a new nuclear membrane forms telophase has ended


  • Organelles divide up and the cell is divided.
  • Creates 2 exact duplicate cells

Cytokinesis proceeds differently in plant cells than in animal cells

Cell Division Phage


102. Meiosis. Characteristic of stages

Meiosis :

  • Meiosis is the reduction of a cell’s chromosome number from diploid to haploid by 2 consecutive cell divisions.

1 (2n4c) → 4 (nc)

  • Meiosis is a special type of cell division necessary for sexual reproduction in eukaryotes
  • The cells produced by meiosis are either gametes (animals) or otherwise usually spores (рlants).
  • Meiosis has halved the number of sets of chromosomes. When two gametes fuse during fertilisation, the number of sets of chromosomes in the resulting zygote is restored to the original number.
  • The steps leading up to meiosis are similar to those of mitosis – the centrioles and chromosomes are replicated.
  • Meiotic division occurs in two stages, meiosis I and meiosis II
  • Meiosis I separates homologous chromosomes, producing two haploid cells (n chromosomes, 23 in humans), and thus meiosis is referred to as a reductional division.
  • In meiosis I, the phases are analogous to mitosis: prophase I, metaphase I, anaphase I, and telophase I

Prophase I  is divided into 5 stages:







  • The first stage of prophase is called stage “thin threads”.
  • In this stage individual chromosomes condense into visible strands within the nucleus


  • The homologous chromosomes align side-by-side in a process of pairing called synapsis.
  • A special structure called a synaptonemal complex forms at the regions where the chromosomes are aligned.
  • The paired chromosomes are called bivalent or tetrad chromosomes.


  • The pachytene -stage, “thick threads”,
  • is the stage when chromosomal crossover (crossing over) occurs. Nonsister chromatids of homologous chromosomes may exchange segments.
  • At the sites where exchange happens, chiasmata form.
  • The exchange of information between the non-sister chromatids results in a recombination of information


the synaptonemal complex degrades and homologous chromosomes separate from one another a little.


  • The nuclear membrane disappears.
  • The chromosomes attached to spindle fibers begin to move.

Metaphase I

  • Tetrads line up along the equatorial plane.

Anaphase I

  • Homologous pairs of chromosomes separate from the tetrad. (Sister chromatids remain together)
  • Homologous pairs of chromosomes move towards the ends of the spindle fibers.

Telophase I

  • Homologous chromosomes reach the ends of the cell
  • The cytoplasm divides
  • Chromosomes usually do not uncoil.
  • Nuclear membrane may re-form


  • Cell plate or contractile ring form and divide cell
  • Cells may enter a period of rest known as interkinesis or interphase II.
  • No DNA replication occurs during this stage.

Meiosis II

Meiosis II is the second part of the meiotic process, also known as equational division.

Meiosis II is similar to mitosis.

Prophase II

  • Spindle fibers form and attach to the centromere (remember the chromosomes are still coiled)
  • If nuclear membrane reformed it breaks down.

Metaphase II

  • Chromosomes line up along the equatorial plane.

Anaphase II

  • Sister chromatids separate.
  • Move on the spindle fibers towards the ends of the cell.

Telophase II

  • Nuclei are reformed


  • Cell plate or contractile ring form and divide cell

Messios 1

Messios 2

103 . Differences and similarity between a mitosis and meiosis

Differences :

Diffrence between

Similarity between a mitosis and meiosis


104. Gametogenesis. Characteristic of stages

Gametogenesis : Sexual reproduction starts in the formation of gametes.

The process of formation of the male and female gametes is known as Gametogenesis .

Characteristic of stages :

  • Spermatogenesis

sperm-formation occurs in the testes of males

  • Oogenesis 

egg-formation occurs in the ovaries of females

Period Gametogenesis 

Period Gametogenesis

Period Reproduction

Period Reproduction

Period Growth 


Period Growth

Period Maturing

Period Maturing


Period Formation 

Period Formations

104. Gametes: structure and functions. Egg and sperm.

Gametes :

Gametes are an organism’s reproductive cells. They are also referred to as sex cells. Female gametes are called ova or egg cells, and male gametes are called sperm. Gametes are haploid cells.

Structure of egg 

Structure of Egg

Structure of Sperm


Structure of Sperm

Function of Egg 

The function of the ovum is to carry the set of chromosomes contributed by the female and create the right environment to enable fertilization by the sperm. Ova also provide nutrients for the growing embryo until it sinks into the uterus and the placenta takes over.

Function of Sperm

It function Fertilization to egg


The head of the nucleus contains an enlarged haploid nucleus, the frontward part of which is enveloped by a cap-like structure, acrosome.

Function of Acrosome

The enzymes form the most of acrosome which helps in the fertilization of the ovum.

Function of mitochondria

Numerous mitochondria are present in the middle piece which creates energy for the movement of tail or flagellum.

Function of Tail

It is responsible for the vigorous movement of sperm towards ovum.


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