INTRODUCTION: CELL
All living forms are composed of microscopic units called "Cells"
A cell is the basic structural and fictional unit of all life forms.
Study of structure and composition of cell is called 'Cytology' Cell was first observed by Robert Hooke na dead cork shoe in the year 1665 He described about this in his book 'Micrographia'
The word cell was derived from the Greek wordCellule which means small rooms
First being cell was delivered by AV. Leeuwenhoek.
The term protoplasm was coined by Purkinje In 1839
Protoplasm was discovered by Telix Dujardin and named as surcode
Its consistency differs under different conditions. It exists in sol get states
N. Grew (1682) proposed the cell concept which states that the cell is the unit of structure of organisms.
Radolf Virchow (1858) proposed that new cells arise from the preexisting cells.
Knoll & Ruska (1932) designed the electron microscope which was employed to study the ultrastructure of cells and various cell organelles.
CELL THEORY
Two biologists 'Schleiden and Schwann' gave the Cell theory' which was later on expanded by Rudolf Virchow'. Cell theory states that
(i) all living organisms are made up of cells.
(ii) cell is the functional and structural unit of life
(iii) all cells arise from pre-existing cells.
⚫ Viruses are the exceptions to cdl theory
TYPES OF CELL & ORGANISM
(a) On the Basis of Number of Cells Organisms can be categorised as
(i) Unicellular organisms : These are the organisms which are made up of single call only This single cell performs all the vital body functions of an organism, eg. Amoeba
(ii) Multicellular organisms: These are the organisms which are made up of mimercia cells. These cells then combine to form a organ and group of organs performing diffe functions forms an organ system which are forms an organism, eg plants and animals
(b) On the basis of type of organisation, cells are of two types:
(i) Prokaryotic cells: These are primitive and incomplete cells. they have less developed
nucleus without nuclear membrane & nucleolus, eg., Bacteria
(ii) Eukaryotic cells. These are well-developed cells. They have advanced nudeus with moder membrane and nucleolus eg., Plants and animal
CELL SHAPE
Cells are of variable shapes and sizes. Their shape is according to the function. Generally cells are spherical but they may be elongated (nerve cell), beanched (pigmented), discoidal (RBC), spindle shaped (muscircell), etc.
CELL SIZE
Size of cells is variable depending upon the type of organism. Some are microscopic while some are viable with naked eyes.
Their sine may vary from 0.2 um to 18 cm.
Size of a typical cell in a multicellular organism ranges from 20 to 30 mm.
The largest cell is an ostrich egg (15 cm in diameter with shell & 8 cm without shell)
The longest cell is the nerve cell (upto 1 m. or more.)
Smallest cells so far known are PFLO eg.,mycoplasma (0.1 m in diameter)
The Haman egg is 0.1 mm in diameter.
COMPONENTS OF CELL
There is an occurrence of division of labour within a cell as they all got certain specific components called organelles cach of them performs a specific function.
The three basic components of all the cells are
(1) (plasma Membrane) (2) Nucleus
(3) Cytoplasm
CELL MEMBRANE
(a) Cell Membrane
Cell membrane a also called Plasma Membrane or Plasma lemma
Plasma membrane name was given by Nageli
It is the limiting boundary of each cell which separates the cytoplasm from its surroundings.
It is found in both plant and animal cells.
It is the outermost covering of a cell in case of animals and lies below the cell wall in case of plants. .It is made up of proteins and lipids where proteins are sandwiched between hilayer of lipids.
Plasma membrane is selectively permeable in nature. It allows or permits the entry and exit of some materials in and out of the cell
Singer and Nicholson gave the laid mosaic model of plasma membrane. According to them it consists of a protein layer sandwiched between two layers of lipids. It is in a quasi-fluid state. It is 75 A thick.
It is flexible and can be folded, broken and reunited
(i) Functions of plasma membrane
(A) It regulates the movement of molecules inside and outside the cell.
(B) It helps in maintaining the distinct composition of the cell.
(ii) Transportation of molecules across the plasma membrane: This can be done by the following ways
(A) Diffusion: Movement of solutes or ions from higher concentration to lower concentration is called diffusion. It does not require energy therefore it is called passive transport.
(B) Osmosis: The movement of solvent or water from higher concentration (solvent) to lower concentration (solvent) through a semipermeable membrane is called osmosis. Or the movement of solvent or water from lower concentration to higher concentration of solution through a semipermeable membrane is called osmosis Osmosis can also be called diffusion of solvents.
Endosmosis: Movement of solvent into the cell is called Endosmosis
Exosmosis: Movement of solvent outside the cell is called Exostosis
(iii) Types of solution on the basis of concentration:
(A) Isotonic solution: When the concentration of the solution outside is equal to the concentration of cytoplasm of the cell, the solution is said to he isotonic.
(B) Hypertonic solution: When the concentration of the solution outside the cell is more than that inside the cell, the solution is said to be hypertonic. Due to this, cell loses water and becomes plasmolysed.
(C) Hypotonic solution: When the concentration of the solution outside the cell is lesser than that of cytoplasm of cell, the solution is said to he hypotonic. Due to this cell swells up and bursts
Mediated Transport
Transport of materials across the plasma membrane with the help of carrier proteins is called mediated transport
Types of mediated transport
Mediated transport is of the following two
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(i) Facilitated transport
(ii) Active transport
(i) Facilitated transport: In this case, transport proteins (e.g. permeates) assist molecules to diffuse through the membrane down the concentration gradient Le from the region of higher concentration is the region of lower concentration across the membrane It is, therefore, termed as facilitated diffusion. No, cellular energy is used in such transport A carrier protein combines with a specific substance (eg glucose) to be transported and moves it down the concentration gradient from one side of membrane to another through a channel formed by it
In liver and red blood cells, facilitated transport moves glucose across the cell membrane by specific carrier protein molecule in both directions depending upon whether glucose concentration is higher inside or outside the membrane
(ii)Active transport: In this case, carrier proteins move substances against the concentration gradient, Le from lower concentration to higher concentration. This uphill transport valves work and always requires energy provided by ATP (adenosine triphosphate)
Mechanism of active transport of materials is described below:
(i) The carrier protein has a binding site for ATP in addition to the binding site for the substrate As the ATP molecule hinders the carrier protein. It is hydrolysed to ADP
(ii) The energy so se line hrings the substrate binding site of the carrier protein to the surface of the membrane. The substrate present in the medias joins the carrier protein at substrate binding site to form carrier substrate complex.
(iii) The substrate band carrier protein undergoes conformational changes and carries the strate through a channel in it to the cytoplasmic side of the membrane
(iv) Now, the form of binding site changes and the substrate is released. The carrier protein gains its original form and is ready to transport another molecule of substrate.
There are many active transport systems in the cell Among these, sodium-potassium exchange pumps are prominent. It maintains sodium and potassium gradients in cells and the surrounding extracellular fluid
Importance of active transport: The Na-K exchange pump plays the following rules
(i) It helps in maintaining a positive charge on the outside of the membrane and negative charge on the inside (resting potential)
(ii) it helps in nerve impulse conduction
(iii) It helps in muscle contraction
(iv) It helps in urine formation in kidney tubules
(V) It helps in salt excretion in marine birds
(vi) It helps in controlling water contents of the cell.
Bulk Transport: Animal cells can also actively take in turn materials in by utilising energy Such materials include macromolecules, lipid droplets and solid particles. hems of this size cannot cross the phospholipid bilayer by diffusion or with the help of transport proteins Special processes are involved in the transport of such large quantities of materials.
These include endocytosis (phagocytosis) and exocytosis.
Endocytosis: The term endocytosis refers to the invagination of a small region of plasma membrane, and ultimately forming an intracellular membrane-bound vesicle. Endocytosis is not shown by plant cells because of their rigid cell wall and internal turgor pressure Depending upon the intake of fluid droplet or solid particles, endocytosis is of two types:
i) Pinocytosis (ii) Phagocytosis
(i) Pinocytosis: The intake of a tiny droplet of extracellular fluid by a cell through the cell membrane is called pinocytosis. It is also, therefore, termed as cell drinking. It was first observed in Amoeba In this process, a small region of plasma membrane invaginates and the thuid droplet passes into the pocket so formed. This pocket is called covels. The pocket derpens and finally nips off as a fluid-filled vacuole called pinosome or pinocytosis vesicle.
(ii) Phagocytosis: Phagocytosis is the intake of solid particles by a cell through a cell membrane. It is also called cell eating. Phagocytosis is the major feeding method in many unicellular organisms (eg Amoeba) and simple metazoa (eg sponges). An area of the plasma membrane in contact with the food particles). The contact induces the cell membrane to put tiny protoplasmic projections the pseudopodia, around the food particle). The pseudopodia meet on the other side of the food particle(s) and fuse. In this way, an internal vacuole. called phagosome, containing food particle(s) is acquired
Exocytosis: Exocytosis is the process that involves fases membrane to extrude its contents to the surrender of membrane of the exocrotic vesicle with the plas medium.
This process is also called cellular vomiting w ephagy and the vesicles that turn out the material are termed as exocytosis vesicles.
Exocytosis process is responsible for
(i) removal of undigested food left in the food ve in the cells.
(ii) secretion of substances such as hormones and enzymes
(iii) replacement of internalised membrane by the fi of exocytotic vedeles with the cell membrane
(b) Cell Wall:
It is the outermost covering of the plant cells.
It is absent in animal cells.
Cell wall is rigid, strong, thick, porous and noo living structure. It is made up of cellulose and hemicelluloses Cell walls of two adjacent a are joined by a layer called middle lamella made up of calcium and magnesium pectate.
Functions of cell wall:
It provides definite shape to the cell
It provides strength to the cell
It is permeable and allows entry of molecules of different sizes
It is antigen specific.
It has the characteristics of repair and regeneration.
(c) Nucleus
Nucleus is the most important cell organs which directs and controls all its cellular activities.
It is called "Headquarter of the cell
It was discovered by Robert Brown in 1831
In eukaryotes a well-defined nucleus is penet while in prokaryotes a well-defined nucleus
absent.
Prokaryotes contain a primitive nucleus.
It has a double-layered covering called a membrane.
Nuclear membrane has pores which regulate the movement of materials is and out of the nucleus.
Besides nuclear membrane, nucleus also co nucleolus and chromatin material and the substance filled inside the nucleus is made plus or karyolymph
Chromosomes or chromatin material count of DNA which stores and transmits hereditary information for the cell to function, grow and reproduce.
Function of the nucleus:
(A) It controls all the metabolic activities of the cell and regulates the cell cycle.
(B) It helps in transmission of hereditary characters from parents to offsprings.
CYTOPLASM
Cytoplasm was discovered by Kolliker in 1862.
It is the site of both biosynthetic and catabolic pathways.
It can be divided into two parts:
(i) Cytosol: Aqueous soluble part contains various fibrous proteins forming cytoskeleton.
(ii) Cell organelles: They are the living part of the cells having definite shape, structure and function bounded by plasma membranes.
ENDOPLASMIC RETICULUM
It is the network of membranes present in the cytoplasm.
It was discovered by Porter, Claude and Fullam.
These are present in all cells except prokaryotes and mammalian erythrocytes.
They are made up of three components:
(i) Cisternae: These are long, flattened, parallely arranged, unbranched tubules. These form successive layers of ER. These are found in cells which are active in protein synthesis and are 40-50 pm in diameter.
(ii) Vesicles: These are round or spherical in shape. They are found in synthetically active cells.
(iii) Tubules: Types of ER
Endoplasmic reticulum is of two types
Smooth ER
-Made of tubules mainly
-Helps in steroid, lipids and polysaccharide synthesis
-Ribosomes are absent
-Helps in membrane biogenesis
Rough ER
-Made of clstenae and vesicles
-Helps in protein synthesis
-Contains ribosomes on its surface
Functions of ER:
(i) It is the only organelle which can move within a cell; so it serves as a channel for the transport of materials between various regions of cytoplasm and nucleus.
(ii) It also functions as a cytoplasmic framework to provide space for some of the biochemical activities. of the cell.
(iii)It forms an endoskeleton It helps in synthesis of fats, steroids, cholesterol, etc.
(iv) It contains secretory proteins.
(v) SER plays a crucial role in detoxification of drugs and poisonous by-products.
Golgi apparatus
Golgi apparatus of a system of membrane bounded vesicles arranged parallel to each other in stacks called Cisternae along with some large and spherical vacuoles. .
It was discovered by Camillo Golgi.
In plants, Golgi apparatus is called Dictyosome.
It is single membrane bounded.
It is absent in prokaryotes, mammalian RBCs and sieve cells.
Functions of Golgi Apparatus:
(i) It helps in formation of lipids.
(ii) It helps in formation of middle lamellae.
(iii) It is secretory in nature,
(iv) It helps in melanin pigment synthesis.
(v) Lipids and proteins synthesised in endoplasmic reticulums are packed at Golgi complex. They provide the site for assembly of new membrane material.
MITOCHONDRIA
It is a rod-shaped structure found in the cytoplasm of all eukaryotic cells except mammalian RBCs.
These are also absent in prokaryotes.
It was first seen by Kolliker in insect cells.
Maximum mitochondria are found in metabolically active cells. .
It is also called 'Power House of the Cell' or the 'Storage Battery:
It is a double membranous structure where the outer membrane has specific proteins. While the inner membrane is folded inside to form chambers called Cristae. 'Cristae' are the infoldings of the inner mitochondrial membrane that possesses enzymes for respiratory cycles like the Kreb Cycle. ATP synthesising units are called Oxysomes oF particles.
Space between inner and outer mitochondrial membranes is called Peri Mitochondrial space. The fluid present in mitochondria is called matrix.
Functions:
(i) Its main function is to produce and store the energy in the form of ATP.
(ii) It is the site of Kreb cycle of respiration.
(iii) Oxysome contains enzymes for ATP production.
(iv) Matrix contains enzymes for Kreb cycle.
RIBOSOMES
Ribosomes are the sites of protein synthesis.
All structural and functional proteins (enzymes) coded by the nuclear DNA are synthesised upon cytoplasmic ribosomes. The DNA codes are transcribed into messenger RNA (mRNA) molecules in the chromosomes of the nucleus. mRNA molecules diffuse out into the cytoplasm and each gets attached to several ribosomes which thus form a group called polyribosome or polyribosomes. In this way each mRNA molecule brings about polymerisation of specific protein molecules, with the help of ribosomes from amino acid molecules found in the Cytosol.
PLASTID
It is double membranous discoidal structure found only in plant cells.
Term plastid was given by Haeckel. .
Besides being discoidal or rhombic in plant cells, they occur in variable shapes like in algae; they can be 'U' shaped, spiral, coiled, ribbon shaped, etc.
(a) Chloroplast has the following two parts:
(i) Grana: It constitutes the lamellar system. These are found layered on top of each other, these stacks are called Grana. Each granum of the chloroplast is formed by superimposed closed compartments called Thylakoids.
Function: They are the sites of light reaction of photosynthesis as they contain photosynthetic pigment chlorophyll. In each thylakoid Quantasomes are present which are called Photosynthetic units. Each quantasome possesses 230 chlorophyll molecules.
(ii) Stroma: It is a granular transparent substance also called matrix. Grana are embedded in it. Besides grana they also contain lipid droplets, starch grains, ribosomes, etc.
Function: This is the site of dark reaction of photosynthesis. Also helps in protein synthesis due to presence of ribosomes.
VACUOLES
These are membrane-bounded regions in the cytoplasm, containing water and other substances.
They are bounded by a single membrane called Tonoplast.
In animal cells vacuoles are smaller in size and numerous, while in plant cells a single large vacuole is found which occupies about 90% of the volume
Functions of Vacuoles:
It helps in maintaining osmotic pressure in a cell.
It stores toxic metabolic products of plant cell.
It contains various coloured pigments like anthocyanins.
LYSOSOMES
(Discovery: Christian de Duve) (Lyso= digestive, some = body)
These are tiny sac-like granules containing enzymes of intracellular digestion.
They are bounded by a single membrane.
They occur in animal cells and a few plant cells.
They contain hydrolysing enzymes called acid hydrolysis.
Function of Lysosomes:
Their main function is autophagy = digestion
They are a kind of waste disposal system.
They help in digesting foreign materials and worn-out cells.
During disturbances in cellular metabolism, i.e. in case of cell damage, lysosomes burst and their enzymes are released into the cytoplasm and they digest their own cell, so they are also called 'Suicidal Bags.
PEROXISOMES
These structures were first described by Rodhin (1954).
In plant cells, they were first observed in germinating seeds by Tolbert (1969).
The term 'peroxisomes' was first used by de Duve and also called uricosomes.
Peroxisomes have ovoid or granular structures, limited by a single unit membrane and have a diameter of 0.1-1.04 μm.
In green leaves of C, plants, peroxisomes carry out photorespiration.
In animal cells they carry out lipid metabolism.
They contain important enzymes as oxidases (peroxide producing enzyme), peroxidases and catalases (which break down toxic peroxides to water and oxygen).
GLYOXYSOMES
Discovery: Beevers and Breidenbach (1967).
They are about 0.5 to 1 μm in size and are surrounded by a single unit membrane.
They are found in plant cells, particularly in germinating fatty seeds, e.g. Ricinus (castor) and groundnut in which fat is being converted into carbohydrates by a process called glyoxylate cycle.
Glyoxysomes contain important enzymes, isocitrate, lyase, maltase and Synthetase along with several others.
Structure of glyoxysomes is similar to peroxisomes.
CYTOSKELETON (CILIA AND FLAGELLA)
In many eukaryotic as well as prokaryotic cells of both plants and animals, a cytoskeleton has been reported in recent years.
The elements of this cytoskeleton are proteins
The cytoskeleton consists of the following two elements within a cell.
(a) Microtubules (b) Microfilaments
.
Cilia and flagella of eukaryotic cells are microscopic, contractile and filamentous.
Cilia is shorter than flagella and are numerous.
MICROTUBULES AND MICROFILAMENTS
(A) Microtubules:
Introduction:
These are cylindrical structures formed by the polymerisation of two-part subunit of globular protein tubulin into helical stacks. .
Ultrastructure:
Microtublues radiate from each end of the cell which helps in the movement of chromosomes.
These are found in many plant and animal cells.
Function of Microfilaments:
Microtubules help in the structure and movement of cilia and flagella.
It also plays a role in cell division.
(B) Microfilaments:
Ultrastructure:
● These are long and helically intertwined polymers. Microfilaments are made up of protein actin.
Function of Microfilaments:
These filaments help in cell movement and in formation of cell furrow and cell plate.
CELL DIVISION
Cell multiplication is needed for the growth, development and repair of the body. Cell multiplies by dividing itself again and again; this process is called cell division.
Cell divisions are of two types:
(a) Mitosis
(b) Meiosis
MITOSIS
Stages of Mitosis:
Interphase, prophase, metaphase, anaphase and telophase are roughly the five stages of mitosis.
(a) Interphase:
The period between one cell division and the next is called interphase, in which the cell is said to be in the resting stage.
Interphase, however, includes three phases, i.e. Gl-phase, S-phase and G2-phase. GI-phase is a resting phase or pre-DNA synthesis phase.
During S-phase, DNA synthesis takes place. G2-phase is again a resting phase and it may be described as a post-DNA synthesis phase.
The main mitosis division takes place during M-phase which involves prophase, metaphase, anaphase and telophase.
(b) Prophase:
Prophase is actually the first and the longest phase in the mitosis cell division.
Chromosomes become visible in the nucleus as short, thick and helically coiled threads.
Each chromosome splits into two chromatids joined at the centromere.
Nuclear membrane dissolves away.
Nucleolus also dissolves away and finally disappears.
(c) Metaphase:
It is the second stage in the mitotic cell division. . Nuclear membrane and nucleolus disintegrate and they are lost completely.
Spindle tubules start appearing, and these tubules get attached to chromosomes at the centromeres.
Chromosomes move actively, become shorter and thicker and arrange themselves in the centre or on the equator of the spindle.
Separation of the two chromatids from each chromosome also begins at the end of metaphase.
(d) Anaphase:
It is the third stage of mitosis.
Chromatids separate from each other at centromeres.
Separated sister chromatids, each with a centromere, are called daughter chromosomes. They move to the ends of opposite poles of the spindle.
Daughter chromosomes appear in V, U or J-shaped during their movement towards the poles.
During the late anaphase stage, the cell starts constricting in the middle region.
(e) Telophase:
Telophase is the last stage of mitotic cell division.
VARIOUS STAGES OF MITOSIS
Chromatids or daughter chromosomes are now at the end of the spindle.
Nuclear membranes and nucleoli reform around each group of chromosomes and thus two new nuclei are reorganised at each pole.
Chromosomes begin to lose their compact structure.
Spindle apparatus disappears gradually.
KARYOKINESIS
Division of nucleus is called karyokinesis and the process of the division of cytoplasm is called cytokinesis.
In animal cells, a circular constriction appears at the equator, the constriction deepens and eventually divides the cell into two.
In plants, there is no constriction. A cell plate or new cell wall forms across the cell resulting in the separation of two daughter cells.
SIGNIFICANCE OF MITOSIS
Mitosis occurs during the growth and development of multicellular plants and animals.
Mitosis ensures that the two daughter cells inherit the same number of chromosomes.
It helps the cell in maintaining proper size.
In unicellular organisms mitosis helps in asexual reproduction during which two or more individuals arise from the mother cell.
If mitosis becomes uncontrolled it may cause tumour or cancerous growth.
MEIOSIS
Meiosis is also called reduction division because the chromosomes in this division are reduced from the diploid to the haploid number.
Meiosis occurs in all organisms which reproduce sexually.
Meiosis produces haploid sex cells from diploid I cells.
Meiosis involves two cell division, viz., meiosis I and meiosis II.
In meiosis I, the replicated homologous chromosomes pair with each other on the spindle, cross over and then separate to either end of the spindle.
On the other hand, in meiosis II, the chromatids of each chromosome move towards centromere, and these chromatids separate each end of the second spindle.
As a result of this process, a diploid cell divides to form four haploid cells.
First Meiosis Division
First meiosis division is actually the reductional division. It consists of prophase I, metaphase I, anaphase I and telophase I.
(a) Prophase I:
Prophase I is the longest phase of meiosis and includes five sub-phases.
(i) Leptotene:
This is the first stage in the first meiosis prophase.
In this stage, the chromosomes appear as separate thin and fine thread-like structures
(ii)Zygotene
Homologous chromosomes come together or arrange themselves side by side in pairs to form bivalents.
This pairing of homologous chromosomes during zygotene in the first meiosis prophase is called synapsis.
(iii) Pachytene:
The bivalents or chromosomes become shorter thicker.
They replicate or split into chromatids but remain linked at the centromeres.
Each bivalent thus now consists of four chromatids
Crossing over between non-sister chromatids of homologous pair takes place.
(iv) Diplotene:
The centromeres of paired bivalents move away from crossing over can also be seen.
The points in the bivalent where the two chromosomes appear to be joined and crossed is called chiasmata.
Chiasmata formation and crossing over are the distinguishing features of diplotene.
(v) Diakinesis:
This is the last stage of first meiosis prophase.
The chromosomes become shortest and thickest.
Terminalisation of chiasmata occurs.
Nuclear membrane starts disintegrating. Nucleolus also disintegrates. Diakinesis is followed by metaphase I.
(b) Metaphase I:
Nuclear membrane disappears completely at the beginning of metaphase I.
Pairs of homologous chromosomes are lined up at the centre.
Spindle apparatus starts appearing. Few spindle fibres get attached with the centromeres of chromosomes.
Metaphase I change into anaphase I.
(c) Anaphase I:
Partners of homologous chromosomes separate completely and move to opposite poles of spindle during anaphase I, which in turn changes into telophase I.
(d) Telophase I:
The separated partners of homologous chromosomes collect at the poles of the spindle and nuclear membranes form around them. Two daughter haploid nuclei are thus formed. The chromosomes lengthen as they uncoil. Nucleoli start reappearing
Second Meiosis Division
Like mitosis, the second meiosis divisions also consists of four phases, i.e. prophase II, metaphase II, anaphase II and telophase II.
Prophase II
In both the haploid nuclei, each chromosome splits up into two chromatids with a single functional centromere. The nuclear membrane and nucleolus disintegrate partially or completely.
Metaphase II:
The chromatids arrange themselves at metaphase plate or spindle. .
Anaphase II:
During anaphase II, the centromere splits. The two chromatids belonging to each chromosomes may now be called chromosomes and pass to the two opposite poles of spindle. .
Telophase II:
The haploid set of chromosomes at two different poles of spindle uncoils and forms chromatin material. Nuclear membrane forms around each haploid set of chromosomes. Nucleolus also reappears.
Significance of Meiosis
Meiosis results in the formation of haploid gametes (sperm and ovum).
The phenomenon of crossing over provides new combinations of chromosomes and hence new combinations of genes and also of characters in offspring.
The four chromatids of a homologous pair of chromosomes are passed on to four different daughter cells. This is called the segregation of chromosomes.
This causes genetic variations in daughter cells. Failure of meiosis leads to the formation of diploid gametes which on fusion form polyploids.
Special Note:
Beside mitosis and meiosis, there is also a third type of division. It is called amitosis. It is a direct division of the nucleus by constriction.
Practicec Exercise-l
1. The first person to observe a cell under microscope was
(a) M. Schleiden
(c) Robert Hooke
(b) T. Schwann
(d) A.V.Leeuwenhoek
2. Cell theory was propounded by
(a) Morgan
(b) Halden
(c) Schleiden and Schwann
(d) Robert Hooke
3. The word cell was coined by
(a)-Robert Hooke
(c) Cuvier
(b) Weismann
(d) Darwin
4. Nucleus discovered by
(a) Robert Hooke
(c) Dujardin
(b) Robert Brown
(d) Purkinje
5. Smallest cells so far known are
(a) Bacteria
(b) Blue-green algae
c) PPLOS
(d) human egg
6. Which of the following is the longest cell of animal kingdom?
(a) Bacteria
(b) Nerve cell
(c) Virus
(d) Muscle cell
7. Which one of the following prokaryotic cell?
(a) Typical paint cell Bacteria
(b) Typical animal cell
(c) Bacteria
(d) None of these
8. What is cytology?
(a) Study of cytoplasm
(b) Study of structure and composition of cell
(c) Study of animal cell only
(d) Study of cell only
9. Who coined term protoplasm?
(a) Leeuwenhoek
(b) Purkinje
(c) Robert Hooke
(d) Robert Brown
10. Cell is
(a) functional unit of life
(b) structural unit of life
(c) hereditary unit of life
(d) all of the above
11. Plasma membrane is made up of
(a) proteins and carbohydrates
(b) proteins and lipids
(c) proteins and nucleic acids
(d) proteins, some nucleic acids and lipids
12. Plant cell wall is mainly composed of
(a) sugars (c) proteins
(b) cellulose (d) lipids
13. ER was discovered by
(a) Robert Brown
(b) Porter
(c) A.V. Leeuwenhoek
(d) Schwann
14. A solution is said to be hypotonic when
(a) concentration of medium is higher than that of the cell
(b)concentration of medium is equal to that of the cell
(c) concentration of medium is lower than that of the cell
(d) none of the above are correct.
.
