Grade 11 Botany Note

Cell Biology

Cell: Cell is the basic structural and functional unit of living organism. Just as bricks is unit structure of house.

Each cell is capable of performing a basic function of life. Such as reproduction, respiration, excretion, growth, and so on.

All the life activates exhibited by living organisms are the combined action of these cells. In unicellular organisms single cell perform all life activates. Hence cell is considered as structural and functional unit of life.

Discovery of the cell:

 

1. In 1665, Robert Hooke identified a cell in a slice of cork. He observed many small, hollow, Honey-comb like compartments.

2. In 1675, Antony van Leuwenhoek, such scientist observed green coloured body in a cell of plant. It is name as chloroplast.

3. In 1831, Antony van Leeuwenhoek, such scientist observed green coloured body in a cell of plant. It is name as chloroplast.

3. In 1831, Robert Brown observed a dense spherical body. It is name as nucleus.

4. In 1838, a German botanist, Matthias Schleiden concluded that all plants are composed of cells.

5. In 1839, the German zoologist Theodor Schwann came to the same conclusion about animals

 

The cell theory:

The combined views of Schleiden and Schwann led to the formulation of the cell theory.

    The cell theory is also known as the cell doctrine or cell principle. It state:
1.All living organisms’ are made up of cells and their products.

2. All cells arise from pre- existing cell.

3. Cells are the structural and functional unit of all living organisms.

4. Cell is hereditary cells.

5. All cells are basically alike in chemical composition and metabolic activities.

6. Life passes from one generation to other generation in the form of a living cell.

 

Exception of cell theory:

It is one of the most fundamental and universally applied theories except in few cases;

1. Virus is the living organism but it lacks the cellular organization.

2. Bacteria, blue green algae are prokaryotic and hence are without true cellular structures.

3. Kollikar (1843)- He observed a jelly like substance named as cytoplasm.

4. Von Mon (1846) - He suggests that cytoplasm and nucleus are called by combine named protoplasm.

5. Purkinje (1839)- Protoplasm (living content)

 

UNICELLULAR AND MULTICELLULAR ORGANISM:

1. Unicellular organisms: Organisms which are made from just one cell are known as unicellular organisms.

In a unicellular organism, the single cell performs all metabolic functions such growth, respiration, digestion, etc.

 

2. Multicellular organisms: Organisms which are made from many cells are called multicellular organisms.

A different cell performs special function. For e.g.;

1. Neuron produces nerve impulse only.

2. Germ cells produce gametes only.

 

STRUCTURE OF CELL:

Cell is a microscopic mass of protoplasm     surrounded by the cell membrane. The protoplasm

1. Cell size: Smallest cell are those of micro-organism. They have the size of 0.1 - 0.5 um.

# a unicellular eukaryotic cell - 1u- 1mm

#a cell of multicellular organism- 5-100um

#Human erythrocytes - 1-8um in diameter

#A striated muscle cell - 1-40mm long and 30-80 u in diameter.

# Bacterial cell- 1u-0.001mm

#Among plants- larger cell occur in algae

# Internodal cell of chara-1 - 10cm in length.

 In general eggs are large sized cell.

#Human egg is slightly over 0.1mm or 100 um in diameter.

#Hen egg is 60x45mm

#Egg of Ostrich is 170x150mm in diameter.

 

2. Cell shape: Depending upon function needs of organism and environmental condition the shape of cells vary widely. They may be spherical, oval, elongated,

Cylindrical, tubular, rod-like, polygonal, discoidal or irregular in shape.

         There are two types of cell on the basis of cell organelles and organization of DNA.

 

1. Prokaryotic cell:

Fig: Prokaryotic cell         

Prokaryotic cell is primitive type of cell and it is found in blue green algae and bacteria. A cell in which genetic material is not enclosed by a membrane to form and organized nucleus but lies freely in the cytoplasm. Blue green algae and bacteria are photoautrophs. Their photosynthesis thylakoid lies freely in cytoplasm but they are not organized chloroplast.The photosynthetic thylakoid are sides of photosynthetic lamellae.Certain lamellae structure known as mesosome which are found to carry on respiration. Ribosome are found lying freely in cytoplasm and it has a sedimentation coefficient of 70 and are known as 70s type. The cell lacks membrane bound cell organelles like mitochondria, vacuole, Golgi body, centrosome, lysosome, plastids, and endoplasmic reticulum.

Nucleus is not true i.e. there is no genetic material which is not enclosed a definite membrane. The chromatin materials lying freely in center of cytoplasm in the form of tangled mass.The DNA of prokaryotic cell is non-histone type and circular in nature.

Cell wall is non-cellulosic and it mainly contains carbohydrate and amino acids. It is mainly made up of murein and peptidoglycan along with mumaric acid. Flagella are present in some bacteria only and bacterial flagella are single stranded.

2. Eukaryotic cell:

Fig: Eukaryotic cell

 

Eukaryotic cell is advanced type of cell and found in algae, fungi, seed plants and animals. They comprises all those cell which have well organized nucleus i.e. chromatin materials enclosed by a definite nuclear envelop. Genetic material contains DNA with histone protein to form well organized chromosomes. Membrane bound cell organelles like endoplasmic reticulum, plastids, mitochondria, chloroplast, lysome, vacuole, etc. all is present. Lysosome are found lying freely in cytoplasm or associated with endoplasmic reticulum. Cell wall if present cellulose .Respiration is carries out by mitochondria and cytoplasm. The respiratory enzyme are present i.e. mitochondria and cytoplasm and they are 80s type.

 

CELL ORGANELLES AND CELL INCLUSIONS:

1. Cytoplasm:

The semi-fluid mass of the protoplasm excluding nucleus is called cytoplasm. It contains a number of living and non- living substances inside them. The living substances inside the cytoplasm which are membrane bound are known as cell organelles. The cell organelles are ER (Endoplasmic Reticulum), plastids, Golgi body, lysosome, ribosome, mitochondria, etc. The non-living substances of cell are known as cell inclusion which is non-membrane bound. They are formed as a result of metabolism in the cell. They are also known as ergastic substances.

They are of different types such as reserve food materials, tannins, resins, gums, oils, latex, alkaloids, minerals, salts, etc.

 

2. Cell wall and its functions:

Cell wall is thick, rigid outer covering of the cell which gives definite shape and also provides protection to the protoplasm. It is non-living in nature and permeable in nature. The chief structural component of cell wall is cellulose which is a chain of many thousands of sugar units. Beside cellulose, pectin components, lignin, cutin, proteins, fats, etc. Its thickness varies in different types of cell from 0.1um - 101 um. The cell wall consists of 3 parts. They are:

1. Middle lamellae: It is thin cementing layer between two adjacent cells or it is found in between two primary wall and is formed during cell division. It is composed of calcium and magnesium.

2. Primary wall: In young enlarging cell the primary wall remains thin and elastic becoming thick and rigid at the approach of maturity. The primary wall contains cellulose, hemi cellulose and peptic component. It also contains a number of micro fibrils embedded in the matrices.

3. Secondary wall: The secondary wall is formed by the deposition of cellulose on the primary wall or it is formed when the cell has stop growing. It is bound in certain mature and highly specialized cell such as tracheid, vessels, and collenchymatous, sclerenchymatous cells. It is composed of 3 layers of micro fibrils laid down one after another. This layer is chemically made up of lignin in addition to cellulose, hemicellulose and peptin.There are some other chemical substances such as cutin, and suberin, waxes, mucilage, silica etc. are deposited on the wall.

Functions of cell wall:

1. It gives mechanical support to the cell and plant as a whole.

2. It gives definite shape.

3. It provides inner content of cell from mechanical injuries and checks the entry of germs.

4. Cell wall of root hairs absorbs water and minerals from the soil.

5. Being permeable in nature, it is capable of absorbing water and help in the movement of water and solutes towards protoplasm.

 

PLASMODESMATA:

The primary cell wall and middle lamella never occur in the form continuous the layer but have a minute aperture through which cells of the tissue maintain cytoplasmic relation with each other. Such cytoplasmic junction or bridge is known as plamodesmata. It permits circulation of fluids various substances can pass from one cell to another through plasmodesmata.

Protoplasm:

The term protoplasm is introduced by purkinjii in 1839. It is defined as physical basis of life because it is fundamental structures of all organisms. it is living in nature and consists of a mixture of organic and inorganic materials. It is colorless, semitransparent, viscous and a slimy in nature. The protoplasm is divided into nucleoplasm (protoplasm of nucleus) and cytoplasm (extra nuclear protoplasm) . It shows intra cellular movement which is known as cyclosis. Water is the chief constituent of all physiological active protoplasm. It consists of 75-90% of the total protoplasm.

In the hydrophytes, it exists up to 95-98% and in the dry seeds only 10% water is found. The remaining part of protoplasm contains two types of constituent, organic and inorganic materials.

The common organic components are carbohydrates, proteins, fats, lipids, nucleic acids.

Protein= 7-20%

Carbohydrates=1%

Lipids=2-3%

Inorganic material=1%

RNA=0.7%

DNA=0.4%

Major elements constituting of protoplasm are:

Carbon=2%

Oxygen=62%

Hydrogen=10%

Nitrogen=3%

Plasma membrane (Bio membrane):

Plasma membrane is thin, transparent electron microscopic which forms the outermost boundary of cytoplasm. It is also known as cell membrane or plasma membrane. Plasma membrane is selectively permeable in nature. It is lipoprotinous i.e. they are chiefly composed of lipids and proteins. Carbohydrate, are also found in the plasma membrane.

 

Structure of membrane:

Several models have been proposed to explain the organization of plasma membrane. A few of them are Danielle Davson model, unit membrane hypothesis, fluid mosaic model etc.

Plasma membrane consists of phospholipids and protein and measures about 75A in diameter. It is a unit membrane i.e. it consists of two lipids layer bounded on either side by monomolecular layer of protein or the two molecules of lipids are sandwich between two molecules of protein. A hydrophilic head of lipid layer project towards the protein layer side the hydrophobic or non-polar region lies towards the inner side of the membrane.

Function of plasma membrane:

1. It controls the passage of materials to and fro the cell.

2. It protects the cell from injury.

3. It is selectively permeable. It controls and co-ordinates the rate of substances transfer and diffusion.

 

Endoplasmic Reticulum(ER)

Extending throughout the cytoplasm there is a network of interconnected channels which are lined by the membrane. The whole of this memberous structure is known as endoplasmic recticulum.They are connected with nuclear membrane and frequently associated with ribosome. When they are associated with ribosome; the ER is called rough ER. The function of rough ER is in synthesizing protein because ribosome functions as protein synthesis. When they are without ribosomes, they are called as smooth ER. The smooth ER has smooth membrane and may also take part in synthesis and storage of fats. ER consists of three types of structures;

1.Cisternae

2.Vesicles

3.Tubules

 

Fig: Endoplasmic reticulum

1. Cisternae:

They are long flattened, sacs like, narrow two layers and unbranched tubules found near the nucleus. They lay one upon the other and are interconnected. Each tubular structure is about 40-50nm in diameter and contains ribosomes on the membrane. They are found in that cell which have synthetic roles. Such as the cell of pancreas, brain etc.

2. Vesicles:

They are oval or rounded sac of 25-500nm in the diameter. The vesicles as small vacuoles and often remain scattered in the cytoplasm.

3. Tubules:

They are tubular and branched structure which may be connected with cisternae and vesicles to form the reticular system (net likes). They have a diameter of 50-100ñm and are present near the cell membrane. They are without ribosomes. They are present more in lipid forming cells.

Function;

1. They act as cell circulatory system and help in the transport of materials.

2. They acts as cytoskeleton and provide the mechanical support and shape to the cell.

3. They help in cell plate formation during the cytokinesis.

 

Plastids;

They are the smaller discoid organelles of cytoplasm and found only in green part of plants but absent in fungi, bacteria and      BGA.

Plastids are three types they are chloroplast, leucoplast and Chromoplast. All the plastids arise from the minute protoplasmic bodies known as pro-plastids. They are found inside the dividing cell.one form of the plastid change into another form. Such as leucoplast into chloroplast and chloroplast into Chromoplast. In young tomato fruit leucoplast gradually change into chloroplast and finally into Chromoplast.

 

Leucoplast:

They are colorless plastid founds into roots and parts where the light generally not reached such parts are grown below the ground. They are mainly concerned with the storage of reserve of various kinds of reserve foods materials such as; carbohydrates, lipids, proteins. Their function is to convert the sugar into starch and insoluble food material for the proposed of storage.

Chromoplast:

They are the colored plastids and contain the variety of pigment other than green. They also contain the color pigments like carotene (orange or brown) and xanthophyll (yellow or orange). They are not photosynthetic due to lacks of chlorophyll. They are found in the colors parts plants such as; fruits, flowers. Generally chloroplast found in the petals of flowers, to make them flowers attractive and invite insects for purpose of pollination.

 

CHLOROPLAST:

They are most common plastids founds in the green parts of plants. They are green pigments so colors beings green. The process of photosynthesis takes place in them. They are founds in green leaves which absorbed carbon dioxide from the air and energy from the sunlight. They not found in that part where the light not exposed.

Fig: chloroplast

 

Each chloroplast is discoid and spherical in nature. It is bounded by two membranes. Internally it is filled with matrix known as stroma in which grana are invaded. The matrix contains: DNA, RNA, ribosomes, and starch etc. each granum consists of grana lamellae placed one above the other like stack of coins. In cross section these lamella are paired to form the sacs like structure and has been known as thylakoids. Some of the grana lamellae of thylakoid granum are connected with thylakoid of other grana by the stroma lamellae or fret membrane. Grana are the site of primary photochemical reaction (light reaction) which contains the chlorophylls. The dark reaction of photosynthesis takes place in sroma structure.

FUNCTION:

1. The most important and fundamental function of chloroplast is photosynthesis and it is also known as kitchen of cell.

2. Leucoplast: storage and other.

3. Chromoplast: attractive and help in pollination.

 

RIBOSOMES:

The ribosomes are very minute spherical structure. They are found associated with ER and are freely lying in the cytoplasm. Each ribosomes contain of two unequal subunits fitting as cap over sphere which represent the larger subunits, i.e. dome in shape while the subunits is smaller size and has cap in shape and fit in larger subunits. Ribosomes are commonly called ribo-nucleo-protein i.e. it is rich inRNA and protein.

In the prokaryotic cell the ribosomal structure results in dissociation of ribosomes with the sedimentation coefficient of 70s into two subunits of 30s and 50s. But in eukaryotic cell, the sedimentation coefficient of 80s into 60s and 40subunits. These subunits play specific role in protein synthesis. The ribosomes found in mitochondria and chloroplast however resembles that prokaryotic cell.

 FUNCTIONS:

1. Ribosomes are considers as protein factories or engine of cell because they are site of protein synthesis.

2. Protein either controls the cellular structure or act as enzymes or controls the cellular functions.

3. Ribosome stores the protein temporarily and store the rRNA which help in protein synthesis.

 

MITOCHONDRIA:

Mitochondria are cylindrical and globular in nature. It is also known as power house of cell because a huge amount of energy liberated. During the aerobic respiration is trapped in the mitochondria in the form of energy reach ATP. The mitochondria have outer and inner membrane. The inner membrane is produced into plate like or finger like projection known as cristae. The matrix inside the inner membrane is known as the mitochondrial matrix. The matrixes contain DNA, RNA, and several enzymes.

On the inner layer of cristae are found small stalked particle known as elementary particle or oxyzome. Each stalked particles contain flat base, elongated stalk and round and globular head. The head part of it produced ATP. It can catalyze the formation of ATP to form ADP and vice versa.

 Mitochondrion is the centres of respiratory activity and contains the several types of enzymes. They are the self-replicating and they are passes from one to another cell.

FUNCTION;

1.Mitochondria is consider as the power house of cell or storage factories of ATP because they it is the site of ATP formation.

2.It synthesizes the amino acid and manufacture of amino acids.

=Release energy during aerobic respiration for the vital activities of cell.

 

NUCIEUS:

Fig: nucleus

Nucleus is the most important and essential part of the cell. It is rounded protoplasmic body which invaded in the cytoplasm. It controls all the cellular activities, initiated the cell division and carries the hereditary activities of the cell. It consists of four type of the structures;

1. Nuclear membrane or envelope

2. Nucleoplasm

3. Chromatin network

4. Nucleolus

NUCLEAR MEMBRANE;

It is the outer membrane of the cell. It is the outer perforated (porous) membranous covering of cell. It is composed of two membranous I.e. outer and inner membranes. These two membranes are separated by perinuclear space. The outer membrane consists of no. of ribosomes, and also connected with ER. While the inner membrane lacks ribosomes. A no of pores occurs in the nuclear envelope, which helps the transfer of substance between the nucleus and cytoplasm. During the cell division, the membrane of nuclear envelops break down into fragments.

 

NUCLEOPLASM:

Nucleoplasm is also known as nuclear sap. It is transparent, semi-fluid matrix. It consists of nucleotide (purine, pyrimidine base, pentose sugar) and enzymes required for the synthesis of nucleic acid and nucleic proteins.

CHROMATIN NETWORK:

It is very fine fibers. It consists of coil of DNA bounds to basic protein known as histone. Chromatin fibers are actually elongated chromosomes. This overlaps one another to form a network. It consists of two parts a smooth lightly staining Euchromatin and rough dark Heterochromatin. Euchromatin contains large amount of DNA and small amount of RNA while Heterochromatin contain large amount of RNA and small amount DNA. The whole of chromatin is not functional, only the parts of Euchromatin which are associated with the acid protein take part in the formation of RNAS.

NUCLEOLUS:

It is the naked rounded and slightly irregular structure which found attach to the chromatin network. It is the place where the ribosomes are formed. Every cell may possess one or more nucleoli. It consists of large amount of DNA, RNA and get stained intensely (dark stain). Structurally it has four constituents; namely chromatin, matrix, fibrils. Nucleolus performs the four functions:

 1. Synthesis of rRNA

 2. Synthesis of nuclear protein

 3. Formation of ribosomes

 4. Formation of spindle fibres

GOLGI BODIES (DICTYOSOME)

In plant, the no. of Golgi bodies varies from few to many. Each Golgi body consists of a pair of a pair of membrane. Which are slightly curve and arranged in parallel rows This slightly curve and flattened lamellar plate like structure is called the cisternae, which are dilated (swollen) at the margins. Near the dilated end of membranes are found small found the spherical vesicles or vacuoles which are pinched off (separated) from the dilated end. Golgi bodies are formed from plasma lemma, nuclear envelope and ER.

FUNCTION:

1. It takes part in the cell plate during the cytokinesis in the plant cell.

2. It helps in the synthesis of pectin substance in the cell wall.

3. It helps in the formation of primary lysosome.

 

LYSOSOMES:

The term lysosome was derived from the two Greek words: lysis-digestive, soma- body. It is electro microscopic, which envelope in intracellular digestive activities. It consists of digestive or hydrolyzing enzymes capable of lysis or digestion. The lysosome is mostly found in animal cell. Particularly in the cell of liver, spleen, brain etc. they are also found in certain plant like fungi, euglena, meristematic cell (root tip cell of maize) they are very minute spherical structure bounded by a membrane and containing some digestive enzymes. They contain about 40 different kinds of enzymes. Out of which some are proteases, nucleases, lipases, glycosylate etc. and some are lytic bodies functioning as suicidal bag (sac). When the membrane is ruptured, it is released out in the cytoplasm causing its disintegration (death). So, it also known as suicidal bag of the cell.

Functions: It acts as intercellular digestion of the substances’, worn out cell organelles.

 

Vacuole:

                    Fig: Vacuole

The vacuole consists of single membrane known as tonoplast or vacuolar membrane which is semi permeable nature. The vacuole arises from ER. The tonoplast surrounds the cell sap. The cell sap is a full of soluble pigments. Small vacuoles fused to form larger at the maturity of plant cell. They are generally found at the distal end of cisternae.

 

Centrosome:  They are non-membranous structure. IT consists of two rod cell and they form astral rays (star like) during cell division and these are known as spindle fibres. They are mostly found in animal cell and lower plants and are absent in seed plants. In plant cell, they occur in the form of microtubules and form spindle fibres at the time of cell division. Centrioles the time off cell division. Centrioles give rise to basal bodies of cilia and flagella.

 

Differences between

Plant cell

Animal cell

1. The plant cell is characterized by the presence of cell outside the cell membrane.

 

2. It consists of plastids.

 

3. Centrioles are usually absent.

 

4. A mature plant cell contains a large central vacuole.                  

 

5. Aster rays are absent.                                                 

6. The spindle fiber is astral type.                                               

7.Cytokinesis occurs by cell plate method.                   

 

8.Mitosis occurs only in the meristimatic cell.

1.The cell wall is absent in animal cell. It is enclosed by the cell membrane.

 

2.There is no plastid.

 

3.Centriole is present.

 

4.Vacuoles are either smaller or absent.

 

5.Centriole forms the aster rays.

 

6.The spindle fiber is astral type.

 

7.Cytokinesis occurs by cleavage method.

 

8.Mitosis occurs in the throughout the body.

 

CELL DIVISION:

When the cell grows and attain the maximum size, it divided into two and then process continuous and resulting in the development of the body of plant and animals. The cell division consists of i.e. nuclear division and cytoplasmic division called cytokinesis. Nucleus carries the hereditary character of the cell. So the nuclear division is the main stage of cell division.

 

CELL CYCLE;

Fig: Cell Cycle

All those changes which occur during the cell division cell growth and collectively known as cell cycle. Each cells are capable to undergoes the cell division and possess through the cell cycle. A cell cycle consists of three main steps;

 

1. Interphase

2.M- phase or mitotic phase

3. Cytokinesis

INTERPHASE:

It is the stage between the end of one cell division and the beginning of another cell division. Metabolically it is very active phase of the cell division, as the cell prepare itself for the next cell division for the no. of the biosynthetic activities, interphase is further divided into three sub-phases

 

GAP ONE OR FIRST GROWTH PHASE;

This phase is characterized by the increase in cell size due to high rate of synthesis. There occurs an active synthesis of RNA, proteins, carbohydrates and lipids.

SYNTHETIC PHASE OR S- PHASE;

This sub phase is characterized by:

1. Replication of DNA molecules

2. Synthesis of histones

GAP- PHASE OR SECIOND GROWTH PHASE\ G2 PHASE:

This sub phase is characterized by;

1. Synthesis of spindle protein.

2. Synthesis and storage of ATP molecules for M-phase.

3. Multiplication of cell organelles (mitochondria, plastids)

M PHASE;

It is followed by the interphase and involves in the division of the nuclear materials (karyokinesis) the division of nucleus occurs in-

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

 CYTOKINENSIS;

After karyokinensis, the cell is ready to divide. The cell division is preferred as cytoplasmic division. It results in the equal distribution of organelles and cytoplasm in each daughter cell.

TYPES OF CELL DIVISION;

A cell may divide by any of the following methods:

1. Direct method (AMITOSIS)

2. Indirect method (MITOSIS) or indirect cell division.

3. Reduction cell division (MEIOSIS)

DIRECT METHOD OR AMITOS:

It is simple and primitive type of cell division. It occurs without the formation of spindle fibers and appears of chromosomes. The nucleus of the cell increases and develops a constriction around middle. The constriction gradually deeper and finally nucleus divide into two daughter nuclei. Cytokinesis also occurs two equal parts.

CELL DIVISION:

Cell division is the process in which the formation of new cells or increase in the no. of cell takes place by the division of pre-existing cell.

MITOSIS OR INDIRECT METHOD:

 Mitosis is the type of cell division in which chromosomes divide to form two daughter nuclei with the same no. of chromosomes as the present in the parent cell. It takes place in the somatic cell or vegetative body of the plant. It is also known as somatic cell division. It occurs in the growing region as in the root tip and shoot tip. Mitosis cell division comprises the following steps;

1. Interphase

2. Karyokinesis

3. Cytokinesis

INTERPHASE:

This phase is also known as resting phase because it was through nucleus inactive but metabolically, very active.

1. In the interphase, nucleus large with intact nuclear membrane and distingly visible nucleolus.

2. Chromosomes appear in the form of diffused, long, coiled and distingly visible chromatin fibers.

3. DNA of chromosomes is duplicated.

4. RNA, protein synthesis takes place.

KARYOKINENSIS:

It is the process of the division of the nucleus. The division of the nucleus comprises 4 steps;

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

PROPHASE:

It is the longest phase of the division and comprises three sub-phases:

  1. Early prophase
  2. Middle prophase
  3. Late prophase

EARLY PRIPHASE;

In the early prophase, the chromatin threads shortened and thicken to form elongated chromosomes.

The elongated chromosomes may show overlapping and their ends are not visible.

MIDDLE PROPHASE;

1. In the middle prophase, chromosomes consist of two longitudinal threads called chromatids.

2. Two chromatids are joined to each other by means of the narrow point called centromere.

1. Nucleolus or nuclei are attached to one to more chromosomes.

2. In the middle prophase, chromosomes shortened and thick further to assume characteristic shape and size.

LATE PROPHASE:

1. Fine fibres start appearing around the nucleus. They consist of microtubules. This helps in the formation of spindle fibres.

2. Nuclear membrane breaks up.

3. Nucleolus disappears completely.

4. The fibres converse in the region of the pole.

METAPHASE;

1. The metaphase is marked by complete dissolution of nuclear membrane and simultaneous appearance of spindle fibres.

2. All the chromosomes held together in the equatorial region by the means of micro tubular fibrils called spindle fibres. These fibres arranged at two opposite poles.

3. The chromosomes are distinctly visible in this phase. Thus they can easily be counted and their size and shape can be determined.

ANAPHASE:
1. The centromere of each chromosomes divide into two so that each chromatids have each own centromere.

2. The two chromatid now start repelling each other and separated completely to become daughter chromosomes.

3. Each chromatid migrates towards the opposite poles by the contraction of spindle fibres and stretching of interzonal fibres.

4. The arms of the chromosomes are directed towards the equatorial regions and the centromere towards the pole.

5. The anaphase ends when all the chromosomes reached the opposite pole. Anaphase divides the nuclear matter in to two equal and similar halves. Thus known as equational division.

TELOPHASE:

In this phase, the groups of chromosomes formed at the end of anaphase recognized themselves in to nuclei.

1. Chromosome elongates and overlaps one another to form chromatid reticulum.

2. Spindle fiber disappears.

3. Nuclear membrane and nucleolus reappear completely.

4. Two daughter nuclei are formed at each pole.

CYTOKINESIS:

After nuclear division, the division of cytoplasm takes place which is known as cytokinesis. The cytokinesis occurs either by cell plate in between the two daughter nuclei or by means of;

A. Cell plate

B. Peripheral furrowing

 In plant cell cytokinesis occur by cell plate method. Small drops let or granular body of Golgi complex and micro tubules are gather in the equatorial region to form cell plate.

 Furrowing method of cytokinesis occur in bacteria, fungi and in animal cell. In his method peripheral furrow occurs gradually in between two daughter nuclei. Later it dippers and when the age of furrow meet in the centre of the cell, the cytoplasm is divided in to two equal halves.

IMPORTANCES:

1. It keeps the chromosomes number constant. Genetic stability in organism so linear heredity of an organism is maintains.

2. Somatic cell is usually produced by mitosis. Therefore it is essential for growth and development of multicellular organism.

3. The daughter cells formed by this division have the same genetic constituent qualitatively and qualitatively as the mother cell.

4. It is the methods of multiplication in unicellular organisms.

5. It is the mechanism for replacing the old and worn out cell.

6. An injury or wound is healed by the repeating mitotic division of the surrounding healthy cells.

MEIOSIS CELL DIVISION:

Meiosis is complicated process of nuclear division in which the chromosome no. reduced to half to form four daughter nuclei; it is because the chromosomes replicate only ones, while they undergoes two divisions. The reduced chromosome no. is expressed as haploid or n. the process of meiosis comprises the two successive division is known as heterotypic or reduction division in this process chromosome no. reduced into half. The second division is homotypic or equational division because the chromosome no. remains same as produced after the end of the first division. It takes place in the reproductive cell. It can be observed in the floral buds when the pollen grains are being formed.

FIRSTMEIOTICDIVISIONORMEIOSISI:
The reduction division is the first division of meiosis. In this division, into two daughter nuclei with reduced or haploid chromosomes no. it can be studied under the four stages;

1. PROPHASE I

2. METAPHASE I

3. ANAPHASE I

4. TELOPHASE I

 

PROPHASE I:

The most significant feature of cell meiosis is prophase I of the meiosis I division. It is a long process distinguished into five successive sub-phases.

1. Leptotene

2. Zygotene

3. Pachytene

4. Diplotene

5. Diakinesis

LEPTOTENE;

1. In leptotene, nucleus enlarges in size and it further increases. Nucleolus also increases.

2. The chromosome become distinct; appear as long fine threads by the condensation of chromatin materials.

3. Each chromosome appears to the longitudinal in shape but each DNA is already duplicated. So that it is formed of two chromatids. However the chromatids are tightly bounded together. So the leptotene chromosomes appear single.

4. The paternal and maternal chromatid arrange separately.

ZYGOTENE;

1. Homologous chromosomes start pairing

2. The pairing of chromosomes is known as synapsis.

3. The pair chromosomes known as bivalent.

PACHYTENE;

1. Pachytene starts when the pairing is complicated.

2. The chromosomes show shorting and thickening.

3. The two component of bivalent coiled around each other by the relational coiling, now, the homologues chromosome of all bivalent split longitudinally into the two sister chromatids. Thus the late Pachytene stage, the bivalent appear four stranded.

4. Each bivalent or chromosome pair is actually made up of four chromatid two of each chromosome.

5. Nucleolus till remain attach at the nuclear organizing region.

DIPLOTENE;

1. The synapsed chromosome dissolved or two partner of bivalent separated completely.

2. The homologous chromosome separated from one another but the still remain attach at the certain point along their length. The point is known as chaismata. The no. of chaismata determine the shape of the chromosomes, when there are two chaismata the chromosome O shaped and when they are many, they loop like.

3. The sister chromatid of bivalent are broken and rejoined crosswise. The phenomenon is known as crossing over.

4. Nucleolus becomes disorganized.

SIGNIFICANCE OF CROSSING OVER:

1. The chaismata point is formed when the paternal and maternal chromatids are attached. So the ex-change of nuclear material takes place.

2. It may leads to variation and transfer of hereditary information.

3. In the process of ex-change the gene may be lost or gain due to mutation occurs in offspring.

DIAKINESIS:

1. Chromosomes become short and thick.

2. Complete exchange of part between two chromatids of bivalent take place.

3. Nuclear membrane disappears completely.

4. Spindle fibre appears around the bivalent.

 

2. MRTAPHASE:

a) Metaphase of meiosis first is marked by the complete disappearance of the nuclear membrane and simultaneously appearance of the spindle fibre.

b) Four stranded chromosomes or bivalent arranged themselves in two parallel rows at the equator.

c) The chromosome or bivalent are attached to the spindle fibre by their centromere.

d) The centromere or bivalent lay equidistance from each other and are directed toward the poles. While it’sarms are generally lie horizontally on the equator.

3. ANAPHASE:

a) The centromere of chromosome doesn’t divide into two and two chromatid of each chromosome remain joined together by centromere.

b) The centromere of homologous chromosome of bivalent repel to each other.

c) Spindle fibre contract as a result of which by bivalent move towards the opposite site.

d) At the late anaphase one homologous of each bivalent move towards the one pole and other to the opposite pole. So to the group of haploid chromosome are formed at each pole. These involve the reduction of chromosome number.

e) The homologous chromosome which moves towards the opposite pole is the chromosome of either paternal or maternal origin.

4. Telophase:

a) Nuclear membrane and the nucleolus are formed around the two groups of chromosomes at each pole.

b) The chromosomes become elongated and become uncoiled and formed the chromatin threads.

c) Spindle fibres disappear.

d) The two newly formed nuclei with the reduced chromosomes are sometimes separated by a cross wall. Thus a dyad formed.

e) Cytokinesis is postponed till the end of second division.

SECOND MEIOSIS CELL DIVISION:

Second meiosis division is a mitotic division and is sometime known as meiotic mitosis. As a result of four haploid nuclei are formed from the four haploid cells.

Meiosis second divided into four stages:

1. Prophase second

a) Very short stage. The chromosome doesn’t undergo the any appreciable change.

b) The nuclear membrane and nucleolus is present but gradually disappear at the late prophase.

c) The chromosome is arranged in zigzag ways.

2. Metaphase second:

a) Nuclear membrane and nucleolus disappear

b) Formation of spindle fibre is complete.

c) The chromosome in the each daughter cell is arranged in the equatorial plane of the cell and their centromere lie at the centre of the spindle.

3. Anaphase second:

a) The centromere of each chromosome divided into two. So that there is one centromere for one chromatid.

b) The two chromatid of a chromosome separate completely and are known as a daughter or new chromosome.

c) Spindle fibre contract as a result of which the daughter chromosome move toward the opposite poles.

d) At the end of anaphase second, four group of chromosomes are produced each group having the haploid no. of chromosome.

4. Telophase second:

a) The four groups of chromosome arranged themselves in to haploid nuclei.

b) Nuclear membrane nucleolus reappears.

c) Chromosome elongated and become thread like.

Cytokinesis:

After the Telophase second, intermediate walls develop between two daughter nuclei of each cell. Division of cytoplasm is followed is either cell plate or furrowing method. Finally 4 cells, each with single haploid cell is formed.

Significance of meiosis:

1. It is essential for sexually reproducing organism because it produces haploid gametes or spore that is essential for sexual reproduction.

2. If mitosis is only method of the cell division after every act of fertilization the chromosome of the individual sill become a double in the subsequence generation. The increase in no. of chromosome is harmful and creates imbalance between the cytoplasm and nucleus, which may ultimately cause several variation and mal formation in organism. So meiosis helps in keeping the no. of chromosome constant in the species.

3. During the Diplotene stage of meiosis first crossing over provides an opportunity for the exchange of gene between the homologous chromosomes. Thus, the chromosome with change the genetic constitution is formed which may cause the mutation in the species. Variation is the cause of evolution. 


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