MITOSIS & MEIOSIS- CELL DIVISION

MITOSIS & MEIOSIS- CELL DIVISION

INTRODUCTION

Cell division is the process by which chromosome and cytoplasm of a parent cell divided into daughter cells. In eukaryotic cell two distinct type of division take pace known as Mitosis and Meiosis.

MITOSIS

A vegetative division, whereby each daughter cell is genetically identical to the parent cell.

MEIOSIS

• A reproductive cell division, whereby the number of chromosomes in the daughter cells is reduced by half to produce haploid gametes.
• Meiosis has two successive division can say Meiosis 1 & Meiosis 2, with a short inter phase between them.
• Meiosis results in four haploid daughter cells by undergoing one round of DNA replication followed by two divisions.

Prokaryotes (bacteria) undergo a vegetative cell division called binary fission, where their genetic material is segregated equally into two daughter cells.

Binary fission may be the means of division by most prokaryotes; however there are alternative manners of division, like budding, that have been observed.

All cell divisions, regardless of organism, are preceded by a single round of DNA replication. The primary concern of cell division is the maintenance of the original cell’s genome. Before division can occur, the genomic information that is stored in chromosomes must be replicated, i.e. DNA replication and the duplicated genome must be separated cleanly between cells.

MITOSIS

• Mitosis occurs only in eukaryotic cells
• The term “mitosis”, coined by Walther Flemming in 1882.
• A vegetative division, whereby each daughter cell is genetically identical to the parent cell. Mitosis takes place in both vegetative and reproductive cell.
• Replicated chromosomes are separated into two new nuclei.
• Cell division gives rise to genetically identical cells in which the number of chromosomes is maintained
• Karyokinesis (division of nucleus) is followed by cytokineses (division of cytoplasm)
• In plant cells the cytokineses is through the formation of cell plate whereas in animal cells, it is through the formation of cleavage furrow.
• The primary result of mitosis and cytokineses is the transfer of a parent cell’s genome into two daughter cells.

PHASES OF MITOSIS

Prophase–

• Both nucleolus and nucleus are prominent
• Formation of thin long and coiled chromosome, i.e. chromonemata
• Each chromosome has two chromatid

Metaphase–

• Nuclear membrane and nucleolus are absent
• Chromosome appears most condensed rod like structure and arranged at Center of the cell
• Thickest, smallest and clearest chromosome hence best stage for study of chromosome
• Centromere exactly on the equatorial line, spindle fibers is attached to the kinetochore of centromere. Centromere is connected by spindle fiber on both sides.

Anaphase–

• Nuclear membrane and nucleolus are absent
• Longitudinal splitting of centromere and comes in “V” shape
• Chromatids move towards pole
• In late anaphase, chromosomes also reach their overall maximal condensation level, to help chromosome segregation and the re-formation of the nucleus

Telophase-

• Nuclear membrane and nucleolus reappear
• Uncoiling of chromosomes
• Both sets of chromosomes, now surrounded by new nuclear membrane, begin to “relax” or decondense. Mitosis is complete. Each daughter nucleus has an identical set of chromosomes.

Cytokineses– Karyokinesis is followed by cytokineses. Cytokineses is not a phase of mitosis but rather a separate process, necessary for completing cell division. In plant cells the cytokineses is through the formation of cell plate whereas in animal cells, it is through the formation of cleavage furrow.

 

Function of Mitosis

• Development and growth of organism- it is the basis of growth of a multicellular body from a single cell.
• Cell replacement
• Regeneration in some organism, e.g. Starfish
• Asexual reproduction

MEIOSIS

1. Special type of cell division in sexually-reproducing organisms used to produce the gametes, such as sperm or egg cells
2. Meiosis has two successive divisions (Meiosis 1 & Meiosis 2) with a short inter phase between them; ultimately result in four cells with only one copy of each chromosome (haploid).
3. In meiosis, DNA replication is followed by two rounds of cell division to produce four daughter cells, each with half the number of chromosomes as the original parent cell.
4. Before undergoing meiosis 1 during the inter phase genetic materials are duplicated due to active DNA replication
5. Meiosis occurs in all sexually-reproducing single-celled and multicellular organisms (which are all eukaryotes), including animals, plants and fungi. It is an essential process for oogenesis and spermatogenesis.
6. Meiosis 1 is reductional division whereas meiosis 2 is equational division or can say simple mitosis, which are further divided into Karyokinesis I and Cytokineses I and Karyokinesis II and Cytokineses II respectively. The preparatory steps that lead up to meiosis are identical in pattern and name to inter phase of the mitotic cell cycle.
   Inter phase is divided into three phases:

• Growth 1 (G₁) phase: This is very active phase in which the cell synthesizes its vast array of proteins, including the enzymes and structural proteins need for growth. In G₁, each of the chromosomes consists of a single linear molecule of DNA.
• Synthesis (S) phase: The genetic material is replicated; each of the cell’s chromosomes duplicates to become two identical sister chromatids attached at a centromere. This replication does not change the ploidy of the cell since the centromere number remains the same. The identical sister chromatids have not yet condensed into the densely packaged chromosomes visible with the light microscope. This will take place during prophase I in meiosis.
• Growth 2 (G₂) phase: G₂ phase as seen before mitosis is not present in meiosis. Meiotic prophase corresponds most closely to the G₂ phase of the mitotic cell cycle. Thus Inter phase is followed by meiosis I and then meiosis I.

Meiosis 1

Meiosis I segregates homologous chromosomes, which are joined as tetrads, producing two haploid cells which each contain chromatid pairs.

Prophase1-

• It is the longest phase and has five distinct sub phases according to appearance of chromosomes. Most of the cytogenetically events like synapses, crossing over etc. takes place in this stage. The sub stages are-

Leptotene-

• Chromosomes appear as long thin thread like structure and each chromosome is longitudinally single with a large number of chromomeres (beads on chromosome)
• Orientation and polarization of chromosomes towards the centrioles take place.

Zygotene-

• pairing of homologous chromosome, i.e. synapsis takes place
• As a result of synapsis, bivalent (each pair of homologous chromosome is called bivalent) is formed.
• Synaptonemal compex appears due to synapsis and controls pairing.
• Nuclear membrane and nucleolus are present.

Pachytene-

• Splitting of chromonemata (coiled filament of the chromosome which bears gene) into chromatids takes place and bivalent has four chromatids now, i.e. tetrad stage of homologous chromosome is found. Dehydration and condensation of chromosomes occur
• The exchange of genetic material between non-sister chromatids of each tetrad, i.e. Crossing over takes place that leads to recombination of genes.
• The crossing point is called chiasmata. Visible as cross like structure. Endonuclease enzyme breaks the chromatids while ligase enzyme unites the broken chromatids.
• Nuclear membrane and nucleolus are present.

Diplotene-

• Repulsion between homologous chromosomes begins
• Terminalization of chiasma starts and synaptonemal complex disappears
• Nuclear membrane and nucleolus are present.

Diakinesis-

• Here end of the prophase1.
• Terminalization is the characteristic feature
• At the end Nuclear membrane and nucleolus disappear. It is the best stage for counting the number of bivalent.

Metaphase1-

• Nuclear membrane and nucleolus are absent
• Chromosomes are exactly at equatorial pate and chaismata are absent.
• Spindle fiber is connected with centromere only at one side.

Anaphase1-

• Centromere does not divide hence no splitting of centromere
• “V” shaped chromosome moves to their poles
• Reduction in number of chromosomes, i.e. n of 2n. here only separation of chromosomes and every nucleus has half number of chromosomes (n).
• Tetrad is divided into diad and each diad has only one pair of chromatids
• Nuclear membrane and nucleolus are absent.

Telophase1-

• Chromosomes reached to their respective poles
• Nuclear membrane and nucleolus are formed later
• Two daughter nuclei are formed

Meiosis 2

Meiosis2 is the second meiotic division, and usually involves equational segregation, or separation of sister chromatids similar to mitosis.

Prophase2-

• Disappearance of the nucleoli and the nuclear envelope again as well as the shortening and thickening of the chromatids. Centrosomes move to the Polar Regions and arrange spindle fibers for the second meiotic division.

Metaphase2-

• Centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles.
• Nuclear membrane and nucleolus are absent.

Anaphase2-

• Longitudinal splitting of centromere

Telophase2-

• Similar to telophase1
• Nuclear membrane and nucleolus reappears.

Meiosis is now complete and ends up with four new daughter cells.

Read more..
CELL STRUCTURE AND FUNCTION
DIFFERENCE BETWEEN GENE AND ALLELE
MENDEL’S LAW OF INHERITANCE
DIFFERENCE BETWEEN PLANT CELL AND ANIMAL CELL
DIFFERENCE BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS
HOMOLOGOUS CHROMOSOMES-AN OVERVIEW & FUNCTION
WHAT BIODIVERSITY IS-MEANING,TYPES&THREATS
TYPES OF THE ECOSYSTEM