CELL DIVISIONCeleste Rodeno-Desingaño

CELL DIVISION

A cellular process by which the parent cell produces daughter cells.

For unicellular organisms, the result of cell division is for reproduction

For multicellular organisms, the result of cell division is for growth and maintenance of body cells

CELL DIVISION 2 Types of cell

division1. Mitosis2. Meiosis

Prior to cell division, the cell undergoes Interphase stage

A. MITOSIS An indirect cell

division exhibited by all somatic cells or body cells as well as immature germ cells consisting of four stages:› Prophase› Metaphase › Anaphase› Telophase

These 4 stages take place before the parent cell completely divide into two daughter cells

Prophase

Preparatory stage that readies the nucleus for the crucial separation of 2 complete sets of chromosomes into 2 daughter nuclei.

The chromosome shorten, thicken and become visible as thick rods.

The chromosomes are longitudinally doubled; each half is called chromatid

Nucleoli and nuclear membrane completely disappear by late prophase. In animal cell, the centriole divides and spindle fibers radiate from the aster

Metaphase The nuclear membrane disappears and spindle soon appears. The chromosomes, which maximally condensed, align at the equatorial plane.

The spindle fibers attach on the opposite sides of the centromere.

Centrioles separate and move to new positions on opposite sides of the nucleus.

Anaphase The centromeres

become functionally double.

Each member of the doubled chromosome begins to move toward the opposite poles.

The poleward movement of the chromosomes is due to the depolymerization of the spindle fibers attached to the centromeres.

Telophase Upon reaching the

opposite poles, the chromosomes regroup into two nuclear structures.

Chromosomes begin to uncoil and lengthen and finally lose their visible identity.

Cytokinesis This process usually follows after mitosis.

In plant, the cytoplasm is divided via cell plate formation where separation starts from the inside of the cell towards the periphery.

In animal, cytoplasm is divided via furrowing or cleavage formation starting from the periphery to the inside of the cell

Cytokinesis Illustration

B. MEIOSIS Reproductive cell division which takes place in the

course of gametogenesis or maturation of gametes.

2 Divisions of Meiosis1. Meiosis I (Reductional Division) Involves the separation of homologous

chromosomes resulting in two cells with haploid chromosome number.

2. Meiosis II (Equational Division) Two haploid cells from Meiosis I proceed to this

process, which involves only the separation of chromatids producing four haploid cells

Meiosis I – Prophase 1 Individual chromosomes come

slowly into view as they coil and become shorter and thicker.

Nucleoli fade from view and finally nuclear membrane disappears and spindle is organized.

Difference between prophase of mitosis with meiosis is that in meiosis the members of each pair of homologous chromosomes move together and come to lie side by side in an intimate association.

Meiosis I – Prophase 1 Chromosomes do not fuse but

often intertwine.

Crossing over › a chromatid from one double

stranded chromosome and a chromatid from the other may break at corresponding points and exchange parts

Chiasma › a discrete point where

homologous chromosomes apper to be joined.

Synapsis› The pairing process of

chromosomes.

Meiosis I – Metaphase 1 The bivalents align at

the metaphase/equatorial plane

The bivalents are so oriented that each homologue is on each side of the metaphase plane with their centromere attached to the spindle fiber.

Meiosis I – Anaphase 1 The univalents (or homologue

in each bivalent) separate from each other.

Movements of the univalents to the opposite poles are due to depolymerization of the spindle fibers.

The homologues segregate from one another so that each anaphase group is composed of haploid number (n) of chromosomes. This process accounts for the reductional phase of meiosis I.

Meiosis I – Telophase 1 The chromosomes regroup and

their coiled structures begin to relax. The nuclear membrane as well as the nucleolus reappear forming 2 haploid daughter cells.

Cytokinesis or Interkinesis follows.

Interkinesis Occurs before the cell proceeds

to the next stage

No replication of the genetic material occurs and hence no new chromatids are formed.

Meiosis II This division proceeds in a manner

similar to mitosis except that there are two haploid cells (from meiosis I) that undergo each stage.› Prophase II – Chromosomes

condense; nucleus and nuclear membrane disintegrate

› Metaphase II – Spindle fibers are formed; chromosomes align at the equatorial plane

› Anaphase II – Sister chromatids of a univalent separate and proceed to the poles as individual chromosomes

› Telophase II – Chromosomes uncoil and lengthen, nuclear membrane and nucleolus reappear.

Cytokinesis follows, forming four cells with haploid chromosome number (e.g. tetrad formation in plant)

MEIOSISOOGENESIS

SPERMATOGENESIS