BASIC BIOLOGYBIO 101/4

Cell division Meiosis

23 Jan 2007

What is meiosis?

Meiosis is a type of cell division. Occurs in gamete cell. There is a single DNA replication, followed by

2 nuclear divisions. Each of the resulting daughter cells has one

half of the number of chromosomes as the parent cell.

From Diploid (2n) Haploid (n)

Occur at sexual reproduction (gamete cells)

Organ Male Female

Animal Testis Ovary

Plant Anther Ovary

Animal Sperm Ovum

Plant Pollen Eggs cell

Will produce haploid (n) gamete cells

Meiosis is divided into two part – Meiosis I– Meiosis II

How it become haploid (n)?

Parent cell with diploid (2n) requires two cell division, result in 4 daughter cells (n).

Continuous processes– Meiosis I

Prophase IMetaphase IAnaphase ITelophase I

– Meiosis II Prophase II

Metaphase IIAnaphase II

Telophase II

Term Homologous chromosome Chromosome with the similar size, shape and the

position of their centromeres. Synapsis Condition of chromosome pairing Bivalent Structure of homologous pair Tetrad Homologous chromosome that have 4 chromatids Sister chromatid Chromatid from same homologous chromosome Chiasma Point at which crossing over occur

Bivalent

Stages of meiosis I

Reduction division It reduces the number of chromosome from 46 (2n)

to 23 (n). The cell divide has homologous pair of

chromosome. One homolog comes from mother (maternal) and

one from father (paternal). When meiosis begins, DNA of each homologous

replicates, forming two chromatids joined at two centromeres.

Prophase I

1. Duplicated chromosomes condense and become visible.

2. Pairing up of homologous chromosomes forming bivalent. This process is called synapsis.

3. Synapsis cause the homologous chromosomes closely associated.

4. Both homologous chromosomes interwine at chiasma and this process is known as crossing over.

5. Crossing over causes exchange the genetic material (DNA) between paternal and maternal chromosomes.

6. Crossing over produces new combination of genes.

7. The genetic recombination from this process greatly

enhance the genetic variation.

8. Spindle fibers are form.

9. Nuclear envelope breaks down.

Metaphase I

1. Homologous chromosomes line up at the equator of the cell.

2. Form double rows of homologous chromosomes.

3. Unipolar spindle attachment to single kinetochore at each homologue.

Anaphase I Homologous chromosomes separate and move to

opposite poles of the cell. Sister chromatids remain attached at their

centromeres.

Telophase I

1. One of each pair of homologous chromosomes is at each pole.

2. Cytokinesis occur.

3. The nuclear envelope reforms and the nucleoli reappear.

Stages in meiosis II Equational division Same like mitosis. Produce 4 daughter cells with haploid (n) number of

chromosomes.

Prophase II The chromosomes are again condensed and visible,

become short and thick.

Nucleolus and membrane nucleus dissapear.

Centriole move to opposite poles.

DNA does not replicate again.

Metaphase II Chromosomes line up at the equator of the cell.

Anaphase II Centomere separation occurs. Chromatids moves to opposite poles.

Telophase II Nuclei formed at opposite poles of each cell. Chromosomes gradually elongate to form chromatin fibers. Cytokinesis occurs. Four haploid cells are produced.

Importance of meiosis For sexual reproduction. Maintains the chromosome numbers constant from

generation to generation. Assures a different genetic make up for the next

generation as a result of crossing over and new combination of genes.

Comparison between mitosis and meiosis Mitosis Meiosis

Dividing cells can be diploid or haploid

Dividing cells are diploid

Occurs in somatic cell in all parts of the body

Only in sexual reproduction cell, gamete cell

DNA replicates once during the S phase of interphase, and nucleus divides once.

DNA replicates once during S phase of interphase but there are two successive nuclear division.

Chromosomes do not associate during prophase.

Homologous chromosomes associated to form bivalen during prophase I.

Chiasma are never formed and crossing over never occurs.

Chiasma form and crossing over occurs during prophase I.

Mitosis Meiosis

Chromosomes form a single row at the equator of the cell during metaphase.

Chromosomes form two rows at the equator of the cell during metaphase I.

Chromatids move to opposite poles during anaphase.

Chromosomes move to opposite poles during anaphase I.

Daughter cells have the same number of chromosomes as the parent cell.

Daughter cells have only half the number of chromosomes found in the parent cell.

In the absence of mutation, daughter cells are genetically identical to parental cell.

Daughter cells are genetically different from parental cell.

2 daughter cells are formed. 4 daughter cells are formed. In male human, 4 sperm cells are formed. In female human, one ovum and 3 polar bodies are formed.