Biology Form 4 Chapter 5 Cell Division

5.2.1 The process of meiosis

1. Meiosis consists of two nuclear division: Meiosis I and Meiosis II.

Meiosis I: Prophase I, Metaphase I, Anaphase I and Telophase I.Meiosis II: Prophase II, Metaphase II, Anaphase II and Telophase II.

2. Meiosis I begins with a single diploid parent cell.At the end of meiosis II, four haploid daughter cells are produced, each of the daughter cell genetically different from the other and parent cell.

5.2.2 The stages of meiosis

1. Meiosis I is also preceded by interphase like mitosis, during which replication of DNA and organelles (example, centriole in animal cell) takes place and energy is built up to be used later.

2. Table 5.2 The stages of meiosis in an animal cell

Phases diagram Phase/ Description

Meiosis I

PROPHASE I The chromosomes begin to condense, become shorter, thicker and clearly visible.

Homologous chromosome pair up to form bivalent through process called synapsis.

Each bivalent consists of a tetrad. A tetrad consists of two homologous chromosomes and each chromosome is made up of two sister chromatid.

Crossing over occurs where the non-sister chromatids exchange segments of DNA. The points at which segments of chromatids cross over are called chiasmata.

The nucleolus and nuclear membrane disappear. Each pair of centrioles migrates to the opposite poles of

the cells the spindle fibres are radiated from it.

METAPHASE I The pairs of homologous chromosomes as tetrad are lined up on the metaphase plate.

Biology Form 4 Chapter 5 Cell Division

Each chromosome is attached to the spindle fibres.

ANAPHASE I The homologous chromosomes are pulled away by spindle fibres from one another to the opposite poles of the cell.

Each chromosome still consists of two sister chromatids.

TELOPHASE I The chromosomes arrive at the poles. Each pole has a haploid daughter nucleus because it

contains only one set of chromosomes. The spindle fibres disappear. The nuclear membrane and nucleolus reapears.

Cytokinesis Cytokinesis occurs simultaneously with telophase I resulting in two haploid daughter cells.

Biology Form 4 Chapter 5 Cell Division

Meiosis II

Meiosis II

PROPHASE II The nuclear membrane of daughter cells disintegrate and nucleolus disappear.

The spindle fibres re-form.

METAPHASE II The chromosomes lined up on the metaphase plate. The sister chromatid of each chromosome is attached to

the spindle fibres at the centromere.

ANAPHASE II The centromere of the sister chromatids separate. The chromosomes are pulled to the opposite poles of the

cell.

TELOPHASE II The nucleolus and nuclear membrane re-form. The spindle fibres break down.

Cytokinesis Four haploid daughter cells are formed. Each of the daughter cells contains half the number of

chromosomes and is genetically different from the parent cell.

These haploid cells will develop into gametes.

5. The Importance of meiosis:(a) To ensures that the diploid number of chromosomes is maintained from one generation to next.

Biology Form 4 Chapter 5 Cell Division

(b) To provide for genetic variation which occurs from(i) Genetic recombination

Crossing over during prophase I results in the exchange of genetic material between non-sister chromatids of a bivalent to form a new combination of genes on a chromosome.

Independent assortmentDuring metaphase I, each pair of homologous chromosomes is arranged independently and randomly at the metaphase plate of the cell. The paternal and maternal chromosomes may be oriented to face either one of the poles.

(ii) The random fertilisation of an ovum by a sperm

5.2.3 Comparison of meiosis I and meiosis IISimilarities 1. Both are nuclear division and cytokinesis.2. Each division consists of 4 phase (prophase, metaphase,anaphase and telophase).3. Spindle formation and the breaking down of nuclear membrane and nucleouli occur during the

prophase of both meiosis I and meiosis II.4. Nuclear membrane and nucleolus are reformed during the telophase of meiosis I and meiosis II.

DifferencesMeiosis I Aspect compared Meiosis II

Occurs prior to meiosis I Interphase Does not occurs prior to meiosis I

Chromosome already replicated, synapsis of homologous chromosomes, formation of chiasma and crossing over between the non-sister chromatids occurs.

Prophase Replication of chromosome, synapsis of homologous chromosomes, formation of chiasma and crossing over between the non-sister chromatids does not occur.

Paired homologous chromosomes align at the metaphase plate.

Metaphase Sister chromatids/ chromosomes align at the metaphase plate.

Separation of homologous chromosomes to opposite poles.

Anaphase Separation of sister chromatids/ chromosomes to opposite poles.

Two non-identical daughter cells are formed.

Telophase Four non-identical daughter cells are formed.

Is halved. Chromosome number

Is maintained.

Does not occur. Spliting of centromeres

Occurs during the anaphase II.

5.2.4 Comparison of mitosis and meiosis

Similarities 1. Both are nuclear division.2. DNA replication & duplication only once.

Biology Form 4 Chapter 5 Cell Division

3. Both start from diploid cells.4. Both follow similar phases: prophase, metaphase, anaphase and telophase.

DifferencesMitosis Aspect compared Meiosis

All somatic cells Type of cell Cells in the reproductive organs

Produces new cells for growth, replacement of old and damaged cells and axesual reproduction.

Purpose Produces gametes for sexual reproduction.

Does not occur. Synapsis Homologous chromosomes pair up to form tetrads or bivalents.

Does not occur. Crossing over Crossing over between the non-sister chromatids occurs during prophase I.

Chromosomes are arranged randomly at the metaphase plate.

Metaphase Homologous chromosomes are line up side by side at the metaphase plate during metaphase I.

Sister chromatids separate and pulled towards the opposite poles.

Anaphase Homologous chromosomes separate and pulled towards the opposite poles during anaphase I.

One Number of cell division

Two

Two Number of daughter cells produced

Four

Diploid (2n) or same number of chromosomes as the parent cell

Chromosomal number of daughter

cell produced

Haploid (n) or half the number of chromosomes of the parent cell

Genetically identical to the parent cell Genetic content Different from the parent cell

No genetic variation in any generation Genetic variation Causes genetic variation from one generation to the next

5.3 Appreciating the Movement of Chromosomes during Mitosis and Meiosis

1. Mitosis and meiosis are regulated in a precise manner so that they are not go awry.

2. During mitosis and meiosis, the failure of homologous chromosomes or sister chromatids to separate normally or to move to opposite poles of the cell result in abnormal number of chromosomes in a cell is known as non-disjunction.

Biology Form 4 Chapter 5 Cell Division

3. Non-disjunction causes genetic disorder such as Down’s syndrome, Turner’s syndrome and Klinefelter’s syndrome.

4. Down’s syndrome is caused by the chromosome 21 pair fails to separate during meiosis and both copies of the chromosome end up in a single egg cell results in the egg cell has 3 copies of chromosomes number 21.

Subsequent fertilization by a sperm (n=23) with the egg cell contains 3 copies of chromosome number 21 (n=24) causing each somatic cell to have a total of 47 chromosomes instead of the normal 46 chromosomes.

Diagram 5.12 The karyotype of Down’s syndrome patient

A Down’s syndrome patient has the following characteristics:(a) mentally retarded(b) weaked cardiovascular system(c) shortened limbs(d) slanted eyes(e) a protruding tongue(f) receding forehead and chin