5.2

5.2 MEIOSIS

7th May 2010

What is meiosis?• The process of nuclear division that reduces

the number of chromosomes in new cells to half the number of chromosomes in parent cell.

2n (diploid) [parent cell]

n (diploid) [new cell]

Necessity of trait inheritance

SEXUAL REPRODUCTION• Involves:– Fusion of TWO haploid gametes

• Results in :– Formation of a DIPLOID zygote.

• Means:– Offspring (child) will inherit

traits from both parents to continue life.

n n

2n

The Significance of MEIOSIS

• Sexual reproduction leads to genetic variation among the offspring.

• Meiosis makes genetic variations possible in three ways:1. Cross-over2. Reduction and fusion of gametes3. Independent (random) assortment

Where does Meiosis Takes Place?

Human• Testes (male)• Ovaries (female)

Plant• Anthers • Ovaries Flowers

Stages of MEIOSIS

Begins with single diploid parent cell.

Prophase I, Metaphase I, Anapphase I, Telophase I

At the end, four haploid daughter cells produced.

Prophase II, Metaphase II ,Anaphase II, Telophase II

Consists of TWO separate divisions:• Meiosis I• Meiosis II

MEIOSIS I MEIOSIS II

Interphase

• Before meiosis begins, genetic material is duplicated.

• At this point, chromosomes are not condensed and are not visible under a light microscope.

MEIOSIS I: Prophase I • Duplicated chromatin condenses. Each

chromosome consists of two, closely associated sister chromatids.

• Cell has two centrosomes, which form the mitotic spindle.

• The pairing is called synapsis.• Nuclear envelop breaks down.• Crossing-over can occur during the

latter part of this stage. Crossing-over point chiasmata

• Diploid cell contains homologous chromosomes. One chromosome in each pair is maternally derived and one is paternally derived.

• Homologous chromosomes lined up at equatorial plate in a pair-wise fashion.

MEIOSIS I: Metaphase I

• Homologous pairs separate with sister chromatids remaining together.

• The centromeres of the chromosomes do not, so each chromosome still consists of two sister chromatids, which now may not be genetically identical due to crossing over.

MEIOSIS I: Anaphase I

• Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

• Interkinesis (a short period of interphase) then follow telophase I.

• DNA replication does not take place during interkinesis.

MEIOSIS I: Telophase I

5.2 SELF TEST

1) _________ cells contain two copies of each chromosome.a) haploidb) diploid c) aneuploid d) homologous

2) In general, gametes are ___________ and somatic cells are ____________. a) diploid, haploidb) haploid, diploidc) diploid, diploid d) haploid, haploid

3) Pairs of matching chromosomes are termed ______________, and each member of the pair is called a _____________. a) paired, haploid b) diploid, haploid c) homologous chromosomes, homolog d) homolog, homologous chromosomes

4) Which of the following represent the correct sequence of steps in meiosis?a) telophase, metaphase, anaphase, prophaseb) anaphase, metaphase, telophase, prophase c) prophase, metaphase, anaphase, telophase d) metaphase, anaphase, prophase, telophase

5) What structures separate from one another during meiosis I? a) sister chromatidsb) homologous chromosomes c) centromeresd) Synapsis

6) During what stage of meiosis I does synapsis occur? a) prophase b) metaphase c) anaphase d) telophase

7) During what stage of meiosis I does crossing over occur? a) prophase b) metaphase c) anaphase d) telophase

8) Human males have a) 22 pairs of autosomes and two X chromosomes b) 22 pairs of sex chromosomes and 1 pair of autosomes c) 22 pairs of autosomes, one X chromosome and one Y

chromosomed) 1 pair of autosomes and two X chromosomes

9) State five similarities and five differences between mitosis and meiosis.

Similarities

1. Both involve cell divisions2. Both start from diploid cells3. Chromosomes duplicate only once4. Both involve the separation of chromatids in anaphase of mitosis and in anaphase II5. Both follow similar phases: prophase, metaphase, anaphase and telophase.

Differences