6.3 Meiosis

Question 1

Describe sexual reproduction

Answer 1

Two gamete, one sperm cell and one egg cell, fuse together at fertilisation to form a zygote, which divides and develops into an new organism.

Question 2

What is meiosis?

Answer 2

A type of cell division in which the nucleus divides twice to produce four genetically different daughter cells - the gametes. Each gamete contains half of the chromosome number of the parent cell - it is haploid.

• meiosis is therefore known as reduction division

Question 3

What are homologous chromosomes?

Answer 3

Diploid cells (2n) contain two of each chromosome (a pair), one from each parent.]

• the chromosomes that make up each pair are the same size and have the same genes, but they could have different versions of those genes (alleles)
• thee pairs of matching chromosomes are called homologous chromosomes

Question 4

Describe the two meiosis divisions?

Answer 4

• meiosis I: the first division is the reduction division when the pair of homologous chromosomes are separated into two cells. Each intermediate cell will only contain one full set of genes instead of two, so the cells are haploid
• meiosis II: the second division is similar to mitosis and the pairs of chromatids present in each daughter cell are separated, forming two more cells. 4 haploid cells produced in total.

Question 5

Describe all the stages in Meiosis 1

Answer 5

Prophase I:

• chromosomes condense, the nuclear envelope breaks down, the nucleolus disappears and the spindle formation begins
• homologous chromosomes pair up, forming bivalents
• the chromatids of the homologous chromosomes entangle resulting in crossing over

Metaphase I:

• homologous pairs of chromosomes assemble along the metaphase plate
• the orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair. So the maternal or paternal chromosomes can end up facing either pole. This is called independent assortment which results in genetic variation

Anaphase I:

• homologous chromosomes chromosomes are pulled to opposite poles and the chromatids stay joined together
• sections of DNA on sister chromatids which became entangled during crossing over, now break off and rejoin resulting in the exchange of DNA. The points at which the chromatids break and rejoin are called the chiasmata
• when exchange occurs this form recombinant chromatids, resulting in different combination of alleles
• this causes genetic variation to arise as the sister chromatids are no longer identical

Telophase I:

• the chromosomes assemble at each pole and the nuclear membrane reforms around each group of chromosomes
• the chromosomes uncoil
• the cell undergoes cytokinesis and divides into tow cells
• the reduction of chromosome number from diploid to haploid it complete

Question 6

Describe all the stage in meiosis II

Answer 6

Prophase II:

• the chromosomes, which still consist of two chromatids, child and condense and become visible again
• nuclear envelope breaks down
• spindle formation begins

Metaphase II:

• the individual chromosomes assemble on the metaphase plate
• due crossing over, the chromatids are no longer identical and independent assortment happens again and more genetic variation is produced

Anaphase II:
- chromatids of individual chromosomes separate and are pulled to opposite poles after division of the centromeres

Telophase II:

• the chromatids assemble at the poles
• the chromosomes uncoil and become chromatin again
• the nuclear envelope forms around each group of chromosomes and the nucleolus becomes visible
• cytokinesis result in the division of the cells, forming four daughter cells in total
• the daughter cells are haploid, genetically different from each other and from the parent cell (due to crossing over and independent assortment)