Question

• Describe what happens to the chromosomes of a cell during each stage of meiosis.
• When do haploid cells first form during the process of meiosis?
• What is a gamete? A zygote? How many chromosomes do they have (n or 2n)? Are they haploid or diploid cells?
• Describe two different aspects of meiosis that contribute to genetic variation between individuals.
• What are the differences between Mitosis and Meiosis?

Answer 1

Answer-

Meosis is reductional division where two nuclear divisions form four daughter cells which are all haploid .

Stages in meosis -l

Prophase l- the chromatin material condenses , nuclear membrane disappears. It is divided into 5 stages

Leptotene , zygotene , pachytene , diplotene , diakinesis.

In leptotene chromatin condensation continues.

In zygotene the chromosomes pair up and form synapsis and are called homologous chromosomes. A synaptonemal complex is formed

In pachytene paired Chromosomes appear as tetrads sand recombination nodules appear for the process of crossing over or recombination where two chromosomes exchange genetic material.

In diplotene chiasmata is formed . Where bivalent Chromosomes separate except at site of crossover.

Diaknesis - chiasmata is terminated ,dissolution if synaptonemal complex , chromosomes are condensed and meotic spindle is assembled.

Metaphase l- bivalent or homologous chromosomes arrange on metaphase plate at centre of the cell. Microtubules from opposite ends attch to Chromosomes.

Anaphasel- the homologous chromosomes separate towards opposite piles while sister chromatids remain attached.

Telophase l- nuclear membrane reappears , and cytokinesis occyrs where two daughter cells are formed .

Meosis ll

Prophase ll- the two daughter cells immediately enter prophase ll wgere nuclear membrane disappears and chromatin condenses.

Metaphase ll chromosomes arrange at equatorial plate at the cell centte and microtubules attach to kinetochores of suster chromatids.

Anaphase ll- centromere splits and each Chromosome separates and moves to opposite poles.

Telophase ll- the two groups of chromosomes have nuclear membrane reappeared. Cytokinesis occyrs and forms a tetrad of cells.

2- haploid cells form first at telophase 1 of meosis 1 where two daughter cells are formed eaxh containing only half the number of chromosomes as parent cell.

3- a gamete is areproductive cell which is haploid that is n . It carries only half set if chromosomes from either the father or the mother. In males the gamete is sperm carrying 23 chromosomes . And in females the gamete is ovum or egg again carrying 23 chromosomes. Gametes are formed from meosis.

A zygote is a result of fertilisation which is fusion of a male and female gamete so 23+23= 46 Chromosomes are present in a gamete so it is diploid or 2n

3- two aspects leading to genetic variation are-

A- recombination or crossing over - here exchange if genetic material occurs between non sister chromatids of homologous chromosomes ,leading to recombination of genetic material on the two Chromosomes.

B- independant assortment . Which occurs in metaphase l of meosis l. This is during rearrangement of homologous chromosomes on metaphase plate.

differences

Mitosis	meosis
Equational division	reductional division
Forms two diploid daughter cells	forms four haploid daughter cells
Single nuclear division	two nuclear divisions
Does not lead to genetic variation	recombination and independent assortment leads to genetic variation
Forms somatic cells and helps in growth	forms gametes ir reproductive cells or gametogenesis
Continous process	dicontinuous
No pairing of homologous chromosomes	chromosomes pair up
Genetically identical daughter cells	all daughter cells are unique
No synaptonemal complex or synapsis	forms synaptobemal complex in prophase 1

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