Question

1. This problem requires you to think about what happens to the amount of DNA during meiosis. Do not get lost worrying about the units!

The haploid germ cells of a sexually reproducing organism are found to contain 15 fg (femtograms=1 X 10^-15g) of DNA each.  Fertilized eggs contain 30 fg before DNA synthesis occurs. How much DNA would you expect to find in the following?

a) A somatic cell in G1?

b) A somatic cell in G2?

c) A germ cell in prophase I of meiosis?

d) A germ cell in prophase II of meiosis?

e)  A germ cell after telophase II and cytokinesis of meiosisII?

2. Complete the following table to compare mitosis and meiosis.

	Mitosis	Meiosis
Function of the process in Humans
Is the DNA duplicated prior? If so how many times?
Number of cell divisions
Number of cells produced at end of process
Number of chromosomes in daughter cells (doubled, halved, same?
Are Tetrads formed?
Genetic Relationship of daughter cells to parent cell

3. Describe three different processes that occur during a sexual life cycle that increase the genetic diversity of offspring.

Answer 1

a):

• gametes and haploid and somatic cells are diploid,
• In G1 phase, the DNA is unreplicated state so the amount of DNA= 2amount of DNA in gametes

= 210 fg

= 20 fg

b)

• Somatic cells in G2
• cell cycle progress as G1, S, G2 and M phase,
• DNA is replicated in S phase so the amount of DNA in G2= amount of DNA in G1 2

= 20 fg 2

= 40 fg

c):

• germ cell in prophase I of meiosis
• DNA is the duplicated state in prophase I so
• amount of DNA in the prophase=amount of DNA in G1 2

   = 20 fg 2

  = 40 fg

d):

• germ cell in prophase II of meiosis
• after meiosis I homologous chromosomes are separated, so the cells become haploid but the DNA is in duplicated state

so amount of DNA in prophase II= 2amount of DNA in gametes

= 210 fg

= 20 fg

e):

• after meiosis II haploid gametes are produced the amount of DNA in gamete=10 fg

3):

• Three different processes that occur during a sexual life cycle to increase the genetic diversity of the offspring are
• Crossing over.
• Independent assortment.
• Recombination.

Crossing over :

• During prophase I of meosis crossing over of the non-sister chromatids of homologous chromosomes take place which exchanges segments of genetic information.
• The process of crossing over is known as synapsis occurs during diplotene stage of prophase I.

Independent Assortment:

• During meosis the orientation of the bivalent along the plane is random and it leads to independent assortment of the genes.
• It takes place during metaphase of meosis.

Recombination:

• It is due to the chromosomal cross over between the paired chromosomes inherited from each parent.
• The genetic material will combine and give new combinations.

All these results in the greater diversity of the genotypes and maintain the stability of the species.

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