Question

The animal Fauna hypotheticus has a diploid chromosome number of 28. It has a nuclear genome consisting of 2 x 10⁹ bp of B-DNA with a G+C content of 60%.

a. Determine the number of chromosomes and DNA molecules in a cell: (i) during G1 of interphase; (ii) at the start of prophase of mitosis; (iii) at the end of anaphase of mitosis; (iv) after cytokinesis of mitosis; (v) at the start of prophase I of meiosis; (vi) at the start of prophase II of meiosis; and (vii) after cytokinesis of meiosis II.

b. Determine the total number of base pairs and the number of adenines found in the nuclear DNA of a cell at : (i) the start of mitosis; (ii) the end of mitosis; (iii) the start of meiosis I; and (iv) the end of meiosis II.

Somatic cells from a eukaryotic organism have 2n=8. Each haploid chromosome set contains approximately 4 m of DNA. The average size of each chromosome during the metaphase of mitosis is 10 mm. How many times is the DNA compacted at metaphase?

Answer 1

ANSWER: a)

During S phase of mitosis DNA replicates and two sister chromatids held at centromere are formed i.e., two molecules of DNA but it considered as one chromosome as it held at centromere.

We should remember during counting no. of chromosome and no.of DNA:

• the number of chromosomes = the number of centromeres
• number of DNA molecules in a cell = number of chromatids in a cell

The number of chromosomes and DNA molecules in a cell are given in the following table.

	the number of chromosomes	Number of DNA molecules in a cell
during G1 of interphase (no. of chromosomes and DNA molecule remains same)	28	28
at the start of prophase of mitosis (DNA molecules double in the S phase while chromosomes remain same as chromatids held at centromere)	28	56
at the end of anaphase of mitosis(seperation of sister chromatids, so no. of chromosomes also double)	56	56
after cytokinesis of mitosis(two daughter cell is formed and each cell has same no. of chromosomes and DNA molecule as of Parent cell)	28	28
at the start of prophase I of meiosis (DNA molecules double in the S phase while chromosomes remain same as chromatids held at centromere)	28	56
at the start of prophase II of meiosis (due to reduction division at meiosis I, the no.of chromosomes and no. DNA reduced to half)	14	28
after cytokinesis of meiosis II (Gametes with half the no. of chromosomes to that of the parent is formed)	14	14

ANSWER

As it is given genome is of 2 x 10⁹ bp and 60% G+C content. So rest 40% will A+T.

According to chargaff’s rule, Adenine and Thymine are present in equal proportion as Adenine binds with thymine through two hydrogen bond in DNA double helix.

Hence, Adenine + thymine = 40% X 2 x 10⁹ bp = 8 x 10⁸ bp

And Adenine = ½ (Adenine + thymine) = ½(8 x 10⁸) base

Adenine = 4 x 10⁸ base

• Start of mitosis from prophase and the end of the mitosis with formation of two diploid daughter cells(cytokinesis).
• the start of meiosis I means the start of Prophase I and the end of the meiosis II is with the formation of four haploid cells.

	total number of base pairs in the nuclear DNA of a cell	the number of adenines found in the nuclear DNA of a cell
start of mitosis (DNA molecules double in the S phase)	2(2 x 109 bp )= 4 x 109 bp	2(4 x 108 ) = 8 x 108
the end of mitosis (two daughter cell is formed and each cell has same no. of base pairs and same no. of adenine as of Parent cell)	2 x 109	4 x 108
the start of meiosis I (DNA molecules double in the S phase)	4 x 109 bp	8 x 108
the end of meiosis II (Gametes with half the no. of base pairs and half the no. of adenine as of Parent cell)	1 x 109 bp	2 x 108 bp