Question

14. For an organism with a diploid number of 30 chromosomes, determine the per cell number of the following: a. chromatids at metaphase of mitosis b. chromosomes at metaphase of mitosis C. centromeres at metaphase of mitosis d. chromatids at anaphase of mitosis e. chromosomes at anaphase of mitosis f. centromeres at anaphase of mitosis 8. tetrads at metaphase I of meiosis h. chromosomes at metaphase I of meiosis i. chromatids at metaphase I of meiosis j. chromosomes at anaphase I of meiosis k. chromatids at anaphase I of meiosis 1. tetrads at metaphase II of meiosis m. chromosomes at metaphase II of meiosis n. chromosomes at anaphase II of meiosis

Answer 1

The organism is diploid having 30 chromosomes means 2n = 30; n=15.

Chromatid : it is one strand of the chromosome. 1 chromosome=2 chromatid. Chromatid are attached to each other by a centeromere. And chromosomes are of 4 types on the basis of position of centeromere in the chromosome. It consist of unwounded DNA or unrapping DNA.

Chromosome : this is formed when 2 chromatid get attached by centeromere and they consist of wounded or wrapped DNA. Present in nucleus that consists of genetic material.

Centeromere: these are specialized DNA sequences of chromosomes that act a binding material of 2 chromatids. Spindle fibres get attached to it during cell division in metaphase of mitosis and meiosis.

Tetrad : tetrad means 4. When 2 homologous chromosomes are joined and exchange genetic material between them by a process called as crossing over or linkage the structure formed is called tetrad. It is also called bivalent because 2 chromosomes are joined and is called tetrad due to 4 chromatids. They are formed in meiosis 1 in prophase 1.

Ans a) mitosis is equal division as a diploid cell after dividing will remain diploid. So, chromatid in metaphase of mitosis = 60.

The splitting of chromosomes occurs in anaphase from the centeromere before it the chromatid are joined. As 1 chromosome = 2 chromatid. 30 chromosomes = 30*2 = 60.

Ans b) chromosomes at metaphase of mitosis = 30. It is equal to the initial number of chromosomes as no replication has occurred yet.

Ans c) centeromeres at metaphase of mitosis = 30. It is equal to the number of chromosomes as each chromosome consist of a centeromere which attaches the 2 chromatid.

Ans d) chromatid at anaphase of mitosis = 60. As in anaphase the splitting of chromosomes occurs from the centeromere. Each sister chromatid that splits consist of its own centeromere. As the cell have 30 chromosomes so the chromatids are 2*30 = 60.

Ans e) chromosomes at anaphase of mitosis = 60. As the splitted chromatids at poles will act as chromosomes for the new forming cells. One pole consist of 30 chromosomes and other also of 30 so total is 60.

Ans f) centeromeres at anaphase of mitosis = 60. As the splitted chromatids are splitted from the centeromere and each is attached to spindle fibre through the centeromere. So the number of centeromere is equal to the number of chromatids = 60.

Ans g) tetrads at metaphase 1 of meiosis = 15. Tetrad is formed when 2 homologous chromosomes joins. As it is a diploid cell having 30 chromosomes, tetrad is formed by homologous pairing and 2 chromosomes join to form a tetrad hence 30/2 = 15. And bivalents will be 30. As bivalents is pair of chromosomes. While tetrad is chromatids. 1 chromosome=2chromatids and 2 chromosomes joined = 4 chromatids so ans is 15.

Ans h) chromosomes at metaphase1 of meiosis = 30. As their is no division occurred so soon. So the chromosome number remain same.

Ans i) chromatids at metaphase1 of meiosis = 60. No division is occured yet and as their are 30 chromosomes so 2*30=60. Here answer appears to be same as in mitosis but they differ in Structure.

Ans j) chromosomes at anaphase 1 of meiosis = 30. Remember the reduction in chromosome number occurs in meiosis 2. In it the separation of homologous chromosomes occurs. No changes in chromosome number and chromatids.

Ans k) chromatids at anaphase 1 of meiosis = 60. As no change in the number occurs in meiosis 1. Changes in number occurs in meiosis 2.

Ans l) tetrads at metaphase2 of meiosis = 0. No tetrads are formed in metaphase2 of meiosis. Exchange of genetic material occurs in prophase 1 of meiosis only. So no tetrads are formed in metaphase2 of meiosis.

Ans m ) chromosomes at metaphase2 of meiosis = 15. As the meiosis 1 at the end produces cell with half the number of initial chromosomes. 2n =30 will result in n=15 in meiosis 2 and at end of meiosis 1 also we get n= 15 only.

Ans n) chromosomes at anaphase 2 of meiosis = 30. As n=15 initially but in anaphase splitting occurs from the centeromere and the 15 chromosomes are splitted and 30 chromosomes are obtained. Here double the number of initial chromosomes are produced but half at one pole and half at other pole goes in 2 different cells. As this is how 1 cell produces 2 n=15 cells and the other cell also produces n=15 cells. This is how at the end of meiosis 4 haploid cells are produced from diploid cells.

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