Consider a hypothetical (2N) diploid organism with a chromosome number of 8. A.

Question

Consider a hypothetical (2N) diploid organism with a chromosome number of 8. A. How many pairs of homologous chromosomes are there? B. What is the haploid number of chromosomes? C. During metaphase of mitosis how many chromatids are present in the cell? D. During anaphase of meiosis I how many telomeres are present in the cell? Go to the Genetics Home Reference website using the following link: http://ghr.nlm.nih.gov/chromosomes A. How many genes are estimated to be present on human chromosome 13? B. Draw human chromosome 13. Indicate the correct chromosome structure and the position of the RB1 gene causative for a cancer of the eye known as retinoblastoma. C. What is the genetic term used to describe the structure of chromosome 13? D. Select How do geneticists indicate the position of a gene?' (Located below the illustration of chromosome 13) Use the descriptions of 'cytogenetic location' and 'molecular location' to explain the advantage of knowing a gene's molecular

Explanation / Answer

Please find the answers below:

Answer 1: Telomeres are short nucleotide repeat sequences found at the terminal positions of the chromosomal arms. These telomeres represent the site of lagging strand synthesis and thus, prevent shortening of the chromosomes due to un-even replication of the leading and lagging strand. Further, these telomeres also prevent the ends of chromosomes to fuse to each other and thus, prevent clumping of chromosomes in the nucleus. In a normal chromosome, the telomeres will be found at all the four endings of the chromatids and thus, one can find 4 telomeres per chromosome. In a normal diploid cell, if there are 10 chromosomes present in 2n state, then there will be 10*2*4 or 80 telomeres in the cell. As the ploidy of cell changes, the number of telomeres per chromosome also changes with cell cycle.

Thus, in a cell having 2n = 8, the number of telomeres will differ at different stages of cell cycle and the answers can be found as below:

Part A: Since the ploidy of cell is 2n=8, there will be 8 pairs of homologous chromosomes in the cell.

Part B: The haploid number of cell will be 2n/2 i.e. n = 8. This is because the haploid number is half of the total number of chromosomes in the diploid state.

Part C: During metaphase of mitosis, the chromosomes would have undergone doubling and there would be total 16+16 or 32 chromosomes in the cell. Since each chromosome contains 4 telomeres, there would be total 128 telomeres in this stage.

Part D: During anaphase I of meiosis I, the chromosomes have doubled already but the cell actually contains only 16 chromosomes. This is because meiosis can take place in germ-line cells only which are already haploid in number. Thus, for this cell, the normal value of n will be 8 and during meiosis I, this will be double i.e. 16. Thus, the total number of telomeres during meiosis I will be 16*4 = 64.

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