Quiz 3: Chromosome Transmission During Cell Division and Sexual Reproduction

The word "binary fission" means "division into half." It is a common method of reproduction in prokaryotes and is also observed in few single-celled eukaryotes. It begins with single mother cell and gives rise to two daughter cells by the end. In this process of binary fission, DNA (deoxyribonucleic acid), the genetic material replicates first and segregates apart. Soon after this process, cell undergoes cytokinesis and divides into two daughter cells of same size. As the genetic material is partitioned equally, the two daughter cells will be genetically identical with each other and also with the mother cell. Unless mutation occurs during DNA replication, the daughter cells remain genetically identical with the mother cell. The following diagram depicts the process of binary fission:

A drug named aphidicolin inhibits deoxyribonucleic acid synthesis in eukaryotic cells. It is observed that this drug causes DNA to remain in their G 1 phase, so that DNA replication may stop. If such type of drug is administered, then several changes takes place in G 1 , S, G 2 , prophase, metaphase, anaphase, or telophase of cell cycle. The changes observed in somatic cells when following types of drugs are administered are as follows: A drug inhibits microtubule formation- In this case drug will affect G 2 phase of the cell cycle that is it could not complete its prophasE.A drug that allows microtubules to form but prevents them from shortening- In this case metaphase is affected and the cell may possibly not enter anaphasE.A drug that inhibits cytokinesis- In this case, cell could not divide into two daughter cells, that is telophase is affecteD.A drug that prevents chromosomal condensation- In this case cell's G 2 phase is affected and it could not enter prophasE.

Morgan in his experiment crossed a white-eyed male fly with several red-eyed sisters and obtained all red-eyed offsprings. But the observation of Morgan was somewhat different. He found 1237 red-eyed flies and 3 white-eyed males. The possible reasons for getting 3 white-eyed females are- • First there would have be an abnormal fertilization of female gamete. This might happen when white-eyed male parent makes sperm that carries X chromosome and later on it would have fertilized with a female gamete without any sex chromosome. This would have resulted into X0 white-eyed malE.• Secondly, mutations may occur. Though the chances of mutations are rare but still it may take couple of years to get one white-eyed malE.• Thirdly, there is a possibility that allele that is responsible for white-eye would have occurred in previous generations in a female fly.