1. What do you mean by meiosis?
Ans. Meiosis is a special type of cell division involving a diminution (meion in Greek means reduction or diminution) in the amount of genetic material which comprises two successive nuclear divisions with only one round of DNA replication, which produces four haploid daughter cells from an initial diploid cell.
2. Why the gamete, the product in meiosis is endowed with either the maternal or paternal set of chromosomes but not both?
Ans. A diploid nucleus contains two closely similar version of each of the other chromosomes (except the sex chromosome) one from the male parent (paternal chromosome) and one other chromosomes from the female parent (maternal chromosomes). These two versions are called homologues, which recognise each other and become physically paired before they line up in the spindle. Thus segregation of homologues, or the phenomenon of disjunction results in gamete containing either maternal or paternal genes but not both.
3. Compare meiosis with mitosis or normal cell- cycle.
Ans. The divisional scheme in meiosis follows two successive divisions of the first one meiosis-I comprises prophase-I, metaphase-I, anaphase-I and telophase-I in their regular order. The prophase-I is also subdivided in five substages as leptotene, zygotene, pachytene, diplotene and diakinesis. While the second neiotic division wholly resembles with mitosis. The daughter cells are formed by mitosis inherit both copy of each paternal and maternal chromosomes, while meiosis causes inheritance of either the maternal or paternal chromosomes in daughter cells but not both.
4. What is synapsis?
Ans. In early prophase-I particularly during zygotene the homologues begin to pair side by side along their length, however, nothing a definite mechanism is known about the attractive forces that bring homologues together. This process which either may start one end and proceed along the length or simultaneously along the whole length is known as synapsis. Synapsis facilitates the formation of synaptinemal complex which renders the easiness of genetic recombination.
5. Briefly describe the form and significance of synaptinemal complex.
Ans. The synaptinemal complex, which forms just before pachytene and dissolves just afterward, keeps the homologues together in a bivalent, consists of a long ladder-like protein core on opposite sides of which the two homologues are aligned to form a long linear chromosome pair. A typical synaptinemal complex have central element in between two lateral elements provided a recombination nodule, a large protein containing assemblies of 90 nm diameter at intervals on the synaptinemal complex. Thus synaptinemal complex particularly recombination nodules have been suggested to play a part in genetic recombination.
6. What is chiasmata?
Ans. The points at which exchange of genetic material occurs between nonsister chromatids of homologues as evidenced by more or less X-shaped configuration, is known as chiasmata (plural, chiasmata), the number of which per bivalent and their location can be observed variable in diplotene.
Chiasmata created by each crossover event play a role analogous to that of the centromere in an ordinary mitotic division, holding the maternal and paternal homologues together on spindle until anaphase-I.