Cell Biology Test - 20

Correct Answer :- d

Explanation : Genetic variation comes from crossing over, which may occur during prophase I of meiosis.

In prophase I of meiosis, the replicated homologous pair of chromosomes comes together in the process called synapsis, and sections of the chromosomes are exchanged.

The regular sequence of G₁, S, G₂ (interphase) and M (mitotic) phase is called the cell cycle. A cell undergoing meiosis will first enter the G₁ phase, followed by the S phase, then G₂ phase and finally undergo meiosis. The cells which divide further re-enter the G₁ phase and the cycle continues. The cells which do not divide do not proceed beyond the G₁ phase but start differentiating by entering the G₀ phase.

Crossing over is theexchange of genetic material between non-sister chromatids of homologous chromosomes during meiosis, which results in new allelic combinations in the daughter cells. ... These pairs of chromosomes, each derived from one parent, are called homologous chromosomes.

Chiasmata is the point of contact, the physical link, between two (non-sister) chromatids belonging to homologous chromosomes. At a given chiasma, an exchange of genetic material can occur between both chromatids, what is called a chromosomal crossover, but this is much more frequent during meiosis than mitosis.

Synapsis (also called syndesis) is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during zygotene of Prophase I of Meiosis I. During zygotene stage, synapsis begins with a synaptonemal complex forming between homologous chromosomes. Homologous chromosomes pair up through the process of synapsis to form bivalents.

Zygotene is marked by beginning of lateral pairing of homologous chromosomes at the chromosomal tips and its process along the length of chromosomes that is the formation of synaptonemal  complex.
Pachytene is marked by completion of lateral pairing of homologous chromosomes along the entire length of chromosome and crossing over.
Diplotene is marked by breaking of synaptonemal complex, which holds the synapsed chromosomes together, which in turn causes separation of synapsed chromosomes all along their length except for the point of crossing over (chiasmata).
Diakinesis is marked by separation of homologous chromosomes at chiasmata as well.
So, the correct answer is option D.

The formation of tetrad is a special characteristic of Prophase 1 of meiosis 1.
The homologous pair aligns with each other and gets ready for crossing over.
So, the correct option is 'Meiotic Prophase'.

Crossover is the exchange of genetic material that occurs between two homologous chromosomes (a set of one maternal and one paternal chromosome that pair up with each other inside a cell during meiosis) non-sister chromatids that results in recombinant chromosomes during sexual reproduction.

Meiosis is cell division responsible for production of gametes through reductional division which result in halving of the number of chromosomes. The crossing over that takes place in pachytene (prophase I , meiosis I) results in introduction of variation.

Hence, the correct answer is Option D.

In meiosis, daughter cells are different from parent cell due to the process of crossing over. Crossing over occurs between homologous chromosomes and chromosomal segments are exchanged between homologous chromosomes during crossing over which is the main cause of variation.

So, the correct answer is option A.