NEET Practice Questions (MCQs) with Answers & Solutions

(a) At telophase stage, nuclear membrane vesicles associate with the surface of individual chromosomes and fuse to reform the nuclear membranes, which partially enclose clusters of chromosomes before coalescing to reform the complete nuclear envelope. During this process the nuclear pores reassemble and the dephosphorylated reassociate to form the nuclear lamina. One of the lamina protiens (lamina-B) remains with the nuclear membrane fragments throughout mitosis and may help nuclear reassembly. After the nucleus reforms, the pores pump in nuclear protiens, the chromosomes decondense and RNA synthesis resumes, causing the nucleolus to reappear.

Page 168, XI NCERT

This will occur ${\mathrm{G}}_1$ and the cell will enter ${\mathrm{G}}_0$.

The cell cycle can be halted or delayed at three checkpoints (G1/S, G2/M & late metaphase).

These checkpoints controlled by the Cdk enzymes.

G1/S is the major checkpoint or primary checkpoint at which cell decides whether or not to divide. This checkpoint is therefore major checkpoint at which external signals can influence events of the cycle.

It is the phase during which growth factors affect the cycle and also the phase that links cell division to cell growth and nutrition.

Damage to DNA can halt the cycle at this point, as can starvation conditions or lack of growth factors.

If cells are deprived of appropriate growth factors, they stop at the G₁ checkpoint of the cell cycle. With their growth and division arrested, they remain in this dormant G₀ phase. The ability to enter G₀ accounts for the incredible diversity seen in the length of the cell cycle in different tissues. Epithelial cells lining the human gut divide more than twice a day, constantly renewing this lining. By contrast, liver cells divide only once every year or two, spending most of their time in the G₀.

When a cell stops growing, say due to shortage of nutrients, this will occur in which phase of the cell cycle?

Due to shortage of nutrients, growth factors, DNA damage the cell stop at G1 and enters in G₀ or quiescent stage.

(c) If anaphase promoting complex is defective in a human cell, the chromosomes will not segregate during anaphase of mitosis. APC triggers the transition from metaphase to anaphase by tagging specific proteins for degradation.

Concept Enhancer:

Anaphase stage of mitosis is characterised by two events

(a) Splitting of centromeres and segregation of chromosomes

(b) Movement of chromatids towards the opposite poles. 

(a) In the cycle of cell division, interphase is the longest phase consisting of ${\mathrm{G}}_1$, S, ${\mathrm{G}}_2$-phases. In this phase cell prepares itself for cell division. In S-or synthesis phase DNA duplication (synthesis) takes place.

(a) Polyploid cells have a chromosome number that is more than double the haploid number e.g. Triticum aestivum (wheat) is a hexaploid(6n)).

   (d)
    Leptotene - Condensation of chromatin
    Zygotene - Synapsis of homologous chromosomes
    Pachytene - Crossing over
    Diplotene - Dissolution of synaptonemal complex and appearance of chiasmata
    Diakinesis - Terminalisation of chiasmata

The correct sequence of events of meiosis are

Synapsis in zygotene → Crossing Over in pachytene → Terminalisation of chaismata in diakinesis → Disapperance of nucleolus in diakinesis.

Acrocentric chromosomes have the centromere situated close to one end, giving them a 'rod-like' appearance. Examples include human chromosomes 13, 14, 15, 21, and 22.

S-phase is the synthesis phase in which the cell synthesis a replica of its genome, i.e., DNA replication occurs by DNA polymerase.

DNA replication along with the synthesis of histone proteins results in the duplication of chromosomal material i.e., amount of DNA doubles in each cell.

Amount of DNA remains unchanged during ${\mathrm{G}}_1$ phase or post mitotic gap and/or ${\mathrm{G}}_2$ phase or premitotic phase.