Botany MCQs for NEET — Practice Questions with Answers

(c) Each pair of homologous chromosome carrying one maternal and one paternal chromosome of similar type is called bivalent. Each chromosome has two sister chromatids and a centromere. Thus, bivalents possesses four chromatids, two centromeres. This bivalent with four chromatids is called pachytene tetrad (quadrivalent).

(c) 1 x 10⁵ $\stackrel{35 min}{\to }$  2 x 10⁵ $\stackrel{70 min}{\to }$ 4 x 10⁵

$\stackrel{105 min}{\to }$ 8 x 10⁵ $\stackrel{140 min}{\to }$ 16 x 10⁵ $\stackrel{175 min}{\to }$ 32 x 10⁵

(b) During late prophase, specialised structures called kinetochores develop on either surface of the centromere. Chromosomal fibres get attached to kinetochore.

Chromomeres are beaded structures on the chromosomes which are found particularly at the prophase-I (particularly at leptotene) of meiosis-I.

Chromocentre is developed due to the fusion of centromeric regions of all the chromosomes of a cell. Centrosome is found in animal cells(absent in higher plant cells). It is found near the nucleus. Each centrosome is made up of two centrioles and each centriole is a cylindrical structure with a diameter of 1500-1800${\mathrm{A}}^{°}$ and is made up of nine fibrils.

Aleurone layer is the outermost cell layer of the endosperm. The ploidy of the endosperm is triploid. If the root cell which is diploid has 42 chromosomes, so the haploid cell would have 21 chromosomes and triploid would have 3 X 21=63 chromosomes.

(d) In zygotene of prophase-I, homologous chromosomes pair up. This proces is called synapsis. One chromosome of the pair is from the male parent and one from the female parent.

(b) Metaphase can be characterised by the chromosomes that are least coiled which show maximum condensation and are shortest in length. It is the best stage to study the structure, size and number of chromosomes in a cell. Idiogram/karyotype of chromosomes is prepared at metaphase.

(b) Colchicum autumnale provides an alkaloid called colchicine which is used in plant breeding for doubling the chromosome number. Treatment with 0.1% colchicine inhibits spindle formation so that chromatids fail to separate during anaphase.

(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.