NEET Practice Questions (MCQs) with Answers & Solutions

telocentric is absent in human beings

Refer NCERT,Heading 8.5.10, para 3, page 140

(d) Recombination takes place in meiosis but still Meiosis maintains the chromosome number constant. It produces haploid gametes by reducing the chromosome number to half. Crossing over produces new combination of linked genes and is major source of genetic variation.

Also, distribution of bivalents which is at random in metaphase-I provides the secondary source of genetic variation in the organisms and is essential for speciation and evolution.

(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.