NEET Practice Questions (MCQs) with Answers & Solutions

Crossing over is an enzymatic process occurring during the pachytene stage of prophase-l. The enzyme involved in this process is called recombinase which aids in the recombination of genes between homologous chromosomes.
During zygotene stage, homologous chromosomes pair up by process called synapsis and form a complex bivalent structure.
Diplotene is marked by the dissolution of synaptonemal complex and chaima formation. While diakinesis is marked by terminalisation of chaismata (i.e., chaismata shifts towards periphery of chromosome.)

(C) Homologous chromosomes pair up. This process is called synapsis. Each pair is called bivalent, chromosomes line up around the equator of the spindle, attached by their centromeres to the spindle fibres, which are microtubules. The microtubules forming spindles can be of two types, i.e, kinetochore microtubule and non-kinetochore microtubule.

Axoneme is the central core of a cilium or flagellum consisting of a central pair of filaments surrounded by nine other pairs. Equatorial plate is the plane located midway between the poles of a dividing cell during the metaphase stage of mitosis or meiosis. It is formed from the migration of the chormosome to the centre of the spindle.

The pachytene stage of prophase-I of meiosis I is characterized by the appearence of recombination nodules, the sites at which crossing over occurs between non-sister chromatids of the homologous chromosomes. Crossing over is the exchange of genetic material between two homologous chromosomes. It is also an enzyme mediated process and the enzyme involved is called recombinase. 

In metaphase of mitosis, spindle fibres attach to kinetochore of chromosomes. Chromosomes are moved to spindle equator and get alligned along metaphasic plate through spindle fibres to both pole.

(a) Astral rays arises from centriole. In the stage of prophase, the chromosomes begin to curl up and become thicker and appear thread-like. At the same time, the cytoskeleton becomes like a net of fibres extending all over the cell. This network of fibres is called the spindle and each fibre is called an astral ray.(Each centriole with its astral rays is referred to as a aster.) The centrioles move apart along the fibers of the spindle until they reach opposite sides of the cell- the position of the centrioles at opposite sides of the cell become the sides of the spindle. Finally, the nuclear membrane starts to break up.

(d) The presence of plastid is a feature of plant cells but animal cells are devoid of them. Even then they function properly and divide mitotically similar to plant cells.

(a) : Chromosome replication occurs once but meiosis has two M-phases each with its own Karyokinesis and Cytokinesis. As a result chromosome number is halved. The transition period between M-phase II (meiosis II) is short and without DNA replication. It is called interkinesis.

(a) : Colchicine is an alkaloid derived from the crocus, Colchicum autumnale. It inhibits spindle formation in cells during mitosis so that  chromosomes cannot separate during anaphase, thus inducing multiple sets of chromosomes. It does not affect cytokinesis.

(d): S-phase is known as synthetic phase. In this stage replication of DNA takes place on the template of the existing DNA and thus the amount of DNA per cell doubles. If the initial amount of DNA is denoted as 2C, then it increases to 4C.

(b): Cell cycle is divided into 2 phase. (i) Interphase (a period of preparation for cell division), (ii) M phase (the actual period of cell division). The interphase cell is metabolically quite active. Interphase is the long non dividing phase further divided into G₁, S and G₂. It occupies 75 to 90 % of entire cell division time.