Botany MCQs for NEET — Practice Questions with Answers

(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.

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.