Problem 113 - Objective Biology for NEET

Question

Which is the correct statements from the following: I. Synapsis of homologous chromosomes takes place during prophase I of meiosis. II. Division of centromeres takes place during anaphase I of meiosis. III. Spindle fibres disappear completely in telophase of mitosis. IV. Nucleoli reappear at telophase I of meiosis. (a) I only (b) III only (c) I and II only (d) I, III and IV only

Step-by-Step Solution

Answer

Based on the step-by-step analysis of the statements, the correct option is (d) I, III and IV only.

Step 1: Analyzing statement I

Synapsis of homologous chromosomes does indeed occur during prophase I of meiosis. This means statement I is correct.

Step 2: Analyzing statement II

During anaphase I of meiosis, it's not the centromeres that divide, but the homologous chromosomes that separate. Thus statement II is incorrect.

Step 3: Analyzing statement III

In the telophase of mitosis, the spindle fibres do disappear completely. So, statement III is correct.

Step 4: Analyzing statement IV

At telophase I of meiosis, the nucleoli do not reappear. Hence, statement IV is incorrect.

Key Concepts

Synapsis of Homologous Chromosomes

Synapsis is a pivotal process in meiosis, crucial for accurate genetic recombination and variety. During prophase I of meiosis, which marks the beginning of this reduction division, homologous chromosomes come together in a pairing process known as synapsis. Imagine each chromosome as a long string of genetic information, and during synapsis, these strings line up closely along their entire length.

Homologous chromosomes are pairs, one inherited from each parent, that carry genes for the same traits at corresponding locations. While synapsis occurs, crossing over can take place, where sections of DNA are swapped between homologous chromosomes. This exchange leads to genetic diversity in gametes. Synapsis is so essential because it ensures that each gamete receives a unique combination of genes, setting the stage for genetic variety in offspring.

Anaphase I of Meiosis

Anaphase I is the stage in meiosis where things literally start moving apart. Unlike mitosis, where sister chromatids are separated and pulled to opposite poles, anaphase I of meiosis is characterized by the separation of homologous chromosomes. That's your pair of chromosomes that were huddling together during synapsis.

What's critical here is not the centromeres dividing as they would in mitosis, but the actual homologs being pulled apart while still composed of two chromatids. The centromeres hold the chromatids together until anaphase II. This step ensures that the resulting cells - which will eventually become sperm or egg cells - have just one chromosome from each original pair. This reduction in chromosome number, known as haploidy, is vital for sexual reproduction because it allows for the restoration of the diploid number when two gametes fuse during fertilization.

Telophase of Mitosis

The grande finale of mitosis, telophase, is where the cell almost crosses the division finish line. Following the mad dash of the chromosomes to opposite poles during anaphase, telophase is the time of de-escalation and restoration. Spindle fibers, those microtubule threads that were oh-so-important for dragging chromosomes around, disassemble and disappear. You can think of this as cleaning up after a big party.

At the same time, two new nuclear envelopes begin to form around the two sets of separated sister chromatids, now at opposite poles. Chromatids decondense into more diffuse, less visible chromatin, and nucleoli – those little spots within the nucleus where ribosomes are assembled – make their comeback. The cell is nearly ready to divide completely, moving into cytokinesis, which is the physical separation into two distinct daughter cells, each with a nucleus ready for another round of cellular shenanigans.