31 Years NEET Previous Year Questions: Cell Cycle & Cell Division - 2 - NEET MCQ

G₁, or first growth phase is followed by S phase or synthesis phase. DNA replication occurs in S phase and DNA amount doubles up i.e., a cell with 2C DNA in G, phase will now have 4C DNA. G₂, phase is second growth phase where DNA content remains 4C.

(a) During S or synthesis phase, replication or duplication of chromosomal DNA and synthesis of histone proteins takes place. During this time the amount of DNA per cell doubles. 

During cytokinesis in animal cells, a ring of actin filaments forms at the metaphase plate. The ring contracts, forming a cleavage furrow, which divides the cell in two. In plant cells, Golgi vesicles coalesce at the former metaphase plate, forming a phragmoplast.

Therefore A is the correct option.

The correct option is Option B.

A somatic cell that has just completed the S phase of its cell cycle has replicated its DNA but has not yet undergone cell division (mitosis). Therefore, it has the same number of chromosomes as before S phase but twice the amount of DNA, because each chromosome has been replicated into two sister chromatids.

• B (Synapsis) happens in the zygotene stage (early prophase I).
• A (Crossing Over) occurs in the pachytene stage (middle of prophase I).
• C (Terminalisation of Chiasmata) happens in the diplotene and diakinesis stages (later prophase I).
• D (Disappearance of Nucleolus) occurs during the diakinesis stage (end of prophase I).

Thus, the correct sequence is: B → A → C → D

1. Synapsis (B) - Early Prophase I:

• Stage: Zygotene (a sub-stage of prophase I)
• Description: During the zygotene stage, homologous chromosomes begin to pair up through a process called synapsis. Each chromosome aligns with its homologous chromosome, forming pairs known as bivalents or tetrads. These chromosomes are physically paired through a protein structure called the synaptonemal complex.

2. Crossing Over (A) - Late Prophase I:

• Stage: Pachytene (a sub-stage of prophase I)
• Description: After synapsis, during the pachytene stage, crossing over occurs. This is where homologous chromosomes exchange segments of genetic material at specific sites called chiasmata. This exchange increases genetic diversity in the resulting gametes. The chromosomes are now considered recombined.

3. Terminalisation of Chiasmata (C) - End of Prophase I:

• Stage: Diplotene and Diakinesis (final stages of prophase I)
• Description:
  • During the diplotene stage, homologous chromosomes begin to separate slightly but remain connected at the chiasmata where crossing over occurred.
  • In the diakinesis stage, the chiasmata move toward the ends of the chromosomes, a process called terminalisation. The chromosomes are fully condensed, and the homologous chromosomes are now ready to align at the metaphase plate in the next phase.

4. Disappearance of the Nucleolus (D) - Late Prophase I:

• Stage: Diakinesis (last sub-stage of prophase I)
• Description: As the chromosomes condense, the nucleolus (the region in the nucleus where ribosomal RNA is produced) begins to disappear. This marks the transition to the later part of prophase I, preparing the cell for the upcoming stages of metaphase, anaphase, and telophase.

(a) Meiosis-II does not complete untill fertilization occurs in females (in human being). 

(b) Attachment of microtubules to chromosomes is mediated by kinetochores, which actively monitor spindle formation and prevent premature anaphase onset during mitosis. 

(a) Meiosis is a process (discovered by German biologist, Oscar Hertwig) where a single cell divides twice to produce four cells containing half the original amount of genetic information. These cells are our sex cells - sperm in males, eggs in females. Crossing over is the process by which two chromosomes, exchange some distal part of their DNA. This process occurs in the Pachytene stage of Prophase I of meiosis.

(d) Telomerase production is increased in cancer. Telomerase has been examined in hundreds of studies as a potentially sensitive biomarker for screening, early cancer detection, prognosis or in monitoring as an indication of residual disease. 

(d) Synapsis is the pairing of two homologous chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I, Zygotene of meiosis.