Sample 1Easy
DNA synthesis or replication in the cell cycle takes place during the
- A
G1 phase
- B
G2 phase
- C
S phase
- D
M phase
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The answer is C.The S or synthesis phase is the period when DNA replicates, doubling the amount of DNA per cell. The number of chromosomes does not change.
- A.
In G1 the cell is metabolically active and grows but does not replicate its DNA.
- B.
In G2 proteins are synthesised in preparation for mitosis while growth continues.
- D.
M phase is the division itself, after the DNA has already been replicated.
NCERT: Class 11 Biology, Chapter 10 (Cell Cycle and Cell Division), Phases of Cell Cycle
Sample 2Medium
A figure shows a nucleus during prophase I of meiosis. Each pair of homologous chromosomes appears as a bivalent whose two homologues are pulling apart along most of their length, but they stay joined at a few points, forming X-shaped structures. The synaptonemal complex has dissolved. Which substage is shown, and what are the X-shaped points?
- A
Pachytene; recombination nodules
- B
Zygotene; synaptonemal complex
- C
Diakinesis; terminalisation of the centromere
- D
Diplotene; chiasmata
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The answer is D.Diplotene is marked by the dissolution of the synaptonemal complex and the tendency of the recombined homologues to separate, except at the sites of crossing over. These X-shaped points are the chiasmata.
- A.
In pachytene the homologues stay closely paired and crossing over occurs at recombination nodules. The homologues separating at chiasmata marks the next substage.
- B.
Zygotene is when homologous chromosomes begin to pair and the synaptonemal complex forms. In the figure it has already dissolved.
- C.
In diakinesis the chiasmata terminalise and the nuclear envelope breaks down. The X-shaped points are chiasmata, not centromeres.
NCERT: Class 11 Biology, Chapter 10
Sample 3NEET level
In which stage of prophase I of meiosis does the synaptonemal complex dissolve, while the recombined homologous chromosomes stay attached at X-shaped chiasmata?
- A
Zygotene
- B
Pachytene
- C
Diplotene
- D
Diakinesis
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The answer is C.Diplotene begins with the dissolution of the synaptonemal complex, and the homologous chromosomes of each bivalent start to separate except at the sites of crossing over, the chiasmata. The synaptonemal complex forms in zygotene, and crossing over happens in pachytene.
- A.
The synaptonemal complex forms in zygotene; it does not dissolve.
- B.
Crossing over happens in pachytene, while the synaptonemal complex is still intact.
- D.
Diakinesis follows diplotene and shows terminalisation of chiasmata.
NCERT: Class 11 Biology, Chapter 10 (Cell Cycle and Cell Division), Meiosis I – Prophase I