Independent assortment refers to
Answer: A
Which member of each pair goes to which pole is decided independently for every pair, so the combinations multiply. Crossing over is the separate mechanism that shuffles within a chromosome. The two mechanisms act at different scales and multiply together.
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Consider the following statements comparing mitosis and meiosis.
1. Mitosis produces two cells, meiosis four.
2. Crossing over normally occurs in meiosis but not mitosis.
3. Both halve the chromosome number of the parent cell.
Answer: B
The first two are correct. The third applies only to meiosis, since mitosis produces daughters with the same chromosome number as the parent. Mitosis maintains the number while meiosis reduces it. Only one of the two divisions is reductional.
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Non-disjunction is the failure of
Answer: C
It can happen in either meiotic division and leaves a gamete with an extra chromosome or one too few. Trisomy 21 arises this way and becomes more likely with maternal age. Eggs arrested for decades appear to segregate less reliably over time.
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State whether the following is true or false.
Non-disjunction in meiosis I affects all four resulting gametes, while non-disjunction in meiosis II affects only two.
Answer: D
An error in the first division is inherited by both secondary cells and so by all four products, whereas an error in the second affects only the two cells descended from the one that erred. The stage at which the error happened can therefore be inferred from the products.
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Trisomy 21 results from
Answer: A
A gamete carrying two copies instead of one produces a zygote with three. Chromosome 21 is small, which is part of why trisomy of it is survivable when most others are not. Trisomies of larger chromosomes are almost always lost early in pregnancy.
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The spindle fibres attach to a chromosome at the
Answer: B
This protein structure assembles on the centromere and captures microtubules from a pole. Each sister chromatid has its own, which is how the two come to be pulled in opposite directions. Attachment at the kinetochore is what the spindle checkpoint monitors.
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State whether the following is true or false.
In mitosis the two kinetochores of a chromosome attach to opposite poles, while in meiosis I they attach to the same pole.
Answer: C
Correct answer: True
This is the mechanical difference that makes the first meiotic division reductional. In mitosis the two kinetochores of a replicated chromosome face opposite poles, so the sister chromatids are pulled apart. In meiosis I they are fused to face the same pole, so the sisters travel together and it is the two homologues, held by chiasmata, that are separated instead. The mitotic arrangement is restored for meiosis II.
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A cell has 8 chromosomes. How many chromosomes are present in each daughter after mitosis, and after meiosis?
Answer: D
Mitosis conserves the chromosome number, so each daughter has 8, while meiosis halves it, so each of the four products has 4. Setting the two divisions side by side is the clearest way to see what actually distinguishes them.
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Binary fission in bacteria differs from mitosis chiefly in that it
Answer: A
The single circular chromosome is replicated and the copies are drawn apart as the cell elongates, with no spindle involved. A bacterium has no nuclear envelope to dismantle in the first place. The process is correspondingly quicker and simpler than eukaryotic division.
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Assertion (A): Bacteria divide much more rapidly than eukaryotic cells.
Reason (R): Binary fission requires no spindle assembly and no nuclear envelope breakdown.
Answer: B
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the cause. Binary fission needs no spindle, no nuclear envelope to dismantle and rebuild, and no condensation of chromosomes, so much of what occupies a eukaryotic cell for hours is simply absent. A bacterium can divide in twenty minutes against a typical eukaryotic cycle of a day, and it can even begin the next round of replication before the current division is complete.
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