Question

an animal’s somatic cells contain 2n chromosomes. what happens to the number of chromosomes in the an
the number is reduced to n by mitosis.
the number is reduced to n by meiosis.
the number is increased to 4n by mitosis.
the number is increased to 4n by meiosis.

Explanation:

1. Recall the processes of mitosis and meiosis:

• Mitosis is a cell division process that results in two daughter cells with the same number of chromosomes as the parent cell. For a somatic cell with \(2n\) chromosomes, mitosis will produce daughter cells with \(2n\) chromosomes (it is used for growth, repair, etc.). So options about mitosis reducing to \(n\) or increasing to \(4n\) are incorrect.
• Meiosis is a cell division process that occurs in the formation of gametes (sex cells). It involves two rounds of division and results in cells with half the number of chromosomes of the parent cell. A somatic cell with \(2n\) chromosomes will undergo meiosis to produce gametes with \(n\) chromosomes (this is to ensure that when gametes fuse during fertilization, the normal diploid number \(2n\) is restored).

1. Analyze each option:

• Option 1: Mitosis does not reduce the chromosome number to \(n\), so this is wrong.
• Option 2: Meiosis reduces the chromosome number from \(2n\) (in somatic cells that give rise to gametes) to \(n\) in the resulting gametes. This is correct.
• Option 3: Mitosis does not increase the chromosome number to \(4n\) in somatic cells (it maintains the \(2n\) number). The \(4n\) stage in mitosis is a temporary stage during the cell cycle (S - phase replication) but the final daughter cells have \(2n\)), so this is wrong.
• Option 4: Meiosis does not increase the chromosome number to \(4n\); it reduces it, so this is wrong.

Answer:

B. The number is reduced to \(n\) by meiosis. (Assuming the second option is labeled as B, as per the standard multiple - choice labeling. If the original options had different labels, adjust accordingly based on the correct option text "The number is reduced to \(n\) by meiosis".)