Question

Assume that the diploid number of chromosomes in a cell is \(2 \mathrm{n}=8\). Suppose that in undergoing meiosis one pair of homologs fails to disjoin at the first meiotic division and both homologs of the pair proceed to the same pole. (a) How many chromatids would be present in each daughter cell resulting from meiosis I? (b) If the second meiotic division divides all the sister chromatids, how many chromatids would be present in each of the four resulting gametes?

Short answer

In this scenario, after meiosis I, one daughter cell would have 12 chromatids, while the other daughter cell would have 4 chromatids. After meiosis II, two of the resulting gametes would have 6 chromatids each, and the other two gametes would have 2 chromatids each.

Step-by-step solution

Analyzing the number of chromosomes and chromatids in the diploid cell

A diploid cell has a chromosome number of 2n = 8. Each chromosome consists of two chromatids, which means that the total number of chromatids in the diploid cell is 16.

Meiosis I

The cell undergoes meiosis I, but one pair of homologous chromosomes fails to disjoin and both homologs proceed to the same pole. This means that one daughter cell will receive both homologous chromosomes, resulting in an abnormal number of chromatids. In a normal meiosis I, each daughter cell would receive half of the diploid chromosome number, which is n=4. Since the diploid cell has 8 chromosomes, one daughter cell would receive 6 chromosomes whereas the other one would receive only 2 chromosomes after the non-disjunction event.

Counting chromatids in daughter cells after meiosis I

Based on the number of chromosomes in each daughter cell after meiosis I: - The daughter cell that received 6 chromosomes would have a total of 12 chromatids (Remember that each chromosome has 2 chromatids) - The daughter cell that received 2 chromosomes would have a total of 4 chromatids So, the answer for part (a) is 12 chromatids in one daughter cell and 4 chromatids in the other daughter cell.

Meiosis II

During the second meiotic division (meiosis II), the chromatids separate, with each daughter cell receiving one chromatid from each original chromosome.

Counting chromatids in gametes after meiosis II

Since each chromatid in the resulting gametes comes from one original chromosome, the total number of chromatids in each of the four gametes would be: - Two gametes coming from the daughter cell with 12 chromatids would receive 6 chromatids each - Two gametes coming from the daughter cell with 4 chromatids would receive 2 chromatids each So, the answer for part (b) is 6 chromatids in two of the gametes and 2 chromatids in the other two gametes.

Key concepts

Chromosome Non-disjunction

Chromosome non-disjunction occurs when chromosomes do not separate properly during cell division. In meiosis, this can happen during anaphase I or II, leading to an unequal distribution of chromosomes in the resulting cells.
Imagine that chromosomes are like clothes in a suitcase, and during meiosis, the suitcase is supposed to equally divide the clothes between two suitcases (daughter cells). Non-disjunction is like when one suitcase accidentally gets more clothes than the other.
This issues can lead to gametes with an incorrect number of chromosomes, potentially leading to disorders if fertilization occurs.

Chromatids

Chromatids are the two identical halves of a duplicated chromosome. They're held together by a structure called a centromere.
Visualize a chromosome as an "X" shape; each linear part is a chromatid. When a cell prepares to divide, the DNA condenses into these X-shaped pairs. Each half of the X represents a chromatid.
During meiosis I, homologous chromosomes, each consisting of two chromatids, separate. By meiosis II, the chromatids themselves are pulled apart, leading to single chromatid chromosomes in each gamete.

Gametes

Gametes are the reproductive cells containing half the genetic material of an organism. In humans, these are sperm and eggs.
During meiosis, cells divide to form gametes, reducing the chromosome number by half. This ensures that when fertilization occurs, the resulting zygote has the proper diploid number.
Even with non-disjunction, gametes still form but may have too many or too few chromosomes, potentially affecting development if fertilization occurs.

Diploid Number

The diploid number is the total number of chromosomes in a diploid cell, typically symbolized as 2n. For humans, this number is 46.
Diploid cells contain two complete sets of chromosomes, one from each parent. This is crucial for sexual reproduction, ensuring that each parent contributes equally to the offspring.
During meiosis, the diploid number is halved, forming haploid gametes. This process maintains genetic stability across generations.