If a diploid cell has 18 chromosomes, how many chromatids will it have at the bivalent stage?

The correct option is A

9

The correct option is A.

Explanation of the correct option:

1. When an organism's cells contain two full sets of chromosomes, with one chromosome from each parent present in each pair, the organism is said to be diploid.
2. Since humans are diploid, the majority of their cells have 23 chromosomal pairs.
3. One of a chromosome's two identical halves that have undergone replication in order to facilitate cell division is referred to as a chromatid.
4. The centromere, a constrictive area of the chromosome, serves as the connection between the two "sister" chromatids.
5. In a tetrad, a bivalent is one set of sister chromatids on a pair of chromosomes.
6. A tetrad is an association of two identical chromosomes (4 sister chromatids), kept together by at least one DNA crossover.
7. The two identical or homologous chromosomes link together during cell division, particularly during the zygotene stage of meiotic prophase I, to produce the bivalent or tetrad structure.
8. Given that two chromosomes make up one bivalent and that the cell is in the diploid state (2n) and possesses 18 chromosomes, the number 18 will equal 9 when divided by 2.
9. As a result, the cell's 18 chromosomes will divide into 9 bivalents.

Final answer:

At the bivalent stage, a diploid cell will have 9 chromatids.