Question

A bivalent consists of:

• A. two chromatids and one centromere
• B. two chromatids and two centromeres
• C. four chromatids and two centromeres
• D. four chromatids and four centromeres.

Answer 1

The Correct Option is C

A bivalent consists of four chromatids and two centromeres. To understand why this is the correct answer, we must delve into the process of meiosis where bivalents are involved.

1. Meiosis Overview: Meiosis is a type of cell division that reduces the chromosome number by half, creating four haploid cells from one diploid cell. This process occurs in two stages: Meiosis I and Meiosis II.
2. Formation of Bivalents: During Prophase I of Meiosis I, homologous chromosomes pair up, aligning closely to exchange genetic material through a process called crossing over. This pairing of homologous chromosomes is referred to as synapsis, and the resulting structure is called a bivalent or tetrad.
3. Composition of a Bivalent: Each homologous chromosome consists of two sister chromatids connected by a centromere. Therefore, a pair of homologous chromosomes (a bivalent) consists of:
   • 4 chromatids (2 chromatids per homologous chromosome)
   • 2 centromeres (1 centromere per homologous chromosome)
   Hence, the composition is four chromatids and two centromeres.
4. Eliminating Incorrect Options:
   • Two chromatids and one centromere: This describes a single chromosome, not a bivalent.
   • Two chromatids and two centromeres: This does not account for both homologous chromosomes fully, as it would imply only one chromatid per homologous chromosome.
   • Four chromatids and four centromeres: This suggests that each chromatid has its own centromere, which is not the case. Two chromatids share a common centromere.
5. Conclusion: Thus, the correct description of a bivalent is "four chromatids and two centromeres."