A bivalent is also called a(n) __ because it is composed of four chromatids.
The correct answer and explanation is:
The correct answer is tetrad.
A bivalent is also known as a tetrad because it consists of four chromatids. This term is used during the process of meiosis, specifically in prophase I, where homologous chromosomes pair up. Each homologous chromosome consists of two sister chromatids, so when the two chromosomes pair, a structure of four chromatids is formed, which is referred to as a tetrad.
The tetrad plays a crucial role in the process of genetic recombination. During meiosis I, homologous chromosomes exchange genetic material in a process called crossing over. This occurs when the chromatids of homologous chromosomes overlap and break, and then reattach to the opposite chromosome. The exchange of genetic material leads to genetic variation in offspring, which is fundamental to evolution and biodiversity.
Tetrads are visualized under a microscope during the meiotic process, and their formation marks the critical step where genetic variation is introduced. The formation of the tetrad and subsequent crossing over ensure that each gamete (egg or sperm) contains a unique set of genetic instructions, which contributes to genetic diversity.
In summary, a bivalent is called a tetrad because it contains four chromatids. This structure is formed by the pairing of two homologous chromosomes, each with two sister chromatids, and is crucial for the process of genetic recombination, which is essential for sexual reproduction.