Question

Find out the number of bivalents in zygotene stage, if there are 60 chromosomes in \[{G_1}\] phase.
A. 60
B. 30
C. 15
D. 45

Answer 1

Hint: In the zygotene stage, bivalents or tetrads are formed. Each bivalent is formed by four chromosomes. So, the number of bivalents can be calculated by dividing the number of the chromosome by four.

Complete answer: In meiosis I or reductional division, prophase I is divided into five stages: leptotene, zygotene, pachytene, diplotene, and diakinesis. In the zygotene stage, the homologous chromosomes or non-sister chromatids pair up to become bivalents or tetrads. It is also known as zygonema. This synapsis can form up and down the chromosomes allowing numerous points of contact called 'synaptonemal complex', this can be compared to a zipper structure, due to the coils of chromatin. According to the information provided in the question, if in the \[{G_1}\] phase, there are 60 chromosomes then, in the S phase, the chromosome number gets doubled therefore, 120 chromosomes are present at the end of the S phase. Each bivalent is formed of four chromosomes, therefore;
  $\dfrac{{120}}{4} = 30$ bivalents
So, 30 bivalents are formed in the zygotene stage.
Hence, the correct answer is option B.

Additional information: Meiosis is also known as the reductional division. It consists of two phases; meiosis I and meiosis II. Recombination is the most important event that occurs in meiosis, which leads to variations in organisms.

Note: The zygotene is a stage of prophase I that is preceded by pachytene and followed by leptotene. Before zygotene, the chromosomes start to condense and form a long thread-like structure in the nucleus.