Problem 86 - Objective Biology for NEET

Question

Bivalents are formed during (a) Diplotene (b) Pachytene (c) Zygotene (d) Leptotene

Step-by-Step Solution

Answer

The bivalents are formed during the Zygotene stage of Prophase I in meiosis.

Step 1: Understanding Terminologies

First, it is essential to understand the terms used. Bivalents are pairs of homologous chromosomes that are formed during meiosis. They involve the two chromosomes lining up side by side, which is a necessary part of genetic recombination during meiosis.

Step 2: The Stages of Meiosis

Leptotene, Zygotene, Pachytene, and Diplotene are sub-stages of prophase I in meiosis. Leptotene is the stage where chromosomes become visible, but pairing has not started yet. Zygotene is when the synaptonemal complex forms and chromosomes begin to pair up, a process known as synapsis. Pachytene is when synapsis completes and crossing over occurs. Diplotene is when the homologous chromosomes start to separate but remain attached at crossover points (chiasmata).

Step 3: Identifying When Bivalents are Formed

Knowing the occurrences at each stage, it can be recognized that bivalents (pairing of homologous chromosomes) are formed during Zygotene stage, when synaptonemal complex forms, and the homologous chromosomes begin to pair up.

Key Concepts

Meiosis Stages

Meiosis is a fundamental process in sexual reproduction that assures the genetic diversity of organisms. It involves two consecutive cell divisions: Meiosis I and Meiosis II, both consisting of several distinct stages.

During Prophase I of Meiosis I, the chromosomes begin to condense, making them visible under a microscope. This initiation stage is termed Leptotene. Chromosome pairing starts in the following stage, known as Zygotene, where the infrastructure for genetic recombination is created. This sets the stage for Pachytene, where the homologous chromosomes are fully synapsed, and crossing over occurs, exchanging genetic material between chromatids. The fourth stage, Diplotene, involves the dissolution of the synaptonemal complex, and chiasmata become visible, while homologous chromosomes start to separate but still remain attached at the crossover points. The stages following Diplotene are Metaphase I, Anaphase I, Telophase I, and finally Meiosis II, which resembles a typical mitosis ensuring that each of the four daughter cells has a haploid set of chromosomes.

Synaptonemal Complex

The synaptonemal complex is a ladder-like protein structure that forms between homologous chromosomes during Prophase I of meiosis. Its primary role is to facilitate the paring of homologous chromosomes, a process called synapsis. This is crucial as it is the physical basis for genetic recombination, as it maintains the alignment of homologs so that crossing over can occur accurately.

Detailed studies reveal that the complex is composed of two lateral elements (that attach to each chromosome) and a central element, all held together by transverse filaments. Defects in this complex can lead to chromosomal abnormalities due to improper recombination. Understanding the precise time and function of the synaptonemal complex formation, namely during Zygotene, is key to grasping how genetic diversity is achieved during meiosis.

Genetic Recombination

Genetic recombination during meiosis is a process whereby the DNA molecules of homologous chromosomes are shuffled, creating new combinations of alleles. This event primarily occurs during the Pachytene stage of Prophase I and is facilitated by the synaptonemal complex. The process involves the breakage and rejoining of DNA, which leads to exchange of genetic material between non-sister chromatids.

Recombination is not just a source of genetic diversity; it also plays a critical role in the proper segregation of chromosomes during the first division of meiosis. The resulting recombinant chromosomes contain genes from both parents, and this variability is essential for the evolution of species. Because of genetic recombination, siblings from the same parents tend to have different genetic make-ups, except for identical twins.

Chiasmata

Chiasmata (singular: chiasma) are the microscopically visible X-shaped exchange points where crossover has occurred between homologous non-sister chromatids during genetic recombination. They become evident during the Diplotene stage of Prophase I, as the synaptonemal complex disassembles and the homologous chromosomes begin to pull apart from each other.

Chiasmata are vital to the meiotic process as they maintain the connection between homologous chromosomes up to Metaphase I, ensuring proper alignment and segregation during cell division. They are physically manifested proof of crossing over, a fundamental mechanism behind genetic diversity. With chiasmata presence, chromosomes are guaranteed to assort independently, resulting in the probabilistic distribution of maternal and paternal chromosomes to the gametes.