Question

In which stage of meiosis, the chromosome is thin, long and thread-like? (a) Leptotene (b) Zygotene (c) Pachytene (d) Diakinesis

Short answer

The correct answer is (a) Leptotene.

Step-by-step solution

Understand Meiosis

Meiosis is a type of cell division that produces four daughter cells, each with half the number of chromosomes of the parent cell. It occurs in two major phases: Meiosis I and Meiosis II. Both of these phases comprise different stages with distinctive characteristics.

Characteristics of Leptotene

In the Leptotene stage, chromosomes start to condense, but they are still thread-like and long. This is the first stage of Prophase I of Meiosis I.

Characteristics of Zygotene

During the Zygotene stage, the chromosomes continue to condense and synapsis begins. Synapsis is the pairing of two homologous chromosomes. However, they do not appear thread-like in this stage.

Characteristics of Pachytene

In the Pachytene stage, the chromosomes become shorter and thicker. The sister chromatids are clearly visible, and crossing over may occur.

Characteristics of Diakinesis

In the Diakinesis stage, the chromosomes are fully condensed and the nuclear envelope disintegrates, marking the end of Prophase I.

Identify the Correct Answer

Based on the aforementioned characteristics of the different stages of Meiosis, it can be inferred that the chromosomes are thin, long, and thread-like during the 'Leptotene' stage of Meiosis.

Key concepts

Leptotene

In meiosis, during the Leptotene stage, chromosomes begin a journey of transformation. At this early stage, they are long, thin, and thread-like, which is their least condensed form. Leptotene marks the very beginning of Prophase I in the first meiotic division. Here, chromosomes start to condense from more diffuse forms. This stage is crucial for preparing chromosomes for the intricate processes that follow, such as pairing and crossing over. Though it might seem uneventful, this is the starting point where chromosomes are set to undergo significant changes in meiosis.

Chromosome Structure

Chromosome structure is fundamental to understanding meiosis. Each chromosome is made up of DNA tightly coiled many times around proteins called histones, which support its structure. This coiled format ensures that a large amount of genetic material fits inside the cell. In eukaryotic cells, chromosomes are found inside the nucleus.

• During meiosis, chromosomes undergo various structural changes.
• These changes enable processes like genetic recombination and cell division.

Understanding their structure helps us grasp how chromosomes can condense, align, and separate during the stages of meiosis.

Cell Division

Cell division is a life-essential process that allows organisms to grow, repair tissues, and reproduce. Meiosis is a specialized type of cell division that reduces the chromosome number by half. It results in four daughter cells, each with a unique set of chromosomes. These cells are often germ cells, such as sperm and eggs in animals, or spores in plants.

• Meiosis consists of two sequential stages: Meiosis I and Meiosis II.
• It plays a key role in maintaining genetic diversity through recombination.

The precise control and timing of each stage facilitate successful cell division and genetic variance.

Prophase I

Prophase I is one of the most detailed stages of meiosis. It introduces critical changes that set the stage for genetic diversity. During this phase, chromosomes condense and homologous chromosomes pair up in a process called synapsis.

• This stage is further divided into sub-stages: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
• Each sub-stage has unique characteristics, such as the beginning of synapsis in Zygotene and the possibility of crossing over in Pachytene.

Prophase I is essential for ensuring that homologous chromosomes can exchange genetic material, adding to genetic diversity in offspring.

Synapsis

Synapsis is a critical event in meiosis, occurring during Prophase I. It involves the pairing of two homologous chromosomes. This pairing is essential because it allows the chromosomes to align correctly and facilitates genetic recombination, or crossing over.

• Synapsis begins during the Zygotene stage and is completed by the Pachytene stage.
• This process ensures that each daughter cell receives a complete set of chromosomes.

By helping chromosomes align properly, synapsis plays a key role in the accurate separation of chromosomes during meiosis, preventing genetic disorders.