How often have we heard relatives comparing our looks to that of our parents? ‘You have your father’s nose’, ‘your eyes are just like your mother’s’, these are comments that we have all heard from someone or the other in our lives. Do you know why? This is because we are born by the fusion of gametes produced by our father and our mother. These gametes carry chromosomes from the respective parent and as they fuse, a zygote is formed which receives a combination of maternal and paternal chromosomes and ultimately grows into an offspring, which in this case is us. But how are these gametes formed? Gamete formation requires a special type of cell division which allows only half the chromosomes of each parent to enter the gametes. Do you know why? This is because the chromosome number in each and every cell of an organism is fixed based on the species it belongs to. If gametes carry the same number of chromosomes as all the other cells of a parent, then fusion of the male and female gametes will result in the formation of a zygote that will have double the number of chromosomes compared to that of the parents. This will disturb the genetic integrity of the species and the zygote will no longer belong to the same species. Thus, to maintain the chromosome number of the gametes at half of that of the parent cells, a special reduction division, known as meiosis is carried out. Meiosis occurs in two phases, the first of which ensures the reduction in the chromosome number. In this article we will discuss the first meiotic division or Meiosis I in detail.
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Meiosis is a type of cell division that results in the formation of four haploid daughter cells from one diploid parent cell. This means that during meiosis the chromosome number in the daughter cells is half of that of the parent cell. Hence this type of division is also known as reduction division.
Meiosis occurs in diploid sexually reproducing organisms for the purpose of gamete formation. In sexually reproducing haploid organisms, it occurs right after the formation of a zygote to give rise to a haploid gametophytic body that produces haploid gametes by mitosis.
Meiosis involves two successive cell divisions, meiosis I and meiosis II, which follow in close sequence with only a single cycle of DNA replication during the S phase of the interphase of cell cycle. The cells undergoing meiosis are known as the meiocytes.
The first meiotic division, also known as the reduction division or heterotypic division results in the formation of two daughter cells with haploid number of chromosomes.
The second meiotic division, also known as the homotypic division is a mitotic division in which each of the two haploid daughter cells undergo mitosis to form four haploid daughter cells.
Meiosis I is initiated after the chromosomes of the parent cell have undergone replication to form identical sister chromatids during the S phase of the cell cycle. The reduction in chromosome number in the daughter cells occurs during this phase because the homologous chromosomes of the parent cell separate during this phase and enter two different daughter cells. Meiosis I can further be divided into two phases - Karyokinesis I (division of the nucleus) and Cytokinesis I (division of the cytoplasm). Karyokinesis I is further divided into Prophase I, Metaphase I, Anaphase I and Telophase I.
Fig: Events of Meiosis I
This phase involves all the nuclear changes that result in the division of the diploid parent nucleus into two haploid daughter nuclei. Let us discuss the stages involved in the karyokinesis of meiosis I.
Prophase I during meiosis I is the longest phase of meiotic cell division and is much more complex compared to prophase of mitosis. This phase is further subdivided into 5 stages -
Fig: Events of Prophase I of Meiosis I
The nucleus in this stage is large and shows the appearance of long fine thread-like chromosomes formed by the condensation of chromatin material. In this stage, each chromosome appears to be longitudinally single but its DNA is already duplicated, such that it is formed of two chromatids. However, the chromatids are tightly wound together and not visible as separate entities.
The chromosomes exhibit a beaded appearance due to the presence of darkly stained regions known as chromomeres which represent the highly coiled regions of DNA each of which correspond to a gene. The position, number and the size of chromomeres are constant and identical in homologous chromosomes.
During this phase, shortening of chromosomes occurs by further coiling which makes them distinctly visible. The homologous chromosomes of each pair are brought close together and paired up. This pairing is known as synapsis.
Fig: Homologous chromosomes
Pairing can be proterminal (beginning from the end), procentric (beginning from the centre) or random (starting at many points and moving towards the ends). Pairing is very specific and involves point to point or gene to gene pairing across the length of the chromosome. Each pair is called a bivalent at this stage and the number of bivalents is half the number of chromosomes in the cell.
Fig: Bivalent formation
A fibrillar, ladder-like structure known as the synaptonemal complex is formed at the centromeric region of the homologous chromosomes to stabilise the pairing until crossing over is completed.
The nucleolus increases in size and the centrioles start moving apart.
The paired chromosomes coil further during this stage to thicken, shorten and become more visible. The sister chromatids become more distinct due to the formation of a longitudinal furrow. Thus, each bivalent now consists of four chromatids and is called a tetrad.
Fig: Tetrads visible during pachytene stage
Pachytene is also characterised by exchange of chromatid segments between the non-sister chromatids of each tetrad. This is known as crossing over. During this process, breaks are introduced in the non-sister chromatids of a tetrad at identical points with the help of the endonuclease enzyme. The broken segments of the non-sister chromatids interchange, that is, the broken segment of one chromatid joins with the non-sister chromatid of its tetrad with the help of the ligase enzyme. The point of interchange and rejoining appears like an X-shaped structure known as the chiasma. There can be more than one chiasmata in a tetrad. Crossing over helps in creating new combinations of alleles in the chromosomes and thus helps to bring about variations in gametes.
Fig: Crossing over
By the end of pachytene, crossing over is completed and the non-sister chromatids are left linked at the site of crossing over.
GIF: Pachytene stage
In diplotene, the synaptic forces of attraction between homologous chromosomes lapse, the synaptonemal complex is dissolved and the homologous chromosomes start separating. But the non-sister chromatids remain in contact at the chiasmata. Simultaneously, the chromosomes become increasingly shorter and thicker by coiling. As each bivalent becomes increasingly shorter, the chiasmata moves away from the centromere and approaches the ends. This phenomenon is known as terminalisation.
Fig: X-shaped chiasmata
This phase can last for months or years in the oocytes of some vertebrates.
Fig: Human Oocyte
During this phase, the chromosomes continue to contract such that bivalents are the thickest at this stage. They start appearing as round and darkly stained bodies due to further terminalisation. During the later part of this phase, the centriole and centrosphere divide and move towards the opposite poles and the nuclear membrane and nucleolus disappears.
The chromosomes arrange themselves at the equatorial plate with their arms rested at the equator and the centromeres facing towards the poles. The spindle fibres are formed between the two centrioles and the chromosomal spindle fibres attach to the kinetochore at the centromere of each chromosome. The two chromosomes of a bivalent attach to two different spindle fibres, each belonging to different poles.
Fig: Metaphase I
The spindle fibres contract and pull the centromeres of homologous chromosomes towards opposite poles. Thus, the homologous chromosomes are separated and reach the opposite poles due to contraction of the spindle fibres (disjunction). By the end of anaphase, two groups of haploid chromosomes are formed at each pole of the cell.
Fig: Anaphase I
The chromosomes which have reached the poles will uncoil and form a chromatin net. Nuclear membrane and nucleolus reappear and form two haploid daughter nuclei.
Fig: Telophase I
This results in the division of the cytoplasm and formation of two haploid daughter cells. Each daughter cell receives only one member of a bivalent and hence haploid number is maintained in daughter cells. However, cytokinesis may be postponed till the end of the second meiotic division and the daughter nuclei formed at the end of meiosis I may immediately enter the second meiotic division.
Meiosis I helps to reduce the number of chromosomes in the daughter cells to one half (diploid to haploid). Reduction in number of chromosomes is essential for sexual reproduction. Random shuffling of the maternal and paternal chromosomes in the daughter cells and crossing over during meiosis I helps in introducing new genetic combinations in the daughter cells and thus helps to bring about variations in offspring.
Q1. Which of the following options correctly represents the order in which the following events occur during meiosis I?
Solution: The correct order in which the given events occur is - synapsis, followed by crossing over, terminalisation and disjunction of the homologous chromosomes. Synapsis involves the pairing of the homologous chromosomes during zygotene. Crossing over occurs during pachytene when segments are exchanged between the non-sister chromatids of the paired homologous chromosomes. Terminalisation occurs during diplotene and involves moving of the chiasmata away from the centromere and towards the ends. Disjunction of the homologous chromosomes occurs during anaphase I during which the homologous chromosomes are separated and reach the opposite poles due to contraction of spindle fibres attached to their centromeres. Thus, the correct option is b.
Q2. The chromosome appears as a single, long, thin thread-like structure during
Solution: During leptotene, the nucleus is large and shows the appearance of long, thin thread-like chromosomes formed by the condensation of chromatin material. Thus, the correct option id d.
Q3. Crossing over during pachytene occurs between
Solution: Pachytene is characterised by exchange of chromatid segments between the non-sister chromatids of each tetrad which is made up of two homologous chromosomes, each having their sister chromatids. This is known as crossing over. During this process, breaks are introduced in the non-sister chromatids of a tetrad at identical points with the help of the endonuclease enzyme. The broken segments of the non-sister chromatids interchange, that is, the broken segment of one chromatid joins with the non-sister chromatid of its tetrad with the help of the ligase enzyme. Thus, the correct option is c.
Q4. The bivalents start appearing as tetrads during
Solution: During zygotene, the homologous chromosomes are brought together and they form pairs known as bivalents. During pachytene, the paired chromosomes coil further during this stage to thicken, shorten and become more visible. The sister chromatids become more distinct due to the formation of a longitudinal furrow. Thus, each bivalent now consists of four chromatids and is called a tetrad. Thus, the correct option is c.
Q1. Which is the longest phase of Prophase I?
Answer: Pachytene is the longest stage of prophase I. This phase is characterised by crossing over between the non-sister chromatids of the homologous chromosomes.
Q2. What is interkinesis?
Answer: Interkinesis is the brief interphase between the first meiotic division and the second meiotic division. Its duration varies from one species to another.
Q3. What is meant by sporogenic meiosis?
Answer: In plants, the haploid spores give rise to haploid gametophytes which produce gametes by mitosis. The haploid spores are produced from the zygote in some algae and spore mother cells in other plants by the process of meiosis, known as sporogenic meiosis.
Q4. What is nondisjunction?
Answer: The failure of separation of homologous chromosomes during meiosis is called nondisjunction of chromosomes. It results in an abnormal number of chromosomes in the gametes and can lead to chromosomal disorders.
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