8.2
Meiosis I
Meiosis I, also known as reductional division, is the first cell division of meiosis that produces two haploid cells from a diploid cell. Meiosis I is divided into four distinct stages – prophase I, metaphase I, anaphase I, and telophase I.
Prophase I consists of five substages – leptotene, zygotene, pachytene, diplotene, and diakinesis.
In leptotene, the chromosomes begin to condense, appearing as a thread-like, beaded structure where each chromosome consists of a pair of sister chromatids held together by cohesin complexes.
At the onset of zygotene, homologous chromosomes pair to form tetrads.
The pairing, also known as synapsis, occurs lengthwise where each gene segment of paternal chromosomes pairs with its counterpart on the maternal chromosome.
A protein based ladder-like structure called the synaptonemal complex forms between the paired chromatids. This complex aids pairing and recombination of the chromosomes.
In pachytene, the exchange of equal chromosome segments or crossing over occurs between two homologous chromosomes. The chromosomes connect at the point of the crossover called chiasma.
During diplotene, the synaptonemal complex breaks down and the paired chromosomes begin to dissociate, with the chiasma becoming visible.
Diakinesis, the final step of prophase, is characterized by further condensation of the chromosomes, dissolution of the nuclear envelope and the formation of a meiotic spindle.
In metaphase I, the paired homologous chromosomes are randomly positioned at the equator and the centromere of each chromosome in a tetrad attaches to the opposite poles of the spindle. The distribution of the chromosomes into two different cells is arbitrary and independent of other chromosomes. This is also known as an independent assortment of the chromosomes, and it helps to produce unique gametes.
In anaphase I, the centromere holding the duplicated chromosomes together does not get divided, and therefore, the whole chromosome with a pair of chromatids moves to the opposite poles of the cell, creating two haploid sets of the genome.
In the final stage, telophase I, chromosomes decondense and a nuclear membrane forms around each haploid genome.
During cytokinesis in animal cells, the cell membrane pinches in forming two haploid daughter cells.
Meiosis I plays a vital role in creating genetic diversity in the species through genetic recombination and independent assortment of chromosomes.
Meiosis is the division of a diploid cell into haploid cells forming sperm and eggs in animals through differentiation. Meiosis I is the first stage of meiosis, where the genetic recombination of homologous chromosomes and the reduction of the ploidy level by half occurs.
Prophase I is the most extended and complex step of meiosis I characterized by synapsis, chromosome pairing, and recombination of the homologous chromosomes. This process is facilitated by a proteinaceous structure called the synaptonemal complex. The assembly of the synaptonemal complex starts with the formation of the scaffold between two homologous chromatids. These scaffolds are known as the axial element, and they extend along the entire length of the chromosomes. This is followed by pairing and stabilizing the chromosomes along the axial elements with the help of transverse filaments and the middle central elements.
In most organisms, synapsis requires the formation of the double-strand breaks in the DNA created by the dedicated enzyme called Spo11. Two homologous chromosomes may cross over during the repair of these double-strand breaks. Each chromosome undergoes at least one cross-over during the prophase I stage.
In metaphase I, the paired homologous chromosomes are aligned on the equator and then start moving to the opposite poles of the cell during anaphase I. In a process called nondisjunction, the paired chromosomes fail to segregate during anaphase I, leading to the production of daughter cells with an abnormal number of chromosomes. Abnormalities in chromosome numbers can cause genetic diseases such as Down syndrome. Down syndrome is the most common disease caused by nondisjunction of chromosome number 21 during maternal meiosis. This results in an impediment in the physical and cognitive growth of the affected individuals.
Meiosis I, also known as reductional division, is the first cell division of meiosis that produces two haploid cells from a diploid cell. Meiosis I is divided into four distinct stages – prophase I, metaphase I, anaphase I, and telophase I.
Prophase I consists of five substages – leptotene, zygotene, pachytene, diplotene, and diakinesis.
In leptotene, the chromosomes begin to condense, appearing as a thread-like, beaded structure where each chromosome consists of a pair of sister chromatids held together by cohesin complexes.
At the onset of zygotene, homologous chromosomes pair to form tetrads.
The pairing, also known as synapsis, occurs lengthwise where each gene segment of paternal chromosomes pairs with its counterpart on the maternal chromosome.
A protein based ladder-like structure called the synaptonemal complex forms between the paired chromatids. This complex aids pairing and recombination of the chromosomes.
In pachytene, the exchange of equal chromosome segments or crossing over occurs between two homologous chromosomes. The chromosomes connect at the point of the crossover called chiasma.
During diplotene, the synaptonemal complex breaks down and the paired chromosomes begin to dissociate, with the chiasma becoming visible.
Diakinesis, the final step of prophase, is characterized by further condensation of the chromosomes, dissolution of the nuclear envelope and the formation of a meiotic spindle.
In metaphase I, the paired homologous chromosomes are randomly positioned at the equator and the centromere of each chromosome in a tetrad attaches to the opposite poles of the spindle. The distribution of the chromosomes into two different cells is arbitrary and independent of other chromosomes. This is also known as an independent assortment of the chromosomes, and it helps to produce unique gametes.
In anaphase I, the centromere holding the duplicated chromosomes together does not get divided, and therefore, the whole chromosome with a pair of chromatids moves to the opposite poles of the cell, creating two haploid sets of the genome.
In the final stage, telophase I, chromosomes decondense and a nuclear membrane forms around each haploid genome.
During cytokinesis in animal cells, the cell membrane pinches in forming two haploid daughter cells.
Meiosis I plays a vital role in creating genetic diversity in the species through genetic recombination and independent assortment of chromosomes.
From Chapter 8:
