[Biology Class Notes] on Meiosis 1 Stages and Process Pdf for EXAM

Sexual reproduction in organisms takes place through the process of fusion of male and female gametes that are produced in the form of sperm and egg respectively. Gametes are normally haploid in nature which means that they only contain about half the number of chromosomes. This genetic content tends to make them different from all the other types of body cells that are present. It is the process of meiosis that produces these haploid cells.

Meiosis is the process of producing gametes—sex cells, or sperm and eggs in the human body. In a human being, the haploid cells made in meiosis are sperm and eggs. At the point when a sperm and an egg participate in fertilization, the two haploid arrangements of chromosomes create a complete diploid set: another genome.

The process of Meiosis is divided into two parts: meiosis I and meiosis II which are further separated into Karyokinesis I and Cytokinesis I and Karyokinesis II and Cytokinesis II respectively. The preparatory steps that lead up to meiosis are similar in pattern and name to interphase of the mitotic cell cycle. Interphase is partitioned into three stages:

In this very active phase, the cell synthesizes its vast range of proteins, including the enzymes and structural proteins it will require for development and growth.

The hereditary material is replicated; each of the cell’s chromosomes duplicates to become two indistinguishable sister chromatids joined at a centromere. This replication does not change the ploidy of the cell since the centromere number continues as before. The indistinguishable sister chromatids have not yet consolidated into the thickly bundled chromosomes visible with the light microscope, which occurs during prophase I in meiosis.

G2 stage as observed before mitosis is absent in meiosis.

Interphase is followed by meiosis I and after that meiosis II. Meiosis I isolate homologous chromosomes, each still made up of two sister chromatids, into two daughter cells, subsequently decreasing the chromosome number by half. In the process of meiosis II, sister chromatids decouple, and the resultant daughter chromosomes are segregated into four daughter cells. 

For diploid organisms, the daughter cells resulting from meiosis are haploid and contain just a single duplicate of every chromosome. In some organisms, cells enter a resting phase known as interkinesis that occurs between meiosis I and meiosis II.

Meiosis I and II are each classified into prophase, metaphase, anaphase, and telophase stages. Accordingly, meiosis incorporates the phases of meiosis I (prophase I, metaphase I, anaphase I, telophase I) and meiosis II (prophase II, metaphase II, anaphase II, and telophase II). 

Meiosis produces gamete hereditary diversity in two ways: (1) Law of Independent Assortment of homologous chromosome matches along with the metaphase plate during metaphase I and introduction of sister chromatids in metaphase II, this is the resulting division of homologs and daughter chromatids during anaphase I and II, it permits an arbitrary and free distribution of chromosomes to every daughter cell (and at last to gametes). The physical trade of homologous chromosomal regions by homologous recombination during prophase I result in a new combination of DNA inside chromosomes.

During meiosis, specific genes are transcribed to a higher extent. notwithstanding solid meiotic stage-explicit articulation of mRNA, (for example selective utilization of preformed mRNA), directing a definitive meiotic stage-specific protein expression of genes during meiosis. Thus, both transcriptional and translational controls decide the broad restructuring of meiotic cells required to complete meiosis. 

Phases of meiosis

In each round of division, cells experience four phases: prophase, metaphase, anaphase, and telophase. 

Prophase I 

Prophase I is the longest stage of meiosis. During the phase of prophase I, homologous chromosomes pair and exchange DNA (homologous recombination). This frequently results in a chromosomal hybrid. The new combination of DNA made during hybrids is a critical source of hereditary variation and results in new mixes of alleles, which might be beneficial. 

The combined and duplicated chromosomes are called bivalents or quadruplicates, which have two chromosomes and four chromatids, with one chromosome originating from each parent. The method of pairing the homologous chromosomes is called synapsis. At this stage, non-sister chromatids traverse at areas called chiasmata. Prophase I has historically been divided into sub-stages which are named as per the appearance of chromosomes. 

Leptotene

The main phase of prophase “I” is the leptotene stage, otherwise called leptonema, from Greek words signifying “thin threads”. In this phase of prophase, I, singular chromosomes—each comprising two sister chromatids—progress toward becoming “individualized” to shape strands inside the nucleus. The two sister chromatids closely associate and are outwardly distinct from each other. During leptotene, horizontal components of the synaptonemal complex gather. Leptotene is of a very short span and progressive condensation and winding of chromosome strands occur.

Zygotene

The zygotene stage, otherwise called zygonema, from Greek words signifying “matched threads”, happens as the chromosomes approximately line up with one another into homologous chromosome pairs. In some organisms, this is known as the bouquet stage as a result of the manner in which the telomeres bouquet toward one side of the core. At this stage, the synapsis of homologous chromosomes happens, encouraged by the gathering of the central component of the synaptonemal complex. 

Pairing is achieved in a zipper-like style and may begin at the centromere (pro-centric), at the chromosome ends (terminal), or at some other portion. Individuals of a pair are equivalent in length and in the position of the centromere. Therefore, pairing is very specific and definite. The paired chromosomes are known as bivalent or quadruplicate chromosomes.

Pachytene

The pachytene stage, otherwise called pachynema, is from Greek words signifying “thick threads”. At this point, a quadruplicate of the chromosomes has been known as a bivalent. This is the phase when homologous recombination, including chromosomal hybrid (traverse), happens. At this stage, the Non-sister chromatids of homologous chromosomes may exchange segments over regions of homology. 

Diplotene

Diplotene, also known as diplonema, is a Greek word signifying “two threads”, At this stage, the synaptonemal complex degrades and homologous chromosomes separate from each other a bit. The chromosomes themselves uncoil a bit, permitting some transcription of DNA. Be that as it may, the homologous chromosomes of each bivalent remain firmly bound at chiasmata, the areas where crossing-over happened. The chiasmata stay on the chromosomes until they have disjoined at the progress to anaphase I. 

Diakinesis

Chromosomes assemble further during the diakinesis stage; from Greek words signifying “moving through”. This is the main point in meiosis where the four sections of the quadruplicates are really distinguishable. Regions of crossing-over entangle here, successfully covering, making chiasmata distinguishable. Apart from this perception, the rest of the stage intently takes after prometaphase of mitosis; the nuclear disappear, the nuclear membrane disintegrates into vesicles, and the meiotic spindle starts to form.

Synchronous procedures

During these stages, two centrosomes, containing a pair of centrioles in animal cells, relocate to the two poles of the cell. These centrosomes work as microtubule organizing centers nucleating microtubules, which are basically cell ropes and poles. The microtubules attack the nuclear region after the nuclear membrane breaks down, connecting to the chromosomes at the kinetochore. The kinetochore functions as a motor, pulling the chromosome along the appended microtubule toward the starting centrosome, similar to a train on a track.

Metaphase I

The protein complex cohesin-holds sister chromatids together from the time of their replication until anaphase. In mitosis, the power of kinetochore microtubules pulling in opposite directions creates tension. The cell senses this strain and does not progress with anaphase until all the chromosomes are appropriately bi-situated in meiosis, building up strain requires something like one hybrid for each chromosome pair notwithstanding cohesin between the sister chromatids.

Anaphase I

Kinetochore microtubules become short, pulling homologous chromosomes (which comprise a couple of sister chromatids) to opposite poles. Nonkinetochore microtubules extend, pushing the centrosomes separated apart. The cell stretches for division down the center. Unlike in mitosis, just the cohesin from the chromosome arms is degraded while the cohesin encompassing the centromere stays secured. This enables the sister chromatids to stay together while homologs are segregated.

Telophase I 

The primary meiotic division adequately closes when the chromosomes touch base at the poles. Each Daughter cell presently has half the number of chromosomes however every chromosome comprises a pair of chromatids. The microtubules that make up the spindle network breakdown and another nuclear membrane encompasses every haploid pair. The chromosomes uncoil once again into chromatin. Cytokinesis, the pinching of the cell wall in animal cells or the development of the cell wall in plant cells, happens, completing the formation of two daughter cells. Sister chromatids stay connected during telophase I.

Mitosis

This is a process in which there is a single cell that divides into two similar daughter cells that are identical to each other with the help of cell division. 

Meiosis 2 

In this process, the sister chromatids within the two daughter cells are separated which results in the formation of haploids. This process is similar to mitosis, however, it is seen that each cell that divides will only get a single set of homologous chromosomes.

Difference between meiosis 1 and meiosis 2:

Facts about meiosis

1. Meiosis takes part in the formation of eggs and sperm in eukaryotic organisms

2. There are four daughter cells that are provided due to the division of one cell which will be the parent cell twice.

This is all about the phases and properties of Meiosis I. Study the stages properly and understand what happens in a stepwise way. Learn how this complex cell division process occurs in higher-order animals.