Introduction
In this section, you will learn what is meiosis. In addition, to know what is meiosis you will also understand the major phase of meiosis. Meiosis is a type of cell division that takes place in all multicellular organisms. Haploid germ cells are produced as a result of meiosis. This type of cell division also is responsible for the transfer of genetic material (DNA) from generation to generation. It means meiosis maintains the total DNA amongst organisms belonging to the same species. Now we discuss our topic i.e. what is meiosis?
What is meiosis? Who discovered meiosis?
Meiosis is a type of cell division in which a diploid cell (parent cell) produces four haploid cells (daughter cells). This cell division only occurs in multicellular organisms, e.g. in higher animals, and plants, in their germ line cells. As for meiosis concerns it differs from mitosis. Now you will learn who discovered meiosis.
Discovery of meiosis
In the beginning, meiosis was demonstrated for the first time by German Biologist Oscar Hertwig in 1876. Oscar discovered meiosis in sea urchins. After about 7 years it was again described at the chromosomal level by the Belgian zoologist Edouard Van Beneden in 1883. Beneden discovered meiosis in Ascaris roundworm eggs. After learning the discovery of meiosis you will be able to understand the phase of meiosis. At the end of the article, you will also understand some of the major differences between the two cell division mitosis and meiosis.
Phases
During meiosis, a cell undergoes two divisions, the stages of this cell division as in mitosis. The phases of the first division are prophase 1, metaphase 1, anaphase 1, and telophase 1. Prophase 1 of meiosis has some differences from that of the prophase of mitosis. Prophase is the longest phase in both types of cell division. Before learning prophase 1 let’s understand the concept of meiosis 1.
Before learning prophase 1 let’s understand the concept of meiosis 1.
Meiosis 1
Meiosis 1 is called heterotypic division. It is also called reductional division because in meiosis 1 the number of chromosomes is reduced to half. Before the start of meiosis 1, interphase takes place which is nearly the same as in mitosis. In the S phase, the amount of DNA becomes double remaining the number of chromosomes constant. After interphase karyokinesis (a division of the nucleus) and then cytokinesis (a division of the cytoplasm) occur. Karyokinesis is further divided into 4 stages, prophase 1, metaphase 1, anaphase 1, and telophase 1 like in mitosis. We know that meiosis is a double division, therefore for meiosis 1 the name of its stages as prophase 1, metaphase 1, anaphase 1, and telophase 1, and for meiosis 2 the names of its stages are prophase 2, metaphase 2, anaphase 2 and telophase 2. Now let’s learn and understand the process of prophase 1.
prophase 1 meiosis 1
When prophase 1 starts, the chromatin which is the complex of DNA and protein (histone in higher organisms) condenses to produce chromosomes (chroma: color, soma: body). Prophase 1 is the most complicated stage and the longest phase of meiosis 1 as compared to the prophase of mitosis. The pair of chromosomes known as chromatids remain attached at a central point called the centromere. In meiosis, prophase is further divided into five sub-phases, Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis. In prophase crossing over and genetic recombination between non-sister chromatids take place, which consequences in the genetically unidentical, haploid daughter chromatid cell.
Leptoptene or leptonema
The word Letptotene derives from two Greek words Leptos; thin and tainia ; band. Leptotene is also called Leptonema. During leptotene, condensation of chromatin material begins. Chromosomes look like thin and elongated threads. Chromosomal ends converge to a specific site inside the nucleus, particularly where the centromeres are present, and sometime this sight looks like a bouquet, therefore this stage is called the bouquet stage.
Zygotene or zygonema
After the end of leptotene, the second stage of prophase 1 starts which is called Zygotene or Zygonema. The word zygotene comes from two Greek words, Zygo = pair; tainia = band. In zygotene the pairing of homologous chromosomes takes place. The pairing of homologous chromosomes is also called synapsis (synapsis means union). One chromosome comes from the mother and the other from the father and is then paired with each other. There are three types of synapsis.
- Procenrtic synapsis.
In this type, the pairing of homologous chromosomes takes place from the centromere and goes to the ends of chromosomes. - Proterminal synapsis
This type of synapsis is completely opposite to the pro centric In the preterminal synapsis type the pairing of homologous chromosomes starts from the terminal ends and goes to the center of homologous chromosomes. - Random or Localised synapsis
In random or localized synapsis the pairing of homologous chromosomes takes place from different points of chromosomes.
In all three types of synapsis, the pairing of homologous chromosomes is very specific and the homologous chromosomes become attached to each other by a protein-carrying framework generally called synaptonemal complex. The main function of the synaptonemal complex is to stabilize the pairing of homologous chromosomes and also help in the crossing over. The pair of homologous chromosomes forms as a result of synapsis is called bivalent or tetrad. The bivalent is not yet visible in zygotene, it becomes visible in Pachytene.
Pachytene or Pachnema
The word pachytene is derived from two Greek words: Pachus = thick; tainia = band. This is the longest stage of leptotene and zygotene. In this stage, the condensation of chromosomes continues and the chromosomes become usually thick and short and now the bivalent becomes clearly visible. The chromatids of the same chromosome are sister chromatids, while the chromatids of the other chromosome of the same structure are called non-sister chromatids. The chromatids of the non-sister chromosomes come close to each other and are spirally attached to others.
Diplotene or diplonema
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Diplotene or diplonema word diplotene comes from two Greek words Diplo’s = two; tainia = band. In this stage, un-pairing or de-synapsis of the homologous chromosome begins. In this process, the chromatids of paired chromosomes remain attached at some specific points which are chiasmata (singular chiasma). The word chiasma is derived from a Greek word that means cross piece. Chaimeta is the site where crossing over takes place. In diplotene, the nuclear membrane and nucleolus start to disappear. One thing you should keep in mind is that in oocytes of some vertebrates, the diplotene continues for months or sometimes for years.