Definition of meiosis: Meiosis is basically the type of cell division due to which that one parent cell divides into four daughter cells. And the number of chromosomes in each daughter cell is half the number of chromosomes of the parent cell. Phases of Meiosis briefly describe below.
Why Meiosis is Important?
Meiosis, from the Greek word “meioun”, meaning “to make small,” refers to the specialized process by which germ cells divide to produce gametes. Because the chromosome number of a species remains the same from one generation to the next, the chromosome number of germ cells must be reduced by half during meiosis. Meiosis is highly ubiquitous among eukaryotes as it can occur in single-celled organisms like yeast as well as multicellular ones like humans. The process of meiosis is very essential in ensuring genetic diversity through sexual reproduction. In humans, two distinct types of daughter cells are produced by males and females (sperm and egg cells respectively).
Phases of Meiosis:
The process of meiosis takes place in two phases
- Meiosis I
- Meiosis II
Phases of Meiosis 1 and Meiosis 2
During meiosis I, a cell is divided into two, and in meiosis II, further division takes place, resulting in a total of four haploid cells. Before the start of the process, the parent cell goes through a stage of preparation called the interphase. During this phase, the parent cell synthesizes more DNA and proteins, thereby increasing the overall size and mass of the cell.
Meiosis stages Diagram
Meiosis I Definition
During the cell division after the Interphase meiosis, I take place. after Interphase, I, where proteins are grown in G phase and chromosomes are replicated in the S phase. Following this, four phases occur. Meiosis I is known as reductive division, as the cells are reduced from being diploid cells to being haploid cells.
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Phases of Meiosis I
- Prophase I
of meiosis is considered as the most complicated and longer step in the whole process. In this stage, the chromosomes start to condense and pair up with its homologue. It is divided into five phases:
Phases of Prophase I
- Leptotene (first phase) is characterized by the condensation of the chromosomes wherein they become visible as chromatin. (Greek words “lepto” and “tene” which mean “thin” and “ribbon”).
- Zygotene (Greek words “zygo” and “tene” which mean “union” and “thread“) occur when homologous chromosomes begin to form an association called a synapse which results to pairs of chromosomes that have four chromatids.
- Pachytene (Greek words “pachy” means “thick”) is the phase where the crossing over between pairs of homologous chromosomes occurs. The structure formed is referred to as the chiasmata.
- Diplotene (“diplo” means “double”) is the phase where the separation of the homologous chromosomes is starting but they remain attached through the chiasmata.
- Diakinesis (Greek word “dia” means across and “kinesis” means “motion”) is the when the homologous chromosomes continue to separate as the chiasmata move to the opposite ends of the chromosomes.
2. Metaphase I
It is the stage when the homologous pairs of chromosomes randomly align at the metaphase plate. Such configuration becomes the source of genetic material as the chromosomes from the male and female parents appear similar but are not exactly identical.
3. Anaphase I
It is the Separation of the pairs of homologous chromosomes of different daughter cells towards opposite ends. Before the pairs can separate, crossovers between chromosomes must be resolved and meiosis-specific cohesins must be released from the arms of the sister chromatids. Failure to separate the pairs of chromosomes to different daughter cells is referred to as nondisjunction.
4. Telophase I
Telophase I in which chromosomes are finally separated at the different sides of the cell. In addition, the chromosomes return to their uncondensed forms as the nuclear membrane is reformed. The division of the cytoplasm (referred to as cytokinesis) occurs simultaneously resulting in two haploid daughter cells.