References As a general rule, all cells come from pre-existing cells and the primary mode to how this happens is through the process of cell division. In this process, the parent cell divides into daughter cells while at the same time passing its genetic material across generations. In eukaryotic organisms, two kinds of cell division exist: mitosis and meiosis. On one hand, mitosis is concerned in the production of daughter cells that are mere copies of each other.
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References As a general rule, all cells come from pre-existing cells and the primary mode to how this happens is through the process of cell division. In this process, the parent cell divides into daughter cells while at the same time passing its genetic material across generations.
In eukaryotic organisms, two kinds of cell division exist: mitosis and meiosis. On one hand, mitosis is concerned in the production of daughter cells that are mere copies of each other. On the other hand, meiosis involved the production of the reproductive cells that bear unique genetic characteristics.
At the level of the genes, sexual reproduction is focused on the fusion of the paternal and maternal genes that result in a new combination of genes.
And this is where the process of meiosis becomes important. In this article, discover how meiosis occurs in living cells, its different stages, and its significance in the survival of eukaryotic organisms.
What is Meiosis? Meiosis is a type of cell division that involves the reduction in the number of the parental chromosome by half and consequently the production of four haploid daughter cells. This process is very essential in the formation of the sperm and egg cells necessary for sexual reproduction. When the haploid sperm and egg fuse, the resulting offspring acquires the restored number of chromosomes.
Meiosis is highly ubiquitous among eukaryotes as it can occur in single-celled organisms like yeast as well as multi-cellular 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.
Stages of Meiosis Contrary to mitosis, meiosis involves the division of diploid parental cells both paternal and maternal into haploid offspring with only one member of the pair of homologous chromosome from the parents. Meiosis I Just like in mitosis, a cell must first undergo through the interphase before proceeding to meiosis proper. It increases in size during G1 phase, replicates all the chromosomes during S phase, and makes all the preparations during the G2 phase.
Like the usual mitosis, the first meiotic division is divided into four different stages: prophase I, metaphase I, anaphase I, and telophase I. By far, prophase I of meiosis is considered as the most complicated step in the whole process. As compared to mitotic prophase, the prophase of meiosis is definitely longer. In this stage of meiosis I, the chromosomes start to condense and pair up with its homologue.
Basically, the first meiosis begins with a very long prophase that is divided into five phases: leptotene, zygotene, pachytene, diplotene, and diakinesis. Leptotene Leptotene is the first stage of prophase during meiosis I. This phase is characterized by the condensation of the chromosomes wherein they become visible as chromatin. During this stage, homologous chromosomes begin to form an association called a synapse which results to pairs of chromosomes that has four chromatids.
Pachytene This is the phase where the crossing over between pairs of homologous chromosomes occurs. The structure formed is referred to as the chiasmata. Diplotene In this phase, the separation of the homologous chromosomes is starting but they remain attached through the chiasmata. This is when the homologous chromosomes continue to separate as the chiasmata move to the opposite ends of the chromosomes. Metaphase I. In this stage, the homologous pairs of chromosomes randomly align at the metaphase plate.
Anaphase I. The homologous chromosomes become separated as they are pulled toward the opposite ends of the cell. However, the sister chromatids remain attached to their pair and do not move apart. Telophase I. Like in the telophase of mitosis, the chromosomes finally are 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 with telophase I, resulting to two haploid daughter cells. It is important to note that the cells that undergo meiosis II are the daughter cells produced during meiosis I.
Prophase II is almost similar to mitotic prophase. In prophase II, the nuclear envelope disintegrates as the chromosomes condense. Aside from that, the centrosomes separate with each other while the spindle fibers try to catch the chromosomes. Metaphase II. Just like in meiosis I, meiosis II is when the chromosomes align at the metaphase plate because of the attachment of the spindle fibers to the centromeres of chromosomes.
The segregation of different types of chromosome is what creates the difference between the two metaphases of meiosis. Anaphase II.
Unlike anaphase I, which involves the separation of homologous chromosomes, anaphase II is the separation of sister chromatids. In this stage, chromosomes each with a chromatid are separated from each other as they move toward the opposite poles of the cell.
Telophase II. Just like the usual telophase, the cell cytoplasm divides equally and shortly after is reformed. The result of telophase II is the formation of four haploid cells having each set of chromosomes. However, it is important to note that the four daughter cells are not identical with each other due to the events in meiosis I i.
Meiosis prophase 1
Overview[ edit ] Although the process of meiosis is related to the more general cell division process of mitosis , it differs in two important respects: recombination meiosis shuffles the genes between the two chromosomes in each pair one received from each parent , producing recombinant chromosomes with unique genetic combinations in every gamete mitosis occurs only if needed to repair DNA damage; usually occurs between identical sister chromatids and does not result in genetic changes chromosome number ploidy meiosis produces four genetically unique cells, each with half the number of chromosomes as in the parent mitosis produces two genetically identical cells, each with the same number of chromosomes as in the parent Meiosis begins with a diploid cell, which contains two copies of each chromosome, termed homologs. First, the cell undergoes DNA replication , so each homolog now consists of two identical sister chromatids. Then each set of homologs pair with each other and exchange genetic information by homologous recombination often leading to physical connections crossovers between the homologs. In the first meiotic division, the homologs are segregated to separate daughter cells by the spindle apparatus.
Substages of Prophase I
The chromosomes become visible by using electron microscopy, which can distinguish between sister chromatids . Each sister chromatid is attached to the nuclear envelope and are so close together that they can be mistaken for only one chromosome . This is a very short stage of Prophase 1. Zygotene Zygotene is the sub-stage where synapsis between homologous chromosomes begins.