🔎 Meiosis – definition and explanation

Introduction

General view of meiosis. During interphase, the genetic material is duplicated and the phenomenon of crossing over (represented by the recombination of red and blue chromosomes) occurs. During meiosis During reduction, homologous chromosomes are divided into two separate cells. Then during equational meiosis, as during mitosisthese are the chromatids of each chromosome who separates. The result is four haploid (n) cells.

There are two types of cell division in the human body. world living: mitosis, which ensures the birth of cells identical to the mother cell during multiplication asexual and meiosis which leads to the formation of sex cells or gametes for playback.

In animals, meiosis is a process that occurs during gametogenesis (spermatogenesis or oogenesis), that is, during the development of gametes (sperm in males and eggs in females) in so-called diploid species.

In plants, as a result of meiosis, spores are formed, which, as a result of mitosis, give rise to the haploid generation (i.e. pollenleaf base of mosses, etc.) From diploid cells (cells containing 2n chromosomes – inmana normal cell contains 2n = 46 chromosomes (hence 23 pairs), and gamete contains n = 23 chromosomes in two divisions. In haploid species (such as Sordaria macrospora), meiosis occurs after fertilization split an egg (with 2n chromosomes). But besides this role separationmeiosis plays an important role in mixing genetic (gene mixing) due to two mixing mechanisms: interchromosomal mixing and intrachromosomal mixing.

Thus, during meiosis quantity DNA inside a cell develops during time.

Thus, each cell will split its genetic heritage (contained in chromosomes) into two parts so that only half of its genes can be passed on to its daughter cells.

Takes place in several stages formation A together two cell divisions, sequential and inseparable.

Plan

Second division: equational meiosis

Equational meiosis consists of simple mitosis, except number chromosomes, which is equal to n.

• Prophase II: Phase identical to prophase I, but short-lived because the chromosomes remain compact.
• Metaphase II: Chromosomes are located on the equatorial plate.
• Anaphase II: The chromatids of each chromosome separate and migrate to opposite poles of the cell.
• Telophase II: The cell divides into two parts, thus forming four cells. No chromosomes with one chromatid.

At the end of the second meiotic division we move from 2 mother cells with n bichromatid chromosomes to 4 daughter cells with n monochromatid chromosomes.

First division: reduction meiosis

Prophase I

Prophase I is divided into five stages that correspond to five characteristic chromatin states: leptotene, zygotene, pachytene, diplotene and diakinesis.

1. Leptotene: the beginning condensation chromatin and attachment of telomeres (ends of chromosomes) to the membrane nuclear.
2. Zygotene: the beginning of pairing of homologous chromosomes (synapsis) with the help of the synaptonemal complex (or synapton) and convergence telomeres. The synaptonemal complex is a complex structure consisting of a central element SYCP1, which forms a homodimer connected to two lateral elements. The lateral elements are actually cohesins SMC1, SMC3 forming a heterodimer held by hREC8 hHR21. Cohesins are located on both sides by transverse threads, to which the chromatin of each chromosome zone is attached, participating in the subsequent phenomenon of interlacing (or crossing). Eat organization “bouquet” of chromosomes. A set of two homologous chromosomes is called a tetrad (because there are 4 chromatids) or a bivalent (because there are 2 chromosomes).
3. Pachytene: strict pairing of homologous chromosomes and the appearance of recombination nodes and late nodes that allow crossing over (exchange between homologous chromatids). This phase is of great importance in the mixing of chromosomes.
4. Diplotene: Desynapsis (separation of homologous chromosomes), but the chromosomes remain attached at several points where two of the four chromatids appear to intersect (chiasma). For the proper functioning of meiosis, each chromosome requires at least one average 2-3. Chromatin decondenses and large loops form, ensuring a high transcription rate. This stage of prophase I can last for several years.egg.
5. Diakinesis: recondensation of chromatin and separation of telomeres from the nuclear envelope. Sliding of chiasmata to telomeres (terminalization of chiasmata). Eventually the nuclear membrane disappears.

Metaphase I

Pairs of homologous (bivalent) chromosomes are located on opposite sides. For each bivalent, the centromeres are located on both sides and at equal distances from the equatorial plane. Their orientation occurs randomly: this phenomenon is called “independent segregation”. This division allows the second degree diversification of daughter cells: interchromosomal mixing.

Anaphase I

Each chromosome moves away from its counterpart and migrates toward the pole. opposite, extended by kinetochore microtubules (microtubules attached to the kinetochore at the centromere) due to depolymerization of tubulin. There are no split centromere, this is due to the fact that separase destroys hREC8 (and, therefore, the construction of cohesins), but is ineffective at the centromere, given that Sga1 protects hREC8 there.

Telophase I

Nuclear membranes reappear in each cell, resulting in the formation of two haploid cells with n chromosomes and two chromatids (bichromatid chromosomes) (n chromosomes, 2n DNA). The cell is divided into two parts thanks to a contractile ring consisting of actin and myosin.