Everyone knows that with sexual reproduction, a new organism arises as a result of the fusion of two gametes (sex cells). Gametogenesis, or the formation of generative cells, occurs through a specific division called meiosis. What is the essence of this process, what are its stages, we will tell in this article.
A bit of general knowledge
For most heterosexual organisms on our planet, sexual reproduction is characteristic. In this case, the gametes have a half chromosome set, which is called haploid (n). As a result of the fusion of gametes, a zygote is formed, in which diploidy is restored, and the set of chromosomes is designated 2n, which is the essence of meiosis (briefly).
For example, Drosophila (fruit fly) has only 4 chromosomes - this is a diploid set. Gametes in her nucleus have only 2 chromosomes. In humans, in each cell in the nucleus there are 46 chromosomes, and in the gametes (egg and sperm) - 23 each.
Butthe restoration of diploidy during sexual reproduction is only a small part of what the essence of meiosis is.
Chromosomes and chromatids
To understand the following material, it is important to understand the difference between the two.
Chromosomes (the designation n is used) are carriers of genetic material, but simply these are DNA (deoxyribonucleic acid) molecules, repeatedly spiralized and located in the nucleus of cells of eukaryotic (having a nucleus with a membrane sheath) organisms. In the form in which we are used to seeing them in textbooks and reference books (the photo above shows human chromosomes), they become noticeable only during interphase, before cell division, when they have already doubled.
But the chromatids (denoted with) - this is just the structural part of the chromosome, which has already gone through the process of replication (doubling) in the interphase before cell division. A chromatid is one of two copies of DNA that are connected at this moment by a special constriction (centromere).
As long as two chromatids are connected by a centromere, they are called sister chromatids. And only during the sexual division of cells (meiosis) do they separate and represent independent units of hereditary material, and if crossing over occurred between them (more on that later), then they underwent changes in the gene sequence.
All chromosomes are different in shape and size within one homologous (identical) pair. The entire set of chromosomes in cells of the same species is called a karyotype. So, in humans, the karyotype is 46 chromosomes,of which 22 pairs are homologous or autosomes, and 23 pairs are sex chromosomes (X and Y). In human gametes (sperm and egg) there is a half (haploid) set of chromosomes - 23 autosomes and 1 sex chromosome (X or Y).
Just meiosis provides such a set in gametes.
Special cell division
Specific division with the formation of germ cells - meiosis (from the Greek word Μείωσις, which means reduction) is a set of two consecutive cell divisions, as a result of which the nucleus divides twice, and the chromosomes only once. Due to this, there is a decrease (reduction) of the chromosome set in gametes by half, which, when they merge, restores the diploidy of the zygote. This is its biological significance.
Meiosis (its phases) in all living organisms occurs in the same way:
- The first division (reduction), after which the number of chromosomes is halved.
- The second division (equational) occurs as a simple division (mitosis). It is also called leveling.
First meiotic division
During the preparation of a cell for division (interphase) in the nucleus, the number of chromosomes doubles (there are 4 n), which is typical for cells that divide by simple division (mitosis). In the cells of the precursors of gametes (in humans, spermatocytes and oocytes), such doubling does not occur in the interphase, and the cell starts meiosis with a set of 2n chromosomes and passesthe following steps:
- Prophase I. At this stage, the chromosomes become denser and closer together. Conjugation (adhesion) of homologous chromosomes (one pair) occurs, during which crossing over occurs. This process is characteristic only for meiosis (what is the essence, we will describe below). Then the chromosomes are separated, the shell of the cell nucleus is destroyed, and the division spindle begins to form.
- Metaphase I. The spindle fibers attach to the centromeres of the chromosomes, and they themselves are located along the division equator opposite each other, and not along the same line (as in mitosis).
- Anaphase I. Spindle threads stretch the chromosomes to the poles. Briefly, the meaning and essence of meiosis lies in this phase of division - the poles have chromosomes in the amount of n.
- Telophase I. At this stage, nuclear envelopes are formed. In animals and some plants, further division of the cytoplasm occurs and two daughter cells are formed.
The formed cells enter into interphase, which is either very short or absent.
Second meiotic division
Meiosis II has the same phases:
- Prophase II. Chromosomes condense, nuclear membranes disappear, and the fission spindle begins to appear (photo above).
- During metaphase II, the formation of the spindle continues, and the chromosomes are located along the division equator.
- Anaphase II. Chromosomes are stretched to the poles of the cell (photo below).
- Telophase II. Nuclear membranes are formed, the cytoplasm is divided betweentwo cells.
With this division, the number of chromosomes does not change, but each of them consists of only one chromatid (structural unit). This is the essence of meiosis II. Cells are formed with a haploid set of chromosomes in each (n).
Biological significance of meiosis
What it is, it has already become clear:
- Meiosis is a perfect mechanism that ensures the constancy of the karyotype (number of chromosomes) of a species that is inherent in sexual reproduction.
- Due to two consecutive divisions of meiosis, the number of chromosomes in gametes becomes haploid and it becomes logical to restore diploidy when they merge (fertilize) with the formation of a zygote with the original diploid karyotype.
- It is meiosis that provides such property of organisms as variability. In prophase I - due to crossing over, and in anaphase I - due to the fact that homologous chromosomes with different genes can end up in different gametes.
What is Crossover
Let's return to prophase I of meiosis. It is at this moment, when the homologous chromosomes have approached and almost stuck together, that an exchange between them of any site can occur. It is this exchange that is called crossing over, which literally translated from English (crossing over) means crossing or crossing.
In other words, one part of a chromosome can swap places with the same part of another chromosome from the same pair. This mechanism provides recombinative genetic variability of organisms. shufflinggenes leads to increased biodiversity within a single species.
Life cycle and meiosis
Depending on which stage of the life cycle meiosis occurs, there are three types of meiosis in biology:
- Initial (zygote) occurs immediately after fertilization in the zygote. This type of meiosis is typical for organisms with a predominance of the haploid phase in the life cycle. These are fungi (ascomycetes and basidomycetes), some algae (chlamydomonas), protozoa (sporozoa).
- Intermediate (spore) meiosis occurs during the formation of spores in organisms with a uniform alternation of diploid and haploid forms. These are higher spores (mosses, club mosses, horsetails, ferns), gymnosperms and angiosperms. Among animals, this type of meiosis is characteristic of marine protozoa foraminifera.
- Final (gametic) meiosis is inherent in all multicellular animals, fucus seaweed and some protozoa (ciliates). In these organisms, the diploid phase predominates in the life cycle, and only gametes have a haploid set of chromosomes.
Summarize
Students get acquainted with the essence of meiosis in the 6th grade when studying protozoa, algae, and moving on to the study of plant biology. This key concept of general biology and the mechanisms of formation of germ cells (gametes) allows us to understand the commonality of all life on our planet, to understand the various life cycles of plants and animals.
In addition, it is meiosis that we should beare grateful for the intraspecific diversity of the biological species Homo sapiens. During the study of biology in subsequent classes, students continue to study the phases of sexual division, and when they get acquainted with genetics, the laws of heredity and variability.
Studying the mechanisms of various cell division allows us to understand the uniqueness and expediency of the laws of nature, which have been formed over billions of years of evolution on a single planet of the solar system. And we were lucky to be born on it.
