Everyone knows that with the sexual method of reproduction, a new organism arises as a result of the fusion of two gametes (germ 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 describe in this article.
Some general knowledge
Most of the heterogeneous organisms on our planet are characterized by sexual reproduction. In this case, 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, in Drosophila (fruit fly), only 4 chromosomes are a diploid set. Gametes in her nucleus have only 2 chromosomes. In humans, there are 46 chromosomes in each cell in the nucleus, and 23 in the gametes (egg and sperm).
But the 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 the carriers of genetic material, and simply these are DNA (deoxyribonucleic acid) molecules that are repeatedly helical and located in the nucleus of eukaryotic cells (having a membrane-coated nucleus) of organisms. In the form in which we are accustomed to seeing them in textbooks and reference books (the chromosomes of a person are shown in the photo above), they become noticeable only during the interphase, before cell division, when they have already doubled.
But chromatids (denoted by c) - this is precisely the structural part of the chromosome, which has already passed the process of replication (doubling) in the interphase before cell division. Chromatide is one of two copies of DNA that are connected at this moment by a special constriction (centromere).
While the two chromatids are linked by centromere, they are called sister. And only during the sexual division of cells (meiosis) they are separated and represent independent units of hereditary material, and if crossing over occurred between them (more about later) then they underwent changes in the sequence of genes.
All chromosomes are different in shape and size within the same homologous (identical) pair. The entire set of chromosomes in cells of the same species is called a karyotype. So, a person has a karyotype - 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
A specific division with the formation of germ cells - meiosis (from the Greek word μείωσις, which means reduction) - is a combination of two consecutive cell divisions, as a result of which the nucleus divides twice, and chromosomes only once. Thanks to this, the chromosome set in gametes is reduced (reduced) by half, which, when merged, restores the zygote diploidy. This is its biological significance.
Meiosis (its phases) in all living organisms occurs according to the same scheme:
- 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.
The first meiotic division
During the preparation of cells for division (interphase), the number of chromosomes in the nucleus doubles (they become 4 n), which is typical for cells that divide by simple division (mitosis). In gamete precursor cells (in humans, spermatocytes and oocytes), this duplication does not occur in the interphase, and the cell proceeds to meiosis with a set of 2n chromosomes and goes through the following steps:
- Prophase I. At this stage, the chromosomes become denser and closer. Conjugation (clumping) 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 disconnected, the membrane of the cell nucleus is destroyed, and the fission spindle begins to form.
- Metaphase I. Filament spindle threads are attached 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. Filament spindle strands stretch the chromosomes to the poles. Briefly, the meaning and essence of meiosis lies in this phase of division - at the poles there are chromosomes in the amount of n.
- Telophase I. At this stage, nuclear membranes are formed. In animals and some plants, cytoplasm is further divided and two daughter cells are formed.
The resulting cells enter an interphase, which is either very short or completely absent.
The second meiotic division
Meiosis II has the same phases:
- Prophase II. Chromosomes become denser, nuclear membranes disappear, the fission spindle begins to appear (photo above).
- During metaphase II, spindle formation continues, and the chromosomes are located along the equator of division.
- Anaphase II. Chromosomes stretch to the poles of the cell (photo below).
- Telophase II. Nuclear membranes are formed, the cytoplasm is divided between two cells.
In 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 with a haploid set of chromosomes are formed in each (n).
The biological significance of meiosis
What it consists of, 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 characterized by sexual reproduction.
- Due to two successive divisions of meiosis, the number of chromosomes in gametes becomes haploid and it becomes logical to restore diploidy when they merge (fertilization) with the formation of a zygote with the original diploid karyotype.
- It is meiosis that provides such a 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 be in different gametes.
What is crossing over
Let us return to the prophase I of meiosis. It is at this moment, when the homologous chromosomes come together and almost stick together, that there can be an exchange between them at any site. It is this exchange that is called crossing over, which, literally translated from English (crossing over), means crossing or crossing.
Simply put, one part of a chromosome can swap places with the same part of another chromosome from the same pair. This mechanism provides recombinant genetic variation in organisms. Shuffling genes leads to an increase in biodiversity within one species.
Life cycle and meiosis
Depending on at what stage of the life cycle meiosis occurs, three types are distinguished in biology:
- The initial (zygotic) occurs immediately after fertilization in the zygote. This type of meiosis is characteristic of organisms with a predominant haploid phase in the life cycle. These are mushrooms (ascomycetes and basidomycetes), some algae (chlamydomonas), and protozoa (sporozoans).
- Intermediate (spore) meiosis occurs when spores are formed in organisms with a uniform alternation of diploid and haploid forms. These are the highest spore (mosses, crowns, horsetails, ferns), gymnosperms and angiosperms. Among animals, this type of meiosis is characteristic of marine protozoa foraminifera.
- The final (gamete) 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 learn about the essence of meiosis in grade 6 when studying protozoa, algae, and moving on to studying 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 living things on our planet, to understand the different life cycles of plants and animals.
In addition, it is meiosis that we should be thankful for the intraspecific diversity of the biological species Homo sapiens. Throughout 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 that 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.