Sexual reproduction involves the formation of new individuals not from parts of the parent organism, as in asexual reproduction, but from a zygote formed by the fusion of male and female reproductive cells. Sexual reproduction in nature occurs in most species and has advantages over asexual reproduction, since it combines the hereditary material of parent organisms.
Gametes
Sex cells, or gametes, differ in structure from other cells in the body.
Gametes have a halved amount of hereditary information (the number of chromosomes). This is achieved through meiosis, a special type of division characteristic of developing gametes.
In plants, the organs in which the development of germ cells (gametogenesis) occur are called gametangia. The plant itself on which gametes develop is called a gametophyte.
Female gametes are called eggs, and male gametes are called sperm or spermatozoa (if they have a flagellum).
Rice. 1. Sex cells.
In male animals, gametes develop in gonads called testes, and in female animals - ovaries.
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Gametes vary in size and ability to move between species. In mammals and humans, eggs are large and immobile, while sperm are small and motile.
Fertilization
Mature gametes can combine with gametes of the other sex. This process is called fertilization and in different animals it occurs in two forms:
- external fertilization , occurring outside the body (amphibians, fish);
- internal when gametes meet inside the female's body.
A fertilized egg (zygote) has a full set of chromosomes, half from the father and half from the mother.
Dioecy and hermaphroditism
In some species of plants and animals, both male and female gametes develop in the body of one individual. Such species are called hermaphrodites.
Examples of hermaphroditic species are:
- sea bass;
- large pond snail;
- earthworm;
- bull tapeworm.
If a species has separate male and female organisms, which is the case in most cases, then these animals are said to be dioecious.
When male and female organisms of the same species have noticeable differences in external structure or color, the species is said to be characterized by sexual dimorphism.
Rice. 2. Sexual dimorphism.
Types of sexual reproduction
In addition to sexual reproduction itself, with the fusion of germ cells there are other types:
- parthenogenesis;
- fusion of single-celled organisms;
- conjugation.
In parthenogenesis, offspring develop from unfertilized eggs.
Parthenogenesis occurs in
- ants;
- aphids;
- bees;
- crucian carp, etc.
During parthenogenesis, there is no exchange of hereditary material and all offspring are similar to the maternal organism.
Conjugation is sexual reproduction without the formation of germ cells. Characteristic, for example, of algae. Cells of different individuals grow together for a while and exchange genetic material.
In unicellular algae, fusion of entire parent cells occurs, followed by division into 4 cells.
Rice. 3. Sexual reproduction of algae.
In plants, sexual reproduction is usually combined with vegetative reproduction. For example, onions are usually propagated by shoots - bulbs, but onions are also characterized by sexual reproduction, they bloom and form seeds after pollination.
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Within one of the types of modern plants - green algae - there are species that can be arranged in a certain sequence, showing which path evolution could take. Yet even after studying these species, it remains unclear whether sexual reproduction actually first arose in green algae and whether the evolution of sex actually occurred in this way.
The simplest green algae, such as Protococcus, reproduce only asexually - by simple cell division. In most other green algae, when they reproduce asexually, a vegetative cell develops into one or more specialized reproductive cells called zoospores. Each zoospore is equipped with one or more flagella and is well adapted to ensure the spread of the species.
The vegetative cell of Chlamydomonas, found in ponds, lakes and moist soil, has two flagella and is protected by a strong cellulose wall. Each cell contains one cup-shaped chloroplast, which contains a pyrenoid that takes part in the formation of starch; in addition, the cell has an “eye” (red pigment spot) and two contractile vacuoles lying near the bases of two flagella.
During asexual reproduction, such a cell divides to form 2 to 8 zoospores inside the cellulose wall. As a result of the rupture of this wall, the zoospores are released and swim in different directions, representing independent plants. Sexual reproduction occurs from time to time: the cell divides to form 8 to 32 gametes, which resemble zoospores or adult cells, but are smaller in size. Two of these gametes fuse, starting at the origin of the flagella, to form a zygote. At first, the zygote has 4 flagella (2 from each gamete), but over time they disappear. The cell becomes rounded, produces a thick cell wall, and in this state can withstand long periods of unfavorable external conditions. As soon as conditions again become suitable for growth, the zygote divides meiotically to form four cells, the cell wall cracks, the cells thus released develop flagella and become independent plants.
Sexual reproduction of Chlamydomonas is extremely primitive, since its gametes are unspecialized cells that are no different in appearance from zoospores and adult cells. In most Chlamydomonas species, both fused cells are identical in size and structure; This form of reproduction is called isogamy. In some species, gametes of two genera are formed, of which some are larger than others, but they all have flagella; As a result of the fusion of two gametes of different sizes, a zygote is formed. This form of reproduction is called heterogamy.
Another primitive type of sexual reproduction is observed in Spirogyra, which consists of long threads of haploid cells connected at their ends. In autumn, when reproduction usually occurs, the two threads are located side by side, parallel to each other, and the cells lying one against the other form dome-shaped outgrowths directed towards each other. These outgrowths enlarge, merge and turn into a tube connecting both cells to each other. One of the cells becomes round, slowly flows through the tube and connects with another cell. The nuclei of both cells then fuse and fertilization is complete. The resulting cell, or zygote, is surrounded by a thick cell wall and can spend the winter in this form. In spring, the zygote divides meiotically and forms four haploid nuclei, of which three nuclei degenerate. The fourth nucleus is preserved and, after rupture of the thick wall, divides mitotically, giving rise to a new haploid thread. Sexual reproduction of Spirogyra is primitive, since it involves unspecialized cells (any cell of one thread can merge with a cell of an adjacent thread), which are no different from each other (isogamy).
Using the example of another filamentous green alga, Ulothrix, we can trace the next proposed step in the evolution of sexual reproduction. In this plant, each haploid vegetative cell in a filamentous chain contains one chloroplast, shaped like a collar, and several pyrenoids. When one cell divides, 8 zoospores are formed, each of which carries 4 flagella; zoospores are released and subsequently give rise to new threads. One of the cells of the filament can undergo several divisions, as a result of which numerous small identical gametes are formed, reminiscent of zoospores, but differing from them in the presence of two flagella instead of four.
As in Chlamydomonas, two of these free-swimming forms fuse to form a zygote, which initially has four flagella. After swimming for some time, the zygote loses its flagella, secretes a thick cell wall, and in this form is able to withstand cold and drying out. Subsequently, the zygote divides meiotically and gives rise to four cells. The latter are eventually freed from the wall of the old zygote and develop into new threads. Thus, in Ulothrix, sexual reproduction is isogamous and occurs by the fusion of two identical cells, but these cells are specialized and different from ordinary vegetative cells.
Sexual reproduction, observed in another filamentous algae, Oedogonium, appears to represent the third stage of evolution. The cells that merge to form a zygote are not the same: one of them is a round, immobile egg cell rich in reserve substances, the other is a small, motile sperm. Sexual reproduction by the fusion of dissimilar gametes, called heterogamy, is characteristic of most higher plants. Any vegetative cell can turn into either an oogonia, a cell that forms an egg, or an antheridium, which produces sperm. The cells that form the eggs are large and spherical; their protoplasm extends from the solid cell wall and forms a round, immobile egg cell, overflowing with nutrients.
As a result of repeated division of another vegetative cell, a number of short disc-shaped cells are formed that produce sperm. In this case, the protoplasm of each of these cells divides and produces two small spermatozoa, bearing a corolla of flagella at the anterior end. The sperm swims towards the egg, attracted by the chemicals it secretes. Through a gap in the cell wall, the sperm penetrates the egg and fuses with the latter. Both the egg and the sperm are haploid, and when they fuse, a diploid zygote is formed. The zygote secretes a thick cell wall around itself, and in this form it can endure periods unfavorable for growth. Eventually, the zygote undergoes meiosis to form four haploid cells, each of which bears a corolla of flagella at the anterior end and resembles zoospores, cells used for asexual reproduction. Zoospores of both sexual and asexual origin germinate, divide and produce a new thread of Oedogonium.
The last stage of the evolution of sexual reproduction can be found in the study of other algae, for example Volvox, as well as higher plants and animals, in which specialized gametes are produced only by specialized cells of the body - the reproductive organs, and not by vegetative cells, as in Ulothrix or Oedogonium. Volvox is a colonial algae shaped like a hollow ball, which consists of cells each bearing two flagella and connected to neighboring cells by thin filaments of protoplasm. One such colony can contain up to 40,000 cells, most of which are identical and perform only vegetative functions. Small, motile spermatozoa with two flagella are formed only by special organs - antheridia (this term is also used to designate the organs of higher plants that produce spermatozoa). A single large immobile egg cell is formed inside a special organ - the oogonia.
Having left the antheridium, the motile sperm swims up to the egg; as a result of their fusion, a diploid zygote is obtained; A thick cell wall is formed around the zygote, which can protect it from adverse external influences. During germination, the zygote divides meiotically to form haploid cells. The latter, after a series of mitotic divisions, give rise to a new colony. Some Volvox species have both antheridia and oogonia in the same colony; in other species, each colony has either only antheridia or only oogonia, and depending on this it is called “male” or “female”. In such forms, the evolution of sexual reproduction has reached the stage of sexual differentiation.
Evolution thus proceeded in different directions, each of which led to a different kind of specialization. The first direction is the transition from the formation of identical gametes (isogamy) to the formation of different gametes (heterogamy); this provides clear advantages that contribute to the survival of the species: the large number and mobility of sperm ensure their meeting with the egg, and the large size and nutrient reserves of the egg provide nutrition to the zygote until it becomes capable of independent nutrition. The second direction of evolution is the specialization of the cells of a colony or multicellular body, so that some cells perform only vegetative functions, while others perform only reproductive functions. The third direction of development led to the differentiation of the sexes. In the primitive plants considered, the same plant can reproduce both sexually and asexually, depending on environmental conditions. The fourth direction of evolution led to the retention of a fertilized immobile egg in the body of the mother plant. In the most highly organized algae and in all higher plants, there is a clearly defined and regular alternation of the generation of plants that reproduce sexually and the generation of plants that reproduce asexually - by spores. The occurrence of such a change of generations is the result of the fifth direction of evolution, the beginning of which should be sought in green algae. Thus, the “sea lettuce” Ulva is represented by plants of two genus, identical in size and structure. However, some of these plants are diploid sporophytes, forming haploid zoospores by meiosis, which develop into haploid gametophytes. These latter (second form) produce gametes that fuse to form a diploid zygote, which develops into a diploid sporophyte.
The development cycle of a given species is understood as the biological developmental processes that occur during the period between a certain stage in the life course of an organism and the same stage in the life course of its offspring. Bacteria, blue-green algae and protococcal algae, which reproduce by simple division, have an extremely simple development cycle. Filamentous green algae, such as Ulothrix, are colonies of haploid cells that reproduce asexually for most of their development cycle. Mitotic divisions of these cells lead to the formation of either new vegetative filament cells, or haploid gametes, or - in asexual reproduction - haploid zoospores, which after a series of divisions give rise to new haploid colonies. The only diploid cell is the zygote, which divides meiotically to form haploid vegetative cells.
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The reproductive organ systems of various animals are built according to a fundamentally uniform plan, although with numerous variations. The gonads and their ducts can be single, paired or multiple. Spermatozoa produced in the testes are released through ducts into the external environment; they are often suspended in seminal fluid secreted by the accessory glands of the reproductive system.
Alexander Markov
Sperm can unite in small dense schools, which allows them to overtake single competitors. This is an example of mutual assistance with elements of altruism, because out of the entire flock, only one sperm will eventually be able to fertilize the egg. As it turned out, in American hamsters Peromyscus maniculatus, in which females mate with several males in a row, sperm are able to distinguish relatives from strangers and unite mainly with “their own”. In the closely related monogamous species P. polionotus, spermatozoa lack such selectivity, which is fully consistent with the predictions of the theory of kin selection.
Konstantin Popadin
Females of most animal species actively choose males for themselves, guided by such non-adaptive traits as tail size (in peacocks), song and coloring (in birds), the severity of which is associated with the “quality” of the male. However, when the female has no choice (for example, in the case of forced insemination), she may invest different amounts of energy in the offspring according to the quality of the male.
Richard Dawkins
Dawkins's book shows in a popular and intelligible form how highly organized Complexity can arise from primordial Simplicity, without the participation of any higher intelligent being. The watchmaker mentioned in the title of the book is taken from the famous treatise of the 18th century theologian William Paley, who argued that watches cannot appear spontaneously and spontaneously, but only as the fruit of the mind and efforts of a conscious being (the watchmaker); thus, even more complex (than watches) living beings can be created only by the will and mind of the Creator. Dawkins shows in his book that natural selection, operating on spontaneous variations of simple initial forms, over hundreds and thousands of generations, can generate equally impressive complexity. The book also shows specific mechanisms that implement such increasing selection and provides answers to frequently asked questions regarding evolution.
Secrets of gender [Man and woman in the mirror of evolution] Butovskaya Marina Lvovna
Sexual reproduction: paths of evolution
Sexual reproduction did not arise immediately in the process of evolution. The first simple single-celled creatures such as amoebas, flagellates (green euglena), ciliates (slipper ciliates), radiolarians (sunflower) reproduced by simple cell division into daughter cells.
In the process of evolution, some protozoa began to practice two types of reproduction, alternating them depending on the time of year and environmental conditions: asexual and a primitive version of sexual. For example, there are two generations in the life cycle of foraminifera. One of them has a single (haploid) set of chromosomes, the second has a double (diploid) set. The asexual generation (called agamonts) is formed by multiple divisions of the mother cell. Daughter cells, having an amoeboid shape, leave the mother's shell, grow and each secretes its own shell around itself. They give rise to another generation (called gamonts), which reproduces sexually. Gamonts divide many times, resulting in the formation of small cells with flagella (these are called gametes). The gametes are released into the water and copulation occurs there. In foraminifera, copulation occurs between gametes of the same shape and size (isogamous copulation). As a result of the fusion of gametes, a zygote is formed, from which an asexual generation is formed. This method of reproduction is the most primitive form of the sexual process. In this case, there are no female and male gametes, and therefore, sex as a phenomenon is also absent.
Some protozoa, such as ciliates, reproduce asexually by dividing the cell in two. In this case, the nucleus divides mitotically, as during the division of any cell of a multicellular organism. However, ciliates also have a unique form of the sexual process: conjugation. Before conjugation, the nucleus divides meiotically to form four haploid nuclei. Three of them dissolve, and one again divides in two. This time through mitosis. Stationary and migrating nuclei are formed. During the process of conjugation, two ciliates join in pairs and exchange migrating nuclei. After such an exchange, in each ciliate the stationary nucleus merges with an alien migrating nucleus. Then the individuals disperse. The sexual process in ciliates cannot be considered a form of reproduction, since there is no increase in the number of individuals.
The most important step towards the formation of sexes should be considered the emergence of anisogamy: the formation of gametes of different sizes - large and small (we have already discussed why this became possible). Anisogamy has been described in some species of flagellates (Chlamydomonas, Volvox). Large gametes in Volvox lack flagella and resemble an egg in appearance. Although flagellates produce gametes of different sizes during sexual reproduction, it is not possible to say anything about their sex, since one individual can produce gametes of different sizes. Consequently, the production of gametes of different sizes in itself does not yet indicate the formation of the phenomenon of sex.
Paradoxically, it turns out that sexual reproduction occurs earlier in evolution than sex. The formation of sexes was preceded by a long period of evolution, when, through a special process (meiosis), the formation of germ cells with a single set of chromosomes already occurred. The sexual process that arose in some protozoa (ciliates) was not yet an integral part of sexual reproduction.
The life cycle of protozoa can be characterized only by an asexual type of reproduction (from one cell division to another), or only by a sexual one (from zygote to zygote); Alternation of sexual and asexual reproduction (metagenesis) may also be observed. Although you can read in biology textbooks that protozoa have both asexual and sexual reproduction, they still do not have sex. Speaking about the processes of sexual reproduction in protozoa, it is more correct to talk about the presence of different sexual types in them (for example, in the slipper ciliate). The sexual type of these organisms is determined solely by genetic material and does not in any way affect the external appearance of the individuals. For example, in a number of mold species there are up to 13 sexual types.
An uninitiated reader may shrug his shoulders and say: “What difference does it make - sexual types or gender?” However, there is a difference, and a significant one. It is impossible to compare sexual types in different animal species with each other, since they do not have universal attributes of sex. On the contrary, sex (sexuality) in bisexual species is determined by a set of fixed characteristics and is represented by two standard options: male and female. Females always produce relatively large, nutrient-rich, immotile eggs and have specialized reproductive organs to produce them. Males produce small, nutrient-poor, and highly motile sperm, which are formed in specific male reproductive organs.
From the book Animal Life Volume I Mammals author Bram Alfred Edmund2. Sexual reproduction In more complexly organized animals, either exclusively, or at least predominantly, the sexual method of reproduction is practiced, although the rudiments of this form of reproduction of offspring, so to speak, a hint of it, are also found in lower animals
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Reproduction- the ability of living organisms to reproduce their own kind. There are two main reproduction method- asexual and sexual.
Asexual reproduction occurs with the participation of only one parent and occurs without the formation of gametes. The daughter generation in some species arises from one or a group of cells of the mother’s body, in other species - in specialized organs. The following are distinguished: methods of asexual reproduction: division, budding, fragmentation, polyembryony, sporulation, vegetative propagation.
Division- a method of asexual reproduction characteristic of unicellular organisms, in which the mother is divided into two or more daughter cells. We can distinguish: a) simple binary fission (prokaryotes), b) mitotic binary fission (protozoa, unicellular algae), c) multiple fission, or schizogony (malarial plasmodium, trypanosomes). During the division of the paramecium (1), the micronucleus is divided by mitosis, the macronucleus by amitosis. During schizogony (2), the nucleus is first divided repeatedly by mitosis, then each of the daughter nuclei is surrounded by cytoplasm, and several independent organisms are formed.
Budding- a method of asexual reproduction in which new individuals are formed in the form of outgrowths on the body of the parent individual (3). Daughter individuals can separate from the mother and move on to an independent lifestyle (hydra, yeast), or they can remain attached to it, in this case forming colonies (coral polyps).
Fragmentation(4) - a method of asexual reproduction, in which new individuals are formed from fragments (parts) into which the maternal individual breaks up (anneli, starfish, spirogyra, elodea). Fragmentation is based on the ability of organisms to regenerate.
Polyembryony- a method of asexual reproduction in which new individuals are formed from fragments (parts) into which the embryo breaks up (monozygotic twins).
Vegetative propagation- a method of asexual reproduction in which new individuals are formed either from parts of the vegetative body of the mother individual, or from special structures (rhizome, tuber, etc.) specifically designed for this form of reproduction. Vegetative propagation is typical for many groups of plants and is used in gardening, vegetable gardening, and plant breeding (artificial vegetative propagation).
| Vegetative organ | Method of vegetative propagation | Examples |
|---|---|---|
| Root | Root cuttings | Rosehip, raspberry, aspen, willow, dandelion |
| Root suckers | Cherry, plum, sow thistle, thistle, lilac | |
| Aboveground parts of shoots | Dividing bushes | Phlox, daisy, primrose, rhubarb |
| Stem cuttings | Grapes, currants, gooseberries | |
| Layerings | Gooseberries, grapes, bird cherry | |
| Underground parts of shoots | Rhizome | Asparagus, bamboo, iris, lily of the valley |
| Tuber | Potatoes, sunflower, Jerusalem artichoke | |
| Bulb | Onion, garlic, tulip, hyacinth | |
| Corm | Gladiolus, crocus | |
| Sheet | Leaf cuttings | Begonia, gloxinia, coleus |
Sporulation(6) - reproduction through spores. Controversy- specialized cells, in most species they are formed in special organs - sporangia. In higher plants, spore formation is preceded by meiosis.
Cloning- a set of methods used by humans to obtain genetically identical copies of cells or individuals. Clone- a collection of cells or individuals descended from a common ancestor through asexual reproduction. The basis for obtaining a clone is mitosis (in bacteria - simple division).
Sexual reproduction is carried out with the participation of two parent individuals (male and female), in which specialized cells are formed in special organs - gametes. The process of gamete formation is called gametogenesis, the main stage of gametogenesis is meiosis. The daughter generation develops from zygotes- a cell formed as a result of the fusion of male and female gametes. The process of fusion of male and female gametes is called fertilization. An obligatory consequence of sexual reproduction is the recombination of genetic material in the daughter generation.
Depending on the structural features of the gametes, the following can be distinguished: forms of sexual reproduction: isogamy, heterogamy and oogamy.
Isogamy(1) - a form of sexual reproduction in which gametes (conditionally female and conditionally male) are mobile and have the same morphology and size.
Heterogamy(2) - a form of sexual reproduction in which female and male gametes are motile, but female gametes are larger than male ones and less mobile.
Oogamy(3) - a form of sexual reproduction in which female gametes are immobile and larger than male gametes. In this case, female gametes are called eggs, male gametes, if they have flagella, - spermatozoa, if they don’t have it, - sperm.
Oogamy is characteristic of most species of animals and plants. Isogamy and heterogamy occur in some primitive organisms (algae). In addition to the above, some algae and fungi have forms of reproduction in which sex cells are not formed: hologamy and conjugation. At hologamia single-celled haploid organisms merge with each other, which in this case act as gametes. The resulting diploid zygote then divides by meiosis to produce four haploid organisms. At conjugation(4) the contents of individual haploid cells of filamentous thalli merge. Through specially formed channels, the contents of one cell flow into another, a diploid zygote is formed, which usually, after a period of rest, also divides by meiosis.
Go to lectures No. 13“Methods of division of eukaryotic cells: mitosis, meiosis, amitosis”
Go to lectures No. 15"Sexual reproduction in angiosperms"
The formation of germ cells, as a rule, is associated with the passage of meiosis at some stage of the life cycle of the organism. In most cases, sexual reproduction is accompanied by the fusion of sex cells, or gametes, which restores a double set of chromosomes relative to gametes. Depending on the systematic position of eukaryotic organisms, sexual reproduction has its own characteristics, but, as a rule, it allows the combination of genetic material from two parent organisms and makes it possible to obtain offspring with a combination of properties not found in the parental forms.
The effectiveness of combining genetic material in offspring obtained as a result of sexual reproduction is facilitated by:
- chance meeting of two gametes;
- random arrangement and divergence to the division poles of homologous chromosomes during meiosis;
- crossing over between homologous chromosomes.
This form of sexual reproduction, parthenogenesis, does not involve the fusion of gametes. But since the organism develops from a germ cell (oocyte), parthenogenesis is still considered sexual reproduction.
In many groups of eukaryotes, the secondary disappearance of sexual reproduction has occurred, or it occurs very rarely. In particular, the department of deuteromycetes (fungi) unites a large group of phylogenetic ascomycetes and basidiomycetes that have lost the sexual process. Until 1888, it was assumed that among terrestrial higher plants, sexual reproduction was completely lost in sugarcane. The loss of sexual reproduction has not been described in any group of metazoans. However, many species are known (lower crustaceans - daphnia, some types of worms) capable of reproducing parthenogenetically under favorable conditions for tens and hundreds of generations. For example, some species of rotifers have been reproducing only parthenogenetically for millions of years.
In a number of polyploid organisms with an odd number of sets of chromosomes, sexual reproduction plays a small role in maintaining genetic variability in the population due to the formation of unbalanced sets of chromosomes in gametes and descendants.
The ability to combine genetic material during sexual reproduction is of great importance for the selection of model and economically important organisms.
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✪ Mitosis, meiosis and sexual reproduction
✪ Forms of reproduction of organisms. Sexual reproduction
✪ Biology lesson No. 11. Types of reproduction.
Subtitles
I touched on this a little in the video about genetic variation in populations, but I think everyone knows that all of us (and when I say we, I mean humans, frankly, most eukaryotic organisms) are all the result of sexual reproduction. So, this is the very first cell that can turn into Salman, we know that this is the very first cell, or better if it is the nucleus of this first cell. I can draw the whole cell and its entire structure, but let's focus on the nucleus. It contains 23 pairs of chromosomes. 46 chromosomes, 23 from my father and 23 from my mother, so 1, 2 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 , 20, 21, 22, 23 from my father. And it should be noted that the latter helps determine my gender; my gender completely depends on it. This is my Y chromosome. And I have 23 homologous chromosomes, or one chromosome that is homologous to each of the others, so I have 23 chromosomes from my mother, 1, 2, 3, 4, 5 - well, you get the idea. I'll draw many chromosomes and then draw the X chromosome, which is the sex-determining chromosome from the mother. We learned earlier that each of these pairs are homologous chromosomes that encode the same gene from the father and one from the mother. So, the first cell that can become me is my mother's fertilized egg. So, my mother's egg. My mother's egg. I'll draw the entire egg. Like this. Next I will look at DNA, so my mother's DNA contains 23 chromosomes. They don't have pairs, that's the key point. So here are 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and 23 X- chromosome. So, X chromosome. This combination is from my mother, this is from my mother, and the sperm is from my father. Let's point this out. Let's put it here. I will draw the sperm quite large next to the egg, although in reality it is not so. This is a kind of nucleus of the egg, but let's say that it is a sperm, it has a tail that helps to swim and it contains 23 chromosomes. So 1, 2, 3, 4, ... 15, 16, 17, 18 ... 22nd and one Y chromosome. I'll highlight the Y chromosome in a different color. This union, this fertilization, which occurs between the sperm and the egg: the sperm penetrates the egg, a zygote is formed, that is, the fertilized egg of my mother, containing all the DNA of the father and mother. So, all the DNA of the father and mother. This is the very first cell that was formed from this fertilized egg; it is called a zygote. This is the result of the fusion of two gametes. Two gametes. So this is a gamete and this is a gamete. The egg and sperm are gametes. I'm doing all this to... to bring you up to speed, I already introduced you to this when we talked about genetic diversity in a population. Look, this is my complete set of chromosomes. There are 23 pairs, and each pair is a pair of homologous chromosomes. So, 23 pairs and each of them is a pair of homologous chromosomes. They code for the same thing, one came from the mother, the other from the father, and these are 46 separate chromosomes. 23 from mother and 23 from father. Each gamete contains only 23 chromosomes, or half the full set. A gamete contains only 23 chromosomes, or half the full set. All I'm talking about now: 46, or 23 pairs of individual chromosomes, is a distinctive feature of people. If we consider other species, they may have 10 chromosomes or even 5 chromosomes. But something is typical for all organisms that reproduce sexually: their gametes contain half the set of chromosomes, unlike the zygote or the organism itself. Half the set of chromosomes is called the haploid set. This means that it contains half of the chromosomes. It's easy to remember: the word "haploid" in English begins the same way as the word "half". The haploid set of people is 23 chromosomes. If we call such a set haploid, then what do we call the complete set of chromosomes? It is called the diploid set. Diploid. It’s also not difficult to remember, because di- is a prefix that means “double”; we have a double set of chromosomes. This is the haploid and these are the diploid sets. These are sets of human chromosomes. Sometimes, when talking about sets of chromosomes, you can see the designation N chromosomes. It comes up quite often and I want you to get used to it. Look, let's say there is some kind of organism, or, in fact, any organism at all, it has a set of chromosomes. If the diploid set is 2N, then the haploid set, which is half of the diploid set, is N. The diploid set in humans is 46 chromosomes, and N is 23. A fertilized egg, ordinary somatic cell, or body cell has a diploid set of chromosomes, and a sex cell , I will explain further, has a haploid set of chromosomes. So, gametes, that is, a sperm or egg contain half the set of chromosomes. They merge and a zygote is formed, which is my first cell and contains a diploid set of chromosomes. I want to deviate a little from the topic because I have some very interesting things to say. We are talking about natural selection, and you will be surprised that it is still happening today, although our society does not live in the wild and we do not encounter predators, we have food and everything we need. The process of fertilization itself is a difficult competition, because the sperm that reaches the egg and fertilizes it is one in 200 million. About one in 200 million. About...about that. One in 200 million. So let's continue further. About 200-300 million sperm simultaneously strive for the egg. From the moment of birth, we are the result of competition. From the moment of birth, we are the result of competition between these males, one might say, that is, male gametes, or sperm. Some of them could have mutations, as a result they did not know where to swim, they chose the wrong direction, perhaps some had abnormal tails that did not allow them to swim quickly, so we are already the result of selection between them under these conditions. If we have some serious mutations in some of these sperm, they have less chance, especially if the mutations affect their ability to swim. The likelihood that they will win the race for the egg is small. So we are already the result of a race of 280 million organisms, if we count each sperm as an organism, and we are the result of a winning combination. Sometimes we feel lost on this planet. We are one of 6 billion people, but we are already the result of a very great achievement. Now, let's go back and talk a little about our zygotes and how they turn into humans, how people produce gametes that then fertilize the gametes of other people to form... to form zygotes. This is a very interesting topic. Its general idea is this: the very first cell is the mother’s egg, which was fertilized by the father’s sperm, that is, the zygote. It's called a zygote. After successful fertilization, it contains 2N, or a diploid set of chromosomes in the case of a person, which I am, and I have 46 chromosomes. Then this cell right here begins to divide and split again, and again, and again. We will make a series of videos about this process, which is called mitosis. Literally, mitosis is the division of a cell to form its copies. First, the number of cells doubles and new cells appear. I'd rather picture them like this, because in reality, when a cell divides, the new cells are no larger in size than the mother cell. So now each of them contains 2N chromosomes (or 46) in the case of humans, they continue to divide over and over again. In the end, I'll show it like this. They keep sharing. I'll talk a little more about this initial colony of cells, but I won't go into detail right now. 2N From a genetic point of view, all these new cells are copies of the original cell. They continue to divide, and a huge number of cells are formed, an unimaginable number of cells, all of them are copies of the first cell and all contain 2N chromosomes, that is, a diploid set. They contain all my genetic material, but depending on their environment and interactions with each other, they begin to differentiate. They all contain 2N chromosomes, that is, they all have a diploid set. And mitosis is the continuous process of dividing one cell into two, dividing these two cells into four, and so on. Afterwards they begin to differentiate. Perhaps these cells will eventually differentiate and become the basis of my brain. These cells will differentiate right here and give rise to my heart. And these cells will be the basis of my lungs and so on. Ultimately a human being is formed. But it doesn't have to be a person. This is a process that is true for all species, as we said before. Let me draw a man. I'll try to cope. Now we will talk about many cells. So we're people, and I'll draw a very simple drawing, just an outline of a person. I was in art class in high school and this drawing doesn't fully show off my artistic abilities. I'm drawing just to give you a general idea. The cells keep dividing and you end up with a human being, and you can't even see the cells on that scale. So, most of the cells in the human body, mine or yours, are the result of mitosis, which begins in the zygote, and then the cells continue to divide, again and again. They began to differentiate. I mentioned that some of them turned into brain cells. Some of them go into heart cells. The process of differentiation itself is really interesting, and we'll talk about it in more detail when we look at stem cells, embryonic stem cells, and, and maybe we'll talk about the controversies on this topic. Now I will talk about how gametes are formed. How do cells with a haploid set of chromosomes form in me? Before breeding? And what happens in your genitals? Men have sex cells, some of these cells will turn into sex cells. Reproductive cells are part of your reproductive organs. So let's note that these are sex cells. In men they are located in the gonads, that is, here. In women, reproductive cells are located in the ovaries. These are germ cells, they are formed as a result of mitosis. I will draw sex cells. Since the sex cell is formed as a result of mitosis, it has 2N chromosomes, that is, it is a diploid cell, that is, it contains a diploid set... a set of chromosomes. But these cells have a distinctive feature: germ cells are able to continue to mitotically divide to form other germ cells identical to the first. Or they can divide in another way - meiosis occurs. Meiosis is very important for the formation of gametes. Let's indicate it. Meiosis. And if meiosis of a germ cell occurs, four cells are formed, not two. (There will be a separate video about the mechanism of this process.) We’ll look at it a little later. So, these new cells contain a haploid set of chromosomes. Men produce sperm. So, these are spermatozoa. Women produce eggs. The sperm and egg are gametes. This is interesting to discuss because in the last videos I talked a lot about mutations and what happens to species. But let's think about what happens if a mutation occurs in some cell here, some in a somatic cell... a mutation occurs in a somatic cell, can this mutation somehow affect my children? Will the mutation continue in my children? No. What happens in this cell has no effect on the germ cells. It's just a random mutation. But it may affect my ability to reproduce. For example, God forgive me, it could be cancer or something like that, especially if it starts at a young age and goes into the final stage, it may affect your ability to reproduce, but it will not affect the DNA that you pass on to your descendants . So, if there is a serious mutation here that can affect your functioning or lead to cancer and begin to reproduce, it will not be passed on to your children. Only characteristic features or changes that have occurred in the germ cells are transmitted. Therefore, if a mutation occurs in the germ cell or during the process of meiosis, when there are significant DNA recombinations in the cell due to crossover, (we looked at this in the video about variability), then new forms or new variants can be formed that can be passed on to children. I want to focus on this because we are talking about mutations, but there are different types of mutations. Some mutations cannot be passed on to children; these are mutations in somatic cells. Some of them are truly minor and therefore do not affect the overall functioning of the body, but mutations in germ cells, recombinations or variations during meiosis are passed on to offspring. But be careful here. Remember, this is a serious competition. So, out of all of them, about 280 million sperm that move towards the egg at the same time, some may have mutations. Let's label the mutations with different colors. This is a purple mutation. It's blue. In order for a mutation to be passed on to offspring, a sperm with such a mutation must win the race. So, at the level of sexual reproduction, selection already occurs, where the best qualities win, I mean that the sperm must be strong enough to win this very, very difficult competition. It is unlikely that a sperm with a mutation that in any way disfigures it or affects its ability to move, affects the behavior, and that such sperm will successfully fertilize an egg. That's all I wanted to convey to you. So, the main idea is partly contained in the terms “haploid” and “diploid”. It's very confusing at first, but it's just half the normal set of chromosomes. For humans, this number is 23. 23 chromosomes. And the cells that have half the set of chromosomes are our gametes, in men - sperm, in women - eggs. But everything else, all our somatic cells are diploid, that is, they have a full set of chromosomes, they all contain copies of our DNA. This is why DNA analysis is so interesting, since a complete set of DNA can be obtained from any cell in the body. We receive all the genetic information that describes this organism. See you in the next video.
