of living organisms is known that they breathe, eat, reproduce and die, this is their biological function.But due to what it's going on?Due blocks - cells, which also breathe, eat, reproduce and die.But how does this happen?
The structure of cells
house consists of bricks, blocks or logs.And the body can be divided into elementary units - cells.All the diversity of living things is because of them, the difference lies only in their number and type.Of them is muscle, bone, skin, internal organs - so much they differ in their appointment.But regardless of what functions are performed by one or the other cell, they are arranged similarly.First of all, any "brick" is a shell and located in the cytoplasm with its organelles.Some cells do not have nuclei, they are called prokaryotic, but more or less developed organisms composed of eukaryotic having core in which the genetic information is stored.
organelles located in the cytoplasm, are varied and interesting, they perform important functions.In animal cells secrete endoplasmic reticulum, ribosomes, mitochondria, Golgi apparatus, centrioles, lysosomes and propulsion elements.With them, and all the processes that ensure the functioning of the body.
cell activity
As has been said, all life eats, breathes, reproduces and dies.This is true both for the whole organism, that is, people, animals, plants and so on. D., And for the cells.It's amazing, but each "brick" has a life of its own.Due to its organelles it receives and recycles nutrients, oxygen, removes all the excess out.Itself the cytoplasm and endoplasmic reticulum transport function is performed, the mitochondria are responsible including respiration and energy supply.Golgi complex has been the accumulation of waste products and output cells.The rest of the organelles are also involved in complex processes.And at some stage of their life cycle, the cell begins to divide, that is, the process of reproduction.It is worth considering in more detail.
process of cell division
Reproduction - one of the stages of development of a living organism.The same applies to the cells.At a certain stage of the life cycle they are part of the state when they become ready to breed.Prokaryotic cells are simply divided in two, extended, and then forming a partition.This process is simple and almost completely studied by the example of rod-shaped bacteria.
with eukaryotic cells the situation is more complicated.They breed in three different ways, called amitosis, mitosis and meiosis.Each of these paths has its own characteristics, it is inherent in a particular type of cell.Amitosis considered the most simple, it is also called a direct binary fission.If there is a doubling of the DNA molecule.However, division spindle is not formed, so that this method is more energy economical.Amitosis observed in unicellular organisms, while the multicellular tissue multiply by other mechanisms.However, it is sometimes observed, and where reduced mitotic activity, e.g., in mature tissues.
sometimes directly recovered as a kind of division of mitosis, but some scientists believe it is a separate mechanism.The course of this process, even in the old cells is rare.Next will be considered meiosis and its phases, mitosis, and the similarities and differences between these methods.Compared with the simple division they are more complex and sophisticated.This is especially true reduction division, so that the characteristics of the phases of meiosis is the most detailed.
important role in dividing cells have centrioles - specific organelles, usually located close to the Golgi complex.Each structure consists of 27 microtubules, grouped by three.The whole structure has a cylindrical shape.The centrioles are directly involved in the formation of spindle cells in the process of indirect division, which will be discussed further.
Mitosis
Persistence cells varies.Some live a couple of days, but some can be attributed to long-lived as their complete change happens very rarely.And almost all of these cells reproduce via mitosis.Most of them extends between periods dividing an average of 10-24 hours.Mitosis itself takes a short period of time - in animals approximately 0.5-1 hour, and the plants about 2-3.This mechanism ensures the growth of the cell population and the reproduction of their identical genetic filling units.So the continuity of generations observed at the elementary level.The number of chromosomes is unchanged.This mechanism is the most common form of reproduction in eukaryotic cells.
value of this kind of division is large - a process that helps to grow and regenerate tissues, thereby is the development of the whole organism.Furthermore, it mitosis underlies asexual reproduction.And another feature - the movement of cells and replacing already obsolete.Therefore, I believe that due to the fact that the difficult stage of meiosis, and its role is much greater wrong.Both of these processes have different functions and in their important and irreplaceable.
Mitosis consists of several phases, differing in their morphological features.Condition in which the cells are being prepared for indirect indexing is called interphase and the process itself is divided by 5 steps, to be considered in greater detail.
Phase of mitosis
Being in interphase, the cell prepares for division: the synthesis of DNA and proteins.This stage is divided into several, during which there is growth of the entire structure and chromosome doubling.In this state, the cell stays up to 90% of the total life cycle.
remaining 10% takes a direct division is divided into five stages.During mitosis, plant cells also released preprofaza, which is absent in all other cases.The formation of new structures, the core moves to the center.Formed preprofaznaya tape, mark the proposed location of the future division.
In still other cells mitosis process is as follows:
Table 1
| Name step | Feature |
| Prophase | kernel grows in size, the chromosomes it spiralizuyutsya become visiblemicroscope.In the cytoplasm formed division spindle.Often there is a disintegration of the nucleolus, but it does not always happen.The content of the genetic material in the cell remains unchanged. |
| prometaphase | There disintegration of the nuclear membrane.Chromosomes begin active but chaotic traffic.Ultimately, all of them come into the plane of a metaphase plate.This stage lasts up to 20 minutes. |
| Metaphase chromosomes line up along the equatorial plane of the spindle about an equal distance from both poles.Number of microtubules holding the entire structure in a stable state, reaches a maximum.Sister chromatid repel each other, while maintaining the connection only in the centromere. | |
| Anaphase | The shortest stage.Chromatids separate and repel each other in the direction of the nearest pole.This process is sometimes referred to as separate and isolated anaphase A. Further there is a discrepancy themselves poles division.In some of the simplest cell division spindle with increases up to 15 times.And this sub-step is called anaphase B. The duration and sequence of processes at this stage is variable. |
| Telophase | After differences chromatids to opposite poles stop.There decondensation of chromosomes, that is, they increase in size.It begins reconstruction of nuclear shells future daughter cells.Spindle microtubules disappear.Forming the core, RNA synthesis is resumed. |
After the division of genetic information occurs or cytokinesis cytokinesis.This term refers to the formation of the bodies of daughter cells from the body of the parent.This organelles, usually divided in half, although there may be exceptions, forms a partition.Cytokinesis is not isolated in a separate phase, as a rule, considering it as part of telophase.
So, the most interesting processes involved chromosomes that carry genetic information.What is it and why are they so important?About
chromosomes
Not having a clue about genetics, people know that a lot depends on the quality of the offspring of parents.With the development of biology, it became clear that the information about a particular body is stored in each cell, and part of it is passed on to future generations.
At the end of the 19th century were discovered chromosomes - structures consisting of a long DNA molecule.This is made possible with the improvement of microscopes, and even now you can see them only during division.Most often credited with the discovery of the German scientist W. Fleming, who not only organize all that has been learned before him, but also contributed: he was one of the first to investigate the cellular structure, meiosis and its phases, as well as coined the term "mitosis".The concept of "chromosome" was proposed later other scientists - a German histologist D. Waldeyer.
chromosome structure at the time when they are clearly visible, is quite simple - they are the two chromatids joined by the centromere in the middle.It is a specific sequence of nucleotides and plays an important role in cell proliferation.Ultimately chromosome appeared to be in prophase and metaphase, when it may be best to see, like the letter H.
In 1900 were discovered Mendel's laws describing the principles of the transmission of hereditary characteristics.Then it became clear that the chromosomes - this is exactly what is transmitted with the help of genetic information.In the future, the scientists conducted a series of experiments to prove it.And then it became a subject of study and the impact on them has kotroe cell division.
Meiosis
Unlike mitosis, the mechanism ultimately leads to the formation of two cells with chromosome 2 times smaller than the original.Thus the process of meiosis is the transition from the diploid to the haploid phase, the first case of nuclear fission, and in the second - the entire cell.Restoration of the complete set of chromosomes occurs as a result of further fusion of gametes.Due to the decrease in the number of chromosome, this method is still defined as the reduction-division cells.
meiosis phase studied and well-known scholars such as V. Fleming, E. Strasburgrer VI Belyaev and others.The study of this process in the cells of both plants and animals, is still going on - so it is complicated.Initially, this was considered the embodiment of the process of mitosis, but almost immediately after the opening he still was isolated as a separate mechanism.Characteristics of meiosis and its theoretical value were first adequately described by Augustus Wiseman in 1887.Since then, the study of meiosis progressed greatly, but the findings have not yet been refuted.
Meiosis should not be confused with the germ line, although both of these processes are closely related.In the formation of sex cells of both mechanisms are involved, but between them there are some major differences.Meiosis occurs in two stages of division, each of which consists of four main phases, between them there is a short break.The duration of the whole process depends on the amount of DNA in the nucleus and the structure of the chromosomal organization.In general, it is much more prolonged in comparison with mitosis.
Incidentally, one of the main causes of a significant species diversity - is meiosis.The set of chromosomes in meiosis result is divided into two parts, so that there are new combinations of genes, primarily potentially increase flexibility and adaptability of organisms as a result of receiving a certain set of characteristics and qualities.
Phases of meiosis
As already mentioned, reduction of cell division is conventionally divided into two stages.Each of these stages is divided by 4. And even the first phase of meiosis - prophase I in turn subdivided into five separate stages.Since the study of this process is continued in the future can be highlighted and others.Now there are following phases of meiosis:
Table 2
| Name step | Feature |
| first division (pressure reducing) | |
| Prophase I | |
| leptotena | In another way, this stage is called the stage of fine threads.Chromosomes appear in the microscope as a tangle.Sometimes isolated proleptotenu when individual strings more difficult to discern. |
| zygote stage coalescing strands.Homologous, i.e. similar to each other in morphology and genetically, the pairs of chromosomes are merged.In the course of the merger, ie conjugation formed bivalents or tetrads.So called fairly stable complexes of the pairs of chromosomes. | |
| Paquita | Stage thick filaments.At this stage of the chromosome ends spiralizuyutsya and DNA replication, formed chiasm - the point of contact parts of chromosomes - chromatids.There is a process of crossing over.Chromosomes overlap and share certain portions of the genetic information. |
| diplotene | Also called stage double strands.Homologous chromosomes in bivalents repel each other and remain connected only chiasm. |
| diakinesis | At this stage the odds bivalents at the nuclear periphery. |
| Metaphase I | shell core collapses, forming division spindle.Bivalents moved to the center of the cell and line up along the equatorial plane. |
| Anaphase I | bivalent break, after which each chromosome of the pair is moved to the closest pole of the cell.Divided into chromatids occurs. |
| Telophase I | The process of chromosome segregation.Is the formation of certain nuclei of daughter cells, each - with a haploid set.Chromosomes dispiralized formed nuclear envelope.Sometimes there is cytokinesis, ie the division of the cell body. |
| second division (equational) | |
| Prophase II | condensation of chromosomes, the cell center is divided.Destroyed the nuclear envelope.Formed division spindle, perpendicular to the first. |
| Metaphase II | in each daughter cell chromosomes line up along the equator.Each of them consists of two chromatids. |
| Anaphase II | Each chromosome is divided into chromatids.This part of the costs to the opposite poles. |
| Telophase II | received odnohromatidnye dispiralized chromosomes.Formed nuclear envelope. |
So, it is obvious that phase meiotic division is much more difficult than the process of mitosis.But, as already mentioned, this does not diminish the biological role of indirect division, because they have different functions.
way, meiosis and its phases are observed and some protozoa.However, generally, it includes only one notch.It is assumed that such a single-stage form later developed into a modern, two-step.
differences and similarities between mitosis and meiosis
At first glance it seems that the differences between these two processes are clear, it's totally different mechanisms.However, a deeper analysis reveals that the differences mitosis and meiosis are not so global, they eventually lead to the formation of new cells.
First of all, is to talk about what is common in these mechanisms.Essentially only two matches: in the same sequence of phases, and in that, before the two kinds of division, DNA replication occurs.Although regard meiotic prophase I before this process is not fully completed, ending at one of the first substages.
