Ciclo celular | Los secretos del cuerpo humano

CELLULAR CYCLE

Welcome to a new entry! Today's topic is cell reproduction. How do cells divide? How is it that the number of chromosomes does not double from one generation to another? Is mitosis a myth?

In the first place, the cell cycle comprises the period of time that goes from the moment a cell is formed, by division of a pre-existing one, until it divides to give rise to two new cells. Furthermore, it can last from 8 hours to 100 days.

The interface is the period of time that elapses between two successive divisions. During this stage there is a great metabolic activity, there is an increase in the size of the cell and the duplication of DNA. In addition, it is mainly divided into three phases:

G1 phase: in this phase there is the synthesis of the proteins necessary for the cell to increase in size and new organelles are formed (ribosomes, mitochondria ...) At the end of G1 a moment of no return is distinguished, the restriction point ( point R), from which it is impossible to stop the following phases from happening (S, G2 and M). However, some cells remain at rest and therefore do not divide. This process is called G0.
S phase: DNA duplication occurs, that is, the synthesis of DNA and histones, so that when later, during mitosis, DNA condenses to form chromosomes, these instead of having one chromatid, have two.
G2 phase: in this stage the genes that encode the proteins necessary for the cell to divide are transcribed and translated and the centrioles are duplicated.

Source: own image.

On the other hand, cell division is the process by which, from a mother cell, 2 daughter cells are formed with the same chromosomal endowment as the parent. This includes the division of the nucleus or mitosis, also called karyokinesis, and the division of the cytoplasm or cytokinesis.

Among the causes that trigger cell division are an increase in the size of the cytoplasm or in temperature and the action of certain chemicals such as hormones, oxygen or certain nutrients.

MITOSIS

Mitosis is the process of cell division by which a stem cell gives rise to two daughter cells with the same genetic endowment as that of the mother. During mitotic division, a series of well-defined morphological changes occur in the cell that allow the process to be divided into a series of stages.

Prophase : chromosomes begin to become visible, and they spiral out, becoming shorter and thicker. The diplosome of the centrosome is duplicated and they migrate to opposite poles, appearing between them the achromatic spindle. Finally, the nuclear membrane and nucleolus fragment until they disappear at the end of this phase.

Metaphase : in this stage is when the chromosomes are best visualized since they reach their maximum degree of packaging. In addition, kinetochoric microtubules elongate by polymerization and push chromosomes into the equatorial plane of the spindle, where they form the equatorial or metaphase plate.

Anaphase : each chromatid, which is already a daughter chromosome or an anaphase chromosome, moves towards opposite poles by shortening (by depolymerization) of the kinetochoric microtubules.

Telophase : in it, the chromosomal microtubules disappear when the daughter chromosomes have already reached the poles. The chromosomes begin to decondense and around each group the nuclear membrane reappears. Finally, the nucleoli appear again.

Simultaneously, the division of the cytoplasm, cytokinesis , occurs differently in animal and plant cells.

In animal cells : strangulation occurs due to the fact that in the equatorial region a groove covered by microfilaments appears, forming a contractile ring that narrows until it separates into two daughter cells.
In plant cells: it is carried out by the formation of a cellulosic septum called a fragmoplast which does not close completely but is perforated by plasmodesmata.

The biological significance of mitosis is one of conservative reproduction, since the same genetic material is maintained from one generation to another. It can occur in haploid or diploid beings. In unicellular beings it is the way to create new individuals and, in multicellular beings, to generate new cells to repair injured cells for the growth of the being or its development.

In the following image, you can see a diagram of the stages of mitosis.

Source: own image.

MEIOSIS

Meiosis is a reductive division, in which the cells responsible for forming gametes undergo a series of transformations aimed at reducing the number of chromosomes to half that of the mother cell. In this way, once a new individual is formed, through the union of two gametes in a process of fertilization, its number of chromosomes will be identical to those of its parents.

Meiosis consists of two successive divisions:

FIRST MEIOTIC DIVISION.

PROFASE I : is the longest and most complex. It is subdivided into five phases: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis.
- Leptotene : the chromosomes become visible.
- Zygotene : synapse is produced, which consists of homologous chromosomes pairing with each other through the formation of a protein structure called the synaptonemic complex, which pairs each gene with its homologue.
- Pachytene : crossover between non-sister chromatids, which consists of the breakdown and exchange of fragments between non-sister chromatids of homologous chromosomes. In addition, recombination nodules can be observed, which are the protein structures responsible for crossing over.
- Diplotene : homologous chromosomes begin to separate, although they remain united by the points where the crossover has taken place, the chiasmata.
- Diakinesis : the chromosomes are condensed to the maximum. In each bivalent, the 4 chromatids (tetrads) and the chiasms can be seen. In addition, the nuclear envelope and nucleolus begin to disappear, the achromatic spindle is formed, and kinetochoric microtubules begin to form.
METAPHASE I : the bivalents are located in the equatorial plane of the cell to form the metaphase plate.
ANAPHASE I : the chiasmas are broken, separating the bivalents and each homologue moves to an opposite pole of the cell.
TELOPHASE I : homologous chromosomes reach the poles and two haploid cells are obtained each with a pair of homologous chromosomes. Nucleolus and nuclear membrane reappear.

Next, cytokinesis occurs, obtaining 2 daughter cells with half the chromosomes that the mother cell had and with 2 chromatids on each chromosome.

SECOND MEIOTIC DIVISION.

PROPHASE II : the nuclear envelope is broken, the chromosomes are duplicated and the mitotic spindle is formed.
METAPHASE II : the chromosomes are located on the spindle forming the equatorial plate.
ANAPHASE II : the centromeres are broken, and each chromatid migrates to an opposite pole.
TELOPHASE II : chromosomes are decondensed and surrounded by a nuclear envelope.

Simultaneously with telophase, cytokinesis occurs. The end result is the formation of four genetically distinct haploid cells.

Finally, the biological meaning of meiosis is to allow the conservation of the chromosome number of each individual and, therefore, of the chromosome number of each species. In addition, the genetic variability of individuals and species increases when crossing over.

Next, I attach a diagram with the different phases of meiosis.

Source: own image.

Finally, it is worth highlighting the BIOLOGICAL CYCLES , which are three:

Haplon cycle: the zygote is divided by meiosis and haploid adults are formed that generate haploid gametes through mitosis, which by fertilization originate the zygote.

Diplonte cycle: the zygote divides by mitosis to generate a diploid adult, which will give rise to haploid gametes, which will give rise to a diploid gamete by fertilization.

Diplohaplonte cycle: the diploid individual (sporophyte) generates by meiosis. These haploid spores give rise to haploid organisms (gametophytes) which are the ones that generate gametes. After fertilization, the gametes form the diploid zygote, which divides by mitosis, giving rise to the diploid sporophyte.

Source: own image.

ACTIVITIES.

2. How are the terms cytokinesis and karyokinesis different?

Cytokinesis is the physical separation of the cytoplasm into two daughter cells during cell division, and occurs at the end of telophase. However, karyokinesis, or mitosis, is the process of division of the nucleus. As a result, two daughter cells are formed with the same number of chromosomes and the same genetic information as the mother cell.

3. What are the differences between sister chromatids and homologous chromosomes?

Sister chromatids are the identical copies formed by DNA replication of a chromosome, with both copies joined by a common centromere.

However, homologous chromosomes are a pair of chromosomes with the same length, shape, and set of genes, which are delivered by different parents. These pair up within a cell during meiosis.

Sister chromatids are generally identical because they are derived from an original chromosome, while homologous chromosomes may or may not be the same as each other because they are derived from different parents.

4. It is known that, in eukaryotes, each chromatid is made up of a single DNA molecule. Indicate the number of DNA molecules in the following cells of a species with 2n = 6:

A sperm: 3 DNA molecules.
A cell in mitotic metaphase: 6 DNA molecules.
A cell in G1 period: 3 DNA molecules.
A cell in G2 period: 6 DNA molecules.
A cell in prophase of the second meiotic division: 3 DNA molecules.

7. Define the following concepts: G1 period; homologous chromosome; crossover and haploid.

The G1 period is the stage between DNA division and synthesis. In it, the cell carries out biosynthetic processes of cellular material, fundamentally the synthesis of proteins so that the cell increases in size and the formation of new organelles. Some cells remain in a state of rest and do not divide: in this case, the phase is called G0 (which would be equivalent to the G1 phase). The transition period between the G1 and S phases is called the restriction point.

Homologous chromosomes are a pair of chromosomes with the same length, shape, and set of genes, which are delivered by different parents. These pair up within a cell during meiosis.

Crosslinking is the breakdown and exchange of fragments between non-sister chromatids that takes place during the pachytene phase of prophase I of meiosis. As a consequence of this crossover, a gene recombination occurs, that is, an exchange of genes between homologous chromosomes.

That a cell is haploid means that it has half the chromosomes of the rest of the somatic cells that make up the organism, which are diploid. Therefore, it is made up of 23 chromosomes. This is possible thanks to meiosis, since from a stem cell, four haploid cells originate so that when fertilization occurs, a diploid zygote is formed.

8. In relation to meiosis, answer the following questions:

Why is the first meiotic division said to be reductional?

The first meiotic division is said to be reductive because from a diploid cell two daughter cells are obtained with half the chromosomes and, therefore, haploid cells.

What is the biological significance of meiosis?

The biological meaning of meiosis is that the female (ovules) and male (sperm) gametes contain half the chromosomes of the rest of the somatic cells so that, when fertilization occurs, the resulting embryo has the correct genetic load (2n ) and not twice as many as their parents.

9. Compare mitosis with meiosis in terms of:

Types of cells involved.
Anaphase of mitosis and anaphase of the first meiotic division.
Result of the process.

First, mitosis occurs in somatic cells and meiosis in germ cells. On the other hand, in the anaphase of mitosis the chromatids separate, while in the first meiotic division the pairs of homologous chromosomes separate. Finally, the result of mitosis is the formation of two cells identical to each other and to the parent, and meiosis gives rise to four haploid cells, which are the gametes.

PAU ACTIVITIES

12. Describe the most relevant cellular events of prophase I of meiosis. Discuss the genetic and evolutionary consequences of prophase I of meiosis. (Option B- June 2001)

The most relevant cellular events of prophase I of meiosis are the following. First, the chromatin condenses and chromosomes are formed, which are made up of two sister chromatids. Next, synapse takes place, which is a process in which homologous chromosomes pair with each other through the formation of a protein structure called the synaptonemic complex, which pairs each gene with its homolog. These two homologous chromosomes joined together are called bivalent. Subsequently, the crossover occurs where the chromatids of the homologous chromosomes exchange fragments of genetic material and are joined by those points that are called chiasmas. Finally, the nuclear membrane disappears and the bivalents are free in the cytoplasm.

The genetic and evolutionary consequences of meiotic prophase I are that homologous chromosomes pair up and then separate to give rise to two daughter cells with half the information of the parent cell. Furthermore, the different distribution possibilities in the segregation of parental chromosomes in the first meiotic division contributes to genetic variability.

15. In view of the diagram, answer the following questions: (Option A-June 2001)

Indicate what moment of the cell cycle the diagrams above represent, what the numbers indicate, and describe the cellular phenomena that occur in A, B, and C.

The moment of the cell cycle that the diagrams represent is mitosis. Figures A, B, C and D represent prophase, figure E the metaphase, F the anaphase, G the telophase and, finally, H the cytokinesis, that is, the division of the cytoplasm.

On the other hand, the number 1 indicates a prophasic chromosome, the 2 indicates the centrioles and the 3 the achromatic spindle.

In figures A, B and C prophase takes place. In Figure A, prophase begins and chromatin is decondensed. In B, the duplication of the centrioles occurs and, finally, in C both centrioles migrate to the opposite poles of the cell.

Say if the pictures correspond to an animal or plant cell. Indicate, reasoning the answer, two characteristics on which it is based.

The drawings correspond to the mitosis of an animal cell since it has centrioles, does not have a cell wall and is dividing by strangulation.

16. In view of the image, answer the following questions: (Option A- June 2003)

What stage of mitosis does it represent? What do arrows A, B and C indicate? Is it an animal or plant cell? Give reasons for the answer. Describe in detail the cellular phenomena that occur at this stage.

This image represents the beginning of the telophase of mitosis, since the sister chromatids of the chromosomes are located at the poles of the cell and the contractile ring is being formed for the division of the cytoplasm by strangulation.

Arrow A indicates the chromatids, B the achromatic spindle and C the contractile ring.

It is an animal cell, since it does not have a cell wall, it has a spherical shape and a division of the cytoplasm is taking place by strangulation mediated by a contractile ring.

Describe the cellular phenomena that take place in the remaining stages of mitosis. Explain what is the biological significance of it.

Mitosis begins with prophase. The 2n chromatids begin to condense. The nucleoli disappear. Centrosomes form and move away towards opposite poles of the cell. The nuclear envelope disappears and kinetochores are formed in each chromatic.

Next, the metaphase occurs. In this stage, the chromosomes adhere to the mitotic spindle through the kinetochores, which are structures found in the telomere. In this way, we find an equatorial plate. The mitotic spindle is composed of the centrosomes, polar microtubules, and kinetochoric fibers.

Then anaphase occurs. The chromosomes are drawn toward the poles of the cell by the mitotic spindle of the centrosomes. The chromosomes that are carried away are anaphasic chromosomes, that is, only one chromosome of each metaphase chromosome.

Finally, in telophase we already find two sets of identical chromatids at each pole of the cell. The nuclear membrane and nucleolus are formed again and the genetic material is decondensed.

I hope you have found it useful and see you in the next post! ;-)