1. What is the cell cycle?
The cell cycle is the existence of a cell from the moment of its formation during the division of the mother cell until its own division (including this division) or death. The cell cycle consists of interphase and mitosis (cell division).
2. What is called interphase? What main events occur in the G 1 -, S- and G 2 -periods of interphase?
Interphase is the part of the cell cycle between two successive divisions. During the entire interphase, chromosomes are non-spiralized and are located in the cell nucleus in the form of chromatin. As a rule, interphase consists of three periods:
● Presynthetic period (G 1) – the longest part of the interphase (from 2 – 3 hours to several days). During this period, the cell grows, the number of organelles increases, energy and substances are accumulated for the subsequent doubling of DNA. During the G 1 period, each chromosome consists of one chromatid. The set of chromosomes (n) and chromatids (c) of a diploid cell in the G 1 period is 2n2c.
● During the synthetic period (S), DNA doubling (replication) occurs, as well as the synthesis of proteins necessary for the subsequent formation of chromosomes. During this same period, the doubling of centrioles occurs. By the end of the S period, each chromosome consists of two identical sister chromatids connected at the centromere. The set of chromosomes and chromatids of a diploid cell at the end of the S-period (i.e. after replication) is 2n4c.
● During the postsynthetic period (G 2), the cell accumulates energy and synthesizes proteins for the upcoming division (for example, tubulin to build microtubules, which subsequently form the spindle). During the entire G 2 period, the set of chromosomes and chromatids in the cell is 2n4c.
At the end of interphase, cell division begins.
3. Which cells are characterized by the G 0 period? What happens during this period?
Unlike constantly dividing cells (for example, cells of the germinal layer of the epidermis of the skin, red bone marrow, the mucous membrane of the gastrointestinal tract of animals, cells of the educational tissue of plants), most cells of a multicellular organism take the path of specialization and, after passing through part of the G 1 period, pass during the rest period (G 0 -period).
Cells in the G0 period perform their specific functions in the body; metabolic and energy processes occur in them, but preparation for replication does not occur. Such cells, as a rule, permanently lose their ability to divide. Examples include neurons, lens cells, and many others.
However, some cells that are in the G0 period (for example, leukocytes, liver cells) can leave it and continue the cell cycle, going through all periods of interphase and mitosis. Thus, liver cells can again acquire the ability to divide after several months of being in a period of rest.
4. How is DNA replication carried out?
Replication is the doubling of DNA, one of the reactions of template synthesis. During replication, special enzymes separate the two strands of the original parent DNA molecule, breaking the hydrogen bonds between complementary nucleotides. Molecules of DNA polymerase, the main replication enzyme, bind to the separated strands. Then the DNA polymerase molecules begin to move along the mother chains, using them as templates, and synthesize new daughter chains, selecting nucleotides for them according to the principle of complementarity.
As a result of replication, two identical double-stranded DNA molecules are formed. Each of them contains one chain of the original mother molecule and one newly synthesized daughter chain.
5. Are the DNA molecules that make up homologous chromosomes the same? In the composition of sister chromatids? Why?
DNA molecules in sister chromatids of one chromosome are identical (have the same nucleotide sequence), because they are formed as a result of replication of the original mother DNA molecule. Each of the two DNA molecules that make up sister chromatids contains one strand of the original mother DNA molecule (template) and one new, daughter strand synthesized on this template.
The DNA molecules in homologous chromosomes are not identical. This is due to the fact that homologous chromosomes have different origins. In each pair of homologous chromosomes, one is maternal (inherited from the mother), and the other is paternal (inherited from the father).
6. What is necrosis? Apoptosis? What are the similarities and differences between necrosis and apoptosis?
Necrosis is the death of cells and tissues in a living organism, caused by the action of damaging factors of various natures.
Apoptosis is programmed cell death regulated by the body (so-called “cellular suicide”).
Similarities:
● Necrosis and apoptosis are two types of cell death.
● Occurs at all stages of the body’s life.
Differences:
● Necrosis is random (unplanned) cell death, which may be caused by exposure to high and low temperatures, ionizing radiation, various chemicals (including toxins), mechanical damage, impaired blood supply or innervation of tissues, or an allergic reaction. Apoptosis is initially planned by the body (genetically programmed) and regulated by it. During apoptosis, cells die without direct damage, as a result of their receiving a specific molecular signal - an “order to self-destruct.”
● As a result of apoptosis, individual specific cells die (only those that have received the “order”), and entire groups of cells usually undergo necrotic death.
● During necrotic death in damaged cells, membrane permeability is disrupted, protein synthesis stops, other metabolic processes stop, the nucleus, organelles and, finally, the entire cell are destroyed. Typically, dying cells are attacked by leukocytes, and an inflammatory reaction develops in the area of necrosis. During apoptosis, the cell breaks up into separate fragments surrounded by plasmalemma. Typically, fragments of dead cells are absorbed by white blood cells or neighboring cells without triggering an inflammatory response.
And (or) other significant features.
7. What is the significance of programmed cell death in the life of multicellular organisms?
One of the main functions of apoptosis in a multicellular organism is to ensure cellular homeostasis. Thanks to apoptosis, the correct ratio of the number of cells of different types is maintained, tissue renewal is ensured, and genetically defective cells are removed. Apoptosis seems to interrupt the infinity of cell divisions. Weakening of apoptosis often leads to the development of malignant tumors and autoimmune diseases (pathological processes in which an immune reaction develops against the body’s own cells and tissues).
8. Why do you think that in the vast majority of living organisms the main keeper of hereditary information is DNA, and RNA performs only auxiliary functions?
The double-stranded nature of the DNA molecule underlies the processes of its self-duplication (replication) and the elimination of damage - repair (the undamaged strand serves as a matrix for restoring the damaged strand). Being single-stranded, RNA is not capable of replication, and its repair processes are hampered. In addition, the presence of an additional hydroxyl group on ribose (compared to deoxyribose) makes RNA more susceptible to hydrolysis than DNA.
The G1, S and G2 phases of the cell cycle are collectively called interphase. A dividing cell spends most of its time in interphase as it grows in preparation for division. The mitosis phase involves nuclear separation followed by cytokinesis (division of the cytoplasm into two separate cells). At the end of the mitotic cycle, two different ones are formed. Each cell contains identical genetic material.
The time required to complete cell division depends on its type. For example, cells in the bone marrow, skin cells, stomach and intestinal cells divide quickly and constantly. Other cells divide as needed, replacing damaged or dead cells. These types of cells include cells from the kidneys, liver, and lungs. Others, including nerve cells, stop dividing after maturation.
Periods and phases of the cell cycle
Scheme of the main phases of the cell cycle
The two main periods of the eukaryotic cell cycle include interphase and mitosis:
Interphase
During this period, the cell doubles and synthesizes DNA. It is estimated that a dividing cell spends about 90-95% of its time in interphase, which consists of the following 3 phases:
- Phase G1: the period of time before DNA synthesis. During this phase, the cell increases in size and number in preparation for division. in this phase they are diploid, meaning they have two sets of chromosomes.
- S-phase: stage of the cycle during which DNA is synthesized. Most cells have a narrow window of time during which DNA synthesis occurs. The chromosome content doubles in this phase.
- Phase G2: the period after DNA synthesis but before the onset of mitosis. The cell synthesizes additional proteins and continues to grow in size.
Phases of mitosis
During mitosis and cytokinesis, the contents of the mother cell are evenly distributed between the two daughter cells. Mitosis has five phases: prophase, prometaphase, metaphase, anaphase and telophase.
- Prophase: at this stage, changes occur both in the cytoplasm and in the dividing cell. condenses into discrete chromosomes. Chromosomes begin to migrate to the center of the cell. The nuclear envelope breaks down and spindle fibers form at opposite poles of the cell.
- Prometaphase: the phase of mitosis in eukaryotic somatic cells after prophase and preceding metaphase. In prometaphase, the nuclear membrane breaks down into numerous “membrane vesicles,” and the chromosomes inside form protein structures called kinetochores.
- Metaphase: at this stage, the nuclear one completely disappears, a spindle is formed, and the chromosomes are located on the metaphase plate (a plane that is equally distant from the two poles of the cell).
- Anaphase: at this stage, the paired chromosomes () separate and begin to move towards opposite ends (poles) of the cell. The fission spindle, which is not connected to the spindle, extends and lengthens the cell.
- Telophase: At this stage, the chromosomes reach new nuclei, and the genetic content of the cell is divided equally into two parts. Cytokinesis (eukaryotic cell division) begins before the end of mitosis and ends shortly after telophase.
Cytokinesis
Cytokinesis is the process of separation of the cytoplasm in eukaryotic cells that produces various daughter cells. Cytokinesis occurs at the end of the cell cycle after mitosis or.
During animal cell division, cytokinesis occurs when the contractile ring forms a split furrow that pinches the cell membrane in half. The cell plate is built, which divides the cell into two parts.
Once the cell has completed all phases of the cell cycle, it returns to the G1 phase and the entire cycle repeats again. The body's cells are also capable of entering a state of rest, called the Gap 0 (G0) phase, at any point in their life cycle. They can remain in this stage for a very long period of time until signals are given to move through the cell cycle.
Cells that contain genetic mutations are permanently placed in the G0 phase to prevent them from replicating. When the cell cycle goes wrong, normal cell growth is disrupted. Can develop that gain control of their own growth signals and continue to reproduce unchecked.
Cell cycle and meiosis
Not all cells divide through the process of mitosis. Organisms that reproduce sexually also undergo a type of cell division called meiosis. Meiosis occurs in and is similar to the process of mitosis. However, after a complete cell cycle, meiosis produces four daughter cells. Each cell contains half the number of chromosomes of the original (parent) cell. This means that the sex cells are . When haploid male and female sex cells come together in a process called , they form one called a zygote.
Being in high concentration, it prevents the activation of protein kinases CDK4 or CDK6 by cyclins D1, or. Under these conditions, the cell remains in the G0 phase or early G1 phase until it receives a mitogenic stimulus. After adequate stimulation, the concentration of the p27 inhibitor decreases against the background of an increase in the intracellular content of cyclins D. This is accompanied by activation of CDK and, ultimately, phosphorylation of the pRb protein, release of the associated transcription factor E2F and activation of transcription of the corresponding genes.
During these early stages of the G1 phase of the cell cycle, the concentration of p27 protein is still quite high. Therefore, after cessation of mitogenic stimulation of cells, the content of this protein is quickly restored to a critical level and further passage of cells through the cell cycle is blocked at the corresponding G1 stage. This reversibility is possible until the G1 phase in its development reaches a certain stage, called the transition point, after which the cell becomes committed to division, and the removal of growth factors from the environment is not accompanied by inhibition of the cell cycle. Although from this point on the cells become independent of external signals to divide, they retain the ability to self-control the cell cycle.
Early in the cell cycle, healthy cells can recognize and respond to DNA damage by arresting cell cycle progression in the G1 phase until the damage is repaired. For example, in response to DNA damage caused by ultraviolet light or ionizing radiation, the p53 protein induces transcription of the p21 protein gene. Increasing its intracellular concentration blocks the activation of CDK2 by cyclins E or . This arrests cells in late G1 phase or early S phase of the cell cycle. At this time, the cell itself determines its future fate - if the damage cannot be eliminated, it enters into
InterphaseG1 follows the telophase of mitosis. During this phase, the cell synthesizes RNA and proteins. The duration of the phase is from several hours to several days. G0. Cells may exit the cycle and be in the G0 phase. In the G0 phase, cells begin to differentiate. S. During the S phase, protein synthesis continues in the cell, DNA replication occurs, and centrioles separate. In most cells, the S phase lasts 8-12 hours. G2. During the G2 phase, RNA and protein synthesis continues (for example, the synthesis of tubulin for mitotic spindle microtubules). Daughter centrioles reach the size of definitive organelles. This phase lasts 2-4 hours. Mitosis During mitosis, the nucleus (karyokinesis) and cytoplasm (cytokinesis) divide. Phases of mitosis: prophase, prometaphase, metaphase, anaphase, telophase (Fig. 2-52). Prophase. Each chromosome consists of two sister chromatids connected by a centromere; the nucleolus disappears. Centrioles organize the mitotic spindle. A pair of centrioles is part of the mi-
In order for a cell to fully divide, it must increase in size and create a sufficient number of organelles. And in order not to lose hereditary information when divided in half, she must make copies of her chromosomes. And finally, in order to distribute hereditary information strictly equally between two daughter cells, it must arrange the chromosomes in the correct order before distributing them to the daughter cells. All these important tasks are accomplished during the cell cycle.
The cell cycle is important because... it demonstrates the most important: the ability to reproduce, grow and differentiate. Exchange also occurs, but it is not considered when studying the cell cycle.
Definition of the concept
Cell cycle - this is the period of life of a cell from birth to the formation of daughter cells.
In animal cells, the cell cycle, the period of time between two divisions (mitoses), lasts on average from 10 to 24 hours.
The cell cycle consists of several periods (synonym: phases), which naturally replace each other. Collectively, the first phases of the cell cycle (G 1, G 0, S and G 2) are called interphase , and the last phase is called .
Rice. 1.Cell cycle.
Periods (phases) of the cell cycle
1. The period of the first growth G1 (from the English Growth - growth), is 30-40% of the cycle, and the rest period G 0
Synonyms: postmitotic (occurs after mitosis) period, presynthetic (passes before DNA synthesis) period.
The cell cycle begins with the birth of a cell as a result of mitosis. After division, the daughter cells are reduced in size and have fewer organelles than normal. Therefore, a “newborn” small cell in the first period (phase) of the cell cycle (G 1) grows and increases in size, and also forms the missing organelles. There is an active synthesis of proteins necessary for all this. As a result, the cell becomes full-fledged, one might say, “adult”.
How does the growth period G1 usually end for a cell?
- The entry of the cell into the process. Due to differentiation, the cell acquires special characteristics to perform functions necessary for the entire organ and organism. Differentiation is triggered by control substances (hormones) acting on the corresponding molecular receptors of the cell. A cell that has completed its differentiation drops out of the division cycle and is in rest period G 0 . Exposure to activating substances (mitogens) is required for it to undergo dedifferentiation and return to the cell cycle.
- Death (death) of the cell.
- Entering the next period of the cell cycle - synthetic.
2. Synthetic period S (from English Synthesis - synthesis), makes up 30-50% of the cycle
The concept of synthesis in the name of this period refers to DNA synthesis (replication) , and not to any other synthesis processes. Having reached a certain size as a result of passing through the period of first growth, the cell enters the synthetic period, or phase, S, in which DNA synthesis occurs. Due to DNA replication, the cell doubles its genetic material (chromosomes), because An exact copy of each chromosome is formed in the nucleus. Each chromosome becomes double and the entire chromosome set becomes double, or diploid . As a result, the cell is now ready to divide the hereditary material equally between two daughter cells without losing a single gene.
3. The period of the second growth G 2 (from the English Growth - growth), is 10-20% of the cycle
Synonyms: premitotic (passes before mitosis) period, postsynthetic (occurs after the synthetic) period.
The G2 period is preparatory to the next cell division. During the second period of G 2 growth, the cell produces proteins required for mitosis, particularly tubulin for the spindle; creates energy reserves in the form of ATP; checks whether DNA replication is complete and prepares for division.
4. The period of mitotic division M (from English Mitosis - mitosis), is 5-10% of the cycle
After division, the cell enters a new G1 phase and the cell cycle ends.
Cell cycle regulation
At the molecular level, the transition from one phase of the cycle to another is regulated by two proteins - cyclin And cyclin-dependent kinase(CDK).
To regulate the cell cycle, the process of reversible phosphorylation/dephosphorylation of regulatory proteins is used, i.e. addition of phosphates to them followed by elimination. The key substance regulating the entry of a cell into mitosis (i.e., its transition from the G 2 phase to the M phase) is a specific serine/threonine protein kinase, which is called maturation factor- FS, or MPF, from the English maturation promoting factor. In its active form, this protein enzyme catalyzes the phosphorylation of many proteins involved in mitosis. These are, for example, histone H1, which is part of chromatin, lamin (a cytoskeletal component located in the nuclear membrane), transcription factors, mitotic spindle proteins, as well as a number of enzymes. Phosphorylation of these proteins by the maturation factor MPF activates them and initiates the process of mitosis. After completion of mitosis, the PS regulatory subunit, cyclin, is marked with ubiquitin and undergoes breakdown (proteolysis). Now it's your turn protein phosphatase, which dephosphorylate proteins that took part in mitosis, thereby transferring them to an inactive state. As a result, the cell returns to the interphase state.
PS (MPF) is a heterodimeric enzyme that includes a regulatory subunit, namely cyclin, and a catalytic subunit, namely cyclin-dependent kinase CDK, also known as p34cdc2; 34 kDa. The active form of this enzyme is only the dimer CZK + cyclin. In addition, CZK activity is regulated by reversible phosphorylation of the enzyme itself. Cyclins received this name because their concentration changes cyclically in accordance with periods of the cell cycle, in particular, it decreases before the start of cell division.
A number of different cyclins and cyclin-dependent kinases are present in vertebrate cells. Various combinations of two enzyme subunits regulate the initiation of mitosis, the beginning of the transcription process in the G1 phase, the transition of the critical point after completion of transcription, the beginning of the DNA replication process in the S period of interphase (start transition) and other key transitions of the cell cycle (not shown in the diagram).
In frog oocytes, entry into mitosis (G2/M transition) is regulated by changes in cyclin concentration. Cyclin is continuously synthesized in interphase until the maximum concentration is reached in the M phase, when the entire cascade of protein phosphorylation catalyzed by PS is launched. By the end of mitosis, cyclin is quickly destroyed by proteinases, also activated by PS. In other cellular systems, PS activity is regulated by varying degrees of phosphorylation of the enzyme itself.
