General Characteristics and Significance
Mitosis is a process in which chromosomes coil tightly, the mitotic spindle forms, and genetic material is duplicated and evenly distributed along with the cytoplasm.
Biological Role of Mitosis:
- Maintaining a constant set of chromosomes across cell generations.
- Guaranteeing the accurate transfer of hereditary information.
- Increasing cell count, which forms the basis for organism growth and development.
- Ensuring tissue regeneration processes.
- Providing the basis for asexual reproduction in organisms.
Phases of Mitosis
Mitosis is preceded by interphase—a preparatory period during which the formed nucleus is clearly visible. Mitosis itself is divided into four consecutive stages:
- Prophase ($2n4C$)
This is the longest phase. Chromatin condensation (coiling) occurs, and chromosomes become visible. The nuclear envelope and nucleoli break down. The mitotic apparatus begins to form: centrioles migrate to opposite poles of the cell, and spindle microtubules stretch between them.
- Metaphase ($2n4C$)
Attachment of chromosomes to the spindle fibers via kinetochores (centromeres) is completed. The two-chromatid chromosomes align strictly along the cell equator, forming the metaphase plate.
- Anaphase ($4n4C$)
Spindle fibers shorten. Centromeres divide, and sister chromatids separate from each other, becoming independent single-chromatid chromosomes. They migrate to opposite poles. At this point, the cell temporarily contains two diploid sets.
- Telophase ($4n4C \rightarrow 2n2C$ in each)
Chromosomes reach the poles. The mitotic apparatus disintegrates, and chromosomes uncoil. New nuclear membranes form around them, and nucleoli are restored.
Cytokinesis
Cytokinesis is the process of cytoplasmic division that begins in telophase after the disintegration of the mitotic apparatus. As a result, fully functional daughter cells are formed.
The mechanism of cytokinesis depends on the cell type:
- In animals: division proceeds from the periphery to the center. The plasma membrane invaginates, forming a cleavage furrow or constriction.
- In plants: division proceeds from the center to the periphery. A cell plate forms in the equatorial plane.
Briefly: answers to topical questions
What periods is the cell cycle interphase divided into?
The interphase of the cell cycle is divided into three consecutive periods.
- $G_1$ phase (presynthetic) — the cell contains a normal amount of DNA, and the chromosome consists of a single chromatid ($2n2C$ set).
- $S$-phase (synthetic) — DNA replication and sister chromatid formation take place (genetic material set by the end of the period is $2n4C$).
- $G_2$-phase (postsynthetic or premitotic) — active preparation of the cell for division takes place (genetic material set is $2n4C$).
What proteins make up the microtubules of the mitotic spindle?
The microtubules of the mitotic spindle are made of the protein tubulin. The walls of the microtubules are formed by globular subunits of this protein (heterodimers) arranged in a spiral. During the $G_2$-phase, active synthesis of tubulin proteins occurs for the subsequent formation of the mitotic apparatus.
What is a kinetochore and what are its functions?
A kinetochore is a specialized protein complex located at the centromere region of a chromosome. Its main functions are:
- Connects sister chromatids together.
- Serves as the attachment site for mitotic spindle microtubules.
In metaphase, the centromere kinetochores attach to the fibers of the mitotic apparatus, after which the chromosomes align along the equator of the cell.
Through what mechanisms (motor proteins) do chromosomes separate during anaphase?
The separation of chromosomes in anaphase is driven by changes in microtubule length and the action of motor proteins (translocator proteins). Motor proteins include dyneins and kinesins, which consume ATP energy. Mechanisms of chromosome movement:
- Motor proteins transport the chromosome along the microtubules.
- Kinetochore microtubules shorten, pulling chromosomes toward the poles.
- Polar microtubules elongate.
- Motor proteins slide microtubules of opposite polarity past one another, reducing the overlap zone.
During which period of the cell cycle does DNA replication occur?
DNA replication occurs during the synthetic ($S$) phase of the cell cycle interphase. DNA replication is semiconservative. At the beginning of the S-phase, chromosomes are single-chromatid, and the genetic material set is $2n2c$; after replication, chromosomes become double-chromatid, and the set is $2n4c$.
To memorize the sequence of phases (Prophase, Metaphase, Anaphase, Telophase), the mnemonic phrase "PriMat" or "PRAvilno MAma Anyu Tashchila" is often used.
Class and exam questions
In which phase of mitosis do chromosomes align along the equator?
In metaphase. The so-called metaphase plate is formed.
What chromosome set is characteristic of anaphase?
Anaphase is characterized by a $4n4C$ set, because chromatids have separated to the poles and are considered individual chromosomes, though they are still within a single cell.
How does cytokinesis differ between animal and plant cells?
In animal cells, membrane invagination occurs from the periphery to the center, whereas in plant cells, a cell plate forms from the center to the periphery.
What else to study on this topic
- Dynamics of changes in the chromosome set across phases ($2n4C \rightarrow 4n4C \rightarrow 2n2C$)
- Structure and functions of kinetochores
- Mechanism of formation and contraction of spindle fibers
- Molecular mechanisms of membrane invagination during cytokinesis
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