Cell preparation: Interphase
Before entering active division, the cell goes through a preparatory stage — interphase (interphase). A key event occurs during this period: replication, i.e., the duplication of genetic material.
Initially, the mother cell has a diploid set, where chromosomes consist of a single chromatid. Its formula is written as 2n2c, where n denotes the chromosome set and c denotes the amount of DNA molecules. After replication is complete, each chromosome becomes two-chromatid. The chromosome set remains the same (diploid), but the amount of DNA doubles, so the formula changes to 2n4c. In this form, the cell is ready to begin mitosis. It is worth noting that a similar interphase with replication does not occur before the second meiotic division.
Start of division: Prophase and Metaphase
Actual division begins with prophase (prophase). In this phase, DNA strands tightly coil up—chromosome spiralization occurs. Concurrently, the nuclear envelope breaks down, and the spindle apparatus begins to form. In mitosis, homologous chromosomes remain separate from each other (unlike meiosis, where they pair up and form bivalents). The genetic formula at this stage remains at 2n4c.
The next stage is metaphase (metaphase). Chromosomes consisting of two chromatids move toward the center of the cell and align strictly along its equator, forming the so-called metaphase plate. Spindle fibers attach to the centromeres of each chromosome. In mitosis, chromosomes are arranged in a single row, whereas in the first meiotic division, pairs of homologous chromosomes (bivalents) line up along the equator. The cell formula in mitotic metaphase remains 2n4c.
Segregation and completion: Anaphase and Telophase
The most dynamic stage is anaphase (anaphase). Spindle fibers begin to shorten. The centromeres holding the chromatids together divide. Sister chromatids detach from each other and move toward opposite poles of the cell. From this moment on, each former chromatid is considered an independent chromosome. Consequently, the number of chromosomes inside the cell temporarily doubles, and the total genetic formula reaches 4n4c.
The final stage is called telophase (telophase). The chromosomes reach the poles and begin to despiralize (uncoil). Nuclear envelopes re-form around them. The process ends with cytokinesis, the division of the cytoplasm of the mother cell itself. As a result of mitosis, two full-fledged daughter cells are formed, genetically identical to the original one. There is a transition from the general formula 4n4c to the formula 2n2c in each of the new diploid cells.
Briefly: answers to topical questions
During which period of interphase does DNA replication occur?
DNA replication takes place during the synthesis (S) phase of the cell cycle interphase. In this phase, the replication of nuclear DNA molecules occurs, and sister chromatids are formed. As a result, the genetic set of the diploid cell by the end of this period changes from the initial state of 2n2c to 2n4c (diploid set, two-chromatid chromosomes).
What proteins make up the spindle fibers?
Spindle fibers are formed by microtubules, the walls of which consist of the protein tubulin.
- Tubulin is a protein whose globular subunits are arranged in a spiral, forming microtubules 25 nm in diameter.
Microtubules participate in the formation of spindle fibers and ensure the segregation of chromosomes to the cell poles during nuclear division.
To which chromosome structures do the spindle fibers attach?
Spindle fibers attach to the centromeres of chromosomes, specifically to the kinetochores embedded within them.
- Centromere is a specific DNA region in the central part of a chromosome, forming the primary constriction.
- Kinetochores are specialized protein structures in the centromere region that bind sister chromatids and serve as the direct attachment site for mitotic spindle microtubules.
Which cellular structures disappear in the prophase of mitosis?
During the prophase of mitosis, the nuclear envelope and nucleoli disappear. They undergo fragmentation (breakdown), after which the mitotic apparatus begins to form.
The disappearance of nucleoli is a direct consequence of DNA compaction and the inactivation of ribosomal genes encoding rRNA. This is accompanied by a complete cessation of the cell nucleus's genetic activity and the termination of transcription due to DNA being inaccessible to enzymes.
In which phase of mitosis do the centrioles of the cell center double?
Centrioles of the cell center double not during the phases of mitosis, but during interphase, in the synthesis (S) phase of the cell cycle.
DNA replication of the cell nucleus also occurs in the S phase. During mitotic prophase, centrioles move to opposite poles of the cell, and a microtubule spindle forms between them.
To quickly memorize the order of the phases (Prophase, Metaphase, Anaphase, Telophase), use the acronym «PMAT» (Pro-Met-Ana-Telo).
Class and exam questions
What do the symbols n and c mean in biological formulas?
The symbol n denotes ploidy, i.e., the chromosome set in the cell. The symbol c indicates the number of chromatids or DNA molecules.
In which phase of mitosis does the chromosome number temporarily double?
This happens in anaphase. Centromeres divide, sister chromatids become independent chromosomes, and the cell formula becomes 4n4c.
How does mitotic metaphase differ from metaphase I of meiosis?
In mitosis, individual two-chromatid chromosomes line up in a single row at the equator. In meiosis I, bivalents—pairs of homologous chromosomes facing each other—are located at the equator.
What else to study on this topic
- Mechanisms of DNA replication during interphase
- Structure of centromeres and attachment of spindle fibers
- Conjugation and crossing over processes in prophase I of meiosis
- Features of cytokinesis in plant and animal cells
- Reductional and equational division: a detailed comparison of mitosis and meiosis
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