Interphase — preparation for division
The period of cellular life activity between divisions is called interphase. This stage is extremely important, as it accounts for 90% of the entire cell cycle time. During interphase, the genetic set undergoes regular changes, passing through three main periods:
- Presynthetic period (G1): the initial state of the cell. It has a diploid set of chromosomes and a normal amount of DNA. The genetic formula is 2n2c. At this stage, each chromosome consists of only one chromatid.
- Synthetic period (S): the time of DNA replication. It is at this moment that the sister chromatid is formed. By the end of the synthetic period, the amount of DNA doubles, and the genetic set becomes 2n4c.
- Postsynthetic period (G2): the final stage of preparation for mitosis. The cell maintains the 2n4c set and prepares for the upcoming distribution of genetic material.
Mitosis and chromatid separation
During mitosis, the cell goes through several phases in which two-chromatid chromosomes are distributed between daughter cells.
- In prophase, homologous chromosomes are separate, each consisting of two chromatids (2n4c set).
- In metaphase, individual two-chromatid chromosomes align at the cell equator in a strict single file, forming the metaphase plate.
- In anaphase, a key event occurs: spindle fibers shorten, and centromeres divide. Sister chromatids detach from each other and migrate to opposite poles. From this moment on, they become independent chromosomes. The total genetic formula in the cell temporarily doubles and becomes 4n4c.
- In telophase, two diploid daughter cells are formed, which are completely identical to the mother cell. There is a transition from the 4n4c state to 2n2c in each new cell.
Comparison with meiosis
The behavior of chromosomes and chromatids in meiosis differs significantly from mitosis. For clarity, let us examine the main differences in a table.
| Phase | Mitosis | Meiosis I | Meiosis II |
|---|---|---|---|
| Metaphase | At the equator — individual chromosomes (in a single row). Set: 2n4c | At the equator — bivalents (pairs of homologous chromosomes). Set: 2n4c | Chromosomes align along the equator in a single row. Set: n2c |
| Anaphase | Centromeres divide. Sister chromatids migrate. Set: 4n4c | Centromeres do not divide. Entire homologous chromosomes migrate. Set: 2n4c | Centromeres divide. Sister chromatids migrate. Set: 2n2c |
The second meiotic division (equational division) proceeds similarly to mitosis and includes the same stages: prophase II, metaphase II, anaphase II, and telophase II. However, preparation for it has distinctive features. Interkinesis (the gap between divisions) is very short or absent. During this time, DNA replication is absent because the S phase is not needed — chromosomes already consist of two chromatids. The synthesis of chromosomal proteins is also not required.
The result of the second meiotic division is cytokinesis and the formation of four haploid cells with single-chromatid chromosomes. The number of cells doubles, leading to the formation of early spermatids with a ploidy of 1n1c (haploid amount of DNA and chromosomes).
To avoid confusing anaphases: in mitosis and meiosis II, centromeres divide (sister chromatids move to the poles), whereas in meiosis I, centromeres remain intact (entire homologous chromosomes separate).
Class and exam questions
At what point in mitosis do chromatids become independent chromosomes?
This occurs in anaphase. Spindle fibers shorten, centromeres divide, and sister chromatids migrate to the poles, acquiring the status of independent chromosomes.
Why doesn't DNA duplicate before the second meiotic division?
Interkinesis before meiosis II is very short, and the synthetic period is omitted. DNA replication is unnecessary because after the first division, chromosomes already consist of two chromatids.
How does mitotic anaphase differ from anaphase I of meiosis?
In mitosis, centromeres divide, and sister chromatids migrate to the poles. In anaphase I of meiosis, centromeres remain intact, and entire homologous chromosomes move to the poles.
What else to study on this topic
- Conjugation and crossing over in prophase of meiosis I
- Bivalent formation and the metaphase plate
- Reductional division and the formation of haploid cells
- Features of interkinesis between meiotic divisions
- Formation of early spermatids
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