Cell cycle and its phases
The cell cycle is the period of a cell's life from one division to the next. The mitotic cycle encompasses the sequence of processes from the beginning of one division to the beginning of the next. Cell division ensures biological immortality through the continuous renewal of the cytoplasm and the precise replication of genetic material. Disruptions in this process lead to mutations.
The frequency of division depends on the tissue type: embryonic cells divide every hour, intestinal epithelium 1–2 times a day, hepatocytes about once a year, and nerve cells practically do not divide.
About 90% of the cycle time is occupied by interphase, which is preparation for division. It includes three periods:
- G1 (presynthetic phase) — cell growth and organelle synthesis;
- S (synthetic phase) — DNA replication, chromosome duplication;
- G2 (postsynthetic phase) — cell growth, synthesis of proteins and ATP for division.
Mitosis: phases and chromosome sets
Mitosis is the indirect division of somatic cells in which daughter cells receive the exact same genetic set as the parent cell. Two diploid daughter cells, genetically identical to the original, are formed from a diploid parent cell.
- Prophase: chromosome spiralization, breakdown of the nuclear envelope, migration of centrioles to the poles, formation of spindle fibers. Chromosome set — 2n4c.
- Metaphase: two-chromatid chromosomes align at the equator, spindle fibers attach to the centromeres. Chromosome set — 2n4c.
- Anaphase: centromeres divide, spindle fibers pull chromatids to the poles — each chromatid becomes an independent chromosome. Chromosome set — 4n4c in the cell, 2n2c at each pole.
- Telophase: chromosomes despiralize, the nuclear envelope and nucleolus reform, and cytokinesis (cytoplasmic division) takes place. Chromosome set of the daughter cells — 2n2c.
Biological significance of mitosis
Mitosis ensures the preservation of a constant chromosome set across generations of cells. Its functions include:
- growth and development of a multicellular organism;
- tissue regeneration and wound healing;
- asexual reproduction: hydra budding, reproduction of protozoa, vegetative propagation of plants;
- formation of gametes in the plant life cycle.
Meiosis I: reductional division
Meiosis is a type of division resulting in the formation of four haploid cells from a single diploid cell. In humans, gametes contain 23 chromosomes, and after fertilization, the zygote restores the diploid set of 46. Meiosis consists of two consecutive divisions: meiosis I (reductional division) and meiosis II (equational division).
Prophase I is divided into five stages:
- Leptotene — chromosomes are thin and consist of sister chromatids;
- Zygotene — conjugation of homologous chromosomes, formation of bivalents;
- Pachytene — chromosome spiralization and crossing over (exchange of homologous segments between non-sister chromatids), ensuring gene recombination;
- Diplotene — homologous chromosomes repel each other while remaining held together at cross-over points called chiasmata;
- Diakinesis — maximum condensation, breakdown of the nuclear envelope, formation of the spindle apparatus.
In metaphase I, bivalents align at the equator. In anaphase I, whole two-chromatid homologous chromosomes (rather than chromatids) migrate to the poles. In telophase I, two cells form with a haploid set of two-chromatid chromosomes — n2c.
Meiosis II and outcomes of division
Meiosis II proceeds without an interphase S-period — DNA replication does not occur, otherwise mutations would arise. By mechanism, it is similar to mitosis:
- Prophase II: breakdown of the nuclear envelope, spindle formation. Chromosome set — n2c;
- Metaphase II: chromosomes align at the equator;
- Anaphase II: centromeres divide, sister chromatids migrate to the poles;
- Telophase II: nuclear formation, cytokinesis. Result — four haploid cells with an nc set.
Comparison of mitosis and meiosis
Key differences between the two types of division:
- Mitosis: one division, two daughter cells, chromosome set is preserved (2n → 2n), genetic identity to the parent cell;
- Meiosis: two consecutive divisions, four daughter cells, chromosome set is halved (2n → n), genetic diversity due to crossing over and independent assortment of chromosomes;
- mitosis ensures growth, regeneration, and asexual reproduction; meiosis forms the basis of sexual reproduction and hereditary variability.
Key points
- Interphase includes G1, S, and G2 periods; DNA replication occurs exclusively during the S phase.
- Mitosis consists of prophase, metaphase, anaphase, and telophase; the result is two identical diploid cells.
- In anaphase of mitosis, centromeres divide, and chromatids become independent chromosomes.
- Meiosis comprises two divisions: reductional (meiosis I) and equational (meiosis II).
- Conjugation and crossing over in prophase I ensure gene recombination and hereditary variability.
- In anaphase I, homologous chromosomes separate, whereas in anaphase II, sister chromatids separate.
- Between meiosis I and meiosis II, there is no S-period, and DNA replication does not take place.
- The outcome of meiosis is four genetically unique haploid cells, which form the basis of sexual reproduction.