Cell Division: Mitosis and Meiosis
Cell Cycle Phases
G1 phase: A period of intense metabolic activity. The cell size increases, as do its cytoplasmic structures.
S or synthesis phase: The time when DNA replication occurs.
G2 phase: The period in which the cell prepares to enter division; e.g., chromatin begins to condense.
After phase two, cell division begins.
Example: Escherichia coli can divide roughly every 20 minutes, while human skin cells take about 15 days to divide.
Somatic vs. Sex Cells
Somatic cells: These cells have 46 chromosomes in humans. Examples include epithelial and bone cells. They divide by the process of mitosis.
Sex cells (Gametes): These cells are produced through the process of meiosis.
Sex cells have 23 chromosomes.
Somatic cells have a pair of chromosomes of each class; they are called diploid cells.
Sex cells, which carry a single chromosome of each pair, are called haploid cells.
Meiosis Process
This type of cell division is performed to produce gametes or sex cells, which have half the chromosomes of somatic cells.
Homologous chromosomes, consisting of two chromatids each, come together and pair up. Following this pairing, the four chromatids remain together. Then, the chromatids belonging to each of the homologous chromosomes exchange genetic material (crossing over).
In anaphase, homologous chromosomes migrate toward the poles of the cell. This separation reduces the number of chromosomes in each daughter cell.
Telophase is similar to that of mitosis. The result of meiosis I is the formation of two daughter cells with half the number of chromosomes, each with two chromatids.
The second meiotic division is similar to a normal mitosis, but without DNA duplication. In this division, the sister chromatids of homologous chromosomes separate and migrate towards the poles. All processes are completed to give rise to four daughter cells.
The final result of meiosis is the formation of four daughter cells, with half the chromosomes of the mother cell. These chromosomes are different from those of the parent cell due to the crossover of genetic material.
Gamete Production
Sperm production takes place in the testes, and eggs are produced in the ovaries.
After puberty, and virtually throughout their lives, men produce billions of sperm (a normal adult male ejaculation can contain 300 to 400 million sperm).
Of the two million oocytes with which women are born, only 300 to 400 reach maturity in about 40 years of reproductive life, extending from puberty (12 to 14 years) to menopause (which occurs approximately at 50 years).
Mutations
A mutation is a change in the nucleotide sequence of a cell’s DNA. Mutations occurring in gametes, or cells that give rise to them, are transmitted to future generations.
Cancer
Cancer is a disease in which cells proliferate excessively because of repeated, but anomalous, mitosis. This leads to the formation of an amorphous mass, the tumor, which generates new cells that multiply without relation to the normal tissue surrounding them.
A tumor is called malignant when it grows rapidly, invades nearby tissue, and, through the bloodstream or lymph, can reach other organs, where they reproduce and form new tumors called “metastases” or “secondary tumors.”
Preventive Actions
Preventive action is one of the most useful weapons to combat the disease. Regular health checks are crucial to detect any functional or structural alteration in the body early. If the disease develops, follow the treatment prescribed by your doctor.