Cell Division: Mitosis, Meiosis, and Mutations Explained

Cytokinesis

Cytokinesis begins in anaphase and continues through telophase.

• In animals: The process involves strangling the cytoplasm. Segmentation is a linear division, resulting in the formation of a contractile ring formed by actin filaments.
• In plants: The cell wall prevents segmentation. This is done by forming a new wall inside the cell. This new wall is formed at the equator by the association of Golgi apparatus vesicles and microtubules, and it is called a phragmoplast. It extends laterally to reach the cell wall.

Types of Cell Division

1. Bipartition

   Division of the cell. Two daughter cells originate, equal in size. The process involves karyokinesis and cytokinesis, with its variant being strangulation.

2. Multiple Fission

   Divisions of the cell. The parent cell produces more than two daughter cells. First, the nucleus divides several times, and then the cytoplasm divides, forming as many nuclei as there are.

3. Budding

   A bud is produced in the cytoplasm, into which the nucleus moves, and then it divides. One daughter cell is produced, smaller in size.

4. Sporulation

   The cell surrounds itself with a covering. The nucleus divides several times, each surrounded by a portion of cytoplasm. Several daughter cells are produced and released upon breaking the parent cell's covering.

Meiosis

This is the process by which a diploid cell undergoes two consecutive divisions, producing four haploid daughter cells. If this process did not occur, in the case of sexual reproduction, the number of chromosomes would double indefinitely. Thus, in meiosis, we have a single DNA synthesis during two cell divisions.

5 Stages:

1. Leptotene

   The chromosomes are displayed neatly.

2. Zygotene

   Longitudinal pairing begins between homologous chromosomes through the formation of a structure called the synaptonemal complex, which allows the juxtaposition of each gene with its counterpart.

3. Pachytene

   The two homologous chromosomes form a group called a bivalent. Here, the sister chromatids do not overlap, and exchange of fragments occurs at points called chiasmata. Genetic recombination occurs.

4. Diplotene

   The chromosomes tend to separate, highlighting the chiasmata.

5. Diakinesis

   The chromosomes adopt a more compact configuration. The nuclear membrane and nucleolus disappear, and the spindle forms. Meiosis I and II continue.

Mutations

Mutations are alterations of the genetic material capable of being transmitted to offspring. They can affect somatic or germ cells.

Types:

1. Genetic Mutations

   Alterations in the nucleotide sequence of a gene. They can occur by base substitution, nucleotide insertion, or loss, which produces a shift in the reading frame. Causes include errors in reading transcripts and mutagens (chemical substances, radiation). There is a system of repair enzymes that reviews newly synthesized DNA for lesions and arranges them to reduce error levels. This disadvantage is heritable, so it can be passed down.

2. Chromosomal Mutations

   Changes in the structure of the chromosome: duplication, deletion, inversion, translocation.

3. Genomic Mutations

   Alteration in the number of chromosomes (aneuploidy, euploidy).