Cell Division: Mitosis, Meiosis, and Genetic Inheritance

Posted on Jan 7, 2025 in Biology

Cell Cycle

Cycle CC: 2 Stage: 1. Interphase: Slow phase between 2 mitosis, has 3 phases:

  • G1 Phase (cells synthesize proteins and grow, it may become a quiescent phase, G0, for years or even die).
  • S Phase Following this phase, DNA doubles.
  • G2 Phase Before mitosis, centrioles duplicate.

2nd phase Mitosis and Cytokinesis. 2 phases:

  • Karyokinesis
  • Cytokinesis

In karyokinesis, the following are distinguished:

  1. Prophase: Centrioles are placed on opposite sides, microtubules form two types of fibers: kinetochore fibers, where chromosomes are anchored, and polar fibers that go from pole to pole. The nucleolus disappears, and the nuclear envelope begins to disappear.
  2. Metaphase: The nuclear envelope disappears, and the chromosomes engage with kinetochore fibers. Chromosomes are placed on the midplane. Chromosomes are formed by two chromatids (sister chromatids).
  3. Anaphase: Chromatid pairs separate and target the poles. Each chromatid is called a chromatid or homologous anaphase. Polar fibers lengthen, and kinetochore fibers shorten.
  4. Telophase: The spindle disappears, chromosomes become invisible, the nucleolus appears, and the nuclear envelope reforms.

Cytokinesis: Cytoplasmic material is shared. The action of actin and myosin fibers constricts the cell at the equatorial plate, giving rise to two autonomous daughter cells. This occurs in late telophase. In plant cells, which have a cell wall, a phragmoplast (vesicles and microtubules of the Golgi apparatus) forms at the midplane. The vesicles fuse and give rise to a plate separating the two daughter cells. The middle lamella forms from the vesicles, and fusion occurs between the plasma membrane and the cell wall. In mitosis, one cell gives rise to two identical cells with the same number of chromosomes. It occurs in somatic cells.

Meiosis

Meiosis is cell division in breeding cells and spores. One cell gives rise to four haploid cells (n). In the first stage of meiosis, genetic material is exchanged, and each pair of homologous chromosomes is formed. Half of the chromosomes are obtained, resulting in haploid cells (half the number of chromosomes). These cells are different, providing genetic variability and allowing the perpetuation of species. Meiosis has two stages:

A) Meiosis I

Meiosis I has the following phases:

  1. Prophase I
    • Leptotene: Chromosomes are formed by two chromatids (DNA doubled in the S phase). Strands appear with thickenings called chromomeres.
    • Zygotene: Homologous chromosomes pair up in a process called synapsis. The set of paired homologous chromosomes forms the synaptonemal complex.
    • Pachytene: Homologous chromosomes form tetrads (two chromatids per chromosome). Crossing over occurs (homologous chromosomes exchange genetic material).
    • Diplotene: Bivalents or tetrads separate but remain attached by chiasmata (where genetic material exchange occurred).
    • Diakinesis: Chiasmata remain, and the nuclear envelope disappears.
  2. Metaphase I: Bivalent chromosomes are arranged on the equatorial plate.
  3. Anaphase I: Homologous chromosomes separate, constituting dyads (with two chromatids).
  4. Telophase I: A normal telophase occurs, giving rise to two daughter cells, each with a haploid number of chromosomes.

B) Meiosis II

In meiosis II, there is no DNA duplication. It has the same phases as normal mitosis. In this phase, chromatids and homologous chromosomes separate, resulting in two more cells.

Definitions

  • Locus: The place that a gene occupies on a chromosome.
  • Gene: A DNA fragment.
  • Characteristics: Each of the characteristics of a living being.
  • Haploid: A being possessing a single gene for a characteristic.
  • Diploid: A being that has two genes for the same characteristic. Genes may be the same or different.
  • Alleles: Different varieties of a gene for a character.
  • Homologous chromosomes: Those that have the same loci.
  • Genotype: The set of genes of a being.
  • Phenotype: Observable genes of a being.
  • Homozygote: An individual with identical alleles.
  • Heterozygote: An individual with different alleles.
  • Dominant Inheritance: Occurs when the dominant allele manifests, not allowing the other, called recessive, to manifest.
  • Intermediate Inheritance: Hybrids are intermediate, for example, in *Mirabilis jalapa* (Don Diego) flowers.
  • Codominant Inheritance: When alleles are equipotent (e.g., blood groups).