Cell Division: Mitosis, Meiosis and Interphase

• The cell cycle is a series of events within a cell, preparing it to divide into two daughter cells.
• Multiplication of cells occurs through the division of pre-existing cells.
• Multiplication is essential for embryonic development.
• The process is necessary after birth for growth and replacement of dead cells.
• There are two major events in the cell cycles: interphase and mitosis

Mitosis

• A type of division where the daughter cells must have chromosomes identical in number and genetic content to those of the mother.

Meiosis

• Consists of two successive divisions.
• Reduces the number of chromosomes to half the normal number.
• Genetic information in various gametes produced is not identical.

Interphase

• Long period of time in the cell cycle.
• During interphase, the cell increases in size.
• Content and replicate its genetics material during interphase.

Mitosis

• A shorter period of time compared to interphase.
• The cell divides its nucleus first, then its cytoplasm, resulting in two daughter cells.
• Cells may cease mitosis permanently, such as muscle and neuron cells.
• Cells that leave the cell cycle are considered to be in a stable phase, referred to as the G0 (outside) phase or a resting stage.

Interphase Subdivisions

• Interphase is further subdivided into three phases.

The G1 (Gap) Phase:

• Involves the synthesis of macromolecules essential for DNA duplication.
• Cells synthesize RNA, regulatory proteins, and enzymes essential to DNA replication.
• The cell volume is restored to normal.
• Nucleoli are reestablished.
• Centrioles begin to duplicate themselves and complete in the G2 phase.

The S (Synthetic) Phase:

• DNA is duplicated.
• Autosomal cells contain the diploid (2n) amount of DNA before the S phase, which doubles to 4n in preparation for cell division.
• All necessary nucleoproteins (histones) are manufactured and incorporated into DNA, forming chromatin.
• Cell contains twice the normal amount of DNA.

The G2 (Gap) Phase:

• Cells undergo preparation for mitosis
• The period between the end of DNA synthesis and the beginning of mitosis.
• RNA and proteins are synthesized, essential for cell division.
• Energy is stored for mitosis.
• Tubulin is synthesized for assembly into microtubules required for mitosis.

Mitosis (M)

• Occurs at the conclusion of the G2 phase and completes the cell cycle.
• It is the division of the nucleus called karyokinesis, followed by the division of the cytoplasm called cytokinesis.
• Mitosis is subdivided into four stages: prophase, metaphase, anaphase, and telophase.

Prophase

• The centrosome divides into two regions.
• Each half contains a pair of centrioles, which migrate away from each other to opposing poles of the cell.
• Chromosomes condense and become visible microscopically.
• Chromosomes are scattered randomly throughout the cytoplasm.
• Each chromosome consists of two parallel sister chromatids joined together at one point (the centromere).
• The nucleolus and nuclear envelope disappear.

Metaphase

• Chromosomes become maximally condensed
• Lined up at the equator of the mitotic spindle (metaphase plate configuration).

Anaphase

• Begins when sister chromatids pull apart and migrate toward opposite poles.
• In the late stages, a cleavage furrow begins to form at the plasma lemma during cytokinesis.

Telophase

• The terminal phase of mitosis.
• Complete cytokinesis (division of cytoplasm).
• Disappearance of spindle fibers
• Reconstitution of nucleus and nuclear envelope.
• Unwinding of the chromosomes into chromatin.
• Each daughter cell possesses a diploid (2n) number of chromosomes.
• Each daughter cell resulting from mitosis is identical in its genome.

Meiosis

• Is a special type of cell division resulting in the formation of gametes (spermatozoa or ova).
• The chromosome number has been reduced from diploid (2n) to haploid (1n) number
• Begins at the conclusion of interphase and produces germ cells (ova and sperm).
• This process has two crucial results:
  • Reduction in the number of chromosomes from diploid (2n) to haploid (1n).
  • Recombination of genes, ensuring genetic variability and diversity.
• It is divided into two separate events

Meiosis I (Reductional division)

• Separates the homologous pairs of chromosomes
• Reduces the chromosome number from diploid to haploid.
• In gametogenesis, where germ cells are in the S phase of the cell cycle preceding meiosis, the amount of DNA is doubled to 4n, but the chromosome number remains at 2n (46 chromosomes).

Prophase I:

• Lasts a long time
• Homologous pairs of chromosomes approximate each other, lining up and forming a synapse (tetrad).
• Chiasmata (crossing over sites) are formed, and a random exchange of genetic material occurs between homologous chromosomes.

Metaphase I:

• Characterized by homologous pairs of chromosomes lining up at the equatorial plate.
• Each chromosome is composed of two chromatids.

Anaphase I:

• Homologous pairs of chromosomes migrate away from each other to opposing poles.

Telophase I:

• Similar to telophase of mitosis.
• Nuclei are reformed, and cytokinesis occurs.
• Gives rise to two daughter cells.
• Each cell possesses 23 chromosomes (haploid number).
• Each chromosome is composed of two chromatids.

MEIOSIS II:

• Not preceded by an S phase (without DNA synthesis).
• Similar to mitosis, subdivided into prophase II, metaphase II, anaphase II, and telophase II.
• Chromosomes line up at the equator, the kinetochores attach to spindle fibers, chromatids migrate to opposing poles, and cytokinesis divides each cell into two cells.
• Each cell contains a haploid amount of DNA and a haploid chromosome number.
• Results in total of 4 cells from an original diploid germ cell.
• Each gamete contains its own unique genetic content.

Cell division is essential for life, facilitating development, growth, and repair. The cell cycle consists of interphase and mitosis. Mitosis produces identical daughter cells, while meiosis halves the chromosome number. Interphase involves cell growth and DNA replication.