Mitosis, Meiosis & Gametogenesis

• Mitosis, Meiosis & Gametogenesis are processes to study.

Cell Cycle

• Includes a set of activities regarding cellular growth and cell division.
• Consists of two main phases: interphase and mitosis.
• During the G2 phase, the cell performs "double checks" of the duplicated chromosomes for errors and makes needed repairs.
• During the S phase, each of the 46 chromosomes is duplicated by the cell.
• During interphase Cellular contents -excluding the chromosomes - are duplicated.

Chromosomes

• Each chromosome consists of two chromatids.
• The two chromatids are maintained held together by a centromere.

Mitosis

• Mitosis involves the division of the nucleus into two daughter nuclei.
• It also includes the division of the cytoplasm.

Phases of Mitosis

• Prophase involves the condensation of chromatin filaments to form chromosomes.
• During prophase, the nucleolus and nuclear membrane disappear; synthesis of the mitotic spindle also occurs.
• Metaphase involves chromosomes attached to the mitotic spindle by their centromeres.
• The Metaphase chromosomes align on the equatorial plane of the cell.
• During anaphase, chromatids separate and slowly move toward opposite poles.
• By the end of anaphase, the chromosomes form a group at each cellular pole.
• During Telophase, in animal cells, constriction begins to appear along the equatorial plane and is called cytokinesis.
• Post-telophase events are contrary to those of prophase.

Meiosis

• Meiosis is a sequence of two nuclear divisions: the first division (I) is reductional, and the second division (II) is equational.
• The reason the second division is equational is because the haploid chromosomal number which originated in the previous stage (first meiotic division) remains the

Phases of Prophase I in Meiosis

• During leptotene, the chromatin is visible and consists of two chromatids united by a centromere.
• During zygotene, homologous chromosomes become visible
• Synapsis occurs at the telomeres and centromeres.
• Pairs formed as a result of synapsis are known as bivalents.
• During pachytene, genetic material exchange occurs between chromosomes during crossing over.
• Quiasmas are also formed in pachytene.
• A quiasma is the physical location where exchange of genetic material occurs, also known as "crossing over."
• During diplotene, homologous chromosomes repel each other and begin to separate.
• Though they start repelling, chromosomes remain attached to the quiasmas.
• During diakinesis, the chromosomes are in their most condensed state.
• Terminalization of the chiasmata occurs; they move towards the distal part of the chromosomes, moving away from the centromeres.
• Centromeres attach to the spindle fibers during diakinesis.
• Bivalents start migrating toward the equator due to the spindle fibers' action, and the nuclear membrane breaks, and the nucleolus disappears.
• During metaphase I, homologous chromosomes align on the equatorial plane.
• Reduction of genetic material is undergone during anaphase I.
• At anaphase I, chromosomes are known as dyads or univalents; double-stranded chromosomes are no longer paired.
• During telophase I, chromosomes uncoil.
• The nucleolus and the nuclear membrane reappear.
• Interkinesis can be short or absent. DNA synthesis does not occur, and is considered similar to interphase, but it is not the same.

Meiosis II

• During prophase II, the chromosomes begin to curl and shorten.
• The nuclear membrane breaks, and the dyads attach to the fibres of the mitotic spindle and begin to migrate towards the equatorial plane of the cell
• During metaphase II, the chromosomes (univalents) align at the equator.
• Anaphase II commences when the centromeres have divided, and ends when the chromosomes reach the poles.
• During Telophase II, the chromosomes are at the poles.
• Chromosomes uncoil, the nuclear membrane and nucleolus form, and cell division (cytokinesis) then occurs.

Gametogenesis

• Process of meiotic division producing haploid cells.
• There is subsequent maturation of these cells or functional gametes.
• Gametogenesis encompasses oogenesis and spermatogenesis.

Oogenesis

• Includes multiplication (where germ cells in the ovary divide by mitosis, creating oogonia)
• Takes place before birth.
• Includes growth of the oogonia due to accumulation of reserve material
• The set of egg cell and it's outer covering is called Graafian follicle.
• Oocytes have begun Meiosis I, but are retained in prophase.
• Meiosis is restored during maturation after a female reaches puberty
• Several primary oocytes begin to grow in size and end Meiosis I.
• An oocyte divides into one secondary oocyte (23 chromosomes, 2 chromatids) and 1 polar corpuscle that degenerates.
• For the process to continue, fertilization must occur..

Spermatogenesis

• In humans, spermatogenesis begins at puberty and occurs in the testicles.
• Spermatogonia (immature premeiotic germ cells) proliferate through mitosis
• They differentiate and form the primary spermatocytes.