Cell Cycle Phases: Interphase, Mitosis

Cell Cycle Overview

• The cell cycle is a regulated sequence of events controlling cell growth and division.
• The cell cycle produces two identical daughter cells from one parent cell.
• The two main phases of the cell cycle are interphase and M phase (mitosis).

Interphase

• Interphase involves continuous cell growth and is split into three phases: G1, S, and G2.
• The G1 phase is the longest and most variable phase, starting at the end of M phase.
• During G1, the cell gathers nutrients and synthesizes RNA and proteins for DNA synthesis and chromosome replication.
• The S phase (DNA replication) begins with the initiation of DNA synthesis, lasting about 7.5 to 10 hours.
• During the S phase, the cell's DNA doubles, and new chromatids are formed.
• The G2 phase involves cell preparation for cell division, examining replicated DNA.
• During G2, the cell grows and reorganizes cytoplasmic organelles before mitosis.
• The G2 phase can last from 1 hour in rapidly dividing cells to an indefinite duration in some polyploid cells.

Mitosis (M Phase)

• Mitosis includes karyokinesis (nuclear division) and cytokinesis (cell division), lasting about 1 hour.

Cell Cycle Checkpoints

• Internal quality control mechanisms or checkpoints, represented by biochemical pathways, control the transition between cell-cycle stages.

Phases of Mitosis

• Prophase: Replicated chromatin condenses into visible chromosomes, each with two chromatids held together.
• Sister chromatids are held together by proteins at the centromere.
• In late prophase, the nuclear envelope disintegrates, and the nucleolus disappears.
• A kinetochore protein complex appears on each chromatid, opposite the centromere.
• Metaphase: The mitotic spindle forms from three types of microtubules organized around centrosomes.
• The three types of microtubules in metaphase are astral microtubules, polar microtubules, and kinetochore microtubules.
• Anaphase: Sister chromatids separate due to the breakdown of holding proteins.
• Separated chromatids move to opposite poles via kinetochore microtubules towards the centrosomes.
• Telophase: A nuclear envelope reforms around chromosomes at each pole.
• Chromosomes uncoil and become indistinct, nucleoli reappear, and the cytoplasm divides (cytokinesis) to form two daughter cells.

Meiosis

• Meiosis is two sequential nuclear divisions that produce gametes (sex cells) with half the chromosome number and DNA of somatic cells.
• A zygote (fusion of ovum and sperm) and somatic cells are diploid (2n) with 46 chromosomes in humans, having two copies of each chromosome.
• Homologous chromosomes are similar but not identical, with one set of maternal and one of paternal origin.
• Gametes are haploid (1n), with one member of each chromosome pair.
• During gametogenesis, the chromosome number reduces to the haploid state (23 in humans) through meiosis.

Phases of Meiosis I

• Prophase I: An extended phase subdivided into five stages.
  • Leptotene: Chromosomes start to condense.
  • Zygotene: Homologous chromosomes closely associate (synapsis), forming bivalents (pairs of chromosomes) consisting of four chromatids (tetrads).
  • Pachytene: Crossing over occurs between pairs of homologous chromosomes, forming chiasmata.
  • Diplotene: Homologous chromosomes start to separate but remain attached by chiasmata.
  • Diakinesis: Homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.
• Metaphase I: Similar to mitosis metaphase, but paired chromosomes align at the equatorial plate with one member on each side.
  • Chiasmata are cut, and homologous chromosomes separate completely.
  • Spindle microtubules interact with chromosomes through the kinetochore at the centromere.
• Anaphase I: Sister chromatids remain together by protein complexes and the centromere.
  • A maternal or paternal member of each homologous pair moves to each pole.

Phases of Meiosis II

• After meiosis I, cells quickly enter meiosis II without an S phase.
• Meiosis II is an equatorial division resembling mitosis.
• Sister chromatids separate at anaphase II and move to opposite poles of the cell.
• During meiosis II, the cells progress through prophase II, metaphase II, anaphase II, and telophase II.