The cycle is divided into four distinct phases, G1, S, G2, and M (mitosis).

Cell cycle depicted in circular diagram starting with mitosis and cytokinesis which compose the mitotic phase (formation of 2 daughter cells). Next is G1 or cell growth, then S or DNA synthesis, then G2 or cell growth. G1, S, and G2 compose interphase.

 

Interphase

  • During interphase, the cell undergoes normal processes while also preparing for cell division.
  • For a cell to move from interphase to the mitotic phase, many internal and external conditions must be met.
  • The three stages of interphase are called G1, S, and G2.

 

 

G1 phase

  • The cell is active and accumulating the building blocks of chromosomal DNA and the associated proteins.

 

 

S phase

  • In the S phase (synthesis phase), DNA replication results in the formation of two identical copies of each chromosome (sister chromatids) that are joined by centromere.
  • each chromosome is made of two sister chromatids and is a duplicated chromosome. The centrosome is duplicated during the S phase.
  • The two centrosomes will give rise to the mitotic spindle.
  • Centrioles help organize cell division.

 

G2 phase

  • the cell replenishes its energy stores and synthesizes the proteins necessary for chromosome manipulation. Some cell organelles are duplicated, and formation of spindle fibres.
  • The final preparations for the mitotic phase must be completed before the cell is able to enter the first stage of mitosis.

 

 

The mitotic phase

 

Mitosis

Phases

  • prophase
  • prometaphase
  • metaphase
  • anaphase
  • telophase

 

Prophase: Chromosomes condense and become visible. Spindle fibers emerge from the centrosomes. Nuclear envelope breaks down. Centrosomes move toward opposite poles. Prometaphase: Chromosomes continue to condense. Kinetochores appear at the centromeres. Mitotic spindle microtubules attach to kinetochores. Metaphase: Chromosomes are lined up at the metaphase plate. Each sister chromatid is attached to a spindle fiber opposite poles. Anaphase: Centromeres split in two. Sister chromatids (now called chromosomes) are pulled toward opposite poles. Certain fibers begin to elongate the cell. Telophase: Chromosomes arrive at opposite poles and begin to decondense. Nuclear envelope material surrounds each set of chromosomes. The mitotic spindle breaks down. Spindle fibers continue to push poles apart. Cytokinesis: Animal cells - a cleavage furrow separates the daughter cells. Plant cells - a cell plate, the precursor to a new cell wall, separates the daughter cells.

 

 

Reductional division

Meiosis is composed of two distinctive cell divisions, meiosis I and meiosis II.

 

The first division

  • there is pairing and recombination between homologous chromosomes resulting in variation in the genetic makeup of the gametes.
  • Segregation of the homologues occurs during the first meiotic (reductional) division, reducing the forty-six chromosomes to twenty-three, one from each homologous pair.

 

The second division

  • similar to mitosis with segregation of sister chromatids into daughter cells.

 

Overview of meiosis.

Prophase I: The chromosomes condense, and the nuclear envelope breaks down. Crossing over occurs. Metaphase I: Pairs of homologous chromosomes move to the equator of the cell. Anaphase I: Homologous chromosomes move to the opposite poles of the cell. Telophase I & Cytokinesis: Chromosomes gather at the poles of the cells. The cytoplasm divides. Prophase II: A new spindle forms around the chromosomes. Metaphase II: Chromosomes line up at the equator. Anaphase II: Centromeres divide. Chromatids move to the opposite poles of the cells. Telophase II & Cytokinesis: A nuclear envelope forms around each set of chromosomes. The cytoplasm divides.Overview of meiosis.

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