Science 2

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  The Cell-cycle is composed of the interphase and cell division phases. The bulk of the cell cycle is spent in the "living phase", known as the interphase.
• Interphase is further broken down into 3 distinct phases: G1 (Gap 1), S (Synthesis) and G2 (Gap 2).

  G1 phase - cell increases in size, cellular contents duplicated

  S phase - DNA Replication or DNA Synthesis, each of the 46 chromosomes is replicated by the cell

  G2 phase - cell grows more and organelles develop in preparation for cell division
• G1 -Growth

  S -DNA synthesis

  G2 -Growth and preparation for mitosis

  M - Mitosis (cell division)
• THE CELL DIVISION STAGE

  Alternating with the interphase is the cell division phase. In eukaryotic cells, there are two types of cell division: mitosis and meiosis. Mitosis is a type of cell division that produces two identical daughter cells with the same number of chromosomes. Mitosis is divided into four phases.
• PROPHASE STAGE OF MITOSIS

  →The nuclear membrane and nucleoli are still present.

  →The chromosomes are thicker and shorter because of repeated coiling.

  →At this stage, each chromosome is made up of two identical sister chromatids attached at one point, called the centromere.

  →Spindle fibers are formed
• METAPHASE STAGE OF MITOSIS

  →The nuclear membrane has disappeared while the coiled chromosomes align at the metaphase plate.

  →Spindle fibers are attached to a protein called kinetochore at the centromere of each sister chromatid of the chromosome
• ANAPHASE STAGE OF MITOSIS

  →The paired centromeres of each chromosome separate toward the opposite poles of the cells as they are pulled by both spindle fibers through their kinetochores.

  This liberates the sister chromatids. Each sister chromatid is now a single chromosome
• TELOPHASE STAGE OF MITOSIS

  →The chromosomes are at the opposite poles and start to uncoil.

  →The nucleoli and nuclear membrane reappear while the spindle fibers disappear.

  →There is also cytokinesis (division of the cytoplasm) to form two separate daughter cells immediately after mitosis
• • Meiosis reduces the chromosomes number in half.

  • The Cell undergoes two rounds of cell division (Meiosis I and Meiosis II) to form four daughter cells, each with half the chromosome number as the original parent cell and with a unique set of genetic material as a result of 


  crossing over (a segment of a sister chromatid of one chromosome is exchanged with the same segment of the chromatid of the homologous chromosome
• Prophase 1

  The chromosomes condense, and the nuclear envelope breaks down. Crossing-over occurs
• Metaphase 1

  Pairs of homologous chromosomes move to the equator of the cell.
• Anaphase 1

  Homologous chromosomes move to the opposite poles of the cell.
• Telophase 1

  Chromosomes gather at the poles of the cells. The cytoplasm divides.
• Meiosis II is similar to mitosis. It is consisting of the following stages: prophase II, metaphase II, anaphase II, and telophase II. Each double- stranded chromosome attaches to a spindle fiber then its centromere splits into two and the two strands of the chromosome move to opposite poles. At the end of the cell division, four haploid daughter cells are produced.
• Similarities

  • Occurs in plants and animals

  • Starts with a diploid parent cell

  • Produces new cells

  • Cells undergo DNA replication

  • Same basic steps
• Mitosis

  • Occurs in all organisms (except viruses)

  • Creates all body (somatic) cells like blood cells

  • Involves one cell division

  • Produces two diploid (2n) daughter cells

  • Daughter cells are genetically identical

  • Creates a human cell with 46 chromosomes
• Meiosis

  • Occurs only in plants, animals, and fungi

  • Creates only sex (germ) cells like sperm cells

  • Involves two successive cell divisions

  • Produces four haploid (n) daughter cells

  • Daughter cells are genetically different

  • Creates a human cell with 23 chromosomes