The Cell Cycle: an ordered set of events, culminating in cell growth and division into two daughter cells.

Parts of the cell cycle:

G1 phase:

• Sometimes called Interphase.
  • The formation of two daughter cells from a parent cell in cytokinesis is an energy-consuming process. The time needed by the new cells to replenish their energy causes this "gap".
• After cell energy reserves are restored, the daughter cells begin to grow.
• This is the longest phase of the cell cycle.

• Synthesis means to make.
• After a period of growth, each cell's original DNA is duplicated in the process of replication.

• The replication of DNA is another energy-consuming process for the cell. The time taken by a cell to build up its energy reserves produces this second "gap".

The process by which the DNA is divided into two identical sets before the cell divides.

1. Prophase
   • Early prophase: chromatin coils to form chromosomes, nuclear membrane disappears, and centrioles migrate to opposite poles of the cell.
   • Middle prophase: spindle fibers, microtubules of protein, begin developing.
     • Polar fibers extend across the cell from centriole to centriole.
     • Kinetochore fibers extend from the centromeres of a chromosome to the centrioles.
   • Late prophase: protein fibers called asters radiate from each centriole in animal cells.
2. Metaphase:
   • Kinetochore fibers move the chromosomes to the equator of the cell.
3. Anaphase:
   • The centromere of each pair of chromatids divides, the chromatids separate and are pushed to opposite poles of the cell by the combined efforts of the spindle fibers. This movement is done in one quick motion.
4. Telophase:
   • All spindle fibers disappear, chromatids unwind and stretch to form chromatin, and a nuclear membrane forms around each mass of chromatin.

• Cytokinesis is the division of the cytoplasm of a parent cell. The cell pinches together roughly in the center.
• You may either consider it the end of Mitosis or the beginning of Interphase.
• While each daughter cells gets identical sets of DNA, the cytoplasm and organelles are only roughly divided equally between the two cells.

In any growing tissue, cells will be found in all phases of the cell cycle.

 

Meiosis occurs only in gametes:

It is the process of nuclear division that reduces the number of chromosomes by half.

Two nuclear divisions: These are similar to, but different from the single division of mitosis. The process is complicated, providing more opportunity for genetic variation like crossing-over to occur. For our purposes, this summary of events is sufficient.

Meiosis I DNA replication takes place. A parent cell produces two daughter cells with one member of each original pair of homologous chromosomes. Meiosis II There is no DNA replication. The chromatids of each chromosome separate and each daughter cell divides. At the end of Meiosis, there are four daughter cells from each parent cell. Each daughter cell has half the number of chromosomes of the parent.

Test your knowledgeof cell reproduction.

Concept Understanding: Why are the cells of almost all living things essentially the same size? In what year did Watson and Crick propose the double helix model for the DNA molecule? The cell cycle has four parts. Describe what happens in the cell cycle using one sentence for each of the four parts. What are the two main differences between mitosis and meiosis?

Research Links:

• Biological Macromolecules - IMB Jena WWW Service
• Human Genome Sequencing - National Center for Biotechnology Information
• Cells and Chromosomes - Access Exellence, Genentech
• Human Genes, By Chromosome - Johns Hopkins University School of Medicine

Gametes are sex cells, sperm and egg.