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Posted by Jim True on March 18, 2004 6:04 AM. Last Updated October 22, 2006 9:23 PM

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CH 12: The Cell Cycle

Cell Division

• In a unicellular organism, the division of one cell into two produces new individuals.
• In multicellular organisms, cell division generates growth and allows developoment. It also allows replacement of cells that die from "old age" or from damage.
• Cellular reproduction is complex and involves the EXACT duplication of the DNA "blueprints" so that each new cell gets an exact copy.

Cell Terms

• Genome -- The sum of all genetic material in one cell incorporated in its DNA.
• Chromosome -- ("chromo" -- color + "soma" -- body) -- The organelle that "holds" the DNA. Formed by highly complex arrangement of proteins (60% of content), DNA (35%) and RNA (5%) called chromatin. Prokaryotes are different and do not store this information in chromosomes. The chromosome contains other substances known as chromatin.
• When the cell is not actively dividing chromatin forms very long, thin strands.
• Somatic cells -- all body cells EXCEPT reproductive cells.
• Every eukaryotic organism has a characteristic number of chromosomes in the nucleus of each somatic cell. eg., humans have 46 chromosomes. (Sometimes known as the 'species number').
• Gametes -- specialized cells (reproductive or sex cells) that each contain 1/2 the chromosome content of the somatic cell, e.g. human gametes contain 23 chromosomes.

Chromosome

• Chromosomes consist of several regions:
  • Arms -- most of the chromosomal material is located here. Variable in length.
  • Centromere -- within the arms, a central region of special proteins. (Mere means part).
  • Kinetochore -- a disk of protein on each centromere.
  • Telomere -- a loop-shaped cap of material at each end of of chromosome. Telo means end-part.
  • Sister chromatid -- When exact copies of DNA are made prior to cell division, each original chromosome is bound to an identical duplicate at the centromere, e.g. human has 46 sets of sister chromatids (92 chromosomes bound at 46 centromeres).

Mitosis/Cytokinesis

• Mitosis ("mitos" -- thread, "sis" -- process) Separation of sister chromatids into two sets of chromosomes, each enclosed in its own nucleus, ie. NUCLEAR DIVISION.
• Cytokinesis -- Division of the cytoplasm and organelles other than the nucleus, so that one cell becomes two.
• Neither mitosis nor cytokinesis is found in eukaryotic cells.
• Daughter cell -- the newly produced identical cells from mitosis/cytokinesis. Mitosi is duplication of the nucleus; cytokinesis is the division of the cell.
• Meiosis -- Production of gametes by a double series of cell divisions.
• Mitosis and cytokinesis yields TWO daughter cells, each with the SAME NUMBER of chromosomes as the original cell.
• Meiosis and cytokinesis yields FOUR gametes, each with ONE-HALF the number of chromosomes as the original cell. Will cover in ch 13.

Eukaryotic Cell Cycle

• It is important to note several things about the cell cycle:
  • While it has been divided into distinct phases, each of these phases merges continuously with the next.
  • Some characteristics of each phase may overlap from one phase to the next.
  • Not all cells in a multicellular organism are in the same phase at the same time.
• The cell cycle consists of three main phases:
  • Interphase ("inter" -- between) -- The longest overall phase of the cell cycle, occupying about 90% of the time of one cell cycle. It is the period BETWEEN division activities. Interphase is divided into 3 subphases:
    • Gap Phase 1 (G1) -- Main period of cell growth and development. (the "growth" phase.
    • Synthesis Phase (S) -- All chromosomes (and thus, DNA) are replicated, producing sister chromatids.
    • Gap Phase 2 (G2) -- Some additional cell growth occurs here, but this period also involves preparation for nuclear and cellular division.
      • Mitochondria (all eukaryotes, chloroplasts (plants) and centrioles (many protistans and all animals) all contain short fragments of DNA, which allows them to self-replicate independently of the cell.
      • The sister chromatids are also coiling up, becoming shorter in length (condensing). At this point Interphase is said to be complete.
      • In animals, centrioles duplicate in a region called the centrosome, which also duplicates.
      • Special microtubules extend outward in a radial pattern around the centrosomes known as asters ("aster" -- star)
  • Mitotic ('M') phase -- this period consists of 4-5 relatively short subphases as well:
    • Prophase ("pro" -- before) -- the FIRST phase of mitosis. Most of the main preparations for actual division of the nucleus occur at this time. Considered to start when the chromatids become visible on microscope.
      • Formation of special microtubules called spindle fibers occurs. In animal cells, these fibers are formed at, and extend from, the centrioles. The centrioles begin to move to opposite ends of the cell.
      • The ends of the spindle fibers (and centrioles in animals) are considered to lie at the poles of the cell.
      • The center of the cell at right angles to the spindles is called the equatorial plane, the line along which the cell will actually divide. (or metaphase plate)
    • In animals cells, rays from asters form from centrioles to membrane (absent in plants and fungi).
  • Prometaphase -- At the same time all this is occurring, the nuclear membrane breaks down and disappears. (Most biologists do not consider this a separate step; this step is included as part of prophase).
    • Most spindle fibers run pole to pole, but one fiber attaches to each kinetochore.
    • Attached fibers push and pull the sister chromatids by flexing and contracting, and they begin to move the sister chromatids towards the cell equator.
  • Metaphase ("meta" -- change) -- all sister chromatids are lined up on the cell equatorial plane (also referred to as the metaphase plate).
    • At the end of metaphase, by some signal not yet understood, the spindle fibers shorten. Those attached to the sister chromatids pull all of them apart simultaneously.
    • The shortening of the spindle fibers occurs at kinetochore, where special motor proteins pull the chromosomes along the spindle fiber.
  • Anaphase ("ana" -- up) -- The non-kinetochore spindle fibers shorten, pulling chromosomes (since each now has its own centromere) towards the opposite poles.
    • Under normal conditions, each polar region now has same number of chromosomes as original cell.
    • shape of cell becomes elongated.
  • Telophase ("telo" -- an end, complete) -- the end of the mitotic phase.
    • The spindle fibers break down and disappear.
    • The chromosomes begin to uncoil and disappear from view under the microscope.
    • The nuclear membrane reforms around chromosomes.
• Cytokinesis -- The division of the cytoplasm, producing two new cells.
  • In animal cells, this takes place from the OUTSIDE IN via production of a cleavage furrow
  • In plants and fungi, cell divides from INSIDE OUT by forming a cell plate which becomes the new dividing cell wall.
  • As a result of cytokinesis (when present), two new genetically identical daughter cells are formed.
  • Each is identical to the original cell, and is approximately 1/2 the size of the original cell. These cells will grow to the size of the original during G1 and G2 periods of interphase.

Prokaryotic Cell Division

• Prokaryotic cells divide by binary fission ("bi" -- two; "fission" -- to divide).
• Prokaryotic cells contain a single continuous, extremely looped chromosome.
• This chromosome will begin to duplicate at a site called the origin of replication. As the "new" origin forms it moves to the opposite side of the cell from the original origin by an unknown mechanism.
• As it does so, the cell itself elongates to about twice its original size.
• When the chromosomes has completed duplication, the original and the duplicate lie in opposite halves of the cell.
• At this point, the cell membrane grows inward and pinches the cell into two new cells, each about the same size as the original cell.
• Whereas, the eukaryotic cell cycle ranges from 8-20 hours, growth and binary fission in prokaryotes can occur in as little as 15 minutes!

Regulation of Cell Cycle

• Like most cellular processes, regulation of the events of the cell cycle are controlled by chemical cues.
• These provide specific signals to start and stop specific processes.
• In this control system are specific checkpoint, key control events.
• The most important is the G1 checkpoint, which regulates whether the cell will go ahead and divide.
• If the signal is received to continue at this checkpoint, the cell grows and divides.
• If it does not, the cell remains in a non-dividing state of interphase known as the G0 phase (e.g. muscle and many nerve cells). Some cells may remain in G0 for their entire life, others may be "switched on" and start dividing again (e.g., human liver cells).
• Two types of proteins regulate cell cycle activities.
  • Kinase -- group of enzymes that operate on proteins to activate or inactive them (change shape to make them functional or non-functional).
  • Cyclin -- proteins whose concentrations fluctuate cyclically in the cell.
• Kinases drive the cell cycle, but to do so, they must be attached to cyclins to be activated, thus, they are called cyclin-dependent kinases (CDK's).
• Actions of specific CDK's rise and fall with the rise and fall in cyclin concentrations, thus different activities of the cell cycle are regulated. MPF - Maturation Promoting Factor's.
• A specific CDK is the maturation (or M phase) promoting factor (MPF).
• This regulates the turning on and off of mitosis processes.
• "Signaling" of the events of the cell cycle will rely on both internal and external factors.
• Internal factors include such things as CDK's while external growth factors include proteins released by some cells that cause others to divide.
• The availability of growth factors is what is thought to regulate one important aspect of the division of normal cells.
• Density-dependent inhibition -- When cells become crowded, they cease to divide. This allows normal cells in a multicellular organism to "fill" the region they are supposed to and then stop dividing, except when needed to replace missing or dead cells, e.g. repair of a wound.
  • It is thought that the reduction of available growth factors in the "crowd" inhibits division.
• Anchorage dependence -- observed in animal cells, in order for cells to be able to divide, they must be attached to a substrate, which can be a hard surface such as a petri dish, or other cells in a tissue.