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THERE ARE TWO TYPES OF CELL DIVISION: MITOSIS AND MEIOSIS

MITOSIS

MEIOSIS


If you don't remember the various parts of the cell, please refer to the notes and diagrams in Lecture Three.

Of great importance to understanding cell division is to note what happens to the CHROMOSOMES. Recall that in eukaryotic cells, there are multiple, linear chromosomes, each containing a single strand of double-helix DNA. This is organized around a complex structural matrix of protein and RNA, which we'll see later.

When you've seen diagrams of chromosomes in the past, you've probably seen them represented as an "X". This is not how chromosomes exist in the normal cell cycle. Ordinarily, they exist as diffuse CHROMATIN.

When the cell is preparing to divide, everything inside it doubles, including the chromosomes. But the duplicated chromosomes do not immediately separate from one another. They are still joined at the region known as the CENTROMERE. The two identical copies of the formerly single chromosome, still joined at the centromere, are known as SISTER CHROMATIDS:

These, in turn, are classified by the location of the centromere:

In non-metacentric chromosomes:

  • Note: the centromere is the location of the kinetochore, the physical structure to which special protein fibers called SPINDLE FIBERS will attach during cell division, and eventually pull the duplicated chromosomes apart.

    In humans and many other species, chromosomes can be classified as either SEX CHROMOSOMES or AUTOSOMES (the "regular" chromosomes). In mammals, the sex chromosomes are named "X" and "Y".

    Humans have a total of 46 chromosomes (2 sets of 23, one set from each parent). Here's what they look like, specially photographed and stained.

    If you're observant, you've noticed that there are two chromosomes of each "color" and shape, and they can be arranged to reveal the HOMOLOGOUS PAIRS. One homolog of each pair was "donated" to this human by each of his parents.

    In that photograph, we see what is meant by the term DIPLOID. There are TWO members of every homologous pair, meaning that there are two complete sets of chromosomes in the nucleus of every diploid cell in this diploid human.

    If we were to photograph the chromosomes of the GAMETES of this human, the picture might look something like THIS. The gamete is HAPLOID, and has only ONE set of chromosomes; one member of each homologous pair.


    MITOSIS - A closer look at asexual cell division

    This process has (somewhat arbitrarily) been divided into phases that are easily recognizable under the microscope, if the cell is caught and stained at those particular moments:

    A few important notes on the relevance of mitosis to CANCER

  • G1 Phase is important in cancer studies. During late G1, cells pass through a "G1 checkpoint". The specific "messages" the cell receives from its internal and external environment will cause a healthy, normal cell to either enter a state of suspended animation ("G0" Phase) or, more commonly, proceed into normal mitosis.

  • Cancer cells often have a defect in the GI checkpoint, and rather than going into either G0 or mitosis, they enter a state of wild, unequal division, leading to cancer.

  • Initiation of cell division is controlled by the relative concentrations of two proteins in the cell

  • When cyclin concentration is high,it combines with cdc2 to form a complex with the handy name "CdkG1-cyclin complex".

  • A critical concentration of this complex will initiate MITOSIS.
  • In mammals, a gene known as p53 appears to be a "tumor suppressor gene."

  • The product encoded by the p53 gene is a protein involved in the control of transcription of other proteins--including cyclin and cdc #2!
  • Mutant versions of the p53 are often associated with loss of control over mitosis. A cell with a mutant p53 often becomes immortal and highly proliferative--which means CANCEROUS.


    MEIOSIS - A closer look at sexual cell division

    Meiosis I - Reduction Division


    Meiosis II - Equational (mitotic) Division

    Meiosis I:

  • Prophase I - the physical and chemical events are very similar to those of mitosis, with one *very* important difference: CROSSING OVER.

  • Metaphase I - spindle fibers arrange homologs along the metaphase plate at the cell's equator, this time with homologs lined up TOGETHER.

  • Anaphase I - spindle fibers separate homologs, carrying them to opposite poles, but sister chromatids are still connected at the centromere.


    After meiosis, gametogenesis occurs to make the new haploid cells into gametes.

    The generalized animal scenario:

    MALE:

    FEMALE:



    A few notes of interest. What can happen during sexual reproduction?

  • "POLYSPERMY" is the (abnormal) fusion of more than one sperm with a single egg.

    What is the normal number of chromosomes?

  • EUPLOIDY - is the term used to describe the normal number of chromosomes expected in a given species. (from the Greek eu, meaning "true")

  • ANEUPLOIDY - more or fewer than the normal number of chromosomes per homologous pair (from the Greek an - "away" )

    Two of several ways these can happen:

    Note that AUTOSOMAL ANEUPLOIDIES are far more devastating than sex c'some abnormalities; the latter are often survivable, and some persons with sex c'some abnormalities can lead totally normal lives and produce normal offspring. (We'll return to this later, when we talk about mutations.)