Outline and Chapter notes to accompany chapter 2

                     GENES, CHROMOSOMES, AND DNA
                                                               Jan., 2001

A.  MENDEL OBSERVED PHENOTYPES AND FORMED HYPOTHESES

  INHERITANCE FOLLOWS THE SAME LAWS IN MOST ORGANISMS.

  THE BASIC LAWS OF GENETICS were first established by Gregor Mendel
      in 1865.
    Mendel used peas which could either be crossed or self-fertilized
    Earlier observers looked at many traits at once--
        Mendel focused on one at a time.
    Earlier observers used parents of unknown hereditary background.
        Mendel bred PURE LINES first until they bred true.
    Earlier observers only observed a single generation at a time--
        Mendel extended his observations over several generations.
    Earlier observers failed to quantify their results--
        Mendel counted offspring and established ratios.

  TERMINOLOGY:
    PHENOTYPE = appearance ("pheno-"=visible, as in "phenomenon");
      GENOTYPE = genetic make-up, not always visible, but detectable by
        performing crosses
    ALLELES = variants of a gene.
    HOMOZYGOUS = having two alleles that are alike;
      HETEROZYGOUS = having two unlike alleles
    DOMINANT = showing a phenotypic effect in heterozygous form
      RECESSIVE = showing a phenotypic effect only when homozygous

 MENDEL'S FINDINGS:
    Crosses between PURE LINES produce offspring of one (dominant)
        phenotype only
    Crossing of first generation plants produces 3:1 ratio of dominants
        to recessives in the second generation (F2).
    Explanation of 3:1 ratio in terms of PARTICULATE INHERITANCE.
        "LAW OF SEGREGATION" = dominant and recessive alleles of
            heterozygote separate from one another during meiosis
    "LAW OF INDEPENDENT ASSORTMENT" for 2 genes at a time:  genes at
        different locations are chosen (sampled) independently of one
        another during gamete formation.


B.  THE CHROMOSOMAL BASIS OF INHERITANCE EXPLAINS MENDEL'S HYPOTHESES:

  Genes are located on chromosomes within the nucleus of each cell. 
        Behavior of genes follows behavior of chromosomes.  (This
        includes an exception to independent assortment in the case of
        "linked" genes on the same chromosome.)

  MITOSIS:
        Normal cell division is called MITOSIS;  chromosome
        number doubles and is then halved, so the resulting chromosome
        number remains unchanged in the two daughter cells.
  MEIOSIS:
     Most animal and plant cells are DIPLOID (their chromosomes occur in
        pairs);  the major exceptions are the egg and sperm cells,
        called GAMETES;  gametes are always HAPLOID (their chromosomes
        occur as singletons).
     The cell division that produces haploid cells (such as gametes) is
        called MEIOSIS.  During meiosis, the chromosomes
        double once and divide twice, resulting in four haploid cells
        that each have half of the original chromosome number,
        including one chromosome from each pair.
     The separation of chromosome pairs during meiosis is responsible
        for segregation.
  GENE LINKAGE:
     The independent separation of different pairs of chromosomes is
        responsible for independent assortment.
     Genes on the same chromosome segregate together (LINKAGE), unless a
        chromosomal cross-over brings about their recombination.
  CONFIRMATION OF THE CHROMOSOMAL THEORY:
     Experiments have shown that the inheritance of genes parallels the
        inheritance of visible chromosomes.
     When chromosomes have visible markers at opposite ends, recombination
        of genes (as observed in crosses) is always accompanied by the
        rearrangement of the visible chromosome markers.
        recombination of


C.  THE MOLECULAR BASIS OF INHERITANCE FURTHER EXPLAINS MENDEL'S HYPOTHESES.

  DNA AND GENETIC TRANSFORMATION:
    Griffith's experiment established the phenomenon of BACTERIAL
        TRANSFORMATION:  dead bacteria of a virulent strain called IIIS
        were able to transform the nonvirulent strain IIR into IIIS.
    Avery, MacLeod & McCarty established that bacterial transformation
        required DNA.
    Hershey & Chase demonstrated that BACTERIOPHAGE viruses
        reproduced by using genetic material made of DNA.  The viruses
        injected this DNA, but not their protein, into host bacteria
        during viral reproduction.
  THE CHEMICAL COMPOSITION OF DNA:
    DNA is composed of phosphate groups, deoxyribose sugar, and nitrogenous
        bases of four types (abbreviated A, G, C, and T).
    Chargaff discovered that the amount of adenine and thymine were
        equal (A=T) in DNA from a given species, as were the amounts of
        guanine and cytosine (G=C).
  THE THREE-DIMENSIONAL STRUCTURE OF DNA:
    Rosalind Franklin used X-RAY DIFFRACTION to study the 3-D structure
        of DNA 
    Watson & Crick described double-helix model of DNA structure:
        Building blocks:  phosphate groups, deoxyribose sugar, and
            nitrogen-containing bases (A = adenine, G = guanine, C =
            cytosine, and T = thymine)
        One phosphate + one deoxyribose sugar + one base = a
            NUCLEOTIDE.
        Nucleotides are connected by alternating chain of phosphates &
            sugars.
        There are two strands of nucleotides, arranged in opposite
            directions.
        Base-pairing of (A with T) and (C with G) holds the two strands
            together.
        The two strands are twisted to form a DOUBLE HELIX.
    A gene is a sequence of bases in DNA.  The location of the gene on
        the DNA is called its LOCUS.

  DNA REPLICATION:  DNA is made from DNA, using one strand as a TEMPLATE
    (pattern) to synthesize the missing strand one base at a time.

  TRANSCRIPTION:  DNA unwinds in one area, then part of one
    DNA strand can be used as a template to synthesize a complementary
    strand of RNA, still keeping the message in the language
    of a series of nucleotides.

  TRANSLATION (changing nucleotide language into protein language):
    Within a ribosome, MESSENGER RNA comes together with a
    TRANSFER RNA molecule linked to an amino acid.  A three-base
    sequence (CODON) on the messenger RNA matches a complementary
    sequence (ANTICODON) on the transfer RNA.  The codon thus determines
    which transfer RNA molecule is used, and thus which amino acid is
    the next one to be inserted in a growing polypeptide strand.

  MUTATIONS:
    Mutations are heritable changes in genes or chromosomes.
    Most mutations are SINGLE-GENE MUTATIONS that arise from errors in
        replication or from unrepaired damage to DNA molecules.   
        NOTE:  Some geneticists restrict the term "mutation" to single-
        gene mutations only.
    Single-gene mutations include:
        base-pair substitutions
        frame-shift mutations (additions & deletions)
    CHROMOSOMAL ABERRATIONS, which arise from errors in meiosis, include:
        changes in chromosome number (including polyploidy in plants)
        duplications:  a piece of a chromosome appears twice
        inversions:  a piece of a chromosome turns 180 degrees
        deletions:  a piece of a chromosome is missing
        translocations:  a piece of a chromosome attaches to another
            chromosome

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