Human Chromosomes

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					Genetics: Cells, Chromosomes,
           and DNA
The Cell
           Cell Anatomy to Know:
           1.   Nucleus
           2.   DNA
           3.   Ribosomes
           4.   Mitochondria

           Cell Types:
           1.   Somatic Cell
           2.   Sex Cell
            DNA Exists in Chromosomes
• Humans typically have 23 pairs of chromosomes for a total of 46
   – 1 pair of sex chromosomes
   – 22 pairs are autosomes           The Chromosomes:
 Each Cell Contains a Full Compliment of
           Genetic Information

The frog on the right was cloned from an
  intestinal cell from the frog on the left
Mitosis: Somatic Cells Replicate to Grow or
            Replace Old Cells
 Meiosis: Sex Cells Replicate to Produce
Gametes (Egg and Sperm) for Reproduction
Meiosis Creates Variation: Independent Assortment
                of Chromosomes
   Meiosis Creates Variation: Crossing Over (a.k.a.

• DNA is exchanged
  between the maternal
  and paternal
  chromosomes during
 DNA Mutations: Another Source of Genetic
• A permanent structural alteration in DNA.
• In most cases, mutations either have no effect or cause
   – Mutations with no effect occur because of the redundancy in
• Occasionally a mutation can improve an organism's
  chance of surviving and reproducing. (Read: natural
• Mutations are essential for the survival of organisms.
        Somatic and Germinal Mutations

• Somatic Mutations:
  • Occur during mitosis
  • Are not inherited
• Germinal mutations:
  • Occur in sex cells
    during meiosis
  • Are inherited
                Somatic Mutations
• Occur during mitosis
• Are only transmitted to
  the cells of the body that
  descend from the mutated
• Have been linked to many
  types of cancer
• On average, mutations
  occur at any given locus
  about once in every
  million cell divisions       Note the small island of albino
                               tissue in this man. This is the
                               result of a somatic mutation in
                               embryonic development.
             Down’s Syndrome: Trisomy 21
• Most individuals (95%) with trisomy 21
  have 3 copies of chromosome 21
• Trisomy 21 (Down's Syndrome) occurs in
  1/1,000 births for mothers below the age of
• increases to a frequency of 1/50 for mothers
  over the age of 50.
    Variation in sex chromosome number
• X0=Turner’s
• Y0= Lethal
• XXY=Kleinfelter
• XXX= Super
• Other variations:
               Turner’s Syndrome
• Caused by the absence
  of an X-
• Frequency: 1 in 2,500
  live-born females
• Causes short stature and
  infertility. Sometimes
  also associated with
  malformations of
  internal organs.
                          Kleinfelter Syndrome
• Males with an extra X
• There are also XXYY; XXXY;
  XXXXY ; and XY/XXY variants.
• About 1 in 500 males are diagnosed
  with this genetic anomaly.
• Males may have the inability to
  produce sufficient amounts of
  testosterone and experience infertility.
  Breast development is sometimes
• Social problems, lower self esteem,
  mood swings, and learning disabilites
  are sometimes present

                                             49, XXXXY
               The XYY Syndrome: Born to kill?
• Men with a chromosome constitution 47,XYY.
• Approximately 1 of 1000 men
    – Increased height (7cms above average)
    – High activity levels and a tendency to a delayed
      mental maturation
• Potentially dangerous? A 1961 study said so…
    – Media hype on “super-males” with “criminality”
    – Several court cases…XYY used as a defense
    – Suggestions to screen babies so that adult anti-
      social behavior could be anticipated.

• NOT TRUE!…many more studies showed NO
  correlation between this syndrome and criminal
                                                         It was falsely rumored that
                                                         Richard Spect, the killer of 8
                                                         Chicago nursing students had
                                                         the XYY syndrome
                         Deoxyribonucleic Acid

• The genetic code—it
  carries the instructions for
  making living organisms.
• Double Helix Structure—
  like an immensely long
  ladder twisted into a
  helix, or coil.
• The two halves of the
  “ladder” are joined by
  weak hydrogen bonds.

                                                 Watson and Crick
    The Genetic Code

• A DNA nucleotide is made of a sugar, a
  phosphate, and a BASE.
• The bases are the INFORMATION in the code.
• In base pairing, adenine always pairs with
  thymine, and guanine always pairs with cytosine.

                             A = adenine
                             T = thymine
                             C = cytosine
                             G = guanine
                 What is a Gene?
• Definition: A sequence of
  DNA that codes for a
  particular protein.
• Human are estimated to have
  40,000 genes.
• Language Analogy
   – Letters = Bases
   – Words = Codons
   – Sentences = Gene
          DNA Functions I: Replication

• The DNA unzips and
  the two halves separate
• The empty sections of
  each half are filled in by
  free-floating bases
• Each half is then
  incorporated into a new
          DNA Functions II: Protein Synthesis

• The DNA base sequence is read by
  messenger RNA in the nucleus of
  the cell (mRNA…uracil)
• mRNA leaves nucleus and transfers
  base sequence information to
• In the ribosome, the base sequence
  is read by transfer RNA (tRNA) and
  the amino acids are placed in order.
• A chain of amino acids is a protein!
• The protein is then either secreted
  from the cell or retained in the
  cytoplasm where it become
                                         Ribosomes producing proteins by reading mRNA
                Genetic Code Redundancy

•   Many mutations have no visible
    effect because of the redundancy
    in the genetic code.
    –   Different amino acids may serve the
        same function (Lysine = Arginine).
    –   Different DNA may code for the
        same amino acid (AGG and CGG
        code for the amino acid Arginine).

                                              Sequences of the same color code for
                                              the same amino acid

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