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Information is Stored in the Code Letters of DNA

  • All hereditary information is stored in genes, which are parts of giant DNA molecules
  • Genes code for the amino acids of proteins
  • DNA is the archival copy of the code- kept in nucleus where it is protected & repaired
  • DNA is organized with special proteins into chromosomes
  • For protein synthesis a working copy of the code is made from RNA
  • Overall scheme: DNA -> RNA -> protein
  • Another version: “One gene, one enzyme”

The Code is Based Upon the Structure of DNA

  • DNA has a sugar-phosphate backbone- sugar is deoxyribose
  • DNA also has 4 types of nucleotide base : A, C, G, T
  • A = adenine; C = cytosine; G = guanine; T = thymine
  • Molecule is a double helix: 2 complementary strands where A = T, C = G
    • The term”complementary” refers to the fitting together of 2 molecules like hand and glove
    • In DNA complementary bases make good hydrogen bonds with one another
  • Strands of helix are held together by hydrogen bonds between the bases
    • This allows DNA to unwind for duplication and transcription
    • (S = sugar; P = phosphate; B = base):

A Group of 3 Nucleotide Bases (Triplet) Forms a Code Letter (Codon)

  • Groups of 3 nucleotide bases form code letters (codons)
  • For a 3 letter code made from the 4 nucleotide bases there are 64 different possible arrangements or code letters (codons)
  • Codons tell the cell how to make proteins
  • 1 code letter used for “Start”, 3 used for “Stop”, 61 used to code for the 20 different amino acids (most amino acids have more than 1 code letter)
  • Examples of DNA Codons:
    • Start is coded for by TAC (this codon is also used for methionine)
    • Stop is coded for by ATT, ATC and ACT
    • Valine is coded for by CAA, CAC, CAG and CAT
    • Glutamic acid is coded for by CTT and CTC

For Protein Synthesis a Working Copy of the Code is Made From RNA (Transcription)

  • The RNA copy of the code is complementary:
    DNA Base RNA Base
    A U
    C G
    G C
    T A
  • Note that U replaces T in RNA (U = uracil)
  • RNA leaves the nucleus and goes into the cytoplasm, attaches to a ribosome to make protein (translation)
  • Examples of RNA Codons
    • Start is coded for by AUG
    • Stop is coded for by UAA, UAG and UGA
    • Valine is coded for by GUU, GUG, GUC and GUA
    • Glutamic acid is coded for by GAA and GAG

A Change in a Single DNA Base Can Cause a Mutation

  • Changing a single base will change the codon, usually into one for another amino acid
  • Example: sickle cell anemia
    • A mutation caused a GAG codon to change into a GUG codon in the gene for one of the protein chains of hemoglobin
    • The mutation replaced the glutamic acid amino acid with valine in one position of the protein
    • Valine causes the hemoglobin to stick together so that it precipitates out of solution
    • The precipitated hemoglobin causes damage to the red blood cells and this leads to anemia
    • The mutation also gives some resistance to malaria in individuals with one sickle gene and one normal gene

Mutations Cause “Inborn Errors of Metabolism”

  • Most mutations are harmful
  • Caused by chemical and physical agents which damage DNA (UV light, x-rays, many carcinogenic & mutagenic chemicals)
  • Approx. 600 genetic diseases known
  • Some may be treatable someday by gene therapy (correcting defective gene)

The Genetic Code is Universal

  • All creatures on earth have the same genetic code (a few minor codon exceptions)
  • Evidence that life has arisen only once and that we are all related

Telomerase, Aging and Cancer

  • The ends of chromosomes are called telomeres
  • When DNA is replicated the telomeres are often not duplicated properly and the chromosome becomes a little shorter after each replication
  • Some scientists believe that the gradual shortening of the chromosomes causes cell aging and eventual death (most cells in the body can duplicate only 50-100 times before they die)
  • Cells which divide often (germ cells, stem cells and cancer cells) have high levels of an enzyme called telomerase which allows the telomeres to be duplicated properly
  • Telomerase may make the cells potentially immortal
  • Inhibitors of telomerase may someday be useful in cancer therapy

More Information:

Do you want to know how the structure of DNA was discovered? Just click.

James Bindon of the University of Alabama gives detailed information on sickle cell anemia.

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