DNA: The Genetic Material

1
DNA The Genetic Material

• Chapter 9

2
The Structure of DNA

• Watson and Crick determined that DNA is a
  molecule that is a double helix.
• DNA is made up of subunits called nucleotides.
• Nucleotides are made up of a phosphate group, a
  five-carbon sugar, and a nitrogen base.

3
The Structure of DNA

• The sugar molecule and the phosphate group are
  the same for each nucleotide in a molecule of
  DNA, but the nitrogen base has four different
  kinds.
• Adenine
• Guanine
• Thymine
• Cytosine

Purines
Pyrimidines
4
The Structure of DNA

• Purines are nitrogen bases made of two rings of
  carbon and nitrogen atoms.
• Pyrimidines are nitrogen bases made of a single
  ring of carbon and nitrogen atoms.
• The double helix is held together by weak
  hydrogen bonds between the pairs of bases.

5
Scientists Contributions

• Chargaff in 1949 observed that for each organism
  he studied, the amount of adenine always equaled
  the amount of thymine and guanine equaled
  cytosine.
• Maruice Wilkins and Rosalind Franklin in 1952
  used X-ray diffraction photography to photograph
  fibers of DNA molecules. These photographs
  suggested that the DNA molecule resembled a
  tightly coiled helix and was composed of two or
  three chains of nucleotides.

6
Scientists Contributions

• In 1953, Watson and Crick used this information,
  along with their knowledge of chemical bonding,
  to come up with a DNA model. This model was a
  double helix.

7
Pairing Between Bases

• Base-pairing rules
• Thymine on one strand always pairs with Adenine
  on the other strand.
• Guanine on one strand always pairs with Cytosine
  ont eh opposite strand.
• Complementary Strands
• The sequence of bases on one strand determines
  the sequence of bases on the other strand.