12.2 The Structure of DNA

12.2 The Structure of DNA

The Components of DNA

• DNA (Deoxyribonucleic Acid) is a nucleic acid made up of a nucleotide joined into a long strand or chain by covalent bonds

Nucleic Acids and Nucleotide

• Nucleic acids are made up of smaller subunits, linked together to from long chains
• Nucleotides are the building blocks of nucleic acid.

Nitrogenous Bases and Covalent Bonds

• Nitrogen bases are simply bases that contain nitrogen.
• 4 types of nitrogen -Adenine (A) -Guanine (G) -Cytosine (C) -Thymine (T)
• The nucleotides can be joined together in many order meaning that any sequence of bases is possible

Solving the Structure of DNA

Chargaff's Rule

• Erwin Chargaff, an Austrian American biochemist, discovered that the percentage of adenine (A) and thymine (T) bases are almost equal in any sample of DNA. Same with guanine (G) and cytosine (C).
• (A)=(T) and (G)=(C) "Chargraff's Rule"

Franklin's X-Rays

• British scientist, Rosalind Franklin, used x-ray diffraction to get information about structure of DNA molecules.
• X-shaped pattern shows that the strand in DNA are twisted around each other like coils of a spring, a shape known as helix

X-ray of DNA Diffraction

The Work of Watson and Crick

• James Watson, American biologist, and Francis Crick, a British physicist, were also trying to understand the structure of DNA.
• Using Franklin's x-ray pattern, it allowed Watson and Crick to build a model that explained the specific structure and properties of DNA.
• Their model of DNA was a double-helix in which 2 strands of nucleotide sequences were wound around each other.

The Double-Helix Model

• Double-helix model explains Chargaff's rule of base pairing and how 2 strands of DNA are held together.

Antiparallel Strands

• 2 strands of DNA running in opposite directions.
• Allows the nitrogenous bases on both strands to come in contact at the center of the molecule and allows each strand of the double helix to carry a sequence of nucleotides.

Hydrogen Bonding

• Watson and Crick also discovered that hydrogen bonds could form between certain nitrogenous bases (provided enough force to hold 2 strands together)
• Hydrogen bonds are relatively weak chemical forces

Base Pairing

• Base pairing- nearly perfect fit between (A)-(T) and (G)-(C)
• Watson and Crick's model showed that hydrogen bonds could create a nearly perfect fit between nitrogenous bases along center of molecule.
• Their model explained Chargaff's observations.