DNA

Nucleic Acids

• Review– Nucleic acids store and transmit genetic

information– Genetic information = instructions for making

proteins– Monomers = nucleotides• Phosphate• Nitrogen base• 5 carbon sugar

Eukaryotes vs Prokaryotes

• ALL living things have DNA• In prokaryotes (bacteria), DNA is found in the

cytoplasm• In eukaryotes (anything not bacteria), DNA is

found in the nucleus– Usually found as chromatin (DNA wrapped around

histones)– During cell division, DNA is found in chromosomes

• There are four nitrogen bases of DNA– Guanine– Adenine– Cytosine– Thymine

• There is a fifth nitrogen base called uracil; however, it’s only found in RNA

• Thymine is NOT found in RNA

Nitrogen Bases

DNA Structure

• The 5-carbon sugar (pentose) is bound to a phosphate molecule

• Each monomer (nucleotide) is bound to another by alternating sugar and phosphate molecules by a covalent bond

• This forms the backbone of a nucleic acid

Sugar-Phosphate Backbone

Erwin Chargaff

• Erwin Chargaff observed that the percentage of adenine and thymine are almost equal in any sample of DNA.

• He also noticed that the percentage of guanine matched cytosine.

Rosalind Franklin

• Rosalind Franklin used a technique called X-ray diffraction to get information about the structure of the DNA molecule.

• X-ray pictures showed that the strands are twisted around each other.

• The twisted shape is known as a helix.• She showed that DNA is made of two strands.• She also showed that the nitrogen bases are

near the center.

Rosalind Franklin

James Watson and Francis Crick

• James Watson and Francis Crick were also trying to understand the structure of DNA.

• Watson saw a copy of Rosalind Franklin’s X-ray picture and realized the missing piece to solving the mystery of the structure of DNA.

The Double Helix

• DNA is a double helix• A double helix looks like a twisted ladder

Double Helix of DNA

Anti-parallel

• the two strands of DNA run in opposite directions.

• This is described by biochemists as “antiparallel”

• The nitrogen bases on both strands meet at the center of the molecule.

• Each strand can carry a sequence of nucleotides.

• These bases are arranged like letters in an alphabet system with only 4 letters.

DNA Structure

Hydrogen Bonds

• Hydrogen bonds can form between certain nitrogen bases.

• Hydrogen bonds are weak forces which is very important.

• If the nitrogen bases were held by a strong bond, it might not be possible to pull them apart.

• DNA’s strands have to be able to separate easily to function.

Hydrogen Bonds Between Nitrogen Bases

Nitrogen Base Pairing

• Watson and Crick’s DNA model showed hydrogen bonds could hold the nitrogen bases together.

• Bonds only formed between certain bases.• According to base pairing, adenine paired with

thymine, and guanine paired with cytosine.• Base pairing explains Chargaff’s rule.• For every thymine, there has to be an adenine.• For every cytosine, there has to be a guanine.