Deoxyribonucleic Acid

DNA Structure & Replication

What is DNA?

• Long thin double strands composed of subunits called nucleotides

• The nucleotides are linked in a chain

Nucleotides Have 3 Parts

• Phosphate group

• 5 carbon sugar (deoxyribose)– Phosphates and sugar form the backbone of

DNA

• Nitrogen base – 4 types– Adenine– Guanine– Thymine– Cytosine

Nitrogen Bases continued…

• Adenine and guanine are purines – double rings of carbon and nitrogen

• Thymine and Cytosine are pyrimidines – single rings of carbon and nitrogen

Chargaff’s Rules• Base pairing rules show the arrangement of

nucleotides within a DNA molecule

• Adenine-thymine (A-T) and the reverse

• Cytosine-guanine (C-G) and the reverse

• Base pairs of DNA form a double helix and are complementary to each other

• Chargaff was the scientist who discovered that A and T along with C and G always occurred in the same proportions within DNA

Example

ACTGCCGAAT

TGACGGCTTA

The Double Helix

• Watson and Crick (1953) discovered the structure of DNA to be a double helix or “spiral staircase” of two strands of nucleotides twisting around a central axis.

• Names for DNA…double helix, twisted staircase

How DNA is copied…DNA Replication

• Replication is the process of synthesizing a new strand of DNA

• When does this occur? …before cell division

• Prior to replication, double helix must be unwound– This is done by an enzyme called helicase– Helicase breaks up the hydrogen bonds that

link the bases

How DNA is copied…DNA Replication

• The point at which the DNA separates is called the replication fork (y shape)

• At the replication fork, new enzymes known as DNA polymerases move along each new strand adding nucleotides to exposed bases- forming two new double helices

• DNA polymerases “proofread” to prevent errors

• Typically, there are many replication forks along each linear DNA molecule

Semi-conservative Replication

• Half of original DNA strand is “conserved” or used in the new strand.

Two enzymes to know:

• Helicase – breaks the hydrogen bonds between bases to unwind the DNA

• DNA polymerases – bring new nucleotides into place, proofreads new strands as they form