DNA Structure and Replication

By the end of this lesson, you will be able to describe the double helix structure of DNA, explain the role of base pairing, describe the steps of DNA replication, and explain why replication is semi-conservative.

DNA Structure

DNA (deoxyribonucleic acid) is the molecule that carries genetic information.

Nucleotide components:

1. Deoxyribose sugar (5-carbon)
2. Phosphate group
3. Nitrogenous base: A (adenine), T (thymine), G (guanine), C (cytosine)

The double helix: Watson and Crick (1953), with data from Franklin's X-ray crystallography:

• Two antiparallel strands running $5' \to 3'$ and $3' \to 5'$
• Sugar-phosphate backbone on the outside
• Base pairs on the inside, held by hydrogen bonds: A pairs with T (2 H-bonds), G pairs with C (3 H-bonds)
• The sequence of bases along one strand encodes genetic information

Chargaff's rules: in any double-stranded DNA, %A = %T and %G = %C.

DNA Replication

Replication ensures each daughter cell receives a complete copy of the genome.

Semi-conservative replication: each new DNA molecule contains one original strand and one new strand. Proven by Meselson-Stahl experiment (1958) using heavy nitrogen (¹⁵N).

Key Enzymes

Enzyme	Function
Helicase	Unwinds and separates the double helix
Primase	Synthesizes short RNA primers
DNA polymerase III	Adds nucleotides in 5'→3' direction
DNA ligase	Joins Okazaki fragments on the lagging strand
DNA polymerase I	Removes RNA primers; fills gaps

The Process

1. Origin of replication: specific sequences where unwinding begins
2. Replication fork: area where strands are being separated by helicase
3. Leading strand: continuously synthesized $5' \to 3'$ toward the fork
4. Lagging strand: synthesized in fragments (Okazaki fragments) away from the fork; joined by ligase

Error Correction

DNA polymerase proofreads as it synthesizes, removing incorrect nucleotides.

Mutation rate: ~1 error per billion base pairs per replication — extraordinarily accurate.