DNA: structure and replication

Watson and Crick model of DNA

• DNA (deoxyribonucleic acid) has two long chains twisted round each other like a spiral staircase: a double helix.
• Each chain is made of units called nucleotides. A nucleotide has a deoxyribose sugar, a phosphate and a nitrogen base.
• The sugar and phosphate form the backbone, like the rails of the staircase.
• The bases form the steps. Purines: adenine (A) and guanine (G). Pyrimidines: cytosine (C) and thymine (T).
• A always pairs with T by two hydrogen bonds. G always pairs with C by three hydrogen bonds. This is complementary base pairing.
• The two chains run in opposite directions (antiparallel).
• One full turn is 34 Å long and has 10 base pairs, 3.4 Å apart. The helix is 20 Å wide (1 Å = 10⁻¹⁰ m).

How DNA replicates (copies itself)

1. 1
   The enzyme helicase unwinds the double helix and breaks the hydrogen bonds between the bases.
2. 2
   The two chains separate like a zip opening. Each chain acts as a template (pattern).
3. 3
   Free nucleotides pair with the exposed bases: A with T, G with C.
4. 4★
   The enzyme DNA polymerase joins the new nucleotides into a new chain.
5. 5
   One new chain forms smoothly (the leading strand). The other forms in short pieces called Okazaki fragments (the lagging strand).
6. 6
   The enzyme DNA ligase joins the short pieces.
7. 7★
   Result: two DNA molecules, each with one old chain and one new chain. This is semi-conservative replication.

★ key step

Purines vs pyrimidines

A purine always pairs with a pyrimidine, so the helix keeps the same width all along.

Why DNA matters

• It carries the genetic information from parents to offspring.
• It copies itself exactly before a cell divides.
• It controls the making of proteins, which build and run the body.
• Changes in DNA (mutations) cause variation.
• Every person's DNA is unique, except identical twins, so DNA fingerprinting can identify people.

How to draw the DNA double helix

1. 1
   Draw two wavy lines that cross each other again and again, like a twisted ladder. Label sugar-phosphate backbone.
2. 2
   Join the two lines with short horizontal steps. Label base pairs.
3. 3
   Write A = T on some steps and G ≡ C on others. Label hydrogen bonds.
4. 4
   Mark one full turn with a bracket. Label 34 Å (one turn, 10 base pairs).
5. 5
   Mark the width across the helix. Label 20 Å.
6. 6
   Draw an arrow on each chain, pointing in opposite directions. Label antiparallel chains.