Nucleic Acids: DNA and RNA

Nucleic acids

DNA and RNA are nucleic acids — polymers of nucleotides. They store and transfer the genetic information that codes for proteins.

Nucleotide structure

Each nucleotide has three parts:

• a pentose sugar (deoxyribose in DNA, ribose in RNA);
• a phosphate group;
• an organic (nitrogenous) base.

Nucleotides join by condensation to form a phosphodiester bond (sugar–phosphate backbone).

DNA structure

• A double helix of two antiparallel polynucleotide strands.
• The bases pair by complementary base pairing via hydrogen bonds:
• A–T (2 hydrogen bonds)
• C–G (3 hydrogen bonds)
• The sugar-phosphate backbone is on the outside; the paired bases are inside.
• DNA is a stable store of information: the double helix and H-bonds protect it, and the base sequence carries the code.

RNA structure

• Usually single-stranded and shorter.
• Contains the sugar ribose and the base uracil (U) instead of thymine (so A pairs with U).
• mRNA (messenger) carries the code from DNA to ribosomes; tRNA (transfer) brings amino acids; rRNA makes up ribosomes.

DNA replication (semi-conservative)

Before a cell divides, DNA copies itself:

1. DNA helicase unwinds the helix and breaks the hydrogen bonds between bases.

2. Each strand acts as a template; free nucleotides pair with the exposed bases.

3. DNA polymerase joins the new nucleotides, forming new strands.

4. Each new DNA molecule has one original and one new strand — hence semi-conservative.

The genetic code

• A sequence of three bases (a triplet / codon) codes for one amino acid.
• The code is degenerate (most amino acids have more than one codon), non-overlapping, and universal (the same in almost all organisms).

Worked example

A DNA strand reads TACGGA. What is the complementary strand?

• A–T, T–A, C–G, G–C → complementary strand = ATGCCT. ✓

Common mistakes

• Getting base pairs wrong — A–T and C–G (in RNA, A pairs with U).
• Forgetting DNA replication is semi-conservative (one old + one new strand).
• Confusing the sugars: deoxyribose (DNA) vs ribose (RNA).

Exam tips

• Learn nucleotide structure and the A–T / C–G pairing (with H-bond numbers).
• Describe replication step by step, naming helicase and polymerase.
• State the three code features: degenerate, non-overlapping, universal, and that a triplet codes one amino acid.

Key facts to remember

• Nucleotide = pentose sugar + phosphate + base; joined by phosphodiester bonds.
• DNA = double helix, A–T (2 H-bonds) and C–G (3 H-bonds); RNA = single-stranded, ribose, uracil.
• Replication is semi-conservative (helicase unwinds, polymerase builds); a triplet/codon codes one amino acid — code is degenerate, non-overlapping, universal.