Nucleic acids and protein synthesis: the structure of DNA and RNA; semi-conservative replication; the genetic code; transcription and translation; and the role of ATP.

What this dot point is asking

Eduqas wants you to describe the structure of DNA and RNA, explain semi-conservative replication, describe the genetic code, and explain transcription and translation. This is a Core Concepts statement that underpins inheritance, gene technology and cell division, so it is examined on every paper.

The structure of DNA and RNA

A nucleotide is a pentose sugar bonded to a phosphate group and a nitrogenous base. In DNA the sugar is deoxyribose and the bases are adenine, thymine, cytosine and guanine; in RNA the sugar is ribose and uracil replaces thymine.

DNA is a double helix: two strands run antiparallel (in opposite directions), with a sugar-phosphate backbone joined by phosphodiester bonds and the two strands held together by hydrogen bonds between complementary bases.

RNA is a single, shorter strand. Messenger RNA (mRNA) carries the code from the nucleus; transfer RNA (tRNA) carries amino acids and has an anticodon; ribosomes contain ribosomal RNA (rRNA).

Semi-conservative replication

DNA copies itself before division:

1. DNA helicase unwinds the double helix and breaks the hydrogen bonds, separating the two strands.
2. Each strand acts as a template; free DNA nucleotides pair with the exposed bases by complementary base pairing.
3. DNA polymerase joins adjacent nucleotides by phosphodiester bonds, building a new strand on each template.

Each new molecule contains one original and one new strand, which is why replication is semi-conservative. The Meselson-Stahl experiment with heavy nitrogen is the classic evidence.

The genetic code

Transcription and translation

Transcription (in the nucleus): RNA polymerase unwinds the gene and builds an mRNA copy of the template strand by complementary base pairing (with uracil in place of thymine). The mRNA leaves the nucleus through a nuclear pore.

Translation (at a ribosome): the ribosome reads the mRNA codons. A tRNA with the complementary anticodon brings the matching amino acid. The ribosome joins adjacent amino acids by peptide bonds (using ATP) and moves along codon by codon until a stop codon ends the polypeptide. The chain then folds into its functional shape.

Examples in context

Example 1. Why a single base change can be silent. Because the code is degenerate, a mutation that changes the third base of a codon may still code for the same amino acid, so the protein is unchanged. This links the genetic code straight to mutation and variation.

Example 2. Antibiotics that target translation. Several antibiotics bind bacterial (70S) ribosomes and block translation without harming human (80S) ribosomes, a real application of protein synthesis that Eduqas may set alongside the microbiology content.

Try this

Q1. State which bases pair together in DNA and the number of hydrogen bonds in each pair. [2 marks]

• Cue. Adenine with thymine (two hydrogen bonds); cytosine with guanine (three hydrogen bonds).

Q2. Explain why DNA replication is described as semi-conservative. [2 marks]

• Cue. Each new DNA molecule contains one original (parental) strand and one newly synthesised strand, so half of each molecule is conserved.

Q3. Name the process by which an mRNA copy of a gene is made, and state where it occurs. [2 marks]

• Cue. Transcription, in the nucleus.