The structure of DNA and RNA, the roles of ATP, semi-conservative DNA replication, and the principle of protein synthesis.

What this dot point is asking

WJEC wants you to describe the structure of DNA and RNA, state the role of ATP as the energy currency, explain semi-conservative DNA replication, and outline how DNA codes for proteins through transcription and translation.

Structure of DNA and RNA

DNA is a double helix of two antiparallel polynucleotide strands. The sugar-phosphate backbones run on the outside; the bases project inward and pair by complementary base pairing: adenine with thymine by two hydrogen bonds, and cytosine with guanine by three hydrogen bonds. Because C-G pairs have three hydrogen bonds, DNA with a high proportion of C-G is more stable and needs more energy to separate. RNA is single-stranded, uses the sugar ribose instead of deoxyribose, and uses uracil in place of thymine.

The role of ATP

ATP suits its role because the energy it releases is immediate, in a single reaction, and in a manageable quantity, unlike the slow stepwise breakdown of glucose. It cannot be stored in bulk, so cells make it continuously close to where it is used.

Semi-conservative replication

DNA replicates before cell division so that each daughter cell receives a full copy. DNA helicase unwinds the double helix and breaks the hydrogen bonds, separating the two strands. Each strand acts as a template: free activated DNA nucleotides line up opposite their complementary bases, and DNA polymerase joins adjacent nucleotides by forming phosphodiester bonds. Each daughter molecule ends up with one original and one new strand, which is why replication is semi-conservative, the model confirmed by the Meselson and Stahl experiment.

From DNA to protein

The base sequence of a gene codes for the amino acid sequence of a protein in triplets called codons. In outline, transcription copies a gene into messenger RNA in the nucleus, the mRNA leaves through a nuclear pore, and translation at the ribosome reads the mRNA codons three at a time, with transfer RNA bringing the matching amino acids to be joined into a polypeptide.

Examples in context

Example 1. Meselson and Stahl. Bacteria were grown in heavy nitrogen ($^{15}\text{N}$) then switched to light nitrogen ($^{14}\text{N}$). After one round of replication every DNA molecule was of intermediate density, ruling out conservative replication, and after two rounds there was a mix of light and intermediate DNA. This elegant density experiment is the classic proof that replication is semi-conservative.

Example 2. PCR in forensic science. The polymerase chain reaction copies tiny DNA samples by repeatedly heating to separate the strands, cooling so primers bind, then using a heat-stable DNA polymerase to extend new strands, exactly mimicking semi-conservative replication in a tube. It lets forensic scientists amplify trace DNA from a crime scene, showing the topic's real-world reach.

Try this

Q1. State the three components of a nucleotide. [1 mark]

• Cue. A pentose sugar, a phosphate group and a nitrogenous base.

Q2. Explain why complementary base pairing ensures the two new DNA molecules are identical to the original. [2 marks]

• Cue. Each base pairs only with its complement, so each template strand directs an exact copy of the missing strand.

Q3. A DNA sample contains $28\%$ thymine. Calculate the percentage of cytosine. [2 marks]

• Cue. A equals T so A is $28\%$; A plus T $= 56\%$; C plus G $= 44\%$; cytosine $= 44 \div 2 = 22\%$.