The structure of nucleotides, DNA and RNA, the base-pairing rules, semi-conservative DNA replication, and the role of ATP as the energy currency of the cell.

What this dot point is asking

CCEA wants you to describe the structure of a nucleotide and of DNA and RNA, state the complementary base-pairing rules, explain how DNA replicates semi-conservatively, and describe the structure and role of ATP as the energy currency of the cell.

Nucleotide and DNA structure

DNA is a double helix of two antiparallel polynucleotide strands (running in opposite directions). The bases pair by hydrogen bonding: adenine with thymine (two hydrogen bonds) and cytosine with guanine (three hydrogen bonds). This complementary base pairing means the two strands are exact templates of each other, and the amount of adenine equals thymine and cytosine equals guanine (Chargaff's rule). RNA is single-stranded, contains ribose instead of deoxyribose, and uses uracil in place of thymine. Messenger RNA carries the code from the nucleus to the ribosomes.

Semi-conservative replication

This mechanism was confirmed by the Meselson and Stahl experiment, which grew bacteria in heavy nitrogen ($^{15}\text{N}$) then light nitrogen ($^{14}\text{N}$) and used density centrifugation to show that after one round of replication every molecule was a hybrid of one heavy and one light strand, exactly as semi-conservative replication predicts.

ATP

ATP (adenosine triphosphate) is a phosphorylated nucleotide made of adenine, ribose and three phosphate groups. When the terminal phosphate is removed by hydrolysis, ATP becomes ADP plus inorganic phosphate ($\text{P}_\text{i}$) and releases a small, usable amount of energy. ATP is the immediate energy currency of the cell because it is quickly made and broken down, releasing energy in small manageable amounts to power processes such as active transport, muscle contraction and synthesis. It is constantly recycled: ADP is re-phosphorylated to ATP using energy from respiration.

Examples in context

Example 1. Why a mutation can change a protein. During replication, a mismatched base may slip past proofreading, changing one base in a gene. Because base pairing is specific, that error is then copied faithfully into both daughter strands and all future cells. If the change alters a codon, it can change one amino acid in the protein (as in sickle-cell anaemia, where a single base change replaces glutamic acid with valine in haemoglobin). This shows how the very accuracy of base pairing also locks in any error that escapes repair.

Example 2. ATP turnover in a sprinting muscle. A working muscle uses ATP faster than respiration can supply it, so the small store is exhausted in seconds and must be regenerated continuously. The muscle uses creatine phosphate to rephosphorylate ADP almost instantly, then anaerobic and aerobic respiration to keep the ATP pool topped up. This illustrates why ATP, not glucose, is the immediate energy currency: it releases energy in small, fast, usable amounts exactly where and when it is needed.

Try this

Q1. State the base-pairing rules 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 molecule has one original (conserved) strand and one newly synthesised strand.

Q3. A DNA molecule contains 18 percent guanine. Calculate the percentage of adenine. [2 marks]

• Cue. Guanine equals cytosine (18 percent each, total 36 percent), so adenine plus thymine make 64 percent; adenine equals thymine, so adenine is 32 percent.