Nucleic acids and their functions — Eduqas A-Level Biology

Nucleic acids, primarily DNA and RNA, are the molecules that store and transmit genetic information in all living organisms. This topic explores their structure, function, and the processes by which they control cellular activities. Understanding nucleic acids is fundamental to genetics, molecular biology, and biotechnology, as they underpin how traits are inherited and how proteins are synthesised.

In the WJEC A-Level Biology specification, you will study the detailed structure of nucleotides, the building blocks of nucleic acids, and how they polymerise to form polynucleotide chains. You will learn about the double helix model of DNA, the roles of different types of RNA (mRNA, tRNA, rRNA), and the key processes of DNA replication and protein synthesis (transcription and translation). These concepts are essential for explaining how genetic information flows from DNA to protein, and how mutations can lead to changes in phenotype.

Mastering nucleic acids is crucial for later topics such as gene expression, genetic engineering, and evolution. It also provides a foundation for understanding modern techniques like PCR and DNA sequencing, which are widely used in medicine and research. By the end of this topic, you should be able to describe the molecular structure of nucleic acids, explain how DNA replicates semi-conservatively, and outline the steps of transcription and translation.

• →Nucleotide structure: each nucleotide consists of a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base (adenine, guanine, cytosine, thymine in DNA; uracil replaces thymine in RNA).
• →DNA double helix: two antiparallel polynucleotide strands held together by hydrogen bonds between complementary base pairs (A-T, C-G), with a sugar-phosphate backbone on the outside.
• →Semi-conservative DNA replication: each parental strand serves as a template for a new complementary strand, resulting in two daughter molecules each containing one original and one new strand, as demonstrated by Meselson and Stahl.
• →Transcription: the process by which a specific segment of DNA is used as a template to synthesise a complementary mRNA molecule, catalysed by RNA polymerase, occurring in the nucleus.
• →Translation: the process by which the sequence of codons on mRNA is decoded by tRNA molecules to assemble a polypeptide chain at the ribosome, involving initiation, elongation, and termination.

• Structure of nucleotides (pentose sugar, phosphate, organic base)
• Structure of ATP and its role as an energy carrier
• DNA structure: purines/pyrimidines, complementary base pairing, hydrogen bonding, double helix, antiparallel strands
• Differences between DNA and RNA structure
• Semi-conservative replication mechanism (DNA polymerase, helicase)
• Evidence from Meselson and Stahl experiments
• Genetic code and triplet code for amino acids
• Exons and introns
• Transcription of DNA to mRNA
• Translation of mRNA using ribosomes and tRNA
• One gene-one polypeptide hypothesis
• Polypeptide modification and combination

• 💡Be prepared to explain the semi-conservative model using the Meselson and Stahl experimental evidence
• 💡Ensure you can clearly distinguish between the roles of mRNA and tRNA in translation
• 💡Use precise terminology when describing the triplet code and its relationship to amino acids
• 💡Practice drawing or interpreting diagrams of DNA structure and replication
• 💡Be ready to apply the 'one gene-one polypeptide' hypothesis to explain protein synthesis
• 💡When describing DNA replication, always mention the roles of key enzymes: DNA helicase (unwinds and separates strands), DNA polymerase (adds complementary nucleotides in the 5' to 3' direction), and DNA ligase (joins Okazaki fragments on the lagging strand).
• 💡For transcription and translation, be precise about locations: transcription occurs in the nucleus, translation occurs on ribosomes in the cytoplasm (or rough endoplasmic reticulum). Use the correct terminology: 'codon' on mRNA, 'anticodon' on tRNA.
• 💡In exam questions about base pairing, remember that in DNA, A pairs with T (two hydrogen bonds) and C pairs with G (three hydrogen bonds). In RNA, A pairs with U. This is crucial for calculating percentages of bases.

• Confusing the roles of DNA polymerase and helicase during replication
• Failing to distinguish between introns and exons in eukaryotic DNA
• Incorrectly describing the antiparallel nature of DNA strands
• Confusing the specific roles of mRNA and tRNA in protein synthesis
• Misinterpreting the Meselson and Stahl experimental results
• Misconception: DNA replication is fully conservative. Correction: Replication is semi-conservative; each new DNA molecule contains one original strand and one newly synthesised strand, not two new strands.
• Misconception: RNA is only found in the cytoplasm. Correction: RNA is synthesised in the nucleus (transcription) and then moves to the cytoplasm; some RNA (e.g., small nuclear RNA) remains in the nucleus.
• Misconception: The genetic code is overlapping. Correction: The genetic code is non-overlapping; each nucleotide is part of only one codon, and codons are read sequentially without gaps.