The structure of DNA and RNA, the gene as a sequence of bases coding for a protein, the genetic code, and the stages of protein synthesis (transcription and translation).

What this dot point is asking

CCEA wants you to describe the structure of DNA and RNA, explain that a gene is a sequence of bases coding for a protein, describe the genetic code, and explain the stages of protein synthesis (transcription and translation). This is the molecular foundation of the whole genetics unit: inheritance, gene technology and mutations all depend on understanding how DNA codes for proteins.

DNA, RNA and the gene

The two DNA strands run in opposite directions (antiparallel) and the base-pairing rules mean the sequence of one strand determines the other, which is the basis of accurate copying. RNA (ribonucleic acid) differs from DNA: it is usually single-stranded, contains the sugar ribose instead of deoxyribose, and uses the base uracil in place of thymine. Two kinds of RNA are central to protein synthesis: messenger RNA (mRNA), which carries the copy of the gene from the nucleus to the ribosome, and transfer RNA (tRNA), which carries amino acids to the ribosome and reads the code. The order of bases in a gene therefore determines the order of amino acids in a protein, and so the protein's structure and function.

The genetic code

Because there are four bases and the code reads three at a time, there are 64 possible codons, more than enough for the 20 amino acids, which is why the code is degenerate. The universality of the code is what makes gene technology possible: a human gene can be read correctly by a bacterium, so bacteria can be engineered to make human proteins such as insulin. The triplet, non-overlapping reading also means that inserting or deleting a base shifts the reading frame and changes every codon after it, which links to the mutations dot point.

Protein synthesis: transcription and translation

Protein synthesis happens in two stages. In transcription (in the nucleus), the DNA unwinds and the hydrogen bonds of the gene are broken; one strand acts as a template, and free RNA nucleotides pair with it by complementary base pairing (uracil opposite adenine). RNA polymerase joins them into a strand of mRNA, which then leaves the nucleus and travels to a ribosome. In translation (at the ribosome in the cytoplasm), the ribosome reads the mRNA codons in turn. Each tRNA has an anticodon complementary to a codon and carries the matching amino acid; as the ribosome moves along, the amino acids are joined by peptide bonds in the order set by the codons, building the polypeptide. The finished chain folds into the functional protein.

Examples in context

Example 1. Why the universal code matters for medicine. Because the genetic code is universal, a human gene inserted into a bacterium is read and translated correctly, so the bacterium makes the human protein. This is how human insulin is manufactured for people with diabetes, directly linking the molecular basis of protein synthesis to the health-science focus of the qualification.

Example 2. Haemoglobin from a gene. The gene for a haemoglobin chain is transcribed into mRNA and translated into a precise sequence of amino acids that folds into the oxygen-carrying protein. A change in a single base of this gene can change one amino acid and cause sickle-cell disease, showing how the base sequence determines the protein and how it connects to the mutations dot point.

Try this

Q1. State the complementary base-pairing rules in DNA. [2 marks]

• Cue. Adenine pairs with thymine; cytosine pairs with guanine.

Q2. Explain what is meant by the genetic code being a triplet code. [2 marks]

• Cue. Each group of three bases (a codon) codes for one amino acid.

Q3. State where transcription and translation each occur. [2 marks]

• Cue. Transcription in the nucleus; translation at the ribosome in the cytoplasm.