Protein synthesis & mutation
DNA holds the instructions, but instructions do nothing until they are read and acted on. In this lesson we follow the message all the way from a gene to a finished protein. We meet the idea that a polypeptide is coded for by a gene, and that a gene is just a sequence of nucleotides forming part of a DNA molecule. We learn the features of the genetic code — that it is a triplet code, universal, non-overlapping and degenerate — and we practise reading a sequence in threes. Then we walk carefully through transcription in the nucleus, where RNA polymerase copies the template strand into messenger RNA, and translation at the ribosome, where transfer RNA brings amino acids matched to codons by their anticodons and peptide bonds link them into a chain. Finally we see what happens when the message changes: gene mutations by substitution, deletion and insertion, the frameshift they can cause, and the famous single base substitution that turns the HbA allele into HbS and causes sickle cell.
By the end you should be able to (NSSCAS Biology (AS) 3.2.2):
- State that a polypeptide is coded for by a gene and that a gene is a sequence of nucleotides that forms part of a DNA molecule
- State the features of the genetic code
- Describe how the information in DNA is used during transcription and translation to construct polypeptides, including the role of messenger RNA (mRNA), transfer RNA (tRNA) and the ribosomes
- State that a gene mutation is a change in the sequence of nucleotides that may result in an altered polypeptide
- Explain that a gene mutation occurs by substitution or deletion or insertion of base pairs in DNA and outline how each of these types of mutation may affect the polypeptide produced
- Describe the way in which the nucleotide sequence codes for the amino acid sequence in a polypeptide with reference to the nucleotide sequence for HbA (normal) and HbS (sickle cell) alleles of the gene for the β-globin polypeptide
- Design and make a model of RNA to contrast it with the DNA model
Miss Rachel and Mike talk through the whole topic — with the figure and working drawn live.