mRNA modification (OCR A-level Biology)

This fully-resourced lesson describes the control of gene expression at a post-transcriptional level through the removal of introns during splicing. The detailed PowerPoint and accompanying resources have been designed to cover the second part of point 6.1.1 (b) as detailed in the OCR A-level Biology A specification and also explains how it’s possible for 1 gene to give rise to multiple products as a result of this post-transcriptional modification of mRNA.

The lesson begins with a knowledge recall as the students have to recognise the definition of a gene as a sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain. This description was introduced in module 2.1.3 and the aim of the start of the lesson is to introduce the fact that despite this definition, most of the nuclear DNA in eukaryotes doesn’t actually code for proteins. A quick quiz competition is then used to introduce exons as the coding regions within a gene before students are challenged to predict the name of the non-coding regions and then to suggest a function for these introns. At this point, the students will complete a task that acts as a prior knowledge check where they have to identify the 6 errors in the descriptive passage about the lac operon and its role in the regulation of gene expression in prokaryotes. Moving forwards, pre-mRNA as a primary transcript is introduced and students will learn that this isn’t the mature strand that moves off to the ribosome for translation. Instead, a process called splicing takes place where the introns are removed and the remaining exons are joined together. Another quick quiz round leads to an answer of 20000 and students will learn that this is the number of protein-coding genes in the human genome. Importantly, the students are then told that the number of proteins that are synthesised is much higher than this value and a period of class discussion encourages them to come up with biological suggestions for this discrepancy between the two numbers. The lesson concludes with a series of understanding and application questions where students will learn that alternative splicing enables a gene to produce more than a single protein and that this natural phenomenon greatly increases biodiversity