Topic number 14.3
Polypeptide synthesis –
translation
Protein synthesis – the difference
between prokaryotes and eukaryotes
Aims
By completing this worksheet students should be able to describe the difference between
prokaryotic and eukaryotic protein synthesis. They should also explain why organisms
regulate their gene expression. Students should be able to explain what an operon is and state
that they are found in prokaryotes.
Teacher notes
Students will need knowledge of translation and transcription, although this does not have to
be too detailed. They will need to know the structure of mRNA and tRNA. Students will also
need to know the differences between prokaryotes and eukaryotes, specifically the lack of a
nucleus in prokaryotes.
Students should be reminded to look at both the mRNA produced and the location of the
process for question 1. For question 6 students could be prompted by encouraging them to
consider what happens with the code during translation. They should then realise that amino
acids that are not needed will be included in the final molecule. Students may not realise that
these processes require energy and this information may lead them to realise that carrying out
protein synthesis is not efficient if not needed.
Answers to questions
1 Summarise the differences between protein synthesis in eukaryotes and prokaryotes.
Protein synthesis is more complex in eukaryotes than in prokaryotes. Eukaryotes must modify
the mRNA made during transcription. It must have introns removed and be prepared for the
cytoplasm so it does not break down too quickly. Eukaryotic mRNA can only be made in
small pieces as it must be able to leave the nucleus. Transcription and translation happen in
different places in eukaryotes, with transcription occurring in the nucleus, and translation
occurring in the cytoplasm. In prokaryotes both processes occur in the same place, the
cytoplasm.
2 How are prokaryotic ribosomes different from eukaryotic ribosomes?
Eukaryotic ribosomes are bigger than prokaryotic ribosomes.
3 Why does prokaryotic mRNA have a much shorter lifespan than eukaryotic mRNA?
Prokaryotic mRNA is used immediately, often being translated before it is finished being
made by transcription. Eukaryotic mRNA has to leave the nucleus before it can be translated.
4 Describe a polysome and explain how they are useful.
A polysome is a piece of mRNA with several ribosomes attached carrying out translation.
They are useful because multiple copies of the same polypeptide can be translated at the same
time.
AQA Biology A2 Stretch and challenge teacher notes © Nelson Thornes Ltd 2009
1
Topic number 14.3
Polypeptide synthesis –
translation
5 Why is it important that prokaryotic mRNA does not need any modification after it has been
formed?
Prokaryotic mRNA is translated straight away so it needs to be correct. There is no chance to
modify it once it has been finished, because translation may have already started.
6 Explain why introns must be removed from the mRNA before leaving the nucleus.
Once the mRNA has left the nucleus, it may be translated very quickly. If the introns were left
in, a non-functional polypeptide would be made during translation.
7 Suggest why organisms regulate their gene expression.
Organisms regulate their gene expression so that they do not waste resources making
polypeptides they don’t need or that they can get from their environment. This is more
efficient and less wasteful than making everything all the time. Certain cells do not need
certain genes.
8 What is an operon? Explain why they are important to prokaryotes.
An operon is a group of genes that may work together to produce a certain protein or group
of proteins. Operons are important as they allow prokaryotes to regulate their gene
expression by not producing proteins they do not need. The organism can prevent their
transcription without stopping the transcription of other required genes.
9 Explain how the position of an operon regulator gene affects whether the operon is transcribed
or not.
The regulator gene for an operon must be transcribed before the operon is. In this way the
regulator protein is made before the operon is transcribed so that it can stop it happening in
certain conditions.
AQA Biology A2 Stretch and challenge teacher notes © Nelson Thornes Ltd 2009
2