AQA A2 Level Biology

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

1Summarise 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.

2How are prokaryotic ribosomes different from eukaryotic ribosomes?

Eukaryotic ribosomes are bigger than prokaryotic ribosomes.

3Why 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.

4Describe 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.

5Why 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.

6Explain 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.

7Suggest 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.

8What 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.

9Explain 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.