Topic number 14.3
Polypeptide synthesis
– translation
Protein synthesis – the difference
between prokaryotes and eukaryotes
Aims
Protein synthesis is a vital process for all living organisms. This worksheet looks at the
differences in this process between prokaryotic organisms and eukaryotic organisms. You
will be asked to assess the benefits of the different methods. You will also be presented with
some extra details of the process of protein synthesis.
Turning DNA into polypeptides
The process of protein synthesis has two main stages: transcription and translation.
Transcription is the process of making an mRNA copy of the DNA. Translation is the use of
mRNA and tRNA by ribosomes to synthesise a polypeptide chain. Although the processes
are similar in both prokaryotes and eukaryotes, there are some important differences.
Transcription in eukaryotes is more complicated than in prokaryotes. By far the biggest issue
is that the mRNA formed must be transported out of the nucleus before it can be used by the
ribosomes. Because of this, mRNA molecules must be short so that they can travel through
the nuclear pores. Once in the cytoplasm, mRNA molecules can become degraded and
broken down. In eukaryotes the mRNA molecules are modified while still in the nucleus to
prevent this. The average lifespan of a eukaryotic mRNA molecule is 10 minutes.
In eukaryotes, some of the DNA does not code for polypeptides. Some of these non-coding
regions can be found within coding regions, breaking up the code. They are called introns.
These non-coding introns are transcribed into mRNA but must then be removed. This is also
done before the mRNA leaves the nucleus.
In prokaryotes, transcription occurs in the cytoplasm. This allows the mRNA molecules to be
longer. The mRNA in prokaryotes is less modified as there is no need to protect it from
degradation, as translation happens almost immediately. This means the average lifespan of a
prokaryotic mRNA molecule is only 2 minutes.
Translation is almost identical in prokaryotes and eukaryotes. The biggest difference is in the
ribosomes. Eukaryotes have slightly bigger ribosomes but the structure is very similar.
Translation in prokaryotes can begin before the mRNA molecule has actually been finished,
with ribosomes working at one end and the transcription from DNA occurring at the other.
Owing to the length of the mRNA molecule, more than one ribosome can be translating the
mRNA at any one time. This combination of multiple ribosomes on the mRNA is called a
polysome.
Not all the genes on a piece of DNA may be needed by the organism at a particular time.
Often certain substances will not be made by an organism if they can get them from their
environment. To regulate the expression of certain genes, prokaryotes use a system by which
genes are organised into groups called operons. These are regulated by a gene that is
transcribed before the operon. This gene codes for a molecule that can block the transcription
of the operon under certain circumstances.
AQA Biology A2 Stretch and challenge © Nelson Thornes Ltd 2009
1
Topic number 14.3
Polypeptide synthesis
– translation
Questions
1 Summarise the differences between protein synthesis in eukaryotes and prokaryotes.
2 How are prokaryotic ribosomes different from eukaryotic ribosomes?
3 Why does prokaryotic mRNA have a much shorter lifespan than eukaryotic mRNA?
4 Describe a polysome and explain how they are useful.
5 Why is it important that prokaryotic mRNA does not need any modification after it has been
formed?
6 Explain why introns must be removed from the mRNA before leaving the nucleus.
7 Suggest why organisms regulate their gene expression.
8 What is an operon? Explain why they are important to prokaryotes.
9 Explain how the position of an operon regulator gene affects whether the operon is transcribed
or not.
AQA Biology A2 Stretch and challenge © Nelson Thornes Ltd 2009
2