AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
In more complex organisms, the presence of non-coding DNA and of…
Full specification wording
Using genome projects (A-level only): In more complex organisms, the presence of non-coding DNA and of regulatory genes means that knowledge of the genome cannot easily be translated into the proteome.
In a eukaryote only a small proportion of the genome codes for polypeptides, so knowing the sequence does not give you the proteome.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 4 of 5
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
In a eukaryote only a small proportion of the genome codes for polypeptides, so knowing the sequence does not give you the proteome. Genes are split into exons, which are coding, and introns, which are not, and there are long stretches of non-coding repeated sequences between genes. Looking at the raw base sequence you cannot tell which parts will survive into the mature mRNA, so the sequence cannot simply be translated. Splicing makes this worse in a useful way: the same pre-mRNA can be spliced in different combinations, so one gene may give rise to several different polypeptides, and the proteome is larger than the gene count. Regulatory genes add a further layer, coding for transcription factors and RNA molecules that determine which genes are transcribed in which cell type and when, and epigenetic marks such as DNA methylation silence genes without altering a single base.
What earns the marks
- Eukaryotic genes containing introns or non-coding DNA that is not translated.
- Introns being removed from pre-mRNA by splicing, so the coding regions cannot be identified from the DNA sequence alone.
- One gene producing more than one polypeptide because pre-mRNA can be spliced in different combinations.
- Regulatory genes or transcription factors determining which genes are expressed in a particular cell.
- Epigenetic modification altering expression without altering the base sequence.
Common mistakes and how to put them right
Saying that eukaryotic DNA is entirely non-coding, rather than mostly non-coding.
Saying introns are removed from the DNA, when they are removed from the pre-mRNA.
Assuming the number of genes equals the number of different proteins.
Confusing non-coding DNA with genes that happen not to be switched on in that cell.
Forgetting regulatory genes, so the answer only ever mentions introns.
Exam tips
- Two separate ideas earn marks here - non-coding DNA and splicing, and regulation of expression - so give one of each.
- Say 'pre-mRNA' when you mention splicing; saying introns are cut from DNA loses the mark.
- Use the liver cell and neurone contrast to show that one genome supports many proteomes.
Try the lesson
The start of the Using genome projects (A-level only): In more complex organisms, the presence of non-coding DNA and of regulatory genes means that knowledge of the genome cannot easily be translated into the proteome. lesson, made from checked teaching notes for this specification point.
Have a go first
Which feature of the genome of a complex organism means that the proteome cannot easily be predicted from the genome sequence?
Listen
In a eukaryote only a small proportion of the genome codes for polypeptides, so knowing the sequence does not give you the proteome. Genes are split into exons, which are coding, and introns, which are not, and there are long stretches of non-coding repeated sequences between genes. Looking at the raw base sequence you cannot tell which parts will survive into the mature mRNA, so the sequence cannot simply be translated. Splicing makes this worse in a useful way: the same pre-mRNA can be spliced in different combinations, so one gene may give rise to several different polypeptides, and the proteome is larger than the gene count. Regulatory genes add a further layer, coding for transcription factors and RNA molecules that determine which genes are transcribed in which cell type and when, and epigenetic marks such as DNA methylation silence genes without altering a single base. A strong answer earns marks for these points. eukaryotic genes containing introns or non-coding DNA that is not translated. introns being removed from pre-mRNA by splicing, so the coding regions cannot be identified from the DNA sequence alone. one gene producing more than one polypeptide because pre-mRNA can be spliced in different combinations. regulatory genes or transcription factors determining which genes are expressed in a particular cell. Exam tip: Two separate ideas earn marks here - non-coding DNA and splicing, and regulation of expression - so give one of each.
Key words
Say what each word means before you turn the card.
non-coding DNA
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introns and exons
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splicing
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regulatory genes
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See every definition
- non-coding DNA
- DNA that is not translated, including introns and long stretches of repeated sequences between genes.
- introns and exons
- Genes are split into exons, which are coding, and introns, which are not.
- splicing
- Introns are removed from pre-mRNA, and the same pre-mRNA can be spliced in different combinations.
- regulatory genes
- Genes coding for transcription factors and RNA molecules that determine which genes are transcribed in which cell type and when.
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