Real Science and Other Adventures

VCE Biology – From DNA triplet to amino acid – working out the code

VCE 3&4 Biology: Here we look some exam questions where you need to work out the corresponding DNA or RNA sequence.

This video from yourgenome.org sets the scene.

Question 22 in the 2014 VCAA exam:

In populations of fruit flies, there are individuals that are resistant to the effects of insecticides. Insecticide-resistant fruit flies arose as a result of a mutation. In normal insecticide-susceptible fruit flies, a specific section of mRNA has the sequence GCU, whereas in the insecticide-resistant fruit flies, the sequence is UCU.

Considering the mRNA sequence of the insecticide-resistant fruit flies, the corresponding sequence of nucleotides on the individual’s DNA is

A. AGA

B. AUA

C. CGA

D. TGT

You can immediately exclude (B) because it has U rather than T so it is RNA. Also, make sure you read the question carefully – it asks about the mutated DNA, i.e. the second sequence mentioned, UCU. So answer (C) (which corresponds to the unmutated sequence) is wrong.

What it means by the “corresponding” sequence is the one that’s complementary to the mRNA. I’ve called it the transcribed strand below –  because it’s transcribed into mRNA. UCU in the mRNA is transcribed from the complementary sequence AGA (answer A).

The following table takes you through the steps in working out the nucleotide/amino acid sequence from DNA to protein:

codons triplets

Bear in mind you might have to work out a sequence in more than one step as in the next question.

In the written VCAA exam 2 from 2011, question 4 asks:

A DNA template strand codes for the amino acid serine with any of the following codons.

AGA    AGG    AGT    TCG    AGC

A tRNA molecule that codes for the amino acid serine is

A. AGT

B. UGA

C. TCG

D. UCG

This time, you can immediately exclude A and C because they have T rather than U so they are DNA, not tRNA.

Now you have two options left: (B) UGA and (D) UCG. (Time-saving tip: there now are five different sequences in the question and two possible answers so it’s quicker to work backwards from the two remaining answers, (B) and (D) at this stage.)

DNA has two complementary strands. Do you know which is the template strand that the question refers to? The template strand is the one I’ve named the transcribed strand – the same one that was called the corresponding strand in the other question. It’s a template for making mRNA.

To get from DNA to tRNA, (or the reverse, tRNA to DNA) you need to work out two steps as in the table below. Answer (D) (UCG) is complementary to TCG in the template (transcribed) strand.

codon triplet table

Make up some random 3 letter DNA sequences and practise replicating (DNA->DNA), transcribing (DNA->mRNA) and translating (mRNA->amino acid using a codon chart) as well as going from mRNA->tRNA. Practise going in the reverse direction. Note that using codon charts you are working from the mRNA sequence, not the tRNA (anti-codon) sequence.

EXTENSION

Note the similarity between the top DNA strand sequence and mRNA (both in blue in the table), and between the lower (transcribed) DNA sequence and tRNA (red) (except where U replaces T).

*The term “template” strand is most commonly used to mean what I’ve called the transcribed strand. This is the template on which mRNA is made. The strand of DNA at the top of the table is usually called the “coding” or “sense”  or  even “mRNA-like” strand. This is the one that matches the mRNA sequence (except that it has T instead of U).

If you look up one of the many gene sequence databases you will probably find the DNA strand they list is not the sense/mRNA-like DNA strand (which matches the mRNA except for the T/U difference). If you go to this cystic fibrosis gene database, click on the gene diagram to see some of the sequence.

Is this helpful? I’d welcome your feedback and suggestions.

USEFUL RESOURCES

http://www.yourgenome.org/video/from-dna-to-protein
This video takes you from DNA to protein and on the way you can see how the sequences relate to each other.

http://www.bbc.co.uk/bitesize/higher/biology/cell_biology/rna/revision/1/ – a tutorial on protein production

A codon chart to translate mRNA codons into amino acids – https://en.wikipedia.org/wiki/DNA_codon_table

Or use a codon wheel, which does the same thing – http://learn.genetics.utah.edu/content/molecules/dnacodes/lut.cshtml

VCE = Victorian Certificate of Education – secondary school leaving certificate in Victoria, Australia

VCAA = Victorian Curriculum and Assessment Authority

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This entry was posted on September 29, 2015 by in genomics, VCE Biology and tagged , , , , , , , .
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