Author: Kim Edgell
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Nucleic acid homework 3: Protein synthesis and mutations
Name: ______________________________ %:__________________
Q1.(a)     Draw the general structure of an amino acid.
 
(1)
Table 1 shows mRNA codons and the amino acids coded for by each codon. It also shows some properties of the R group of each amino acid. Table 1

Key to the properties of the R group of each amino acid
 
| No overall change | | Positively charged | | Negatively charged
(b)     The genetic code is described as degenerate.
What is meant by this? Use an example from Table 1 to illustrate your answer.
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A scientist investigated changes in the amino acid sequence of a human enzyme resulting from mutations. All these amino acid changes result from single base substitution mutations.
This enzyme is a polypeptide 465 amino acids long.Table 2 shows the result of three of the base substitutions.
 
Amino acid number | Correct amino acid | Amino acid inserted as a result of mutation
203 | Val | Ala
279 | Glu | Lys
300 | Glu | Lys
(c)     What is the minimum number of bases in the gene coding for this polypeptide?

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(d)     Use information from Table 1 to tick (✔) one box that shows a single base substitution mutation in DNA that would result in a change from Val to Ala at amino acid number 203
 
CAA → CGA |
GUU → GCA |
GUU → GUC |
CAC → CGG |
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(e)     A change from Glu to Lys at amino acid 300 had no effect on the rate of reaction catalysed by the enzyme. The same change at amino acid 279 significantly reduced the rate of reaction catalysed by the enzyme.
Use all the information and your knowledge of protein structure to suggest reasons for the differences between the effects of these two changes.
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Q2.(a)     Messenger RNA (mRNA) is used during translation to form polypeptides.
Describe how mRNA is produced in the nucleus of a cell.
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(b)     Describe the structure of proteins.
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(c)     Describe how proteins are digested in the human gut.
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Q3.(a)     The table shows the mRNA codons for some amino acids.
 
Codon | Amino acid
CUA | Leucine
GUC | Valine
ACG | Threonine
UGC | Cysteine
GCU | Alanine
AGU | Serine
(i)      Give the DNA sequence coding for cysteine.
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(ii)     Name the amino acid coded by the tRNA anticodon UCA.
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(b)     A particular gene is 562 base-pairs long. However, the resulting mRNA is only 441 nucleotides long. Explain this difference.
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(c)     Tetracycline binds to bacterial ribosomes. This is shown in the diagram.
 
Protein synthesis in bacteria is similar to that in eukaryotic cells. Explain how tetracycline stops protein synthesis.
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Q4.(a)     (i)      What is the role of RNA polymerase in transcription?
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(ii)     Name the organelle involved in translation.
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(b)     Figure 1 shows some molecules involved in protein synthesis.
Figure 1
 
Complete Figure 1 to show
(i)      the bases on the DNA strand from which the mRNA was transcribed;
(ii)     the bases forming the anticodons of the tRNA molecules.
(2)
Figure 2 shows the effects of two different mutations of the DNA on the base sequence of the mRNA. The table shows the mRNA codons for three amino acids.
Figure 2
 
(c)     Name the type of mutation represented by mutation 1.
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(d)     Use the information in the table to
(i)      identify amino acid X in Figure 1;
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(ii)     explain how each mutation may affect the polypeptide for which this section of DNA is part of the code.
Mutation 1 _____________________________________________________
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Mutation 2 _____________________________________________________
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