Download Answers to EPQ questions 3 Nucleic acids and protein synthesis

3 Nucleic acids and protein synthesis
Answers to Exam practice questions
1 C
[1]
2 B
[1]
3 a) (Very) large number of hydrogen bonds;
Between complementary base pairs;
Allow between A–T and C–G
[2]
b) In its order of bases;
Three bases for each amino acid;
[2]
4 a) L is guanine;
M is cytosine;
N is adenine;
The full name is required to gain each mark
[3]
b) Action of DNA helicase;
Causes hydrogen bonds to break;
(Hydrogen bonds) between complementary base pairs;
[3]
5 a) Bacterial cell has a single molecule of DNA but humans have many/46 molecules;
Bacterial DNA is circular but human DNA is linear;
[2]
b) Base pairs are A–T and C–G;
So percentage of a base in one strand is same as its complement in second strand;
Total of each strand must be 100%;
If student fails to give an explanation but completes table as:
22, 33, 30, 15
15, 30, 33, 22
allow 2 marks
[3]
6 a) Carbon atom 2/C-2/2;
Has only hydrogen atoms/no hydroxyl group;
[2]
b) Circle drawn around one nucleotide;
Nucleotide chosen contains a purine residue;
Accept any shape as long as it clearly encloses a nucleotide
[2]
c) To bottom of diagram;
Because nucleotides added to free OH group/to carbon 3;
Because enzyme/DNA polymerase is specific/has active site that is only complementary to
this shape;
[3]
© Hodder & Stoughton Limited 2015
3 Nucleic acids and protein synthesis
Answers to Exam practice questions
7 A description that includes any five of the following:
• mRNA carries genetic code;
• In its base sequence;
• From DNA/nucleus to ribosome;
• tRNA carries specific amino acid;
• Anticodon on tRNA;
• Complementary to codon on mRNA;
• Anticodon and codon align (in ‘reading space’ in ribosome);
• rRNA involved in structure of ribosomes;
[5]
Stretch and challenge questions
The following indicates the points that would gain credit.
8 a) • DNA helicase is breaking the hydrogen bonds between the two DNA strands, causing
them to unwind.
• The leading strand is being copied in a 3’ to 5’ direction.
• Since its new strand is antiparallel, it is being developed in a 5’ to 3’ direction.
• DNA polymerase can only add new nucleotides to the 3’ end of the developing chain.
• So the new copy of the leading strand is developed continuously in the same direction as
the movement of the helicase.
• Since the lagging strand lies in the opposite direction to the leading strand, the DNA
polymerase must work in the opposite direction to the movement of the helicase.
• As a result, several DNA polymerase molecules attach to the lagging strand at several
points along its length (the replication forks).
• This results in several fragments (A, B and C) being developed separately.
b) The fragments are eventually joined by DNA ligase.
9 • The enzyme is referred to as DNA-dependent DNA polymerase because it makes a DNA
polynucleotide by pairing DNA nucleotides with complementary bases on an existing singlestranded DNA polynucleotide.
•
It is found in both prokaryotic cells (cells without a nucleus, e.g. bacteria) and eukaryotic cells
(cells with a nucleus, e.g., fungi, plants and animals).
•
There are many different DNA-dependent DNA polymerases in prokaryotic cells and in
eukaryotic cells. Some are found in both types of cell.
•
RNA-dependent DNA polymerase makes a DNA polynucleotide by pairing DNA nucleotides
with complementary bases on an existing single-stranded RNA polynucleotide.
•
RNA-dependent DNA polymerases are naturally found in viruses that carry their genome as
RNA. Examples include the hepatitis B virus and the human immunodeficiency virus (HIV).
© Hodder & Stoughton Limited 2015
3 Nucleic acids and protein synthesis
Answers to Exam practice questions
•
RNA-dependent DNA polymerase is often referred to as reverse transcriptase. It is an enzyme
used in gene technology to make DNA from mRNA (it is often easier to gain large amount of
mRNA from cell, rather than DNA).
© Hodder & Stoughton Limited 2015