5. nucleic acids epqs redo Flashcards
During replication, the two DNA strands separate and each acts as a template for the production of a new strand. As new DNA strands are
produced, nucleotides can only be added in the 5’ to 3’ direction.
Use the figure in part (a) and your knowledge of enzyme action and DNA
replication to explain why new nucleotides can only be added in a 5’ to 3’ direction. (4)
- Reference to DNA polymerase;
- (Which is) specific;
- Only complementary with / binds to 5’ end (of strand);
Reject hydrogen bonds / base pairing - Shapes of 5’ end and 3’ end are different / description of how
different.
(3)
- (Treatment D Antibody binds to cyclin A so) it cannot bind to
DNA/enzyme/initiate DNA replication;
For ‘bind to enzyme’ accept ‘activate’.
Idea of ‘initiate DNA replication’ must be linked to
start not just less replication.
For ‘enzyme’ accept named enzyme.
- (Treatment E) RNA interferes with mRNA/tRNA/ribosome/polypeptide
formation (so cyclin A not made); - In Treatment F added cyclin A can bind to DNA/enzyme (to initiate
DNA replication)
OR
Treatment F shows that it is the cyclin A that is being affected in the
other treatments
OR
Treatment F shows that cyclin A allows the enzyme to bind (to DNA)
OR
(Some cells in D or E) can continue with DNA replication because
they have a different cyclin A allele
OR
(Some cells in D or E) can continue with DNA replication because the
antibody/RNA has not bound to all the cyclin A protein/mRNA
OR
(Some cells in E) can continue with DNA replication because they
contain previously translated cyclin A;
Describe the role of two named enzymes in the process of semi-
conservative replication of DNA. (3)
- (DNA) helicase causes breaking of hydrogen/H bonds (between DNA
strands);
Reject ‘helicase hydrolyses hydrogen bonds’.
2. DNA polymerase joins the (DNA) nucleotides;
Reject if suggestion that DNA polymerase joins the
complementary nucleotides or forms H bonds.
Reject if joining RNA nucleotides or forming RNA.
- Forming phosphodiester bonds;
Replication of mitochondrial DNA (mtDNA) is different from that of nuclear
DNA.
The replication of the second strand of mtDNA only starts after two-thirds
of the first strand of mtDNA has been copied.
A piece of mtDNA is 16 500 base pairs long and is replicated at a rate of
50 nucleotides per second.
How long it would take to copy this mtDNA.
C. 550 seconds;
Give two features of DNA and explain how each one is important in the
semi-conservative replication of DNA.
- Weak / easily broken hydrogen bonds between bases allow two
strands to separate / unzip; - Two strands, so both can act as templates;
may appear in the same feature - Complementary base pairing allows accurate replication;
Adenosine triphosphate (ATP) is a nucleotide derivative.
Contrast the structures of ATP and a nucleotide found in DNA to give two differences. (2)
- ATP has ribose and DNA nucleotide has
deoxyribose;
2. ATP has 3 phosphate (groups) and DNA
nucleotide has 1 phosphate (group);
3. ATP – base always adenine and in DNA
nucleotide base can be different / varies;
The enzymes DNA helicase and DNA polymerase are involved in DNA replication.
Describe the function of each of these enzymes. (2)
- DNA helicase – (unwinding DNA and)
breaking hydrogen bonds / bonds between
chains / bases / strands; - DNA polymerase – joins (adjacent)
nucleotides OR forms phosphodiester bond /
sugar-phosphate backbone;
After Watson and Crick proposed the model of DNA structure, scientists
investigated the possible mechanisms for DNA replication.
Two scientists grew a bacterial population, providing them with a nitrogen source
containing only the heavy isotope of nitrogen, 15 N. As soon as all the DNA in this
population contained 15 N, the scientists changed the nitrogen source to one
containing only the lighter isotope of nitrogen, 14 N. They changed the nitrogen
source at 0 hours.
During the investigation, the scientists measured the size of the population of
bacterial cells.
Correct answer for 2 marks, 0.8376308/0.84/0.8 (hours);;
Accept for 1 mark,
Evidence of 4 x 106 and 3 x 109 (written in any format, for correct readings
from graph)
OR
Evidence of 9.550746785 (correct number of generations)
OR
Evidence of 1.1938443348 (correct generations/ hour)
OR
Evidence of 50.26 (correct generation time in minutes)Evidence of 9.380821784 (correct calculation of number of generations)
OR
Evidence of 1.172602723 (correct calculation of generations/ hour)
OR
Evidence of 51.16822503 (correct calculation of generation time in
minutes)
OR
Evidence of 0.8528037505 (correct calculation of generation time in hours)
Incorrect reading of graph, 106.4 and 109.3
OR 106.3 and 109.2
Accept for 1 mark, calculation carried out correctly
Evidence of 9.633591475 (correct calculation of number of generations)
OR
Evidence of 1.204198934 (correct calculation of generations/ hour)
OR
Evidence of 49.82565445 (correct calculation of generation time in
minutes)
OR
Evidence of 0.8304275742 (correct calculation of generation time in hours)
OR
Evidence of 0.83, with no other working
- (Model) Q and
(Name) Semi-conservative (replication);
Explanation
2. (Model) P (is unsupported because)
There should be two peaks in generation 1
OR(Only) one peak is shown in generation 1
OR
There should be 3:1 (ratio) of peaks in generation 2
OR
There should not be an intermediate/15N 14N peak in generation 1/2/3
OR
The original/generation 0/15N peak should be in generation 1/2/3;
3. (Model) R (is unsupported because)
There should be >2 peaks in generation 2/3
OR
There should be one wide/overlapping peak in generation 3;
Describe the structure of DNA. (5)
- Polymer of nucleotides;
Accept ‘Polynucleotide’
Accept for ‘phosphate’. phosphoric acid - Each nucleotide formed from deoxyribose, a phosphate (group) and
an organic/nitrogenous base; - Phosphodiester bonds (between nucleotides);
- Double helix/2 strands held by hydrogen bonds;
- (Hydrogen bonds/pairing) between adenine, thymine and
cytosine, guanine;
Describe how a phosphodiester bond is formed between two nucleotides
within a DNA molecule. (2)
- Condensation (reaction)/loss of water;
- (Between) phosphate and deoxyribose;
- (Catalysed by) DNA polymerase;
Draw and label a single DNA nucleotide.
Use your knowledge of semi-conservative replication of DNA to suggest:
1. the role of the single-stranded DNA fragments
2. the role of the DNA nucleotides. (3)
Role of single-stranded DNA fragments
1. Template;
2. Determines order of nucleotides/bases;
Role of DNA nucleotides
3. Forms complementary pairs / A – T, G - C
OR
Forms complementary (DNA) strand;
A scientist replicated DNA in a test tube. To do this, he mixed an enzyme with
identical single-stranded DNA fragments and a solution containing DNA
nucleotides.
(b) Name the enzyme used in this DNA replication.
DNA polymerase;
Name the two scientists who proposed models of the chemical structure of
DNA and of DNA replication.
Watson and Crick
OR
Crick and Watson;
Describe the role of DNA polymerase in the semi-conservative replication
of DNA. (2)
- Joins (adjacent DNA) nucleotides;
Reject suggestions that it forms hydrogen bonds or
joins complementary bases.
Reject ‘nucleotide bases’.
2. (Catalyses) condensation (reactions);
3. (Catalyses formation of) phosphodiester bonds (between adjacent
nucleotides);
In the process of semi-conservative DNA replication, the two strands within
a DNA molecule are separated. Each then acts as a template for the
formation of a new complementary strand.
Describe how the separation of strands occurs. (2)
- DNA helicase;
- Breaks hydrogen bonds between base pairs/ AT and
GC/complementary bases
OR
Breaks hydrogen bonds between polynucleotide strands;
Correct answer for 2 marks = 70;;
Accept for 1 mark,
A = 42 and T = 42
OR
420 (total bases in gene)
OR
210 (bases in template strand)
