DNA+RNA - Mark Scheme Answers (study Mind)

Question 1

The events that take place during interphase and mitosis lead to the production of two genetically identical cells. Explain how.

Answer 1

1. DNA replicated
2. (Involving) specific / accurate / complementary base-pairing;
3. (Ref to) two identical / sister chromatids
4. Each chromatid / moves / is separated to (opposite) poles / ends of cell

Question 2

The arrows in Figure 2 show the directions in which each new DNA strand is being
produced.
(d) Use Figure 1, Figure 2 and your knowledge of enzyme action to explain why the
arrows point in opposite directions.

Answer 2

1. (Figure 1 shows) DNA has antiparallel strands / described;
2. (Figure 1 shows) shape of the nucleotides is different / nucleotides
   aligned differently;
3. Enzymes have active sites with specific shape;
4. Only substrates with complementary shape / only the 3’ end can bind
   with active site of enzyme / active site of DNA polymerase.

Question 3

Describe the role of DNA polymerase in DNA replication.

Answer 3

Joins nucleotides (to form new strand).

Question 4

(ii) Other than being smaller, give two ways in which prokaryotic DNA is different
from eukaryotic DNA.

Answer 4

1. Circular / non-linear (DNA);
2. Not (associated) with proteins / histones;
3. No introns / no non-coding DNA.

Question 5

Describe the structure of DNA.

Answer 5

1. Polymer of nucleotides;
2. Each nucleotide formed from deoxyribose, a phosphate (group) and
   an organic/nitrogenous base;
3. Phosphodiester bonds (between nucleotides);
4. Double helix/2 strands held by hydrogen bonds;
5. (Hydrogen bonds/pairing) between adenine, thymine and cytosine, guanine;

Question 6

(a) Describe how a phosphodiester bond is formed between two nucleotides
within a DNA molecule.

Answer 6

1. Condensation (reaction)/loss of water;
2. (Between) phosphate and deoxyribose;
3. (Catalysed by) DNA polymerase;

Question 7

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.

Answer 7

DNA helicase;
2. Breaks hydrogen bonds between base pairs/ AT and
GC/complementary bases

Question 8

(c) Name the protein associated with DNA in a chromosome.

Answer 8

Histone

Question 9

(a) Describe the role of DNA polymerase in the semi-conservative replication
of DNA.

Answer 9

1. Joins (adjacent DNA) nucleotides;
2. (Catalyses) condensation (reactions);
3. (Catalyses formation of) phosphodiester bonds (between adjacent
   nucleotides);

Question 10

Name the two scientists who proposed models of the chemical structure of
DNA and of DNA replication.

Answer 10

Watson and crick

Question 11

Name the enzyme used in this DNA replication.

Answer 11

DNA polymerase

Question 12

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.

Answer 12

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

Question 13

Give two features of DNA and explain how each one is important in the
semi-conservative replication of DNA.

Answer 13

1. Weak / easily broken hydrogen bonds between bases allow two strands to separate / unzip;
2. Two strands, so both can act as templates;
3. Complementary base pairing allows accurate replication;

Question 14

Describe the role of two named enzymes in the process of semi-
conservative replication of DNA.

Answer 14

1. (DNA) helicase causes breaking of hydrogen/H bonds (between DNA strands);
2. DNA polymerase joins the (DNA) nucleotides;
3. Forming phosphodiester bonds;

Question 15

Suggest explanations for the results in the table.

Answer 15

1. (Treatment D Antibody binds to cyclin A so) it cannot bind to DNA/enzyme/initiate DNA replication
2. (Treatment E) RNA interferes with mRNA/tRNA/ribosome/polypeptide formation (so cyclin A not made)
3. In Treatment F added cyclin A can bind to DNA/enzyme (to initiate DNA replication)

Question 16

Function of DNA helicase

Answer 16

(unwinding DNA and)
breaking hydrogen bonds / bonds between
chains / bases / strands

Question 17

Function of DNA polymerase

Answer 17

joins (adjacent)
nucleotides OR forms phosphodiester bond /
sugar-phosphate backbone

Question 18

Adenosine triphosphate (ATP) is a nucleotide derivative.
Contrast the structures of ATP and a nucleotide found in DNA to give two differences.

Answer 18

1. 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;

Question 19

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.

Answer 19

1. Reference to DNA polymerase;
2. (Which is) specific;
3. Only complementary with / binds to 5’ end (of strand);
4. Shapes of 5’ end and 3’ end are different / description of how
   different.