DNA

Question 1

Describe the structure of DNA.

Answer 1

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 2

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

Answer 2

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

Question 3

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 3

Breaks hydrogen bonds between base pairs/ AT and
GC/complementary bases

OR

Breaks hydrogen bonds between polynucleotide strands;

Question 4

Name the protein associated with DNA in a chromosome.

Answer 4

Histone

Question 5

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

Answer 5

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

Question 6

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

Answer 6

Watson and Crick

Question 7

Name the enzyme used in this DNA replication.

Answer 7

DNA polymerase

Question 8

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 8

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;

Question 9

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

Answer 9

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 10

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

Answer 10

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

Question 11

Adenosine triphosphate (ATP) is a nucleotide derivative.

Contrast the structures of ATP and a nucleotide found in DNA to give two
differences.

Answer 11

ATP has ribose and DNA nucleotide has
deoxyribose;

ATP has 3 phosphate (groups) and DNA
nucleotide has 1 phosphate (group);

ATP – base always adenine and in DNA
nucleotide base can be different / varies;

Question 12

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.

Answer 12

Reference to DNA polymerase;

(Which is) specific;

Only complementary with / binds to 5’ end (of strand);

Shapes of 5’ end and 3’ end are different