Biological Molecules Part 2 Exam Questions

Question 1

What are the differences between DNA and RNA?

Answer 1

DNA RNA
Contains deoxyribose Contains ribose
Contains thymine instead of uracil Contains uracil instead of thymine
Double-stranded Single-stranded
Longer Shorter

Question 2

Describe the structure of DNA. [5]

Answer 2

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 held by hydrogen bonds;
5. (Hydrogen bonds) between adenine, thymine and cytosine guanine.

Question 3

DNA is a polymer of nucleotides. Each nucleotide contains an organic base.

Explain how the organic bases help to stabilise the structure of DNA. [2]

Answer 3

1. Hydrogen bonds between complementary bases hold the two stands together;
2. Many hydrogen bonds provides strength.

Question 4

Humans and grasshoppers have very similar percentages of each base in their DNA but they are very different organisms.

Use your knowledge of DNA structure and function to explain how this is possible. [2]

Answer 4

1. Have different genes;
2. (So) bases are in a different sequence;
3. (So) different amino acid (coded for) / different protein.

Question 5

The DNA of a virus is different from that of other organisms. The percentages of complementary bases in a virus’s DNA don’t match up.

Suggest why this is and explain your answer. [2]

Answer 5

1. No base pairing;
2. (So) DNA is not double stranded / is single stranded.

Question 6

Name the type of bond between:
- Complementary base pairs
- Adjacent nucleotides in a DNA strand [2]

Answer 6

1. Hydrogen (bonds);
2. Phosphodiester (bonds).

Question 7

Describe the process of semi-conservative replication of DNA. [5]

Answer 7

1. DNA helicase unwinds DNA/double helix
   OR DNA helicase breaks hydrogen bonds;
2. Both strands act as templates;
3. (Free DNA) nucleotides line up in complementary pairs/adenine with thymine, cytosine with guanine;
4. DNA polymerase joins nucleotides (of new strand);
5. Forming phosphodiester bonds;
6. Each new DNA molecule consists of one old/original/template strand and one new strand.

Question 8

Describe the role of two named enzymes in the process of semi-conservative replication of DNA. [3]

Answer 8

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

Question 9

Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication. [1]

Answer 9

1. Watson and Crick

Question 10

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.

1. Name the enzyme used in this DNA replication. [1]
2. Use your knowledge of semi-conservative replication of DNA to suggest:
   a. the role of the single-stranded DNA fragments
   b. the role of the DNA nucleotides [3]

Answer 10

1. DNA polymerase;
2. a. Template; determines order of nucleotides/bases;
3. b. Forms complementary pairs OR Forms complementary (DNA) strand.

Question 11

Explain how the data from the Meselson and Stahl Experiment supports the semi-conservative hypothesis of DNA replication after the bacteria was replicated once in the lighter nitrogen medium. [3]

Answer 11

1. 1 template/original/old and 1 new;
2. Complementary base pairing;
3. 2 isotopes in molecule;
4. 1 strand with ‘heavy’ N;
5. 1 strand with ‘light’ N;
6. No molecules with only 1 isotope.

Question 12

Explain why DNA polymerase move in opposite directions to each other in DNA replication. [4]

Answer 12

1. DNA has antiparallel strands;
2. Shape of the nucleotides is different;
3. Enzymes have active sites with specific shape;
4. Only substrates with complementary shape can bind with active site of enzyme

Question 13

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule. [2]

Answer 13

1. Condensation (reaction);
2. (Between) phosphate group and deoxyribose;
3. (Catalysed by) DNA polymerase.

Question 14

Name the protein associated with DNA in a chromosomes. [1]

Answer 14

1. Histone

Question 15

Describe how the separation of strands occurs in the process of semi-conservative DNA replication. [2]

Answer 15

1. DNA helicase;
2. Breaks hydrogen bonds between base pairs
   OR Breaks hydrogen bonds between polynucleotide strands.

Question 16

Describe the role of DNA polymerase in the semi-conservative replication of DNA. [2]

Answer 16

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

Question 17

Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA. [2]

Answer 17

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

Question 18

Adenosine triphosphate (ATP) is a nucleotide derivative.

Contrast the structures of ATP and a nucleotide found in DNA atoms give two differences. [2]

Answer 18

1. ATP has ribose and DNA nucleotide has deoxyribose;
2. ATP has 3 phosphate (groups) and DNA nucleotides has 1 phosphate (group);
3. ATP - base always adenine and in DNA nucleotide base can be different.

Question 19

ATP is useful in many biological processes. Explain why. [4]

Answer 19

1. Releases energy in small/manageable amounts;
2. (Broken down) in a one step reaction so makes energy available rapidly;
3. Phosphorylates so makes (phosphorylated substances) more reactive/lowers activation energy (for reactions);
4. Reformed / made again (in a rapid one step reaction).

Question 20

Describe how an ATP molecule is formed from its component molecules. [4]

Answer 20

1 and 2. Adenine, ribose, three phosphates;
3. Condensation (reaction);
4. ATP synthase.

Question 21

Water is used to hydrolyse ATP.

Name the two products of ATP hydrolysis. [1]

Answer 21

1. Adenosine diphosphate and (inorganic) phosphate.

Question 22

Describe the roles of iron ions, sodium ions and phosphate ions in cells. [5]

Answer 22

Iron ions:
1. Haemoglobin binds with oxygen OR Haemoglobin transports oxygen;

Sodium ions:
2. Co-transport of glucose/amino acids (into cells);
3. (Because) sodium moved out by active transport;
4. Creates a sodium concentration/diffusion gradient;
5. Affects osmosis/water potential;

Phosphate ions:
6. Affects osmosis/water potential;
7. In phosphodiester bond/backbone of DNA/RNA / in nucelotides;
8. Used in/to produce ATP;
9. Phosphorylates other compounds (usually) making them more reactive;
10. Hydrophilic/water soluble part of phospholipid bilayer.

Question 23

Explain how the structure of DNA is related to its function. [6]

Answer 23

1. Sugar-phosphate (backbone)/double stranded/helix so provides strength/stability/protects hydrogen bonds;
2. Long/large molecule so can store lots of information;
3. Helix/coiled so compact;
4. Base sequence codes for amino acids/protein;
5. Double stranded so replication can occur semi-conservatively;
6. (Weak) hydrogen bonds for replication/unzipping/strand separation
   OR Many hydrogen bonds so strong/stable.

Question 24

Why is DNA a polymer? [1]

Answer 24

1. Made up of repeat units/monomers