Topic 1 - DNA and Proteins

Question 1

State what is meant by a nucleic acid? (1 mark)

Answer 1

A macromolecule made up of repeating units called nucleotides

Question 2

What shape does a DNA molecule take? (1 mark)

Answer 2

Double Helix

Question 3

What type of secondary interaction holds the DNA molecule together? (1 mark)

Answer 3

Hydrogen Bonding

Question 4

What three subunits make up a nucleotide? (3 marks)

Answer 4

Phosphate, Sugar, Base

Question 5

What are four differences between DNA and RNA (4 marks)

Answer 5

1. DNA is long / RNA is short
2. DNA is double stranded / RNA is single stranded
3. DNA uses ATCG / RNA uses AUCG
4. DNA uses a deoxyribose sugar / RNA uses a ribose sugar

Question 6

What are three differences between prokaryotic and eukaryotic DNA? (3 marks)

Answer 6

1. Prokaryotic DNA is circular / Eukaryotic is linear
2. Prokaryotic has one chromosome / Eukaryotic have many
3. Prokaryotic DNA is unbound / Eukaryotic is bound to globular proteins called histones

Question 7

Why is DNA replication described as semi-conservative? (2 marks)

Answer 7

Both original strands act as a template for the synthesis of a new strand (1)

Resulting molecule(s) contain one original (conserved) and one newly-synthesised strand (1)

Question 8

What is the role of the helicase enzyme in DNA replication? (2 marks)

Answer 8

Breaks the weak hydrogen bonds between bases (1)

Exposes the inward-facing nitrogenous bases (1)

Question 9

What is a gene? (2 marks)

Answer 9

Sequences of nucleotide bases (1) that code for the production of a protein or RNA molecule (1)

Question 10

What is a protein? (1 mark)

Answer 10

A large molecule, made up of smaller molecules called amino acids (1)

Question 11

How many DNA bases make up a codon? (1 mark)

Answer 11

3

Question 12

Why is the genetic code described as degenerate? (1 mark)

Answer 12

Several nucleotide sequences may code for the same amino acid (1)

Question 13

What are three similarities and three differences between DNA replication and transcription?

Answer 13

Similarities:

1. Both use helicase to break hydrogen bonding (1)
2. Both use a polymerase enzyme to synthesise a backbone (1)
3. Both occur in the nucleus (1)

Differences:

1. Replication copies an entire molecule, transcription copies just one gene (1)
2. Replication adds DNA nucleotides while transcription adds RNA nucleotides (1)
3. Replication occurs before cell division while transcription occurs when a protein is needed (1)

Question 13

What are three similarities and three differences between DNA replication and transcription?

Answer 13

Similarities:

1. Both use helicase to break hydrogen bonding (1)
2. Both use a polymerase enzyme to synthesise a backbone (1)
3. Both occur in the nucleus (1)

Differences:

1. Replication copies an entire molecule, transcription copies just one gene (1)
2. Replication adds DNA nucleotides while transcription adds RNA nucleotides (1)
3. Replication occurs before cell division while transcription occurs when a protein is needed (1)

Question 14

Describe the process of RNA splicing (2 marks)

Answer 14

The premature pre-mRNA molecule folds into segments, exposing the introns (1)

An enzyme called spliceosome ‘cuts’ the introns out, leaving only the exons (1)

Question 15

What is the ribosome made up of?

Answer 15

Ribosomal RNA (rRNA) and Protein

Question 16

What are the respective roles of the two subunits within the ribosome (2 marks)

Answer 16

Small ribosomal subunit reads the mRNA codon (1)

Large ribosomal subunit brings the relevant amino acid to join the polypeptide chain (1)

Question 17

Describe the process whereby a polypeptide chain is assembled from an mRNA molecule (4 marks)

Answer 17

The mRNA lodges between the two ribosomal subunits and the first codon is completely covered by the ribosome (1)

The small ribosomal subunit reads the codon, and a tRNA molecule uses the anticodon to bring a specific amino acid to the ribosome (1)

The large ribosomal subunit attaches the amino acid in place (1)

The previous steps repeat until a stop codon is reached, whereby the polypeptide chain moves to the endoplasmic reticulum for modification (1)

Question 18

Describe the process by which a mature mRNA molecule is formed (5 marks)

Answer 18

The helicase enzyme breaks the weak hydrogen bonding, exposing the inward-facing bases (1)

Free RNA nucleotides join via hydrogen bonding between complementary base pairings (1)

RNA polymerase synthesises a new phosphate-sugar backbone (1)

The RNA strand detaches and exits the nucleus through pores in the nuclear membrane (1)

Non-coding segments of the pre-mRNA are removed via RNA splicing to form a mature mRNA molecule (1)

Question 19

What is the primary structure of a protein? (1 mark)

Answer 19

The sequence of amino acids within the polypeptide chain (1)

Question 20

What is the secondary structure of a protein? (1 mark)

Answer 20

The interactions between side-chains through peptide bonding, forming structures such as alpha helices and beta pleated sheets (1)

Question 21

What is the tertiary structure of a protein?

Answer 21

The unique three-dimensional structure that determines the protein’s biological function

Question 22

What is the quarternary structure of a protein?

Answer 22

The arrangement of polypeptide chains into a folded, functional protein

Question 23

Describe enzyme activity using the induced-fit model (4 marks)

Answer 23

The enzyme’s active site is complementary to the substrate, allowing the formation of an enzyme-substrate complex (1)

This complex induces a fit of higher specificity between the active site and substrate (1)

This induced fit places pressure on the bonds within the substrate, weakening them (1)

Therefore less thermal energy is required to break the bonds within the substrate, lowering the activation energy of the reaction (1)

Question 24

State five factors that affect the rate of an enzyme-catalysed reaction (5 marks)

Answer 24

1. Temperature (1) 2. pH (1) 3. Enzyme concentration (1) 4. Substrate concentration (1) 5. Inhibitor Concentration (1)

Question 25

Describe the role of an inorganic ion in enzyme activity (1 mark)

Answer 25

Inorganic ions bind to the enzyme, altering its charge and supporting its function (1)

Question 26

Describe the role of a prosthetic group in enzyme function (1 mark)

Answer 26

Prosthetic groups bind permanently with the enzyme, supporting its function (1)

Question 27

Describe the role of coenzymes in enzyme function (1 mark)

Answer 27

Coenzymes bind temporarily to the enzyme, supporting its function (1)

Question 28

Describe how a competitive inhibitor inhibits enzyme function (3 marks)

Answer 28

Competitive inhibitors are complementary in shape to the enzyme's active site (1) This allows them to bind to the active site, occupying it (1) Therefore, an enzyme-substrate complex cannot form and the enzyme cannot catalyse the reaction (1)

Question 29

Describe how a non-competitive inhibitor inhibits enzyme function (3 marks)

Answer 29

A non-competitive inhibitor binds to an allosteric site on the enzyme (1) This denatures the enzyme's active site so it is no longer complementary to the substrate (1) Therefore, an enzyme-substrate complex cannot form and the enzyme is inhibited (1)

Question 30

Describe what is meant by silencing and activating genes (2 marks)

Answer 30

Silencing a gene describes preventing it from being transcribed (1), while activating a gene allows it to be transcribed (1)

Question 30

Describe how DNA Methylation prevents gene expression (3 marks)

Answer 30

A methyl group is added to a cytosine base within the gene (1) This prevents RNA polymerase from accessing the gene during transcription (1) Therefore, the gene cannot be transcribed, silencing it (1)

Question 31

State the DNA nucleotide that is most often methylated (1 mark)

Answer 31

Cytosine (1)

Question 32

State what is meant by an epigenetic factor (1 mark)

Answer 32

Epigenetic factors are factors that affect gene expression without changing the DNA nucleotide sequence (1)

Question 33

State what is meant by the term mutation (1 mark)

Answer 33

A change in the DNA nucleotide sequence (1)

Question 34

State what is meant by a missense mutation (1 mark)

Answer 34

A mutation that results in the production of a different amino acid (1)

Question 35

State what is meant by a samesense mutation (1 mark)

Answer 35

A mutation that results in the production of the same amino acid (1)

Question 36

State what is meant by a frameshift mutation (1 mark)

Answer 36

A mutation that results in a shift in all subsequent nucleotide bases (1)

Question 37

State what is meant by a nonsense mutation (1 mark)

Answer 37

A mutation that results in the production of a stop codon (1 mark)

Question 38

State four factors (mutagens) that affect the rate of mutation (4 marks)

Answer 38

1. Mutagenic Chemicals (1) 2. Virus (1) 3. Ionising Radiation (1) 4. Non-ionising Radiation (1)

Question 39

What is a DNA profile?

Answer 39

A series of DNA bands that comprise a unique personal DNA fingerprint