AQA A Level Biology- Model Answers
1. DNA Replication: DNA helicase breaks hydrogen bonds. Each strand
acts as a template. Free nucleotides attach. Complementary base
pairing occurs- AT and
CG. DNA polymerase joins nucleotides. Hydrogen bonds reform. Semi-
conservative replication- new DNA molecule contains one old and one
new strand
2. Transcription: DNA helicase breaks hydrogen bonds between DNA
strands. One strand acts as a template. RNA nucleotides attract to
exposed complementary bases on DNA. RNA polymerase joins
nucleotides together. Pre-mRNA spliced to remove introns
3. Translation: Ribosomes are sites of protein synthesis. mRNA strand
attaches to ribosome. Each mRNA codon codes for specific amino acid.
Anti-codons on tRNA match up with codon on mRNA and form
complementary base pairs. Peptide bonds formed between adjacent
amino acids. Polypeptide chain formed
4. Protein Structure: Polymer of amino acids joined by peptide bonds
formed by condensation reaction.
• Primary structure is the order of amino acids. •Secondary is folding
,of polypeptide chain due to hydrogen bonding.
• Tertiary structure is polypeptide chain folded into 3D shape due to
hydrogen and ionic bonding and disulphide bridges.
• Quaternary is 2 or more polypeptide chains
5. Protein Digestion in Human Gut: Hydrolysis of peptide bonds.
• Endopeptidases break polypeptides into smaller peptide chains
• Exopeptidases remove terminal amino acids
• Dipeptidases hydrolyse dipeptidases into amino acids
6. DNA Hybridisation/ Immunology: Break hydrogen bonds. Mix DNA
strand of different species. Temperature required to break hydrogen
bonds indicates relation- ship. Compare DNA sequence of nucleotides.
Compare sequence of amino acids. Inject serum into animal. Obtain
antibodies. Add serum from other species. Amount of precipitate
formed indicates relationship
7. Structure of DNA related to its function: Sugar phosphate backbone
providing strength. Long molecule so can store lots of information.
Helical so compact. Dou- ble-stranded so semi-conservative replication
can occur. Weak Hydrogen bonds for unzipping during replication. Many
hydrogen bonds so stable and strong
,8. Vaccines: Vaccine contains dead pathogen. Macrophage presents
antigen on its surface. T cell with complementary receptor protein binds
to antigen. Stimulates
B cells with complementary antibody on surface. B cell secretes
antibodies. They divide to form clones and memory cells. Secondary
exposure, become active and antibodies produced faster. Pathogen
destroyed before toxins produced or cells damaged. Herd effect means
there are fewer people to pass on disease.
, 9. Active and Passive Immunity: •Active involves memory cells whereas
passive doesn't. •Active involves production of antibody by plasma
cells or memory cells. Passive involves antibody introduced into body
from outside.
• Active is long-term as antibody is produced in response to antigen.
Passive is short term as antibody is broken down.
• Active takes time to work whereas passive is fast-acting.
10.Starch and Cellulose Related to Function: Starch:
• has helical shape so compact
• large and insoluble so osmotically inactive
• Branched so glucose easily released for respiration. Large so can't
leave via cell membrane
Cellulose:
• long, straight unbranched chains of ²-glucose
• joined by hydrogen bonding
• forms fibrils
• provides rigidity and strength
11.Enzyme Mutation: Mutation in nucleotide sequence of gene causes
1. DNA Replication: DNA helicase breaks hydrogen bonds. Each strand
acts as a template. Free nucleotides attach. Complementary base
pairing occurs- AT and
CG. DNA polymerase joins nucleotides. Hydrogen bonds reform. Semi-
conservative replication- new DNA molecule contains one old and one
new strand
2. Transcription: DNA helicase breaks hydrogen bonds between DNA
strands. One strand acts as a template. RNA nucleotides attract to
exposed complementary bases on DNA. RNA polymerase joins
nucleotides together. Pre-mRNA spliced to remove introns
3. Translation: Ribosomes are sites of protein synthesis. mRNA strand
attaches to ribosome. Each mRNA codon codes for specific amino acid.
Anti-codons on tRNA match up with codon on mRNA and form
complementary base pairs. Peptide bonds formed between adjacent
amino acids. Polypeptide chain formed
4. Protein Structure: Polymer of amino acids joined by peptide bonds
formed by condensation reaction.
• Primary structure is the order of amino acids. •Secondary is folding
,of polypeptide chain due to hydrogen bonding.
• Tertiary structure is polypeptide chain folded into 3D shape due to
hydrogen and ionic bonding and disulphide bridges.
• Quaternary is 2 or more polypeptide chains
5. Protein Digestion in Human Gut: Hydrolysis of peptide bonds.
• Endopeptidases break polypeptides into smaller peptide chains
• Exopeptidases remove terminal amino acids
• Dipeptidases hydrolyse dipeptidases into amino acids
6. DNA Hybridisation/ Immunology: Break hydrogen bonds. Mix DNA
strand of different species. Temperature required to break hydrogen
bonds indicates relation- ship. Compare DNA sequence of nucleotides.
Compare sequence of amino acids. Inject serum into animal. Obtain
antibodies. Add serum from other species. Amount of precipitate
formed indicates relationship
7. Structure of DNA related to its function: Sugar phosphate backbone
providing strength. Long molecule so can store lots of information.
Helical so compact. Dou- ble-stranded so semi-conservative replication
can occur. Weak Hydrogen bonds for unzipping during replication. Many
hydrogen bonds so stable and strong
,8. Vaccines: Vaccine contains dead pathogen. Macrophage presents
antigen on its surface. T cell with complementary receptor protein binds
to antigen. Stimulates
B cells with complementary antibody on surface. B cell secretes
antibodies. They divide to form clones and memory cells. Secondary
exposure, become active and antibodies produced faster. Pathogen
destroyed before toxins produced or cells damaged. Herd effect means
there are fewer people to pass on disease.
, 9. Active and Passive Immunity: •Active involves memory cells whereas
passive doesn't. •Active involves production of antibody by plasma
cells or memory cells. Passive involves antibody introduced into body
from outside.
• Active is long-term as antibody is produced in response to antigen.
Passive is short term as antibody is broken down.
• Active takes time to work whereas passive is fast-acting.
10.Starch and Cellulose Related to Function: Starch:
• has helical shape so compact
• large and insoluble so osmotically inactive
• Branched so glucose easily released for respiration. Large so can't
leave via cell membrane
Cellulose:
• long, straight unbranched chains of ²-glucose
• joined by hydrogen bonding
• forms fibrils
• provides rigidity and strength
11.Enzyme Mutation: Mutation in nucleotide sequence of gene causes
