State three differences between DNA and RNA. (3 marks)
DNA is double stranded / RNA single stranded
• DNA has deoxyribose / RNA has ribose
• DNA has thymine / RNA has uracil
What is DNA? (2 marks)
A polymer made of nucleotides (1)
• Contains genetic information (1)
Describe the structure of a DNA nucleotide. (3 marks
• Phosphate group (1)
• Deoxyribose sugar (1)
• Nitrogenous base (1)
Q: Name the four bases found in DNA. (2 marks)
• Adenine
• Thymine
• Cytosine
• Guanine
Q: What is meant by complementary base pairing? (2 marks
• A pairs with T (1)
• C pairs with G (1)
Q: Why is DNA described as a double helix? (2 marks)
Two strands (1)
• Twisted around each other (1)
Name two enzymes involved in DNA replication. (2 marks
Helicase
• DNA polymeras
State the function of helicase. (1 mark
• Breaks hydrogen bonds to separate DNA strands
State the function of DNA polymerase. (2 marks)
Adds complementary nucleotides (1)
• Forms phosphodiester bonds / works 5’ → 3’ (1)
Why are DNA strands antiparallel? (2 marks)
Strands run in opposite directions (1)
• Allows base pairing and replication (1)
State three differences between DNA and RNA. (3 marks)
DNA is double stranded / RNA single stranded
• DNA has deoxyribose / RNA has ribose
DNA has thymine / RNA has uracil
Name the three types of RNA. (3 marks)
• mRNA
• tRNA
• rRNA
Where does transcription occur? (1 mark)
Nucleus
What is transcription? (2 marks)
• Copying DNA into mRNA (1)
• Using complementary base pairing (1
Where does translation occur? (1 mark
• Ribosome / cytoplasm
What is the role of tRNA? (2 mark
• Carries amino acids (1)
• Matches anticodon to mRNA codon (1
Why is ATP needed in protein synthesis? (2 marks)
Provides energy (1)
• For forming peptide bonds / movement of ribosome (1)
Q: Give two structural differences between DNA and RNA. (2 marks)
• DNA is double stranded; RNA is single stranded
• DNA contains thymine; RNA contains uracil
Q: Explain how the structure of DNA allows it to carry genetic information. (4 marks)
Sequence of bases stores information (1)
• Complementary base pairing allows replication (1)
• Double helix protects bases (1)
• Hydrogen bonds allow strands to separate (1)
Q: Explain why ribosomes are described as catalysts. (2 marks)
Speed up protein synthesis (1)
• Are not used up in the reaction (1)
DNA REPLICATION
Purpose: To produce two identical DNA molecules for cell division.
• Location: Nucleus (eukaryotes).
• Semi-conservative: Each new DNA molecule contains one original strand and one newly synthesized strand.
Steps:
• Initiation:
• DNA helicase unwinds the double helix and breaks hydrogen bonds between complementary bases.
• Formation of a replication fork.
• Primer:
• RNA primase synthesizes a short RNA primer complementary to the DNA template.
• Elongation:
• DNA polymerase III adds nucleotides to the 3′ end of the new strand.
• Leading strand: Synthesized continuously.
• Lagging strand: Synthesized in Okazaki fragments discontinuously.
• Complementary base pairing: A–T, G–C.
• Primer removal & replacement:
• DNA polymerase I removes RNA primers and replaces them with DNA.
• Ligation:
• DNA ligase joins Okazaki fragments on the lagging strand.
• Result: Two identical DNA molecules, each with one old strand and one new strand.
OCR tip: Always mention enzymes, direction of synthesis (5′→3′), and semi-conservative nature.
DNA transcription
Transcription (OCR A-level)
• Purpose: To make a complementary RNA copy of a gene (mRNA for protein synthesis).
• Location: Nucleus (eukaryotes), cytoplasm (prokaryotes).
Steps:
• Initiation:
• RNA polymerase binds to the promoter region of the gene.
• DNA unwinds at the gene region.
• Elongation:
• RNA polymerase moves along the template strand, adding complementary RNA nucleotides.
• Base pairing rules: A–U, T–A, G–C, C–G.
• Termination:
• RNA polymerase reaches a terminator sequence and detaches.
• Pre-mRNA is released.
• Post-transcriptional modification (eukaryotes):
• Splicing: Removal of introns, joining of exons.
• 5′ cap and 3′ poly-A tail added to protect mRNA and assist in export from nucleus.
DNA translation
Purpose: To synthesize a polypeptide (protein) from an mRNA template.
• Location: Cytoplasm, at ribosomes.
Steps:
• Initiation:
• mRNA binds to the small ribosomal subunit.
• tRNA with complementary anticodon binds to start codon AUG.
• Large ribosomal subunit joins to form functional ribosome.
• Elongation:
• Ribosome moves 5′→3′ along mRNA.
• tRNAs bring amino acids to the ribosome.
• Peptide bonds formed between amino acids via peptidyl transferase.
• tRNA leaves after delivering amino acid.
• Termination:
• Ribosome reaches a stop codon (UAA, UAG, UGA).
• Polypeptide is released.
• Post-translational modification (if needed):
• Folding into 3D structure, addition of prosthetic groups, etc.
Extracting DNA from plant tissue
Step 1: Chop plant tissue into small pieces
• Finely chop the plant tissue using a scalpel or knife.
Why this is done:
• Increases surface area, allowing chemicals to act more effectively.
• Makes it easier to break open cell walls and membranes.
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Step 2: Add salt and detergent
• Place the chopped tissue into a beaker and add salt solution and detergent.
Why this is done:
• Detergent breaks down lipid bilayers of the cell membrane and nuclear envelope.
• Salt neutralises negative charges on the DNA phosphate backbone, allowing DNA strands to clump together.
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Step 3: Heat to 60 °C
• Place the mixture in a water bath at 60 °C for several minutes.
Why this is done:
• Denatures DNase enzymes that could break down DNA.
• Helps further disrupt cell membranes.
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Step 4: Cool to ice-cold
• Place the mixture in an ice bath.
Why this is done:
• Prevents further enzyme activity.
• Reduces the risk of DNA degradation.
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Step 5: Liquidise
• Blend or grind the mixture carefully.
Why this is done:
• Physically breaks down remaining cell walls.
• Releases more DNA into the solution.
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Step 6: Filter
• Filter the mixture through gauze or filter paper into a test tube.
Why this is done:
• Removes cell debris such as cell walls and organelles.
• Leaves DNA dissolved in the filtrate.
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Step 7: Add protease enzyme
• Add protease to the filtrate and mix gently.
Why this is done:
• Breaks down proteins (e.g. histones) that are bound to DNA.
• Produces a purer DNA sample.
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Step 8: Add ice-cold ethanol
• Slowly pour ice-cold ethanol down the side of the test tube.
Why this is done:
• DNA is insoluble in cold ethanol, so it precipitates.
• DNA appears as white, stringy strands.
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Step 9: Collect the DNA
• Use a glass rod to spool the DNA from the ethanol layer.
Why this is done:
• DNA molecules are long and stick together, making them visible and easy to collect.
