nucleotides and nucleic acids

Describe the structure of a nucleotide. (3 marks)

• Pentose sugar (1)

• Phosphate group (1)

• Nitrogenous base (1)

State two differences between DNA and RNA nucleotides. (2 marks)

• DNA has deoxyribose, RNA has ribose (1)

• DNA contains T, RNA contains U (1)

Describe the difference between purines and pyrimidines. (2 marks)

• Purines have two rings (adenine, guanine) (1)

• Pyrimidines have one ring (cytosine, thymine, uracil) (1)

Explain how nucleotides join to form a polynucleotide. (2 marks

• Phosphodiester bond formed (1)

• Between the phosphate and sugar in a condensation reaction (1)

Explain how DNA’s structure allows it to store information. (3 marks)

• Sequence of bases stores genetic code (1)

• Bases attached to a sugar-phosphate backbone for stability (1)

• Complementary base pairing allows accurate replication (1)

State the role of DNA polymerase. (1 mark)

Catalyses formation of phosphodiester bonds between nucleotides (1)

Describe how tRNA is adapted to its function. (3 marks)

• Has anticodon complementary to mRNA codon (1)

• Carries a specific amino acid (1)

• Folded into cloverleaf shape for stability (1)

State two differences between ATP and a DNA nucleotide. (2 marks)

• ATP has three phosphates, DNA nucleotide has one (1)

• ATP contains ribose, DNA nucleotide has deoxyribose (1)

Explain why DNA is more stable than RNA. (2 marks)

• DNA has deoxyribose (less reactive than ribose) (1)

• DNA is double-stranded with hydrogen bonds for protection (1)

Describe how a substitution mutation may affect a protein. (3 marks)

• Changes one base in DNA (1)

• May change codon → different amino acid (1)

• Could alter primary structure and change protein function (1)

A cell uses ATP to join amino acids during protein synthesis.

Q (3 marks):
Explain how ATP enables this process.

• Hydrolysis of ATP releases energy (1)

• Provides energy for forming peptide bonds (1)

• Coupled reaction makes protein synthesis energetically possible (1)

A single base substitution changes a codon from GAG to GAA.
Both codons code for glutamic acid.

Q (2 marks):
Explain why this mutation has no effect on the protein produced.

• Genetic code is degenerate (more than one codon codes for the same amino acid) (1)

• Therefore the amino acid sequence / primary structure remains unchanged (1)

A scientist finds that a newly discovered microorganism has 70% GC content in its DNA.

Q (2 marks):
Suggest how this helps the organism survive in its environment.

• G–C pairs form three hydrogen bonds (1)

• High GC makes DNA more stable, especially in hot / extreme conditions (1)

Active transport relies on ATP generated by cellular respiration.(3 marks):
Explain why cells with high rates of active transport need efficient DNA replication.

• Replication allows rapid cell division → replacement of worn transport cells (1)

• Mitochondria replicate their DNA to maintain ATP supply (1)

• Incorrect replication reduces enzyme production → less ATP → less active transport (1)

Different cells contain identical DNA but express different genes.

(4 marks):

• Different genes are transcribed → different mRNA produced (1)

• Due to different transcription factors in each cell type (1)

• Leads to production of different proteins (1)

• Proteins determine cell structure and function (1)

A gene mutation causes a change in the primary structure of an enzyme.

(4 marks):
Explain how this mutation could affect both enzyme activity and metabolic pathways in the cell.

• Different amino acid → changes hydrogen/ionic bonding (1)

• Alters tertiary structure → active site changes (1)

• Enzyme cannot bind substrate → reduced rate of reaction (1)

• Pathway slows/stops → less product → disrupted cellular function (1)