1. Draw as simple diagram of the molecular structure of DNA. 5 marks
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 two sugar-phosphate backbones shown
A with T and C with G double helical shape shown antiparallel nature of strands indicated ten base pairs per turn of helix correct hydrogen bonding shown (A=T and C=G)
2. Describe the genetic code. 6 marks
 composed of mRNA base triplets
 called codons
 64 different codons
 each codes for the addition of an amino acid to a growing polypeptide chain
 the genetic code is degenerate
 meaning more than one codon can code for a partiuclar amino acid
 the genetic code is universal
 meaning it is the same in almost all organisms
 (AUG is the) start codon
 some (nonsense) codons code for the end of translation
3. Describe the roles of mRNA, tRNA and ribosomes in translation. 6 marks
 mRNA with genetic code/ codons
 tRNA with anticodon
 tRNA with amino acid attached
 ribosome with two sub-units
 mRNA held by ribosome
 start codon
 two tRNA molecules attached with mRNA on ribosome
 peptide bond between amino acids on tRNA
 polypeptide forms
 continues until a stop codon is reached
 polypeptide is released

4. Explain the process of translation. 9 marks
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 mRNA translated in one direction binding of ribosome to mRNA first/ initiator tRNA binds to start codon
AUG is the start codon second tRNA binds to ribosome amino acid/ polypeptide on first tRNA is transferred/ bonded to amino acid on second tRNA peptide bonds between amino acids movement of ribosome down the mRNA loss of tRNA and new tRNA binds reach a stop codon/ termination polypeptide released tRNA activating enzymes link correct amino acid to each tRNA
(activated) tRNA has an anticodon and the corresponding amino acid attached
5. Explain the structure of the DNA double helix, including its subunits and the way in
which they are bonded together. 8 marks HL
 subunits are nucleotides
 one base, one deoxyribose and one phosphate in each nucleotide
 description/ diagram showing base linked to deoxyribose C1 and phosphate to C6
 four different bases - adenine, cytosine, guanine and thymine
 nucleotides linked up with sugar-phosphate bonds
 covalent/ phosphodiester bonds
 two strands (of nucleotides) linked together
 base to base
 A to T and G to C
 hydrogen bonds between bases
 antiparallel strands
 double helix drawn or described
6. Compare DNA transcription with translation. 4 marks
 both in one direction
 both require energy
 DNA is transcribed and mRNA is translated
 transcription produces RNA and translation produces polypeptides/ protein
 transcription in the nucleus (of eukaryotes) and translation in the cytoplasm/ at ER
 tRNA needed for translation but not transcription

7. Outline how enzymes in the cytoplasm of cells are produced. 8 marks HL
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 synthsized by ribosomes free ribosomes/ribosomes not attached to ER mRNA is translated mRNA binds to the ribosome tRNAs bring amino acids anticodon on tRNA binds to codon on mRNA formation of peptide linkage two tRNAs can bind to the ribosome at once growing polypeptide linked to amino acid on tRNA ribosome moves on down mRNA reference to stop/start codons