DNA GENETICS QUESTION 1 Most of the DNA

DNA GENETICS

QUESTION 1 • Most of the DNA of a human cell is contained in the nucleus. Distinguish between unique and highly repetitive sequences in nuclear DNA. 5 marks

QUESTION 1 - ANSWERS • U=Unique sequences; H= Highly repetitive sequences • U: occur once in genome; H: occur many times; • U: long base sequences; H: short sequences/5– 300 bases; • U: (may be) genes; H: not genes; • U: (may be) translated/coding sequences; H: never translated; • U: small differences between individuals; H: can vary greatly; • U: exons (are unique sequences); H: introns (may be repetitive); • U: smaller proportion of genome; H: higher proportion of genome; • satellite DNA is repetitive; • repetitive sequences are used for profiling; • prokaryotes do not (usually) contain repetitive sequences;

QUESTION 2 • Draw a labelled diagram to show four DNA nucleotides, each with a different base, linked together in two strands. 5 marks

QUESTION 2 - ANSWERS • Award 1 for each of these structures clearly drawn and labelled. • four nucleotides shown in diagram with one nucleotide clearly labelled; • base, phosphate and deoxyribose (shown as pentagon) connected between the • correct carbons and labelled at least once; • backbone labelled as covalent bond between nucleotides correctly shown as 3 to 5 bond; • two base pairs linked by hydrogen bonds drawn as dotted lines and labelled; • two H bonds between A and T and three H bonds between C and G; • adenine to thymine and cytosine to guanine; do not accept initials of bases • antiparallel orientation shown;

QUESTION 3 • Explain the structure of the DNA double helix, including its subunits and the way in which they are bonded together. 8 marks

QUESTION 3 - ANSWERS • subunits are nucleotides • two strands (of nucleotides) linked together • one base, one deoxyribose and one phosphate in each nucleotide • base to base • description/ diagram showing base linked to deoxyribose C 1 and phosphate to C 5 • four different bases - adenine, cytosine, guanine and thymine • nucleotides linked up with sugar-phosphate bonds • covalent/ phosphodiester bonds • A to T and G to C • hydrogen bonds between bases • antiparallel strands • double helix drawn or described

QUESTION 4 • Outline the structure of the nucleosomes in eukaryotic chromosomes. 4 marks

QUESTION 4 - ANSWERS • contain histones • eight histone molecules form a cluster in a nucleosome • DNA strand is wound around the histones • wound around twice in each nucleosome • (another) histone molecule holds the nucleosome(s) together

QUESTION 5 • State a role for each of four different named enzymes in DNA replication. 6 marks

QUESTION 5 - ANSWERS • Award 1 mark for any two of the following up to 2 marks maximum. • helicase • DNA polymerase / DNA polymerase III • RNA primase • DNA polymerase I • (DNA) ligase

QUESTION 5 - ANSWERS • Award 1 mark for one function for each of the named enzymes. helicase • splits/ breaks hydrogen bonds/ uncoils DNA/ unwinds DNA • (DNA) polymerase III adds nucleotides (in 5' to 3' direction) extending existing strand • (RNA) primase synthesizes a short RNA primer (which is later removed) on DNA • (DNA) polymerase I replaces RNA primer with DNA • (DNA) ligase joins Okazaki fragments/ fragments on lagging strand/ makes sugarphosphate bonds between fragments

QUESTION 6 • Explain the process of DNA replication. 8 marks

QUESTION 6 - ANSWERS • occurs during (S phase of ) interphase/in preparation for mitosis/cell division; • DNA replication is semi-conservative; • unwinding of double helix / separation of strands by helicase (at replication origin); • hydrogen bonds between two strands are broken; • each strand of parent DNA used as template for synthesis; • synthesis continuous on leading strand but not continuous on lagging strand; • leading to formation of Okazaki fragments (on lagging strand); • synthesis occurs in 5 3 direction; • RNA primer synthesized on parent DNA using RNA primase; • DNA polymerase III adds the nucleotides (to the 3 end) • added according to complementary base pairing; • adenine pairs with thymine and cytosine pairs with guanine; (Both pairings required. Do not accept letters alone. ) • DNA polymerase I removes the RNA primers and replaces them with DNA; • DNA ligase joins Okazaki fragments; • as deoxynucleoside triphosphate joins with growing DNA chain, two phosphates • broken off releasing energy to form bond;

QUESTION 7 • Explain how the process of DNA replication depends on the structure of DNA. 9 marks

QUESTION 7 - ANSWERS • DNA molecule is double (stranded) • A=T and C=G • hydrogen bonds linking the two strands are weak/ can be broken • the two original strands therefore carry the same information • DNA can split into two strands • the two new strands have the same base sequence as the two original ones • split by helicase • helicase moves progressively down the molecules • backbones are linked by covalent/ strong bonds • strands do not therefore break/ base sequence conserved • reference to semi-conservative replication • base pairing/ sequences are complementary • the strands have polarity • base/ nucleotides added in 5` to 3` direction • the two strands have opposite polarity • discontinuous segments/ Okazaki fragments added to one strand • DNA ligase needed to connect the segments

QUESTION 8 • Describe the genetic code. 6 marks

QUESTION 8 - ANSWERS • composed of m. RNA 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

QUESTION 9 • Discuss the relationship between genes and polypeptides. 5 marks

QUESTION 9 - ANSWERS • originally assumed one gene codes for one polypeptide • (one) gene is transcribed into (one) m. RNA • m. RNA is translated by a ribosome to synthesize a polypeptide • many exceptions to one gene --> one polypeptide found • • • many more proteins made than there are genes some genes do not code for polypeptides some genes code for t. RNA/r. RNA some genes regulate gene expression genetic information transcribed by eukaryotes is edited before it is translated polypeptides may be altered before they become fully functional proteins

QUESTION 10 • Explain briefly the advantages and disadvantages of the universality of the genetic code to humans. 4 marks

QUESTION 10 - ANSWERS • genetic material can be transferred between species/ between humans • one species could use a useful gene from another species • transgenic crop plants/ livestock can be produced • bacteria/ yeasts can be genetically engineered to make a useful product • viruses can invade cells and take over their genetic apparatus • viruses cause disease

QUESTION 11 • Compare the processes of DNA replication and transcription. 9 marks

QUESTION 11 - ANSWERS • both involve unwinding the helix • only one strand copied not both • both involve spearating the two strands • no ligase/ no Okazaki fragments with transcription • both involve breaking hydrogen bonds between bases • DNA or RNA polymerase • both involve complementary base pairing • both require a start signal • both involve C pairing with G • but this signal is different for each • both work in a 5` --> 3` direction • transcripiton has only one starting point • both involve linking/ polymerization of nucleotides • but replication has multiple starting points • replication with DNA nucleotides and transcritpion with RNA nucleotides • replication gives two DNA molecules whilst transcription gives m. RNA • details of ribose/ deoxyribose difference • adenine pairing with uracil instead of thymine

QUESTION 12 • Distinguish between RNA and DNA. 3 marks

QUESTION 12 - ANSWERS • DNA is double-stranded while RNA is single-stranded; • DNA contains deoxyribose while RNA contains ribose; • the base thymine found in DNA is replaced by uracil in RNA; • one form of DNA (double helix) but several forms of RNA (t. RNA, m. RNA and r. RNA);

QUESTION 13 • Describe the roles of m. RNA, t. RNA and ribosomes in translation. 6 marks

QUESTION 13 - ANSWERS • m. RNA with genetic code/ codons • t. RNA with anticodon • t. RNA with amino acid attached • ribosome with two sub-units • m. RNA held by ribosome • start codon • two t. RNA molecules attached with m. RNA on ribosome • peptide bond between amino acids on t. RNA • polypeptide forms • continues until a stop codon is reached • polypeptide is released

QUESTION 14 • Outline the structure of t. RNA. 5 marks

QUESTION 14 - ANSWERS Accept any of the points above if clearly explained using a suitably labelled diagram • t. RNA is composed of one chain of (RNA) nucleotides • t. RNA has a position/end/site attaching an amino acid (reject t. RNA contains an amino acid) • at the 3' terminal / consisting of CCA/ACC • t. RNA has an anticodon • anticodon of three bases which are not base paired / single stranded / forming part of a loop • t. RNA has double stranded sections formed by base pairing • double stranded sections can be helical • t. RNA has (three) loops (somethimes with an extra small loop) • t. RNA has a distinctive three dimensional / clover leaf shape

QUESTION 15 • Outline the structure of a ribosome. 4 marks

QUESTION 15 - ANSWERS • small subunit and large subunit; • m. RNA binding site on small subunit; • three t. RNA binding sites / A, P and E t. RNA binding sites; • protein and RNA composition (in both subunits);

QUESTION 16 • Explain the process of translation. 9 marks

QUESTION 16 - ANSWERS • translation involves initiation, elongation/translocation and termination; • m. RNA binds to the small sub-unit of the ribosome; • ribosome slides along m. RNA to the start codon; • anticodon of t. RNA pairs with codon on m. RNA: • complementary base pairing (between codon and anticodon); • (anticodon of) t. RNA with methionine pairs with start codon / AUG is the start codon; • second t. RNA pairs with next codon; • peptide bond forms between amino acids; • ribosome moves along the m. RNA by one codon; • movement in 5 to 3 direction; • t. RNA that has lost its amino acid detaches; • another t. RNA pairs with the next codon/moves into A site; • t. RNA activating enzymes; • link amino acids to specific t. RNA; • stop codon (eventually) reached;

QUESTION 17 • Compare DNA transcription with translation. 4 marks

QUESTION 17 - ANSWERS • both in 5` to 3` direction • both require ATP • DNA is transcribed and m. RNA is translated • transcription produces RNA and translation produces polypeptides/ protein • RNA polymerase for transcription and ribosomes for translation/ ribosomes in translation only • transcription in the nucleus (of eukaryotes) and translation in the cytoplasm/ at ER • t. RNA needed for translation but not transcription