2018 DNA STQ MS

2018 DNA Structure and Replication STQ MS 2018 / H2 / DHS PRELIM / P2 Q5Question 1(a)ref hydrogen bonds Adenine always base pair with Thymine and Cytosine always base pair with Guanine and vice versa / a pyrimidine on one strand always base-pairs with a purine on the other strandThus width between the 2 sugar-phosphate backbones is constant / maintain 2nmComplementary DNA strands are antiparallel, thus A-T and G-C pairs would be able to maintain their hydrogen bonds/ antiparallel DNA is also more structurally stable than parallel DNAthere is hydrophobic interactions between stacked bases(b)(i)Peptidyl transferasecatalyzes the formation of peptide bonds between amino acids via condensation reaction;(b)(ii)RNA polynucleotide strands / molecules fold into compact 3D shapeforming an active site with a specific shape The shape is maintained by hydrogen bonds between ribonucleotides ad interactions with ribosomal protein(c)It prevents the translocation of ribosomes along the mRNA by binding to the empty “E” site, and so only dipeptides are formed

2018 / H2 / EJC PRELIM / P2 Q42Fig. 4.1 shows part of a DNA molecule. Fig. 4.1(a)(i)Name W to Z. [2]W ………………………………………………………U ……………………………………………………….X ……………………………………………………….Z ……………………………………………………….1.W = Deoxyribose (R: sugar, ribose, pentose)2.U = Phosphate group (R: phosphate head)3.X = Cytosine (R: nitrogenous base, pyrimidine)4.Z = Hydrogen bond4 correct = 2 marks, 2-3 correct = 1 mark, less than 2 correct = 0 marks(ii)Give one advantage of DNA having two strands.………………………………………………………………………………………………………………………………………………………………………………………………………[1]1.Collective/numerous hydrogen bonds between the complementary base pairs stabilthe double helical structure/molecule;2.One strand act as a template for repair during DNA replication;

3.Protects / shields the hydrophobic nitrogenous bases from the hydrophilic medium;(b)Fig. 4.2 shows a linear chromosome undergoing the first round of DNA replication. Fig. 4.2(i)On Fig. 4.2, draw the direction of DNA synthesis for the leading strands using solid arrows () and for the lagging strands using dashed arrows (- - - >). [1] 1.All directions of leading and lagging strands (Okazaki fragments) correctly indicated;2.Lagging strands must show several Okazaki fragments in correct directions;Points 1 and 2 must be present for full credit. Not marking for 5’/ 3’ labels, but arrow heneeded.(ii)State two ways DNA replication is different from transcription.

…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………[2]DNA ReplicationTranscriptionDNA polymerase used to polymerise nucleotides/synthesize new strands;RNA polymerase used to polymerise nucleotides/synthesize new strandsHelicase used to unzip DNA strands;RNA polymerase unzip DNA strandsBoth parental strands used as templates;Only one strand used as templateDNA strands produced;mRNA/tRNA/rRNA strand producedDeoyribonucleotide monomers;Ribonucleotide monomers Daughter DNA molecule double-stranded;mRNA molecule single strandedAVP;AVP(c)Fig. 4.3 is an electronmicrograph showing the transcription of genes for ribosomal RNAs at adjacpositions along the chromosome in a eukaryotic cell. Fig. 4.3(i)Describe and explain the pattern of transcription visible on the part of the DNA coding for ribosomRNA, labelled X in Fig. 4.3.…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………[2]1.The pattern shows shorter strands of rRNA emerging on the left and with longer strands

rRNA seen on the right;2.This is because RNA polymerase transcribes DNA from 3’ to 5’ direction/ synthesisrRNA from 5’ to 3’ direction, hence the rRNA strands on the right (near 5’ end of Dtemplate) were the longest (transcription started earlier); (ora)(ii)Suggest possible roles for the non-transcribed DNA that is found between the ribosomal genes.…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………[2] [Total:10]1.The non-transcribed DNA may be enhancer sequences which are bound by activators which increase the transcription rate;2.The non-transcribed DNA may be silencer sequences which are bound by repressors which decrease the transcription rate;3.The non-transcribed DNA may be promoter sequences which are bound by general transcription factors and RNA polymerase bind to form transcription initiation complex to start transcription;