Topic: DNA

subunits are nucleotides one base, one deoxyribose and one phosphate in each nucleotide description/ diagram showing base linked to deoxyribose C1 and phosphate to C5 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

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 particular 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

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

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 (tRNA, mRNA and rRNA);

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

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

Award 1 for each pair of statements in a table, and 1 for any statement below the table. 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;

DNA molecule is double (stranded) hydrogen bonds linking the two strands are weak/ can be broken DNA can split into two strands 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 A=T and C=G the two original strands therefore carry the same information the two new strands have the same base sequence as the two original ones 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

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

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 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 sugar-phosphate bonds between fragments

Accept any of the points above shown on an annotated diagram. 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 3direction; 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;

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

small subunit and large subunit; mRNA binding site on small subunit; three tRNA binding sites / A, P and E tRNA binding sites; protein and RNA composition (in both subunits);

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

both in 5 to 3 direction both require ATP DNA is transcribed and mRNA 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 tRNA needed for translation but not transcription