Section B Answer one question. (a) Describe and explain the properties of plasmids that allow them to be used in gene cloning. [7] (b) Explain the roles of restriction endonucleases, ligases and reverse transcriptase in genetic engineering. [8]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2020). The mark or mark type is shown in brackets.

1. [7] (a) any seven from: 1 double stranded DNA ; 2 have, restriction site / target sequence for restriction enzyme ; 3 allows gene (for cloning) to be inserted ; 4 small; 5 plasmid can, enter/transform, (host) cell / bacterium ; 6 circular; 7 stable; 8 contain, (named) marker genes / (named) genes for antibiotic resistance ; 9 used to identify, GM / transformed / recombinant, (bacterial) cells; 10 replicate, fast / independently (of host cell replication) ; 11 get many copies of cloned gene ; 12 AVP;
2. [8] (b) any eight from: restriction endonucleases 1 cut DNA; 2 at, restriction site / specific (base) sequence / target sequence ; 3 ref. to palindrome ; 4 give, sticky / blunt, ends ; ligases 5 join gene to, plasmid / (named) vector; 6 seal (sugar-phosphate) backbone ; 7 make, phospho(di)ester / covalent, bonds ; 8 make recombinant, DNA / plasmid / vector ; reverse transcriptase 9 makes cDNA ; 10 from mRNA ; 11 ref. to DNA single-stranded initially; 12 AVP;