Q4. Distinguish between an exonuclease and an endonuclease. (NCERT Exercise 12 (c), page 176.)
Both are nucleases. An exonuclease removes nucleotides from the ends of the DNA; an endonuclease makes cuts at specific positions within the DNA, as restriction endonucleases do at their recognition sequences (NCERT page 166).
Q5. Do eukaryotic cells have restriction endonucleases? Justify your answer. (NCERT Exercise 5, page 175.)
No. NCERT describes restriction enzymes as isolated from strains of bacteria, where they restrict the growth of bacteriophages (page 165). Beyond the NCERT text: these enzymes belong to prokaryotes (bacteria and archaea), which use them to chop up viral DNA that gets in, while a methylating enzyme protects the cell’s own DNA (Wikipedia, “Restriction enzyme”, facts in our words). The restriction system is a defence of prokaryotes, so the expected answer is no (our reasoning from these sources).
Q6. From what you have learnt, are enzymes or DNA bigger in molecular size? How do you know? (NCERT Exercise 3, page 175.)
DNA is far bigger. A restriction enzyme, a protein, ‘inspects’ the length of a DNA molecule and binds to just a six-base-pair stretch of it (NCERT pages 165–166), while DNA runs to millions of base pairs: E. coli has 4.6 × 10 bp (NCERT Section 5.1, page 80), and the 6.6 × 10 bp in a typical mammalian cell would stretch about 2.2 metres (NCERT Section 5.1, page 83). This compares an enzyme with the DNA of a cell; a very short DNA strand, such as a primer, can be smaller than a protein (our reasoning).
Q7. Explain briefly how restriction enzymes act on DNA. (NCERT Exercise 11 (b), page 176.)
A restriction endonuclease finds its specific palindromic recognition sequence, binds to the DNA and cuts each of the two strands in the sugar-phosphate backbone, between the same two bases on the opposite strands, leaving sticky ends; fragments cut by the same enzyme can be joined by DNA ligase to make recombinant DNA (NCERT pages 166–167). Beyond the NCERT text: some enzymes, such as SmaI, leave blunt ends instead (Wikipedia, “Restriction enzyme”, facts in our words).
Q8. What does the origin of replication of a vector control besides the start of replication?
The copy number of the linked DNA: to recover many copies of the target DNA, it should be cloned in a vector whose ori supports a high copy number (NCERT page 169).