Question

1. A temperature sensitive e.coli is discovered that accumulates okazaki fragments and doesnt complete replication when grown at elevated temperatures. Which enzyme and or enzyme function do you suspect has been mutated?

2. Briefly describe the different strategies used by prokaryotes vs. eukaryotes to condense their genome.

Answer 1

Sol. 1) The mutated enzyme must be DNA Ligase.

This is because Okazaki Fragments are formed on the lagging stand during DNA polymerization. The function of DNA ligase here is to link the Okazaki Fragment's backbone to that part of the DNA which has RNA primer. Thus, a long continuos strand is formed. Without the proper functioning of DNA ligase, the fragments would be accumlated and the strand won't be finished.

2)

• Prokaryotes - The DNA is twisted by what is known as supercoiling. Supercoiling means that DNA is either under-wound (less than one turn of the helix per 10 base pairs) or over-wound (more than 1 turn per 10 base pairs) from its normal relaxed state. Some proteins are known to be involved in the supercoiling; other proteins and enzymes such as DNA gyrase help in maintaining the supercoiled structure.
• Eukaryotes - DNA is wrapped around proteins known as histones to form structures called nucleosomes. The histones are evolutionarily conserved proteins that are rich in basic amino acids and form an octamer. The DNA is wrapped tightly around the histone proteins. This nucleosome is linked to the next one with the help of a linker DNA. This is further compacted into a 30 nm fiber, which is the diameter of the structure. At the metaphase stage, the chromosomes are at their most compact, are approximately 700 nm in width, and are found in association with scaffold proteins.