In most organisms, replication of DNA occurs in both directions from an origin,
ID: 134945 • Letter: I
Question
In most organisms, replication of DNA occurs in both directions from an origin, and involves the action of multiple enzymes. For each of the situations described below, predict the consequences for a bacterial cell like E. coli versus a eukaryote like yeast.
a.) The sequence at the origin of replication is mutated so that it is not recognized by the proteins that normally bind it.
b.) The gene for helicase is mutated and a helicase that is inactive at temperatures above 37°C is made.
c.) The gene for DNA polymerase I has a mutation that makes the exonuclease activity of the enzyme very inefficient.
d.) The gene for DNA ligase is mutated so that the DNA ligase enzyme inactive at temperatures above 37°C.
e.) There is only one replication fork per replication origin.
f.) A mutation inactivates telomerase.
Explanation / Answer
a.) The sequence at the origin of replication is mutated so that it is not recognized by the proteins that normally bind it.
In E.coli an AT rich 13mer single origin of replication is present for its single chromosome size of 4.6x106, mutations in it may completely stop the chromosome replication. Where as in yeast having 16 chromosomes and overall genome will have on average 300-400 origin of replications and mutations in all of them is a remote possibility, hence mutation in yeast may affect less severely than E. coli. If all are mutated yeast also severely affected.
b.) The gene for helicase is mutated and a helicase that is inactive at temperatures above 37°C is made.
As both E. coli and yeast can grow optimally at or just below 37 °C mutated helicase does not affect them severely. We should make sure that temperature should not rise above 37 °C
c.) The gene for DNA polymerase I has a mutation that makes the exonuclease activity of the enzyme very inefficient.
In e.coli exonuclease activity domain proof reads the DNA pol I polymerase synthesized DNA, if any mistakes present through exonuclease activity it removes the wrong nucleotide and polymerase activity domain again comes back and places the right nucleotide. If mutation is present for this activity, wrong basepairing happens and consequent replications DNA is mutated. In yeast there is no contemporary enzyme like pol I but polymerase ? and ? having similar DNA exonuclease proof reading and exclusive repair or activity respectively.
d.) The gene for DNA ligase is mutated so that the DNA ligase enzyme inactive at temperatures above 37°C.
As both E. coli and yeast can grow optimally at or just below 37 °C mutated ligase does not affect them severely. We should make sure that temperature should not rise above 37 °C
e.) There is only one replication fork per replication origin.
If this is the case it takes longer time for DNA synthesis (bidirectional DNA synthesis in leading and lagging strand will be absent), E.coli is less severely affected because of smaller genome whereas yeast is severely affected because of its large geneome size compared to e.coli
f.) A mutation inactivates telomerase.
E.coli is not affected because it doesn’t have telomerase enzyme, it does’t required because its having circular chromosome. Whereas in yeast linear dna present on chromosomes, mutation in telomerase severely affects yeast bringing in mutations and sticky chromosomes and it eventually dies without further proliferation.
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