When a DNA mismatch is detected in E. coli, what mechanism allows the cell to determine which was more likely to be the original nucleotide?

Explanation:

DNA replicates with a remarkably low error rate. Nevertheless, when errors do occur, a mechanism exists to detect and correct them.

DNA polymerases get two chances to incorporate the correct base at a given site. The first chance comes when complementary bases are inserted opposite the bases on the template strand by DNA Pol III according to the base-pairing rules, A with T and G with C. The second chance depends upon a second enzymatic activity of both DNA Pol I and Pol III, called proofreading. In DNA Pol III, a separate protein subunit, DnaQ, performs the proofreading function, whereas in DNA Pol I, a single protein performs both polymerization and proofreading.

Proofreading activity occurs if an incorrect base has been inserted because this creates a mismatch in base pairing. Both DNA Pol I and Pol III possess a 3′ →5′ exonuclease activity that can remove such wrongly inserted nucleotides. The polymerase senses the error because incorrect base pairing causes a slight distortion in the double helix. After the removal of a mismatched nucleotide, the polymerase then gets a second chance to insert the correct nucleotide. The proofreading exonuclease activity is distinct from the 5′ →3′ exonuclease activity of DNA Pol I that removes the RNA primer from both the leading and lagging strands. Only DNA Pol I has this latter activity.

one characteristic that is shared by all is a soft body. many

of these invertebrate animals also have an outer shell to protect their

body. oysters, clams, snails, and mussels are all mollusks. so are octopuses, squids, and slugs. mollusks live on land and in freshwater.

c should be the answer