Question

Assume that you have discovered a new chemical mutagen that modifies guanine so that it mispairs with thymine when thymine is in the template DNA strand during DNA replication. However, this mispairing is limited to when the modified guanine is being added to the newly replicating DNA strand. When the modified guanine is in the template DNA strand it always pairs normally with cytosine being added to the growing newly synthesized strand. What type of mutation would you predict would be caused by the new chemical mutagen?

G-to-T base substitutions

A-to-G and A to C base substitutions

A-to-T base substitutions

A-to-C base substitutions

A-to-G base substitutions

Answer 1

The discovered chemical mutagen would cause G-to-T base substitutions. This occurs during DNA replication, where modified guanine erroneously pairs with thymine in the template strand, leading to permanent changes in the base pairing after several rounds of replication.

Step-by-step explanation:

The new chemical mutagen that has been discovered causes guanine to mispair with thymine during DNA replication. Specifically, this happens when thymine is in the template strand and the modified guanine is being added to the new strand. However, when the modified guanine is in the template strand, it pairs normally with cytosine. Considering these rules, the chemical mutagen would likely cause G-to-T base substitutions.

Here's why: during DNA replication, guanine (G) typically pairs with cytosine (C). But if the mutagen causes guanine to pair with thymine (T) instead when guanine is part of the new strand, then after replication, the original thymine in the template DNA (which should have bonded with adenine) will have bonded with modified guanine. In subsequent rounds of replication, this abnormal guanine would return to normal function and pair with cytosine, permanently substituting the thymine for cytosine in the complementary strand. This mutagen-induced pairing error leads to a G-to-T substitution.