If a thymine base in the template strand undergoes a tautomeric shift prior to DNA replication, the daughter strand will have a mispaired base incorporated opposite this thymine. If the mismatch is not repaired, a(n) - base pair will be found at this position following the next round of DNA replication.
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5' - ACGCTT - 3' 3' - TGCTAA - 5' The DNA sequence above is a segment of a DNA molecule from a single cell. There is a mismatched base indicated in red that occurred due to a replication error. a) Describe the process of how this mismatch error can be fixed during the process of DNA replication. b) Assuming this mismatch is not fixed prior to completion of DNA replication, describe the process that can fix this error after replication but before cell division. c) Assume this mismatch is not fixed by either method you described in parts a and b. If the DNA sequence given undergoes replication, what will be the DNA sequences in each of the two resulting daughter cells after cell division? Be sure to include directionality labels on your sequences and make it clear which sequence is in which daughter cell by writing "Daughter cell 1" and "Daughter cell 2". d) Will repair mechanisms in either of the daughter cells fix this error? Explain your answer.
Adi S.
Suppose deamination of Cytosine occurs in eukaryotic DNA during the G1 phase of the cell cycle. Using simpler notation: G/C --> G/U Then the cell enters S phase. The products of replication and later repair (if needed) will be: 1. G/C and G/C, and no repair will be necessary in G2. 2. A/T and A/U, and the products of repair in G2 will be A/T and G/C. 3. A/T and A/T, and no repair will be necessary in G2. 4. G/C and G/U, and the products of repair in G2 will be G/C and G/C. 5. G/C and A/U, and the products of repair in G2 will be G/C and A/T.
Suman K.
(a) The most stable tautomeric form of guanine is the lactam form (or cyclic amide, see Section 17.81 ). This is the form normally present in $\mathrm{DNA}$, and, as we have seen, it pairs specifically with cytosine. If guanine tautomerizes (see Section 18.2 ) to the lactim form, it pairs with thymine instead. Write structural formulas showing the hydrogen bonds in this abnormal base pair. CAN'T COPY THE FIGURE (b) Improper base pairings that result from tautomerizations occurring during the process of DNA replication have been suggested as a source of spontaneous mutations. We saw in part (a) that if a tautomerization of guanine occurred at the proper moment, it could lead to the introduction of thymine (instead of cytosine) into its complementary DNA chain. What error would this new DNA chain introduce into its complementary strand during the next replication even if no further tautomerizations take place?
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