Molecular Biology Exam Questions
and answers graded A+
The sequences of promoters tend to be rich in A and T residues. Suggest why this is so. -
ANS✅✅The promoter is a site for polymerase loading and initiation of transcription. To accomplish
this, the RNA polymerase must form an open complex in which the two DNA strands are separated
over a short distance. Due to the effects of AT-rich versus GC-rich DNA on the overall thermal
stability of DNA, the strand separation is more readily accomplished in sequences that are AT-rich.
The -10 and -35 sequences in bacterial promoters are separated by about two turns of the double
helix. How would transcription be affected if a deletion were introduced in the promoter region that
moved the -35 sequence to the -29 position? - ANS✅✅The deletion would move the -35 sequence
closer to the -10 sequence by half a helical turn of the DNA, putting the two elements on opposite
faces of the DNA duplex. This would dramatically reduce binding of sigma factor to the promoter,
thereby decreasing transcription efficiency.
The gene encoding the E. Coli enzyme B-galactosidase begins with the sequence
ATGACCATGATTACG. What is the sequence of the mRNA transcript specified by this part of the
gene? - ANS✅✅5' - AUGACCAUGAUUACG. The sequence reported for a gene is, by convention,
that of the coding strand, and sequences are always written in the 5' - 3' direction.
The sequence of the consensus -10 region is TATAAT. If two genes, tesA and tesB, have identical
promoter sequences except in the -10 region, where the tesA sequence is TAATAT and the tesB
sequence is TGTCGA, which gene do you expect to be more efficiently transcribed, and why? -
ANS✅✅Assuming the two genes use similar transcription initiation modes, tesA will be more
efficiently transcribed. The tesB -10 sequence deviates more from the consensus sequence, and its
higher in GC content will be more difficult to melt.
Gene A encodes protein A. A genetic engineer excises a promoter sequence for gene A from the DNA
and reinserts it at the other end of gene A, oriented so that an RNA polymerase binding at the
promoter will transcribe across gene A. Will the mRNA synthesized by the RNA polymerase still
possess a sequence that produces a functional protein A? Why or why not? - ANS✅✅No. The two
strands of a DNA molecule are antiparallel and complementary (not identical). When the promoter is
inverted, it will direct RNA synthesis that uses what was originally the coding strand as the template
strand. The mRNA sequence, derived from a different DNA strand and synthesized in the opposite
direction, would be very different from the mRNA produced by the original gene and might not even
contain an open reading frame.
In most organisms, specialized DNA repair systems are closely linked to transcription. Suggest a
biological rationale for this close relationship. - ANS✅✅Errors in the genetic information in actively
and answers graded A+
The sequences of promoters tend to be rich in A and T residues. Suggest why this is so. -
ANS✅✅The promoter is a site for polymerase loading and initiation of transcription. To accomplish
this, the RNA polymerase must form an open complex in which the two DNA strands are separated
over a short distance. Due to the effects of AT-rich versus GC-rich DNA on the overall thermal
stability of DNA, the strand separation is more readily accomplished in sequences that are AT-rich.
The -10 and -35 sequences in bacterial promoters are separated by about two turns of the double
helix. How would transcription be affected if a deletion were introduced in the promoter region that
moved the -35 sequence to the -29 position? - ANS✅✅The deletion would move the -35 sequence
closer to the -10 sequence by half a helical turn of the DNA, putting the two elements on opposite
faces of the DNA duplex. This would dramatically reduce binding of sigma factor to the promoter,
thereby decreasing transcription efficiency.
The gene encoding the E. Coli enzyme B-galactosidase begins with the sequence
ATGACCATGATTACG. What is the sequence of the mRNA transcript specified by this part of the
gene? - ANS✅✅5' - AUGACCAUGAUUACG. The sequence reported for a gene is, by convention,
that of the coding strand, and sequences are always written in the 5' - 3' direction.
The sequence of the consensus -10 region is TATAAT. If two genes, tesA and tesB, have identical
promoter sequences except in the -10 region, where the tesA sequence is TAATAT and the tesB
sequence is TGTCGA, which gene do you expect to be more efficiently transcribed, and why? -
ANS✅✅Assuming the two genes use similar transcription initiation modes, tesA will be more
efficiently transcribed. The tesB -10 sequence deviates more from the consensus sequence, and its
higher in GC content will be more difficult to melt.
Gene A encodes protein A. A genetic engineer excises a promoter sequence for gene A from the DNA
and reinserts it at the other end of gene A, oriented so that an RNA polymerase binding at the
promoter will transcribe across gene A. Will the mRNA synthesized by the RNA polymerase still
possess a sequence that produces a functional protein A? Why or why not? - ANS✅✅No. The two
strands of a DNA molecule are antiparallel and complementary (not identical). When the promoter is
inverted, it will direct RNA synthesis that uses what was originally the coding strand as the template
strand. The mRNA sequence, derived from a different DNA strand and synthesized in the opposite
direction, would be very different from the mRNA produced by the original gene and might not even
contain an open reading frame.
In most organisms, specialized DNA repair systems are closely linked to transcription. Suggest a
biological rationale for this close relationship. - ANS✅✅Errors in the genetic information in actively
