DNA Replication & Repair
During DNA replication, what ensures the strands are copied in opposite directions?
The antiparallel orientation of DNA strands.
One template runs 5′→3′ and the other 3′→5′, requiring opposite synthesis.
In eukaryotes, which process removes introns from pre-mRNA?
Splicing by the spliceosome.
What would happen if a mutation disabled a bacterial promoter sequence?
RNA polymerase couldn’t bind, so transcription wouldn’t start.
If two species share identical Alu insertions, what does that indicate?
They inherited those insertions from a common ancestor.
What marks a protein for destruction by the proteasome? Can proteins refold?
Attachment of ubiquitin molecules. No
Which enzyme opens the double helix ahead of the replication fork?
Helicase.
Where in the cell are ribosomal RNAs made and assembled?
In the nucleolus.
What is the main advantage of grouping bacterial genes into operons?
It allows several related genes to be regulated together.
What are retrotransposons able to do that simple DNA transposons cannot?
They copy themselves through an RNA intermediate.
What happens to a protein if a point mutation changes a single amino acid?
It may alter folding or activity, depending on the site of change.
Why does the lagging strand form short DNA fragments?
DNA polymerase can only add nucleotides 5′→3′, away from the fork.
What is the role of snRNA in mRNA processing?
snRNAs recognize intron-exon junctions during splicing.
In eukaryotes, what allows a distant enhancer to influence a promoter?
DNA looping brings enhancer-bound proteins near the promoter.
Why are coding DNA sequences usually more conserved than introns?
Changes in coding DNA often harm protein function and are selected against.
What is the purpose of adding a poly-A tail to mRNA?
To stabilize the transcript and aid in nuclear export.
What keeps the replication fork from becoming overwound?
Topoisomerase relieves torsional stress by cutting and resealing DNA.
Why can RNA fold into complex shapes while DNA usually stays helical?
RNA is single-stranded and can base-pair with itself.
How does the trp operon differ from the lac operon?
The trp operon is repressible; lac is inducible.
How did the globin gene family expand during evolution?
Through duplication followed by mutation and specialization.
What distinguishes proteomics from genomics?
Proteomics studies expressed proteins; genomics studies DNA sequences.
Why is telomerase required in eukaryotic cells but not in most bacteria?
Eukaryotic chromosomes are linear; bacteria have circular DNA.
What does the sigma factor do in bacterial transcription?
Guides RNA polymerase to the promoter to start transcription.
When lactose is present, what happens to the lac repressor?
It binds allolactose and releases the operator, allowing transcription.
What does “conserved synteny” mean between two species?
Genes appear in the same order on homologous chromosomes.
Why are histone tails important in chromatin regulation? What's the difference between euchromatin and heterochromatin?
They can be chemically modified to open or close chromatin.
Euchromatin: Less condensed, "open" structure. Transcriptionally active, can be transcribed into RNA and proteins
Heterochromatin: Highly condensed, "closed" structure Transcriptionally silent, gene-poor.