Biol 213 Test 3 Study

How does the 2’ hydroxyl group affect RNA function?

RNA contains a 2’ hydroxyl (-OH) group, which makes it more reactive and less stable. This allows RNA to fold and act as a catalyst but prevents it from serving as stable genetic storage like DNA.

Why does DNA use thymine instead of uracil?

Thymine allows repair enzymes to detect and fix cytosine deamination (which creates uracil). If uracil were normal in DNA, such mutations would be ignored.

What was shown by the Avery-MacLeod-McCarty experiment?

They showed that DNA is the molecule responsible for heredity by demonstrating its ability to transform nonvirulent bacteria into virulent strains.

What is the structure of a nucleosome?

~147 bp of DNA wrapped around a histone octamer (2 each of H2A, H2B, H3, H4). This is the first level of DNA packaging in eukaryotic cells.

How do histone modifications affect gene expression?

Histone tail acetylation often increases transcription by relaxing chromatin, while methylation can either activate or repress gene expression.

What’s the difference between euchromatin and heterochromatin?

Euchromatin is loosely packed and transcriptionally active. Heterochromatin is densely packed and transcriptionally silent.

Why are calico cats almost always female?

Calico coloration arises due to X-chromosome inactivation in females. Different patches express either the orange or black fur allele.

What is the role of helicase in DNA replication?

Helicase unwinds the DNA double helix at the replication fork by breaking hydrogen bonds between base pairs.

What does DNA polymerase III do?

DNA polymerase III adds nucleotides during DNA replication, synthesizing the leading strand and Okazaki fragments of the lagging strand.

Why is DNA replication semiconservative?

Each daughter molecule has one parental strand and one newly synthesized strand. Proven by the Meselson–Stahl experiment.

What is the role of single-stranded DNA binding protein (SSB)?

SSB keeps DNA strands separated during replication by preventing re-annealing.

Why is DNA polymerase I needed during DNA replication?

DNA polymerase I removes RNA primers and replaces them with DNA nucleotides on the lagging strand.

What is the function of telomerase?

Telomerase extends the ends of linear chromosomes using an internal RNA template, preventing loss of genetic material during replication.

How does topoisomerase prevent DNA tangling?

Topoisomerases relieve supercoiling strain during DNA replication by cutting and rejoining DNA strands.

Why are double-stranded DNA breaks dangerous?

They can cause loss of genetic information and chromosomal rearrangements since there is no intact template for repair.

What is the sigma factor in bacterial transcription?

Sigma factor binds to RNA polymerase and recognizes promoter sequences (-10 and -35), initiating transcription in bacteria.

What are general transcription factors?

These proteins help RNA polymerase II bind to promoters and initiate transcription in eukaryotes, including TFIID and TBP.

What are promoters and terminators?

Promoters are DNA sequences that mark the start of transcription; terminators signal RNA polymerase to stop transcription.

What is alternative splicing?

Alternative splicing enables multiple proteins to be made from a single RNA transcript by including or excluding specific exons.

What is the role of the CTD of RNA polymerase II?

The carboxy-terminal domain coordinates mRNA processing by recruiting enzymes for capping, splicing, and polyadenylation.

What is an operon?

An operon is a cluster of bacterial genes under the control of a single promoter, allowing coordinated regulation. Not found in eukaryotes.

How do microRNAs regulate gene expression?

miRNAs bind complementary mRNAs and either degrade them or block translation, providing post-transcriptional regulation.

What is the role of the mediator complex?

The mediator integrates regulatory signals from transcription factors and helps activate RNA polymerase II in eukaryotes.

How does DNA methylation affect transcription?

Cytosine methylation usually represses gene expression by compacting chromatin and blocking transcription factor binding.

What's the difference between positive and negative regulation?

Positive regulation involves activators enhancing transcription. Negative regulation involves repressors blocking transcription.

How is the lactose operon regulated?

The lac operon is repressed by the Lac repressor (in absence of lactose) and activated by CAP when glucose is low; lactose inactivates the repressor.