Regulation of Gene Expression in Prokaryotes and Eukaryotes - Genetics

Out of the choices, only methylation of histones is commonly associated with the silencing of genes. Proteins known as histone methyltransferases bind a methyl group to amino acids in the histone, most commonly lysine or arginine. The result is a change in chromatin structure, most commonly blocking transcription sites and preventing expression.

Acetylation of histones is often found in activated genes. Phosphorylation of histones has been seen in DNA regulation, but it is unclear whether or not this modification affects the expression of genes.

Topoisomerases unwind supercoiling of DNA by breaking and rejoining DNA chains.

DNA polymerase III has these two functions:

1. 5'-3' polymerase requiring a 3' hydroxide primer and a DNA template

2. 3'-5' exonuclease proofreading

Both DNA polymerase I and DNA polymerase III are prokaryotic only. DNA polymerase I excises RNA primers with a 5' to 3' exonuclease.

5'-3' exonuclease removal of primers by DNA polymerase I.

DNA polymerase I is prokaryotic only, it degrades RNA primer and fills in the gap with DNA.

Nucleotide excision repair (NER) is used to repair thymine dimers, which are caused by ultraviolet damage. It also repairs bulky DNA adducts caused by carcinogens.

Base excision repair (BER) requires glycosylases.

BER steps:

1. Glycosylases recognize incorrectly paired or damaged bases and 'flip' them out of the DNA chain without disrupting the phosphodiester backbone. The N-glycosidic bond of the flipped base is cleaved, leaving an AP site (site without a base).

2. AP site is removed by AP endonucleases and an AP lyase.

3. DNA polymerase I (prokaryotes) or DNA polymerase beta (humans) replaces the gap with a new base and DNA ligase seals the strand.

DNA control elements are contained within the DNA helix.

DNA control elements:

1. TATA box: 25-35 basepairs (bps) upstream from start site, determines site of transcription and directs RNA polymerase II binding

2. Proximal promoter elements: 200 bps upstream of start and are roughly 20bps long

3. Enhancers are short regions of DNA that can be 50-1500bp long. They can be bound by activators to increase transcription. Can be far from the site of transcription and still be functional

NOT DNA control elements:

Transcription factors: bind to DNA control elements to influence transcription but are not considered control elements themselves.

Transcriptional repressors and activators: proteins coded by one gene that act to regulate transcription

Ultraviolet light causes thymine dimers in DNA.

Alternative splicing can:

1. Retain/remove exons.

2. Retain/remove introns.

3. Truncate/extend at 5' or 3' ends.

4. Have mutually exclusive exons (one or the other, but never both).

The correct order of the processes that initiate transcription are as follows:

1. Polymerase binds to promoter sequence. At this point, the DNA is still bound together.

2. Polymerase melts DNA near transcription site. Hydrogen bonds are broken, opening the DNA.

3. Polymerase catalyzes phosphodiester bond linkage of initial 2 rNTPs (ribonucleotide triphosphates).

During elongation, RNA polymerase advances in the 3'-5' direction down the template, melting and adding rNTPs to growing RNA

The lac operon expresses enzymes used to break down lactose. It would be a pointless waste of energy to make those enzymes if there is no lactose present, or if there was glucose present — as glucose is a preferable source of energy to E. coli. High concentrations of cAMP signify low levels of glucose. cAMP binds to CAP, the activator of the lac operon, allowing CAP to bind to the promoter and then bind RNA polymerase to begin transcription. However, in the absence of lactose, there is also a repressor protein present that will block RNA polymerase from binding to the promoter. Lactose binds to the repressor and inactivates it, causing it to detach from the promoter and allow RNA polymerase to bind. Therefore, the expression of the lac operon is induced by the presence of cAMP and lactose.

By definition, an autosome is any chromosome that is not a sex chromosome. Humans have a diploid genome that consists of 22 pairs of autosomal chromosomes, and one special pair of chromosomes (which is nonautosomal) that determines gender (two X chromosomes = female, one X and one Y chromosome = male).

An important concept of genetics is that DNA contains the blueprint of all genetic information. It is the sequence of DNA that determines an organism's genetic code.

All of the choices are correct. During processing of mRNA, the introns are removed and the exons are retained during splicing. A 5' guanosine cap and a 3' poly-A-tail is also inserted.

The purpose of mRNA processing is to protect it from degradation during its transport to cytosol. It does not involve introns or uracil. In addition, this concept has nothing to do with being double-stranded or single-stranded. Also, since the question states mRNA is being processed, it would imply that this takes place in a eukaryotic organism and hence, simultaneous transcription and translation is not possible.