9.2

In most cases, once a developmental step has been taken by a cell, it is not reversed.

Direct measurements of the transcription rates of multiple genes in different cell types have shown that regulation of transcription, either at the [blank_start]initiation[blank_end] step or during [blank_start]elongation[blank_end] in the [blank_start]promoter-proximal[blank_end] region, is the most widespread form of gene control in eukaryotes, as it is in bacteria.

Transcription from a particular promoter is controlled by DNA-binding proteins that are functionally equivalent to bacterial repressors and activators. However, eukaryotic transcriptional regulatory proteins can often function either to activate or to repress transcription, depending on their associations with other proteins. Consequently, they are more generally called [blank_start]transcription factors[blank_end].

The DNA control elements in eukaryotic genomes to which transcription factors bind are often located much closer from the promoter they regulate than is the case in bacterial genomes.

Transcription-control regions for a conserved gene are also often conserved and can be recognized in the background of nonfunctional sequences that diverge during evolution.

[blank_start]Three[blank_end] different RNA Polymerases catalyze the formation of different RNAs.

All three eukaryotic RNA polymerases contain [blank_start]2[blank_end] (use numbers not text) large subunits and [blank_start]10[blank_end]–[blank_start]14[blank_end] smaller subunits, some of which are common between two or all three of the polymerases.

Not all the subunits are necessary for eukaryotic RNA polymerases to function normally.

The carboxyl end of [blank_start]RPB1[blank_end], the largest subunit of RNA polymerase II, contains a stretch of [blank_start]seven[blank_end] amino acids that is nearly precisely repeated multiple times. Neither RNA polymerase I nor III contains these repeating units. This heptapeptide repeat, with a consensus sequence of Tyr- Ser-Pro-Thr-Ser-Pro-Ser, is known as the [blank_start]carboxy-terminal domain[blank_end]

The primary purpose of gene control in multicellular organisms is the execution of precise [blank_start]developmental[blank_end] programs so that the proper genes are expressed in the proper cells at the proper times during [blank_start]embryologic[blank_end] development and cellular [blank_start]differentiation[blank_end].

Eukaryotes contain three types of nuclear RNA polymerases. All three contain [blank_start]two[blank_end] large and [blank_start]three[blank_end] smaller core subunits with homology to the β′, β, α, and ω subunits of E. coli RNA polymerase, as well as several additional small subunits

RNA polymerase I synthesizes only [blank_start]pre-rRNA[blank_end]. RNA polymerase II synthesizes [blank_start]mRNA[blank_end]s, some of the small nuclear RNAs that participate in mRNA splicing, and micro- and small interfering RNAs (miRNAs and siRNAs) that regulate the translation and stability of mRNAs. RNA polymerase III synthesizes [blank_start]t[blank_end]RNAs, [blank_start]5S[blank_end] rRNA, and several other small stable RNAs

The [blank_start]carboxy-terminal domain[blank_end] (CTD) in the largest subunit of RNA polymerase II becomes [blank_start]phosphorylated[blank_end] during [blank_start]transcription initiation[blank_end] and remains phosphorylated as the enzyme transcribes the DNA template.