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Study Guide

Chapter all: Control of Gene Expression

21 pages197 viewsFall 2016

Department
Biology
Course Code
BIO120H1
Professor
all

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Control of Gene Expression
Many processes are common to all cells, so their genes are present in all the cells of the
body. These include the structural proteins of chromosomes, RNA and DNA polymerases
and many of the proteins that form the cytoskeleton such as actin.
Some RNAs and proteins are found only in specialized cells in which they perform
specific function. For example, hemoglobin is expressed specifically in red blood cells.
A typical human cell expresses 30–60% of its approximately 30,000 genes at some level
at a certain time.
There are about 21,000 protein-coding genes and a about 9000 noncoding RNA genes in
humans.
The level of expression of almost every gene is different from one cell type to another.
Genes in different cell types respond very differently to the same extracellular signal.
Cell regulate the expression of gene and control the production of protein at different
levels. They are:
Transcriptional control: to control when and how often a given gene is transcribed.
RNA processing control: to control the splicing and processing of RNA transcripts.
RNA transport and localization control: to select which completed mRNAs are exported
from the nucleus to the cytosol and determining where in the cytosol they are localized.
Translational control: to selecting which mRNAs in the cytoplasm are translated by
ribosomes
mRNA degradation control: to selectively destabilize certain mRNA molecules in the
cytoplasm,
Protein activity control: selectively activating, inactivating, degrading, or localizing
specific protein molecules after they have been made (Figure 7–5).
The proteins which recognize specific sequences of DNA (typically 5–10 nucleotide pairs
in length) to regulate the function of gene are termed as transcription regulators.
Transcription regulators are often called cis-regulatory sequences, because they bind on
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the same chromosome (that is, in cis) to the genes they control.
Approximately 10% of the protein-coding genes of most organisms are devoted to
transcription regulators, making them one of the largest classes of proteins in the cell
Nearly all transcription regulators make majority of their contacts with the major groove
of the DNA.
Each transcription regulator makes a series of contacts with the DNA. This include
hydrogen bonds, ionic bonds, and hydrophobic interactions.
There are about 20 or so contacts typically formed at the proteinDNA interface and this
interaction is highly specific and very strong (Figure 7–8).
Many transcription regulators form dimers, with both monomers making nearly identical
contacts with DNA (Figure 7–9C).
This arrangement doubles the length of the cis-regulatory sequence recognized and greatly
increases the affinity and the specificity of transcription regulator binding.
Transcription regulators may form heterodimers with more than one partner protein. This
allows the same transcription regulator to be reused to create several distinct DNA-binding
specificities (see Figure 7–9C).
The dimers and heterodimers exist predominantly as monomers in solution, and they are
only observed on the appropriate DNA sequence.
Transcription regulators bind to DNA cooperatively, and the curve describing their
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binding is sigmoidal in shape (Figure 7–10B).
Cooperative binding means that, over a range of concentrations of the transcription
regulator, the cis-regulatory sequence is either nearly empty or nearly fully occupied by
the regulators and never in a state in between.
Nucleosome remodeling, also termed as breathing, alter the structure of the nucleosome,
allowing transcription regulators access to the DNA.
The breathing happens at a much lower rate in the middle of the nucleosome; therefore, the
positions where the DNA exits the nucleosome are much easier to occupy (Figure 7–11).
TRANSCRIPTION REGULATORS SWITCH GENES ON AND OFF
The genome of the bacterium E. coli consists of a single, circular DNA molecule of about
4.6 × 106 nucleotide pairs.
Bacterial DNA encodes approximately 4300 proteins.
In E. coli, five genes code for enzymes that manufacture the amino acid tryptophan.
The genes for tryptophan are arranged in a cluster on the chromosome and are transcribed
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