Chapter 8: Control of gene expression - this is a more sensitive method as it can detect proteins produced in
very small amounts
- All organisms use gene expression; they use genes selectively by
- these techniques have revealed common proteins to all cells
choosing which genes to turn off and which ones to turn on
- known as the housekeeping proteins which include structural
- All cells in an organism have the same genome, using gene
proteins of chromosomes, RNA polymerases, DNA repair enzymes,
expression we achieve cell differentiation
ribosomal proteins and enzymes involved in glycolysis, and many
- Cells get their specialized function depending on what gene is
other proteins present in the cytoskeleton
turned on in that cell
- differentiated proteins also make special proteins that are specific
- Beta cells of pancreas produce insulin and alpha cells make glucagon
to their functions
- B lymphocytes make antibodies and red blood cell make
- gene expression can also be studied by cataloging a cell’s RNA,
haemoglobin
including mRNAs that encode protein
- Typically, a cell only expresses half of its gene
- a typical differentiated cell expresses 5000-15000 protein coding
An overview of gene expression genes out of 21000
- Gene expression is a complex process by which cells selectively A Cell Can Change the Expression of Its Genes in Response to External
direct the synthesis of the many thousands of proteins and RNAs Signals
encoded in their genome
- specialized cells can alter their pattern of gene expression in
- Cell differentiation arises because cells make and accumulate
response to extracellular signals
different sets of RNA and protein molecules; they express different
- exposure of liver cells to steroid hormone cortisol can cause a
genes
dramatic increase in production of several proteins
- cortisol is released by the adrenal glands, during periods of
The Different Cell Types of a Multicellular Organism Contain the Same DNA starvation, stress, and intense exercise
- this signals the liver cells to boost the production of glucose from
- it is proved from experiments where genome from a differentiated amino acids and other small molecules
cell was able to aid in complete development of an organism - proteins that are produced due to cortisol includes enzyme e.g.,
- this means that the nucleotide sequence was not lost in the tyrosine aminotransferase which helps convert tyrosine to glucose
differentiated cell - when the hormone is no longer present, protein level return to
- this shows that DNA in specialized cell types of multicellular normal levels
organisms contain the entire set of instructions needed for a whole - fat cells respond differently to cortisol, the production of tyrosine
organism aminotransferase is reduced
Different Cell Types Produce Different Sets of Proteins - some cells don’t even respond to it
- this is due to cell differentiation
- difference in gene expression between cell types can be seen by
comparing the protein compositions in liver cells, heart and brain
- this was done in the past using gel electrophoresis
- now it is down by mass spectroscopy
, Gene Expression Can Be Regulated at Various Steps from DNA to RNA to - they can be as short as 10 nucleotide sequences, they make simple
Protein switches that respond to a single signal; found mainly in bacteria
- other regulatory sequences can be very long, upto 10000 base pairs
- all the steps in the process of DNA to protein can be regulated
and act as molecular microprocessors that integrate info from
- genes can be regulated by:
variety of signals and dictates how often transcription should take
- controlling when and how often a gene is transcribed
place
- how RNA transcript is spliced or processed
- regulatory DNA sequences need to be recognized by transcription
- which mRNAs are transported out of the nucleus
regulator proteins
- how quickly a certain mRNA is degraded
- once they bind, they act as the switch to control transcription
- which mRNAs are translated into proteins by ribosomes
- bacterias produce several hundred transcription regulators
- how rapidly specific proteins are destroyed after they are
- humans make several 1000
made
- each of these recognizes a different sequence and regulates a
- out of all these steps, control of transcription is the most important
different set of gene
as it prevents from unnecessary intermediates from forming
- proteins recognize specific nucleotide sequences because the
How Transcriptional Switches Work surface of the proteins fits in the DNA double helix of that region
- so because different nucleotide sequences cause different surface
- until 50 years ago, the idea that genes could be switched on and off feature of a DNA double helix, different proteins will bind to them
was revolutionary - mostly, proteins bind into the major groove of the double helix and
- this was found out from studies of who e.coli bacteria adapts to make molecular contacts with the nucleotides in the groove
changes in the composition of their growth medium - each individual contact is weak, but the 10-20 contacts together
- same principles apply to eukaryotes too but because higher ensure that the interaction is specific and strong
organisms are more complex, there are other measures - protein-DNA is one of the strongest and most specific molecular
Transcription Regulators Bind to Regulatory DNA Sequences interaction known in bio
- many transcription regulators bind to DNA helix as dimers
- control of transcription must take place at the step at which the - this doubles the area of contact with the DNA and thus increases
process if initiated the strength and specificity of the protein-DNA interaction
- genes have promoter regions where the RNA polymerase binds and
begins the process of making RNA copy of the gene Transcriptional Switches Allow Cells to Respond to Changes in Their
- promoters have transcription initiation sites where the RNA Environment
synthesis begins - best understood example of gene regulation is in bacteria
- a sequence of 50 nucleotide upstream of the initiation site is - because bacteria have a single circular DNA that has bout 4.6*10 6
required for the RNA polymerase to recognize the promoter nucleotides
- sigma factor in bacteria and GTF in eukaryotes bind to these 50 - this DNA encodes about 4300 proteins
nucleotide site - bacteria regulate their gene expression based on the food sources
- all genes have regulatory DNA sequences that switch on and off a available to them
gene - for example, in E.coli, five genes code for the enzyme that
manufactures amino acid tryptophan
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