Gene Structure and Regulation

1
Gene Structure and Regulation
2
Gene Expression

• The expression of genetic information is one of
  the fundamental activities of all cells.
• Instruction stored in DNA are transcribed and
  processed into RNA molecules
• These RNA molecules have specific roles in how
  the information is translated and expressed as a
  gene product.

3
All cellular life forms have about 60 proteins in
common!

• Most of these proteins are involved in
  translation as ribosomal proteins and tRNA
  enzymes.
• A couple are involved in transcription
• A few are involved in DNA replication and repair.

4
An overview of Gene Structure

Coding Region
3
5

3
5
Start
Stop
Regulatory region
Promoter region
Terminator Region
5
One gene can code for more than one protein!

• The human genome has about 30,000 genes but our
  proteome (the total number of different proteins)
  is much larger.
• How can this occur?
• Many genes can produce more than one protein
  because the mRNA transcript contains different
  combination of exons. This process is called
  alternative splicing.

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The Coding Region in Eukaryotes Contain Introns

• Prokaryotes do not have introns

DNA Template Strand
Intron
Intron
Intron
Exon Exon
Exon
Spliceosome
Pre-mRNA transcript of DNA
Exon Exon
Exon
Introns are spliced out by spliceosomes leaving
only the sequences that will be expressed. This
is an example of RNA processing. The result is a
mature mRNA that will leave the nucleus.
Exon Exon
Exon

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Alternative splicing
Exon 1 Exon 2
Exon 3 Exon 4
Possible mRNAs using different combination of
exons
Exon 1 Exon 2
Exon 3 Exon 4
Exon 1 Exon 2
Exon 4
Exon 2 Exon 3
Exon 4
When each mRNA is translated, a different protein
is produced.
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Some genes are expressed continuously

• These are called housekeeping genes because
  they are required for basic functions of cells
  such as provision of energy, passage of molecules
  across the cell membrane, repair and cell
  division.

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Gene Regulation

• Each somatic cell contains and entire organisms
  genome.
• However, even though the cells in your eye have
  the gene form producing fingernail protein
  (keratin) this gene is not expressed.
• How do genes get switched on and off.

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Types of Genes

• Structural genes
• - these genes express structural and/or
• functional proteins.
• Regulatory genes
• - these genes are short nucleotide
• sequences that express proteins that control
  
• the activity of structural genes by feedback
  
• mechanisms. They are normally found
  upstream from the functional gene.

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An overview of Gene Structure

Coding Region
3
5

3
5
Start
Stop
Regulatory region
Promoter region
Terminator Region
12
Why have cells evolved complex mechanisms to
regulate their genes?

• Cells conserve energy and materials by blocking
  unneeded gene expression.
• If a substrate is absent in the environment why
  produce the enzyme for that substrate!
• Repressor molecules keep the cell from wasting
  energy by making mRNA and enzyme molecules that
  have no use.

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Gene Regulation in Prokaryotes

• Bacteria have groups of genes that are controlled
  together and are turned on/off as required.
• When lactose is added to its growth medium,
  E.coli switches on a gene and make the enzyme B
  galactosidase. This enzyme splits sugar lactose
  to produce the sugars glucose and galactosidase.
• Lactose sugar is rarely encountered by bacteria,
  so the enzyme is not usually produced.
• When lactose is absent a protein attaches to DNA
  and blocks the synthesis of mRNA for B
  galactosidase.
• When lactose is present it binds to the
  repressor. This releases the repressor from the
  DNA so that the gene can be transcribed.

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Lac I P
B - Galactosidase gene
Promoter region
Repressor protein expressed by Lac regulatory gene
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Lac I P
B - Galactosidase gene
Repressor protein occupies the promoter region.
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RNA polymerase
Lac I P
B - Galactosidase gene
When no lactose is present
Transcription cant occur
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Lactose
Lac I P
B - Galactosidase gene
When lactose is present
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Lac I P
B - Galactosidase gene
Lactose removes repressor protein
Transcription can occur
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Lac I P
B - Galactosidase gene
Transcription into mRNA
Translation into an enzyme protein
B-galactosidase
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• The gene is off or on depending on the nutrients
  available to the cell.
• The enzyme is not produced when there is no need
  for it (no lactose substrate)