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Gene Expression and Control

Honors Bio Presentation

Christopher Himmelheber

on 19 February 2014

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Transcript of Gene Expression and Control

Gene Expression and Control
Gene Regulation
Overall process of going from DNA to protein
Two basic steps
All processes rely on base pairing between nucleotides
Genetic code
Used to "translate" nucleotide language into amino acid langauge
Read in sets of three nucleotides called codons
One codon = one amino acid
Special codons (
AUG = start
UAA, UAG and UGA = stop
Code is universal (every living thing uses the same code
Cells have three types of RNA
All are made by transcription
Copying the information in DNA into RNA
Prokaryotes (bacteria and archea)
No nuclear membrane
Transcription and translation happen at same time
Genes are grouped together by function
(fungi, protists, plants, animals)
Genes have intervening sequences (introns) that do not code for information

RNA is processed before being expressed as protein
Transcription occurs in three basic steps...
RNA polymerase enzyme binds to promoter region of DNA
RNA polymerase travels along DNA molecule (which direction??) making RNA strand using base pairing rules

Remember, in RNA T = U
RNA polymerase reaches a termination signal at end of gene

RNA polymerase falls off DNA and RNA is released
Converting the information in mRNA into protein
Key Players
RNA Polymerase
Unwinds DNA helix
Builds new RNA molecule
DNA region to be expressed
How does the cell control what genes are expressed?
Operon model of gene control
Example: Lactose operon
Lac operon is an inducible operon (turned on in presence of lactose)

There are also repressible operons (turned off in presence of molecule)
Operon structure
Repressor protein gene: Binds to operator
Promoter: Site of RNA polymerase binding
Operator: Site of active repressor protein binding
Structural genes: Encode proteins for metabolic pathway
Lactose Present
Lactose binds to repressor protein

Repressor protein shifts to inactive shape, does not bind to operator region

RNA polymerase binds to promoter

Genes for lactose breakdown are expressed
Lactose Absent
Repressor protein is in active state

Binds to operator region

Blocks RNA polymerase from transcribing the genes

No expression of lactose breakdown pathway
Many more options for control mechanisms

1. Transcription initiation
2. Alternate mRNAs produced
3. mRNA stability
4. Protein modification
5. Protein degradation
Every protein begins with a methionine amino acid
Start codon (AUG) codes for met
Initiation complex consists of
Met amino acid
Small ribosomal subunit
Initiation complex binds to mRNA molecule
Start codon on mRNA base-pairs with anti-codon on tRNA
Large ribosomal subunit binds
Active ribosome has two tRNA binding sites
A site
P site
Initiator tRNA is positioned at P site
Ribosome is now ready to begin translation
The second amino acid is brought to the ribosome by its tRNA molecule
Enters ribosome at A site
Codon-anticodon base pairing
Catalytic site forms bond between two amino acids
Amino acid chain is now located in A site
P site now has an empty tRNA
Lac operon in action
Don't worry about protein names or functions
Empty tRNA from P site leaves ribsome
Ribosome translocates one codon along mRNA molecule
tRNA with 2 amino acids is now in P site
A site is empty
Next tRNA with amino acid enters empty A site
Codon-anticodon base pairing occurs
Catalytic site forms bond between two amino acids (val and his)
Amino acid chain is now located in A site
P site now has empty tRNA
Process continues until the stop codon is reached
Eventually, a stop codon will enter the A site
No corresponding tRNA
Complex falls apart
Completed protein is released
Key players
tRNA + amino acids
RNA functions
mRNA - carries genetic information from nucleus to ribosomes in cytoplasm

tRNA - brings correct amino acid (protein monomer) to ribosome
- functions as the "translator" molecule
rRNA - structural component of ribosome
Big Questions
How does the cell convert the genetic information in DNA into protein?

How does the cell control what genes are expressed?
The only one you need to remember is AUG = Start
Other Genetic Code Charts
Full transcript