Biology 1002B Lecture Notes - Lecture 13: Indel, Tryptophan, Reading Frame

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Published on 15 Apr 2013
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Lecture 13: Prokaryotic Gene Regulation
Tom’s lectures this term
o Availability: Drop in: TH@ 1 -3, F @ 2-3 pm RM 301G NCB
o Appointments also by email
o Put “1002B” in subject
Be familiar with chapter 13 and 14
Modern endosymbionts have a “prokaryotic” genetic system
o Mitochondrion and chloroplast has “prokaryotic” genetic systems
Different DNA from nucleus (eukaryotic)
Where is the start and ending codons, which way does translation go, where is the promoter,
where is the SD box, et c.
A typical prokaryotic gene has many types of signals coded in DNA
o The image on the slide
Green part: promoter
Yellow part: SD box
Green: start codon
Red: stop codon
Pink: translation terminator
Which of these signals is transcribed but not translated
o Promoters are not transcribed, they attract the polymerase
o Start codons are transcribed and translated
o Stop codons are transcribed but not translated
o Terminator sequences are transcribed but not translated
What if these signals are changed by mutations?
o What happens to a mutation in promoter
It depends
The RNA polymerase may not recognize the gene or it can increase its efficiency
(positive effect)
o What happens to a mutation in SD box
It depends
It may make it more or less functional
Some effect on the efficiency of translation
Unless the mutation makes a start codon
o What happens to a mutation in start codon
For start codon, all mutations will break it, probably kills the gene, lethal
Cannot make start codon better because its only AUG
o What happens to a mutation in a stop codon
It depends
You can mutate a stop codon into another stop codon
If you turn stop codon into any other codons, then translation will continue until
it reaches the next stop codon
We can find redundant stop codons at the end of the gene to secure a fully stop
o What happens to a mutation in the terminator
It depends
We can enhance the loop, more efficient or make it worse, less efficient
Remember trp (tryptophan UGG)
Substitution mutation may:
o Silent mutation
Mutation that substitute a codon with the different code for the same codon,
no effect at all
o Missense codon
Creating new codons that wasn’t there before, creating a new amino acid
The effect depends, which one is switched for what, close to the original, not
too much effect, different lots of difference
Effect depends on which is switched with which
o Nonsense mutations
Creating a ending codon (stop codon)
More severe because it may end the protein too early, which why they might
not function, for sure not function as the original protein
More severe than missense
Indel mutations can cause a shift in reading frame
o Because of adding or deleting one nucleotide
o Tend to be very severe because all the codons are changed
o Start codon sets the frame of how to read the transcript
o So a shift will change all the codons
o There are three possible reading frames on the template strand
o Can genes overlap?
o Can one mutation affect more than one gene?
What signals would you find in the Trp tRNA gene?
o They only have a promoter and a terminator for transcription, they are not translated
so they have no signals for translations
o Where is the anticodon of Trp on the mRNA and the DNA
On the mRNA it is UGG from 5’ to 3’
The stop codon from 3’ to 5’ is ACC
The mRNA that has been transcribed with the promoter is the 5’ and the
terminator end is 3’
5’ -> 3’ CCA
On DNA 3’ to 5’ is GGT
Functional tRNA base pairs by itself, 2 tRNA forms a functional tRNA
Operons bring several genes under the control of one promoter
o The lac operon is normally off, inducible operon, but with the presence of lactose, it is
turned on and then digest lactose, lac Z,Y,A involves in breaking down of lactose
o Lac I is independent, has its own promoter and it’s called Lac repressor
In absence of lactose, lac repressor binding prevents transcription
o When there’s no lactose, the Lac repressor binds electrostatically onto the operator
Attraction of positive charges on the protein, the negative charge on the DNA,
not covalent not base pair
o Proteins that binds DNA, often binds as a dimer
Creates a loop of double stranded DNA, not a hair pin
Prevents polymerase to transcribe through the DNA loop, through the operon
In presence of lactose, transcription is induced
o Allolactose (inducer) binds onto the repressor and make it inactive
o So transcription continues
o Polycistronic message
1 mRNA codes for 3 genes
PHET lac