Biology 1002B Study Guide - Nonsense Mutation, Alternative Splicing, Missense Mutation

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Published on 20 Apr 2013
School
Western University
Department
Biology
Course
Biology 1002B
Professor
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Prokaryotes
-can transcribe and translate a gene
simultaneously
-promoter is immediately upstream of where
transcription begins and RNA polymerase binds
and transcribes, the same RNA polymerase can
transcribe all the various types of genes
-terminators-signal the end of transcription
-act only after they have been
transcribed after which they base
pair w/themselves and form a hair-
pin loop
-translation occurs throughout the cell
-translation initation does not use scanning, but
rather the rRNA ribosomal subunit finds the region
of the start codon directly by base pairing w/ a
specific RIBOSOME BINDING SITE on the mRNA
which is UPSTREAM of the start codon
-elongation stage of translation is faster in
prokaryotes than eukaryotes
-are able to regulate the production of proteins
very quickly in response to changing
environmental conditions since there is nuclear
envelope and so transcription and translation can
be coupled very closely
Eukaryotes
-transcribe and process mRNA in the nucleus
before exporting it to the cytoplasm for translation
-there are different polymerases for transcribing
for transcribing different genes (RNA polymerase
II- protein coding genes, RNA polymerase I and III
are for non-protein coding RNAs), also the
promoters are also immediately pstream of the
transcription point and are more complex than
prokaryotes
-have other sequences further upstream of the
gene which regulate the rate of transcription
-TATA box- key element of the promoter
- important in transcription inititation
- proteins called transcription factors
bind to the TATA box and recruit
polymerase, only once this binding
occurs will the DNA unwind and
transcription begin
-produce pre-mRNA which is processed in the
nucleus to make translatable mRNA which then
leaves the nucleus
-5’cap- at the 5’ end of pre-mRNA
-the initial attachment site for mRNAs to
ribosomes for translation
-clipping sequence- there is no terminator
sequence and so this clipping sequence at the
3’end signals for pre-mRNA to be cleaved at this
point, and this signals the polymerase to stop
transcription, proteins bind here and cleave it
-3’ end remains and poly(A)tail is added which
enables mRNA to be translated efficiently and be
protected from RNA-digesting enzymes
-introns are non-protein coding sequences which
interrupt the protein-coding sequence
-found in the transcription unit of a protein-
coding gene/RNA-coding sequence
-removed from pre-mRNA
-don’t know much of their origin
-exons- amino acid-coding sequences which are
not removed and are still found in mRNA
-mRNA splicing removes introns and joins exons
together
-translation mostly occurs in the cytoplasm (some
in the mitochondria and chloroplast as well)
-translation involves scanning until the start codon
is reached
-most proteins are in active when ribosomes
release them and many proteins require
chaperons to assist in their folding
-other proteins can be activated by the removal of
a covering segment of the amino acid chain and
are subject to certain tgieers which remove a
segment of the amino acids and covert, eg.
enzymes into active forms
mRNA splicing
-occurs in the nucleus, removes introns, joins exons
-occurs in a spliceosomea complex between pre-mRNA and snRNPs(snurps) (small ribonucleoprotein
particles)
-snRNAs base pair w/mRNA at junctions of intron and exons then snRNPs are recruited and loop out of
the intron and bring the two exon ends closer together at this point the spliceosome cleaves the pre-
mRNA at the junction btwn the 5’end of the intron and the adjacent exon and then the intron loops
back to bond w/itself near it’s 3’end
-the catalystic ability in splicing resides in the RNA component of the spliceosomes and not the
proteins
-some introns can even splice themselves
-RNA molecule that is catalystic= a ribozyme
Introns and Protein Variability
-why have introns if we simply remove them? this seems wasteful
-introns may provide a selective advantage to organisms by increasing the coding capacity of existing
genes through a process called alternative splicing
Alternative splicing
-increases the number and variety of proteins encoded in the cell nucleus w/o increasing the size of the
genome
-different proteins can be made in different tissues from the same DNA gene
Signal Peptides a.k.a tags
-the first part of the polypeptide chain
-signal pathway which sends proteins to the ER, other organelles
-nuclear proteins also have tags which are never removed because these proteins need to reenter the
nucleus each time the nuclear envelope is broken down and reformed during cell division cycle
-this basic system of signals in eukaryotes also works in prokaryotes, only thing is that ER-directing
signals direct the proteins to the plasma membrane as BACTERIA DON’T HAVE ER MEMBRANES
Mutations
-base-pair substitution mutations involve a change of one particular base to another in the genetic
material
-missense mutation- the wrong amino acid is placed in the peptide and the function of the polypeptide
is slightly altered o
-nonsense mutation- mutation changes a sense (amino acid-coding) coding to a nonsense (termination)
codon in the mRNA, this results in a premature ‘stop’ and a shorter-than-normal polypeptide
-silent mutation- base-pair substitution which does not alter the amino acid because the changed codon
still codes for the same amino acid
-frame shift mutation- the resulting polypeptide is usually non-functional because of the significantly
altered amino acid sequence
Chapter 15: control of gene expression
Regulation of Transcription in Prokaryotes
operon-cluster of prokaryotic genes and the DNA sequences involved in their regulation
transcription unit- the cluster of genes transcribed into a single mRNA
operator- short segment to which a regulatory protein binds and this protein is encoded by a gene that
is independent of the operon that it controls aka repressor
lacZ, lacy, lacA
lacZ=galactosidease
lacY=permease enzyme (transports lactose actively into the cell)
lacA=transacetylase enzyme, unknown function
positive gene regulation-lactose is present and glucose is absent and so gene products/proteins are
being made
CAP- recruits RNA polymerase at the CAP site- a DNA sequence upstream of the promoter
CAP= an activator stimulates gene expression it is synthesized in its inactive form and is activated
when cAMP binds to it... cAMP binds to CAP then CAP binds to the CAP site which is a signal on the DNA
(a DNA sequence)
-when glucose is absent cAMP is abundant so ACP is active and will bind to the CAP site
-RNA polymerase requires active CAP bound to the CAP site
Regulation of Transcription in Eukaryotes
in prokaryotes gene expression is COMMONLY REGULATED AT THE TRANSCRIPTION LEVEL w/genes
organized into units called OPERONS
-in eukaryotes genes are sually scattered around genomes and not organized into operons
-reuglation of gene expression is more complicated as nuclear DNA is organized w/histones into
chromatin
histones-positively charged proteins which are complexed w/DNA in chromosomes of
eukaryotes, there are 5 types of histones (H1,H2A,H2B, H3, H4) and the amino acid sequences of these
proteins are similar smong eukaryotes
-histones pack DNA, the most fundamental structure= nucleosome
-also eukaryotic nuclear envelope separates the processes of transcription and translation

Document Summary

Promoter is immediately upstream of where transcription begins and rna polymerase binds and transcribes, the same rna polymerase can transcribe all the various types of genes. Act only after they have been transcribed after which they base pair w/themselves and form a hair- pin loop. Translation initation does not use scanning, but rather the rrna ribosomal subunit finds the region of the start codon directly by base pairing w/ a specific ribosome binding site on the mrna which is upstream of the start codon. Elongation stage of translation is faster in prokaryotes than eukaryotes. Are able to regulate the production of proteins very quickly in response to changing environmental conditions since there is nuclear envelope and so transcription and translation can be coupled very closely. Transcribe and process mrna in the nucleus before exporting it to the cytoplasm for translation. There are different polymerases for transcribing for transcribing different genes (rna polymerase.