Transcript Mutations Tutorial

Gene-Proteins-Mutations
A tutorial exploring how genetic mutations
affect proteins structure and function
Before you begin…
• Read the opening case study (Meeting the Affected Family)
on the first page of lab - Discovering the Genetics and
Molecular Biology of Sickle Cell Anemia .
As you explore this tutorial, think about the question posed
in Part I of the Sickle Cell Anemia-Malaria lab case study:
Meeting the Affected Family:
Why do you think that Dr. Dufall wants a blood sample from
Emily and Chaka, if neither of them has sickle cell anemia? In
other words, what could looking at their hemoglobin tell us
about this genetic condition?
Genes code for polypeptides!
Remember that the hemoglobin protein is comprised of 4
polypeptide chains (2 alpha-globin chains and 2 beta-globin chains).
One gene codes
for the beta globin
polypeptides!
One gene codes
for the alpha
globin
polypeptides!
How do cells use the information encoded in
genes to make proteins…
Cytoplasm
Makes a mRNA copy
(or transcript) of the
gene.
The ends of the mRNA
transcript are modified
to facilitate transport
from the nucleus.
Regions of the mRNA
transcript that don’t
provide instructions for
making a polypeptide
are removed (introns).
The remaining mRNA
sequences (exons) are
spliced back together.
Nucleus
mRNA leaves the
nucleus and
combines with a
ribosome in the
cytoplasm.
The ribosome
“reads” the mRNA
sequence and use
the information to
build a polypeptide
(a chain of amino
acids).
Transcription…in a
little more detail!
First, watch an animation of transcription (click on picture
above) to get a sense of how the whole process proceeds.
Return to this show and continue after viewing the video.
Let’s first examine the structure of
RNA more closely…
DNA RNA
DNA
RNA
Double stranded Single stranded
helix
Nucleotides are:
C,G,A,T
Ribonucleotides
are: C,G,A,U
A DNA
nucleotide:
A RNA
ribonucleotide:
DNA
mRNA
During transcription of mRNA notice that:
• Adenine (DNA) pairs with Uracil (RNA)
• Guanine (DNA) pairs with Cytosine (RNA)
• Cytosine (DNA) pairs with Guanine (RNA)
• Thymine (DNA) pairs with Adenine (RNA)
Initiating
transcription…
RNA polymerase begins to
pry open the DNA and
transcribes the gene using
one of the strands as a
template. Other
transcription factors
facilitate the process and
regulate the expression of
the gene.
New ribonucleotides are
added according to
nitrogenous base pairing
rules (U=A; G=C).
DNA
mRNA Processing
An interesting process happens during
processing. A complex of proteins and a kind of
RNA (snRNA) grabs the mRNA forming
loops…like this!
These complexes (called
splicosomes) chop out
whole sections of mRNA
called intervening
sequences (aka introns)
and splices together the
expressed sequences
(aka exons).
From The Cartoon Guide to Genetics by Larry Gonick
Spliceosomes…
Sequences of snRNA
associated with proteins
that recognize introns by
binding to specific
sequences of RNA, “chop”
out the intron and splice
the exons together!
mRNA processing…
Introns – intervening sequences that are
not expressed.
“5’ Caps” and “PolyA Tails” are
ribonucleotides that
are added to
facilitate export of
mRNA from the
nucleus and to
prevent the
degradation of
mRNA in the
cytoplasm.
Exons – expressed sequences..these are
translated into a polypeptide.
Translating the
genetic code!
Making
polypeptides
from the
instructions now
encoded in
mRNA!
Click on the picture above to watch an animation of the
process of protein translation to get a sense of how this
process occurs.
Remember that polypeptides
are composed of specific
sequences of amino acids!
The DNA sequence for a gene
determines the amino acid
sequence.
Each circle represents and amino acid whose name is
abbreviated with 3 letters. “Ala” in this case stands
for the amino acid Alanine.
An example:
The insulin polypeptide!
A hormone that regulate blood
sugar levels in humans!
Here are the 20
amino acids that
comprise all
polypeptides!
Notice that it is the
“R-groups”
(highlighted in white)
that differ between
each amino acid.
Also notice that
amino acids have
different chemical
characteristics (nonpolar, polar, and
electrically charged
(- and +))
- charge
+ charge
• The mRNA transcript
is read three “letters”
(aka ribonucleotides)
at a time.
• The triplet is called a
codon.
• Codons act as the code that
tells the ribosome the order in
which to link amino acids
during translation.
The genetic
code!
Notice that the codons are
somewhat “redundant”. That
is, several different codons
can code for the same amino
acid.
Also notice that 3 codons
serve as “STOP” codons.
These signal the end of the
gene!
Table of mRNA codons and the amino
acids for which they code!
A mutation can alter the DNA
sequence for a gene which can
cause a change in the mRNA
sequence which can result in a
change in the amino acid sequence
of the polypeptide.
This can alter the structure and
therefore function of the protein!
Types of mutations…
Normal mRNA from the gene
A base-pair substitution, which
replaces one DNA nucleotide
for another can have different
effects on the polypeptide
depending on the nature of the
mutation.
Just so you know…
These changes occurred as
mutations in the DNA, and
resulted in a change in the
mRNA, depicted in the figure.
Types of mutations…
Frameshift mutations…shift
the reading frame during
translation resulting in
many changes in the amino
acid sequence downstream
of the mutation!
Thought Question…
What effect do you
think each of the
mutations discussed so
far would have on the
structure (size & shape)
and function of the
polypeptide?
Normal mRNA from the gene
Back to the case study
Meeting the Affected Family:
Why do you think that Dr. Dufall wants a
blood sample from Emily and Chaka, if
neither of them has sickle cell anemia?
In other words, what could looking at
their hemoglobin tell us about this
genetic condition?
Be sure to complete the
homework for the
Discovering the Genetics
and Molecular Biology lab
in your lab manual prior to
coming to lab.