Transcript Chapter 14 - TeacherWeb

Chapter 14
From DNA to Proteins
Honors Biology Program
Mountain Pointe High School
What are the byssus of a mussels?
Why are these byssus so important?
Archibald Garrod
First to notice that many heritable diseases were related to metabolic
pathway malfunctions.
STEPS OF A METABOLIC PATHWAY:
Action of
enzyme 1
A
Action of
enzyme 2
B
Something has
interfered with the
action of enzyme 3.
C
X
D
Completion of the pathway
is blocked, and C
accumulates.
Garrod hypothesized
that each of his
affected patients had
inherited a single
metabolic defect that
interfered with an
enzyme in a particular
metabolic pathway.
Beadle & Tatum
33 years after
Garrod’s hypothesis,
these scientists
were experimenting
with a common
bread mold that’s
capable of
synthesizing
everything it needs
to survive except for
a few basic
substances.
They discovered that some of the fungal
strains would only grow when supplied
with vitamin B6, others would only grow
in the presence of B12, etc.
Neurospora crassa and other fungal species
Beadle & Tatum
After careful examination, they
discovered that there was a different
defective enzyme in each mutant strain
of the fungus.
In other words, each strain of fungus
possessed an inherited mutation that
corresponded to a defective enzyme.
This evidence supported Garrod’s “one
gene, one enzyme” hypothesis!
Sickle-cell Anemia
The most common lethal genetic disease in African
Americans, it causes normal red blood cells to become sickle
shaped, which causes an incredible variety of health problems
for its victims.
Normal red blood cell
Sickled red blood cell
Sickle-cell Anemia
It was discovered that this disease was
caused by a defect in a protein known
as hemoglobin that is found in red blood
cells.
Normal hemoglobin
is designated HbA.
Abnormal hemoglobin is
designated HbS.
In 1949, these
scientists subjected
molecules of HbA and
HbS to gel
electrophoresis.
In this procedure, an
electric field is used to
move molecules
through a gel.
Molecules are
separated by their
size, shape & surface
charge.
upper buffer
solution
electrode
movement of proteins
Pauling &
Itano
glass tube or
plates containing
gel
gel
lower buffer solution
electrode
power supply
Pauling &
Itano
HbA molecules
carried the
greatest surface
charge and
therefore moved
through the gel
the fastest.
HbS molecules
moved much
slower.
As molecules move
through the gel, they’re
separated into distinct
bands.
Vernon Ingram
Pinpointed the
biochemical difference
between HbA and HbS
Hemoglobin (left) is a
molecule made of 4
polypeptide chains, 2
alpha & 2 beta.
Ingram discovered that
the defect was caused
by an incorrect amino
acid substitution in one
of the beta chains!
```
Hemoglobin
molecule
Beta chain of an HbA molecule
VALINE
PROLINE
HISTIDINE
THREONINE
LEUCINE
GLUTAMATE
GLUTAMATE
Beta chain of an HbS molecule
A beta chain
VALINE
PROLINE
HISTIDINE
LEUCINE
THREONINE
VALINE
GLUTAMATE
The discovery of the difference
between the alpha & beta chains of
hemoglobin meant that…
Two genes must code for hemoglobin,
one for each type of polypeptide chain.
Genes code for all proteins, not just
enzymes.
The amino acid sequences of
polypeptide chains are encoded in
genes.
3 Different Types of RNA
The Three Types of RNA
A messenger RNA molecule
(mRNA)
A ribosomal RNA molecule
(rRNA)
A transfer RNA molecule
(tRNA)
mRNA is a single-stranded
molecule that takes DNA’s
protein-building
instructions out of the
nucleus.
rRNA is the primary
component of ribosomes,
the organelles that actually
make proteins.
tRNA is the molecule
responsible for delivering
amino acids one by one to
a ribosome in the correct
order specified by the
mRNA molecule.
Comparing DNA and RNA
Double-stranded
Deoxyribose sugars
4 nitrogenous bases




Adenine
Cytosine
Guanine
Thymine
Single-stranded
Ribose sugars
4 nitrogenous bases




Adenine
Cytosine
Guanine
Uracil
Like thymine, uracil (at right, in blue) is a
pyrimidine and is capable of pairing with
adenine.
Transcription
sugar-phosphate backbone of one strand
of nucleotides in a DNA double helix
Transcription is the process
of using a portion of the
DNA molecule as a
template to assemble a
molecule of mRNA.
Only a selected stretch of
sugar-phosphate
one DNA strand is used as
backbone of
the other strand
a template.
of nucleotides
Transcription is initiated at a
transcribed DNA
promoter, a DNA base
winds up again
sequence that signals the
start of a gene.
Newly forming
RNA transcript
part of the
sequence of
base pairs in DNA
DNA to be
transcribed
unwinds
The DNA
template at the
assembly site
Transcription
growing mRNA transcript
3’
5’
3’
5’
direction of transcription
5’
3’
A “pre mRNA” strand
Once the enzyme DNA helicase has unzipped the DNA
molecule at the appropriate location…
RNA polymerase adds the required complementary bases to
the exposed bases on one of the DNA strands.
Transcription
unit of transcription in a DNA strand
3’
5’
exon
intron
exon
intron
transcription into pre-mRNA
exon
poly-A
cap
tail
5’
(snipped out)
3’
(snipped out)
5’
Next, the premRNA molecule
must be
modified.
3’
mature mRNA transcript
Before the mRNA transcript is finished, useless sections
known as introns must be snipped out, leaving only exons
remaining.
A nucleotide
known as a “cap”
is attached to the
5` end.
A nucleotide
known as a
“poly-A tail” is
attached to the 3`
end.
Now that the mRNA has been transcribed,
what does its message mean?
The Genetic Code
DNA template
codon
codon
codon
codon
codon
mRNA transcript
Amino acid? Amino acid? Amino acid? Amino acid? Amino acid?
tRNA molecules have a nucleotide triplet known as an
“anticodon” on one end and an attachment site for an
amino acid on the other end.
anticodon
codon in mRNA
anticodon
tRNA MOLECULE
amino acid
amino
acid
amino acid attachment site
attachment site
Ribosomes are composed of two subunits made of
rRNA. These subunits are created in the nucleolus,
travel separately out of the nucleus and only unite
when mRNA messages need to be translated into
proteins.
platform for
chain
assembly
Small ribosomal subunit
tunnel
Large ribosomal subunit
Complete ribosome
Translation
3 Stages of Translation:

Initiation
 tRNA molecule is attached
to small ribosomal subunit.
 mRNA molecule’s START
codon (AUG) matches up
with tRNA anticodon,
attaches to small ribosomal
subunit.
 Large ribosomal subunit
attaches to small subunit.
Binding site for mRNA
P
(first binding site
for tRNA)
A
(second binding
site for tRNA)
Translation
Next is the elongation stage:



Ribosome complex moves along mRNA
molecule.
One by one, tRNA molecules deliver the amino
acids coded for by the mRNA to A site.
Amino acids are linked together by peptide
bonds, tRNA molecules exit P site of ribosome.
Translation
The final stage is termination.
 Ribosomal complex reads
STOP codon on mRNA
molecule.
 No tRNA has an anticodon
that corresponds to a STOP
codon.
 Proteins called release
factors bind to ribosome,
cause enzymes to detach
mRNA & polypeptide chain
from ribosome.
 Ribosomal subunits
separate.
 www.johnkyrk.com/DNAtra
nslation.html
TRANSCRIPTION
Pre mRNA
Transcript
Processing
Unwinding of gene regions of a DNA molecule
mRNA
Mature mRNA transcripts
rRNA
tRNA
Ribosomal subunits
Mature tRNA
TRANSLATION
Cytoplasmic
pools of amino
acids, tRNAs,
and ribosomal
subunits
Synthesis of a
polypeptide chain at
binding sites for
mRNA and tRNA on
the surface of an
intact ribosome
FINAL PROTEIN
Destined for use in
cell or for transport
Mutations
original
a base
base triplet
substitution
in a DNA
within the
strand
As DNA is replicated, proofreading
triplet (red)
enzymes detect the mistake and
make a substitution for it:
POSSIBLE OUTCOMES:
OR
One DNA molecule
The other DNA
carries the original,
molecule carries
unmutated sequence
a gene mutation
Changes in the
nucleotide sequence of
genes are known as
mutations.
The most common types
of gene mutations are:
 Base-pair
substitutions (shown
at left)
 Frameshifts
 Insertions
 Deletions
Remember sickle-cell
anemia?
VALINE
It’s caused by a base-pair
substitution that replaces
the amino acid glutamine
with valine.
Mutations
mRNA TRANSCRIPT
DNA TEMPLATE
ARGININE
ARGININE
GLYCINE
GLYCINE
TYROSINE
LEUCINE
TRYPTOPHAN
LEUCINE
ASPARAGINE
GLUTAMATE
RESULTING AMINO
ACID SEQUENCE
ALTERED mRNA
MESSAGE
A BASE INSERTION
(RED) IN DNA
ALTERED AMINO
ACID SEQUENCE
The example above is a frameshift mutation known as an insertion.
This mutation causes DNA’s message to shift one base to the right.
A deletion would cause a one-base shift to the left.