Making Proteins: Translation Lecture #25 Honors Biology Ms. Day.
Chapter 14 From DNA to Proteins Honors Biology Program Mountain Pointe High School.
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Transcript of Chapter 14 From DNA to Proteins Honors Biology Program Mountain Pointe High School.
Chapter 14 From DNA to Proteins
Honors Biology ProgramHonors Biology Program
Mountain Pointe High SchoolMountain Pointe High School
What are the byssus of a mussels?
Why are these byssus so important?
Archibald Garrod
STEPS OF A METABOLIC PATHWAY:STEPS OF A METABOLIC PATHWAY:
First to notice that many heritable diseases were related to metabolic pathway malfunctions.
A B C X D
Action of enzyme 1
Action of enzyme 2
Something has interfered with the
action of enzyme 3.
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.
Neurospora crassa and other fungal species
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.
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 In other words, each strain of fungus possessed an inherited mutation that possessed an inherited mutation that corresponded to a defective enzyme.corresponded to a defective enzyme.
This evidence supported Garrod’s “one This evidence supported Garrod’s “one gene, one enzyme” hypothesis!gene, one enzyme” hypothesis!
Sickle-cell AnemiaThe 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 AnemiaIt was discovered that this disease was caused by a defect in a protein known
as hemoglobinhemoglobin that is found in red blood cells.
Normal hemoglobin Normal hemoglobin is designated is designated HbAHbA..
Abnormal hemoglobin is Abnormal hemoglobin is designated designated HbSHbS..
Pauling & Itano
In 1949, these scientists subjected molecules of HbA and HbS to gel gel electrophoresiselectrophoresis.
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
glass tube or plates containing
gel
gel
lower buffer solution
power supply
electrode
mov
emen
t of p
rote
ins
Pauling & Itano
HbAHbA molecules carried the
greatest surface charge and
therefore moved through the gel
the fastest.
HbSHbS molecules moved much
slower.
As molecules move through the gel, they’re separated into distinct
bands.
Vernon Ingram
Pinpointed the biochemical difference between HbAHbA and HbSHbSHemoglobin (left) is a Hemoglobin (left) is a molecule made of molecule made of 4 4 polypeptide chainspolypeptide chains, 2 , 2 alpha & 2 beta.alpha & 2 beta.Ingram discovered that Ingram discovered that the defect was caused the defect was caused by an by an incorrect amino incorrect amino acidacid substitution in one substitution in one of the beta chains!of the beta chains!
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VALINE
VALINEVALINE
HISTIDINE
HISTIDINE LEUCINE
LEUCINE
PROLINE
PROLINE
THREONINE
THREONINE
GLUTAMATEGLUTAMATE
GLUTAMATE
A beta chain
Hemoglobin molecule Beta chain of an HbA molecule
Beta chain of an HbS molecule
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 RNAmRNAmRNA is a single-stranded molecule that takes DNA’s protein-building protein-building instructionsinstructions out of the nucleus.
rRNArRNA is the primary component of ribosomesribosomes, the organelles that actually make proteins.
tRNAtRNA is the molecule responsible for delivering delivering amino acidsamino acids one by one to a ribosome in the correct order specified by the mRNA molecule.
A messenger RNA molecule (mRNAmRNA)
A ribosomal RNA molecule (rRNArRNA)
A transfer RNA molecule (tRNAtRNA)
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 UracilUracil
Like thymine, uracil (at right, in blue) is a pyrimidine and is capable of pairing with
adenine.
Transcription
TranscriptionTranscription is the process of using a portion of the DNA molecule as a templatetemplate to assemble a molecule of mRNAmRNA.
Only a selected stretch of selected stretch of one DNA strandone DNA strand is used as a template.
Transcription is initiated at a promoterpromoter, a DNA base sequence that signals the start of a genegene.
sugar-phosphate backbone of one strand of nucleotides in a DNA double helix
sugar-phosphate backbone ofthe other strand of nucleotides
part of the sequence of base pairs in DNA
transcribed DNA winds up again
DNA to be transcribed unwinds
Newly forming RNA transcript
The DNA template at the assembly site
Transcription
Once the enzyme DNA helicaseDNA helicase has unzipped the DNA molecule at the appropriate location…
RNA polymeraseRNA polymerase adds the required complementary bases to the exposed bases on one of the DNA strands.
growing mRNA transcript
direction of transcription
3’5’
3’ 5’
5’
3’
A “pre mRNApre mRNA” strand
Transcription
Next, the pre- mRNA molecule
must be modified.
A nucleotide known as a “capcap” is attached to the
5` end.
A nucleotide known as a
“poly-A tailpoly-A tail” is attached to the 3`
end.
unit of transcription in a DNA strand
exon exon exonintron intron
cap
5’
poly-A
tail3’
transcription into pre-mRNAtranscription into pre-mRNA
(snipped out) (snipped out)
mature mRNA transcriptmature mRNA transcript
Before the mRNA transcript is finished, useless sections known as intronsintrons must be snipped out, leaving only exonsexons
remaining.
5’
5’ 3’
3’
Now that the mRNA has been transcribed, what does its message mean?
DNA template
mRNA transcript
Amino acid? Amino acid? Amino acid? Amino acid? Amino acid?
The Genetic CodeThe Genetic Code
codon codon codon codon codon
tRNA molecules have a nucleotide triplet known as an “anticodonanticodon” on one end and an attachment site attachment site
for an amino acidfor an amino acid on the other end.
tRNA MOLECULE
anticodon
amino acid attachment site
anticodon
amino acid
attachment site
codon in mRNA
amino
acid
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 Large ribosomal subunit Complete ribosome
tunnel
Translation
3 Stages of Translation: InitiationInitiation
tRNA molecule is attached to small ribosomal subunit.
mRNA molecule’s START START codoncodon (AUGAUG) matches up with tRNA anticodonanticodon, 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)
TranslationNext is the elongationelongation stage: Ribosome complex moves along mRNA
molecule. One by one, tRNA molecules deliver the amino amino
acidsacids coded for by the mRNA to A siteA site. Amino acids are linked together by peptide peptide
bondsbonds, tRNA molecules exit P siteP site of ribosome.
Translation
The final stage is terminationtermination. Ribosomal complex reads
STOP codonSTOP codon on mRNA molecule.
No tRNA has an anticodonanticodon that corresponds to a STOP STOP codon.codon.
Proteins called release release factorsfactors bind to ribosome, cause enzymesenzymes to detach mRNA & polypeptide chain from ribosome.
Ribosomal subunits separate.
www.johnkyrk.com/DNAtranslation.html
TRANSCRIPTION
TRANSLATION
Unwinding of gene regions of a DNA molecule
Pre mRNA Transcript Processing
mRNA rRNA tRNA
Mature mRNA transcripts Ribosomal subunits Mature tRNA
Synthesis of a polypeptide chain at
binding sites for mRNA and tRNA on
the surface of an intact ribosome
FINAL PROTEIN
Cytoplasmic pools of amino acids, tRNAs, and ribosomal
subunits
Destined for use in cell or for transport
MutationsChanges in the nucleotide sequence of genes are known as mutationsmutations.The most common types of gene mutations are: Base-pair Base-pair
substitutions (shown substitutions (shown at left)at left)
FrameshiftsFrameshifts InsertionsInsertions DeletionsDeletions
original
base triplet
in a DNA
strand
a base
substitution
within the
triplet (red)As DNA is replicated, proofreading
enzymes detect the mistake and
make a substitution for it:
POSSIBLE OUTCOMES:
One DNA molecule
carries the original,
unmutated sequence
OR
The other DNA
molecule carries
a gene mutation
VALINEVALINE
Remember sickle-cell sickle-cell anemiaanemia?
It’s caused by a base-pair base-pair substitutionsubstitution that replaces the amino acid glutamineglutamine
with valinevaline.
Mutations
ARGININE
ARGININE
GLYCINE
GLYCINE
TYROSINE TRYPTOPHAN ASPARAGINE
LEUCINE LEUCINE GLUTAMATE
DNA TEMPLATE
mRNA TRANSCRIPT
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 insertioninsertion.
This mutation causes DNA’s message to shift one base to the right.
A deletiondeletion would cause a one-base shift to the left.