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.