5/29/2012
Chapter 11 Lecture Notes: Transcription and Translation
Biol 100 - K. Marr
The Flow of Genetic Information: DNA to RNA to Protein  Phenotype
Essentials of
Biology
Sylvia S. Mader
Chapter 11
Lecture Outline
Transcription &
Translation
DNA  RNA  Protein
• Transcription: DNA
copied into mRNA
molecule
• Translation:
ribosomes translate
mRNA into protein—
a chain of amino
acids
• Proteins control
phenotype. How?
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
The function of a gene is to dictate the production of a one ore
more proteins. Why are proteins so important?
A few of the many roles played by proteins:
1.
Enzymes: catalysts for nearly all chemical reactions in cells;
Determine what cells can make and digest
C
G
DNA
double helix
G
C
C
DNA
A
Structural components: muscles (actin and myosin), connective
tissue (collagen, elastin)
4.
Receptors on cell surface for growth factors, hormones, etc.
Hormones: e.g. insulin, growth hormone, prolactin
Transport: e.g. hemoglobin, spindle fibers
6.
Immune system: antibodies
A
• Transcription
3
5
T
C
T
T
A
G G
C C
5
3
Cytoplasm
codon
mRNA
U
C C
 DNA serves as template to
make mRNA.
• Translation
G G
A G A A
U
G G
5.
T
C
T
C
A A
G
G
Transcription
mRNA
3.
T G
T
C C
2.
Figure 11.9
Flow of genetic information
Nucleus
 mRNA directs sequence of
amino acids in a protein.
 rRNA and tRNA assist
A G A A
Translation
A
C C
G G
U C U U
tRNA
anticodon
Polypeptide
The order
of Bases in
a gene
determines
the order
of amino
acids in the
protein it
codes for
Gly
Arg
Thr
Figure 11.8 Structure of RNA
Ribonucleic acid (RNA)
• Contains sugar ribose
• Uses uracil, not thymine
 Uses A, C, and G like DNA
• Single-stranded
• 3 majors types
 Messenger RNA (mRNA)
 Transfer RNA (tRNA)
 Ribosomal RNA (rRNA)
Why is the
order of
amino acids
in a protein
important?
One RNA nucleotide
1
5/29/2012
Transcription: copying DNA into RNA ( 1 of 2)
(a) Parent DNA
Transcription: copying DNA into RNA ( 2 of 2)
(d) Products of transcription
(c) Transcription continues
(b) Transcription begins
RNA
polymerase
Noncoding
strand
Template
Strand
(coding
strand)
Complementary
base pairing
RNA
Polymerase
separates
strands
Nucleotide
joining
New RNA strand
(actually several
hundred base
pairs long)
Parent DNA
totally
conserved
Transcription (Freeman)
Transcription (Campbell)
Figure 11.11 Transcription to form mRNA
G
C
G
G
C
template
DNA
strand
3
G
G
C
C
C
RNA
polymerase
G
T
A
 During transcription,
complementary RNA is made from
a DNA template.
 Portion of DNA unwinds and
unzips at the point of attachment of
RNA polymerase.
 Bases join in the order dictated by
the sequence of bases in the
template DNA strand.
G
G
C
mRNA
This mRNA transcript is
Almost ready to be processed.
Figure 11.12 mRNA processing
• Capping & poly-A tail
provide stability
• Introns (non-coding)
removed
• Exons remain (coding)
• Alternative splicing
produces different
mRNA molecules
leading to different
proteins.
• Mature mRNA leaves
nucleus and associates
with ribosome in
cytoplasm.
5
to processing
Transcription in Eukaryotic Cells:
Differential RNA splicing can result in one gene producing more than one protein
Processing of Eukaryotic RNA
Intron
RNA Processing includes
(a) Gene
•
•
•
Intron 1
Intron 2
Intron 3
Intron 4
Intron 5
Exon 1
Exon 2
Exon 3
Exon 4
Exon 5
Exon 6
Transcription
(b) Primary transcript
Adding a cap and tail
Removing introns
Splicing exons together
– Differential splicing produces
different mRNA molecules
Exon
Gene
(DNA)
Cap
RNA
transcript
with cap
and tail
Intron
Exon
Exon
Transcription + the
Addition of cap and tail
Tail
Introns removed
Exons spliced together
RNA splicing:
(c) Spliced RNA
RNA Processing
(d) Mature RNA
mRNA
Differential splicing
can result in
different mRNA
molecules and,
therefore, different
proteins
Coding sequence
Nucleus
Cytoplasm
Translation
(d) protein
Fig. 7.07
2
5/29/2012
Comparing DNA and RNA
DNA
RNA
Number of
Strands
Sugars
DNA & RNA Structure
Bases
Translation
• Ribosomes read
mRNA to
produce a
protein
• tRNA brings in
amino acids
• Resulting protein
contains the
sequence of
amino acids
originally
specified in the
DNA.
Figure 11.13
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
Transfer RNA: tRNA
Please note that due to differing
operating systems, some animations
will not appear until the presentation is
viewed in Presentation Mode (Slide
Show view). You may see blank slides
in the “Normal” or “Slide Sorter” views.
All animations will appear after viewing
in Presentation Mode and playing each
animation. Most animations will require
the latest version of the Flash Player,
which is available at
http://get.adobe.com/flashplayer.
A portion of an mRNA molecule attached to a tRNA
Codon on mRNA
tRNA
mRNA
1. Acts as a molecular
interpreter
2. Each tRNA carries a
specific amino acid
3. Matches amino acids
with codons in mRNA
using anticodons
Each Codon codes
Specifies a specific
tRNA—amino acid
complex
Amino acid
Figure 11.13 tRNA structure
3
5/29/2012
1. Initiation of Translation
A ribosome translating mRNA into protein
Codon
mRNA
Small
subunit
Ribosomes
• Organelle that makes
protein
• Reads mRNA 5’  3’
• Made of rRNA and
protein
• Consist of 2 subunits
Anticodon
Ribosome
tRNA
mRNA
Large
subunit
Amino
acid
Protein
under
construction
2. Elongation
2. Elongation continues: Translocation of Ribosome
Ribosome
moves
tRNA ejected
Peptide bond forms
3. Termination of Translation
3. Termination continued:
Disassembly of Ribosome
Termination
factor binds
Ribosome
moves
tRNA ejected
tRNA
Peptide bond forms
Polypeptide chain
4
5/29/2012
Figure 11.14
Polyribosome structure and function
Cystic Fibrosis: autosomal recessive
Most common lethal genetic disease
3
mRNA
codon
–
1 in 2000 children is born with CF in U.S.
–
Untreated children die by age 5
–
Ave. life expectancy: ~27 yrs
–
Special diet + daily dose of antibiotic prevent infection
Carriers of CF gene:
–
Hispanics: 1 in 46
5
–
African Americans: 1 in 63
a.
–
Asian Americans: 1 in 150
–
Caucasian of European descent: 1 in 25
•
• Polyribosome – several
and
b. ribosomes attach to 400,000
translate the same piece of mRNA.
CF allele protects against the plague and many viruses
Cystic Fibrosis
Cystic Fibrosis phenotype
Our Goal…
To determine the
connection between
DNA and the
symptoms
associated with
cystic fibrosis
Transcription & Translation of the CRTR Gene in Healthy People


A single faulty protein is connected to the symptoms
In 1989 the gene was mapped to chromosome #7
Figure 11.10
Messenger RNA codons—“genetic code”
Part of a normal CFTR gene:
Second base
U
5’...ATCATCTTTGGTGTT...3’ non-coding strand
Transcribe this portion of the gene.

The whole gene codes for 1480 amino acids in CFTR protein!

What is the order of bases in the resulting mRNA molecule?
2.
Translate this portion of the gene.
–
What is the order of amino acids in the resulting protein?
First base
1.
C
A
phenylalanine (Phe)
UUA
UUG
leucine (Leu)
CUU
CUC
CUA
CUG
leucine (Leu)
AUU
AUC
AUA
isoleucine (Ile)
AUG methionine (Met) (start)
GUU
GUC
G GUA
GUG
valine (Val)
A
C
UCU
UCC
UCA
UCG
serine (Ser)
CCU
CCC
CCA
CCG
proline (Pro)
ACU
ACC
ACA
ACG
threonine (Thr)
GCU
GCC
GCA
GCG
alanine (Ala)
G
UAA stop
UAG stop
UGU
cysteine (Cys)
UGC
UGA stop
UGG tryptophan (Trp)
U
C
A
G
CAU
CAC
CAA
CAG
histidine (His)
CGU
CGC
CGA
CGG
U
C
A
G
AAU
AAC
AAA
AAG
asparagine (Asn)
GAU
GAC
GAA
GAG
aspartic acid (Asp)
UAU
UAC
tyrosine (Tyr)
glutamine (Gln)
lysine (Lys)
glutamic acid (Glu)
AGU
AGC
AGA
AGG
GGU
GGC
GGA
GGG
arginine (Arg)
serine (Ser)
arginine (Arg)
glycine (Gly)
U
C
A
G
Third base
3’...TAGTAGAAACCACAA...5’ coding strand
U
UUU
UUC
U
C
A
G
1. The genetic code is the same for almost all organisms!!
2. In which base do the codons for the same amino acid differ?
3. What is the function of the start and stop codons?
5
5/29/2012
Transcription & Translation of the CRTR Gene in People with CF
CFTR Protein: The cystic fibrosis transmembrane regulator protein
Part of CFTR gene associated with Cystic Fibrosis:
Carbohydrate
Chloride ions
Cytoplasm of cell
lining duct or lungs
CFTR Protein
•
Pumps
chloride ions
(salt) into cells
lining ducts or
the lungs
•
What are the
consequences
when CFTR
doesn’t work?
•
How does a
gene control
the production
of a protein?
5’...ATCATTGGTGTT...3’ non-coding strand
CFTR Protein
3’...TAGTAACCACAA...5’ template strand
1. Transcribe this portion of the gene.

What is the order of bases in the resulting mRNA molecule?
2. Translate this portion of the gene.
–
What is the order of amino acids in the resulting protein?
3. What is different about the gene and the protein in people
with cystic fibrosis?
Explaining the symptoms of CF
•
In CF, the faulty CFTR protein
never makes it to cell membrane
1.
What builds up outside of cells?
Why?
2.
Why salty sweat?
3.
Why does mucus collect in
lungs?
4.
Why respiratory infections?
5.
Why problems with digestion?
6.
Why male sterility?
CFTR Protein: Pumps
Chloride ions into cell
Water
Inside of duct
or
Air sac in lungs
Cell
membrane
Water
Understanding Cystic Fibrosis at the Cellular Level
Chloride ions
outside of cell
How does CFTR protein get from where it’s produced to its home
in the cell membrane?
1.
Where is the CFTR protein produced?
2.
CFTR is a glycoprotein—where does it go for modification?
 How does it get there?
Chloride ions
in cell
3.
How does the modified CFTR protein get to the plasma
membrane?
4.
The defective CFTR protein is recognized at the ER as defective
 Where is the defective CFTR protein sent?
What’s a Gene Mutation?
CF symptoms may be mild or severe
CFTR
Gene
• Any change in the nucleotide sequence of DNA
• Types of Mutations
– Base pair Substitution, insertion or deletion
– Occur during DNA replication
• Mutations may Result from:
1.
Errors in DNA replication (Why uncommon?)
2.
Transposons
• “Jumping genes” – pieces of DNA that move within and between chromosomes
3.
4.
Several hundred different mutations are associated with CF
Some Viruses
Mutagens
• physical or chemical agents that cause errors during DNA replication
 chemicals in cigarette smoke
 Radiation (e.g. U.V. light, X-rays)
• Do gene mutations always affect the protein a gene codes for?
6
5/29/2012
Types of Gene Mutations:
Mutations responsible for Sickle Cell Anemia
Base Pair Substitutions, Insertions or deletions
• Only one amino acid in 146 is incorrect in sickle-cell
hemoglobin!
Normal hemoglobin DNA
• Base pair
substitutions
– May result in
changes in the
amino acid
sequence in a
protein, or
– May be silent
(have no effect)
Mutant hemoglobin DNA
mRNA
mRNA
Sickle-cell hemoglobin
Normal hemoglobin
Glu
mRNA
– Change the
reading frame
of the genetic
message
Protein
Met
Lys
Phe
Gly
Ala
Phe
Ser
Ala
(a) Base substitution
Met
Lys
Val
Types of Mutations: Base Insertions and deletions
• Can have
disastrous
effects
mRNA
Protein
Met
Lys
Phe
Gly
Ala
(b) Nucleotide deletion
Met
Lys
Leu
SUMMARY OF KEY CONCEPTS
Ala
His
Although
mutations
are often
harmful…
– mutations are
the source of
the rich
diversity of
genes in the
living world
– mutations
contribute to
the process of
evolution by
natural
selection
Figure 11.17 Summary of gene expression in eukaryotes
DNA and RNA: Polymers of Nucleotides
Nitrogenous
base
Phosphate
group
Sugar
DNA
Nucleotide
Polynucleotide
7