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Genes Expression or Genes and How They
Work: Transcription, Translation, & More
Chapter 15
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Overview: Central Dogma
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Central Dogma
– DNA  RNA Protein
During polypeptide synthesis, ribosomal
RNA (rRNA) is the site of polypeptide
assembly.
– Transfer RNA (tRNA) transports and
positions amino acids.
– Messenger RNA (mRNA) directs which
amino acids are assembled into
polypeptides.
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The Central Dogma
DNA
Central Dogma
Proposed by Francis Crick in 1958 to describe the flow
of information in a cell.
Information stored in DNA is transferred
residue-by-residue to RNA which in turn transfers the
information residue-by-residue to protein.
RNA
The Central Dogma was proposed by Crick to help
scientists think about molecular biology. It has
undergone numerous revisions in the past 45 years.
Protein
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Central Dogma of Gene Expression
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Transcription Overview
Def: DNA sequence is transcribed into RNA
sequence
– initiated when RNA polymerase binds to
promoter binding site
 moves along DNA strand and adds
corresponding complementary RNA
nucleotide
 disengages at stop signal
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Translation Overview
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Def: nucleotide sequence of mRNA transcript
is translated into amino acid sequence in the
polypeptide
 rRNA recognizes and binds to start
sequence
 moves three nucleotides at a time
 disengages at stop signal
Gene expression - collective of transcription
and translation
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Genetic Code
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How does the order of nucleotides in a DNA
molecule encode the information that
specifies the order of amino acids in a
polypeptide?
The answer came in 1961 through an
experiment lead by Crick.
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Genetic Code
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Crick and colleagues reasoned that there must be
codons or block of info that coded for an amino
acid
They hypothesized that it was most likely 3
nucleotides
– Why 3?
– 2 nucleotides did not have enough combinations
(42 is only 16 possible amino acids)
3
– 3 nucleotides (4 is 64 which is enough to cover
the roughly 20 known amino acids)
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Genetic Code
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Now known
Genetic code consists of a series of
information blocks called codons.
– reading frame (triplet)
 each codes for one amino acid
 highly redundant
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Genetic Code
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carboxyl group
amino group
20 amino acids
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Genetic Code
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Could be punctuated or not
– Punctuated code would have a something
in the code that separates codons
– Non-punctuated code would not
In the following example, O is not a base
pair but the “punctuation.”
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Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Delete 1 base
15_06.jpg
WHYDIDTHEREDBATEATTHEFATRAT?
Hypothesis A : Delete T
unpunctuated WHY DID HER EDB ATE ATT HEF ATR AT?
(Nonsense)
Hypothesis B :
punctuated
WHYODIDOTHEOREDOBATOEATOTHEOFATORAT?
Delete T
O
O
R
B
E
T
F
R
WHY DID HEO EDO ATO ATO HEO ATO AT?
(Nonsense)
Delete 3 bases
Hypothesis A : WHYDIDTHEREDBATEATTHEFATRAT?
unpunctuated Delete T,R,and A
WHY DID HEE DBT EAT THE FAT RAT?
(Sense)
(Nonsense)
WHYODIDOTHEOREDOBATOEATOTHEOFATORAT?
Hypothesis B : Delete T,R,and A
punctuated
O
O
E
T
T
E
T
T
WHY DID HEO DOB OEA OTH OFA ORA?
(Nonsense)
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Genetic Code
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Could be punctuated or not
– Punctuated code would have a something
in the code that separates codons
– Non-punctuated code would not
In the following example, O is not a base
pair but the “punctuation.”
Crick concluded that it is not punctuated as
mutations lead to a different sequence of
amino acids, not nonsense.
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Genetic Code
•
Code is practically universal
– ex: AGA codes for arginine in bacteria,
humans and all other organisms studied
– great evidence that all life has a common
ancestor
– Genes coded in one organism can be
transcribed in another
 SWEET biotechnology
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Genetic Code
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Code is practically universal…but not quite
In 1979 mammalian mitochondria found to
have different “universal code”
– In mitochondrial DNA, UGA is not a stop
codon as it is in “universal code”
– Other codons are different
– Chloroplasts and ciliates (protists) have
minor differences as well
It is thought that the changes to
mitochondria and chloroplasts happen after
their endosymbiotic existence
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More on RNA
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Central Dogma shows
how information is
passed from DNA
RNA Protein
RNA’s structure is
different from DNA
– single stranded not
Double Stranded
– uracil not Thymine
– ribose not
deoxyribose
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RNA Structure
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RNA like DNA, is a nucleic acid. RNA
structure differs from DNA structure.
First, RNA is single stranded—it looks like
one-half of a zipper—whereas DNA is
double stranded.
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RNA Structure
•
The sugar in RNA is ribose; Ribose
DNA’s sugar is
deoxyribose.
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RNA Structure
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Both DNA and RNA contain four nitrogenous
bases, but rather than thymine, RNA contains a
similar base called uracil (U).
•
Uracil
Hydrogen bonds
Uracil forms a
base pair with
adenine in
RNA, just as
thymine does in
DNA.
Adenine
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Transcription
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RNA polymerase
– only one of two DNA strands (template or
antisense strand) is transcribed
– non-transcribed strand is termed coding
strand or sense strand
– In both bacteria and eukaryotes, the
polymerase adds ribonucleotides to the
growing 3’ end of an RNA chain.
 synthesis proceeds in 5’3’ direction
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Transcription Bubble
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Transcription
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Promoter
– Transcription starts at RNA polymerase
binding sites called promoters on DNA
template strand.
Initiation
– Other eukaryotic factors bind, assembling
a transcription complex.
 RNA polymerase begins to unwind DNA
helix.
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Transcription
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Elongation
– Transcription bubble moves down DNA at
constant rate leaving growing RNA strands
protruding from the bubble.
Termination
– Stop sequences at the end of the gene
cause phosphodiester bond formation to
cease, transcription bubble to dissociate,
and RNA polymerase to release DNA.
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Eukaryotic Transcription
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Eukaryotic transcription differs from
prokaryotic transcription:
– three RNA polymerase enzymes
– initiation complex forms at promoter
– RNAs are modified after transcription
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Translation: From mRNA to Protein
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The process of converting the information in a
sequence of nitrogenous bases in mRNA into a
sequence of amino acids in protein is known as
translation.
Translation takes place at the ribosomes in the
cytoplasm.
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Translation: From mRNA to Protein
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The role of transfer RNA
Amino
acid
For proteins to be built, the 20
different amino acids dissolved in
the cytoplasm must be brought to
the ribosomes.
This is the role of transfer RNA
(tRNA)
Each tRNA molecule attaches to
only one type of amino acid.
Chain of RNA
nucleotides
Transfer RNA
molecule
Anticondon
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Translation: From mRNA to Protein
Ribosome
mRNA codon
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Translation: From mRNA to Protein
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The first codon on mRNA is AUG, which codes for the
amino acid methionine
AUG signals the start of protein synthesis.
When this signal is given, the ribosome slides along the
mRNA to the next codon.
Methionine
tRNA
anticodon
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Translation: From mRNA to Protein
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A new tRNA molecule carrying an amino
acid pairs with the second mRNA codon.
Alanine
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Translation: From mRNA to Protein
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The amino acids are joined when a peptide
bond is formed between them.
Methionine
Alanine
Peptide
bond
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Translation: From mRNA to Protein
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A chain of amino acids is formed until the stop codon is
reached on the mRNA strand.
Stop codon
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Translation (in more detail)
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Begins when initial portion of mRNA
molecule binds to rRNA in a ribosome
– tRNA molecule with complimentary
anticodon binds to exposed codon on
mRNA
 some tRNA molecules recognize more
than one codon
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Translation (in more detail)
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Activating enzymes
– tRNA molecules attach to specific amino
acids through the action of activating
enzymes (aminoacyl-tRNA syntheases).
 must correspond to specific anticodon
sequences on a tRNA molecule as well
as particular amino acids
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Translation (in more detail)
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Start and stop signals
– start signal coded by AUG codon
– stop signal coded by one of three
nonsense codons: UAA - UAG – UGA
– What do you think “nonsense codons”
means here?
Initiation
– Polypeptide synthesis begins with the
formation of an initiation complex.
 initiation factors
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Formation of Initiation Factor
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Translation (in more detail)
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Elongation
– After initiation complex forms, large
ribosome subunit binds, exposing mRNA
codon adjacent to the initiating codon,
positioning it for interaction with another
amino acid-bearing tRNA molecule.
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Translation (in more detail)
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Translocation
– ribosome moves nucleotides along mRNA
molecule
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A bit about the peptide bond formation
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A peptide bond (or amide bond) is a covalent chemical
bond formed between two molecules when the carboxyl
group of one molecule reacts with the amine group of the
other molecule, thereby releasing a molecule of water.
This is a dehydration synthesis reaction (also known as a
condensation reaction), and usually occurs between amino
acids.
The resulting C(O)NH bond is called a peptide bond, and
the resulting molecule is an amide.
The four-atom functional group -C(=O)NH- is called a
peptide link
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Translation (in more detail)
Termination
– Nonsense codons are recognized by
release factors that release the newly
made polypeptide from the ribosome.
– There is no tRNA with complimentary
antidcodon to (UAA, UAG, UGA)
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Spliced Gene Transcripts
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DNA sequence specifying a protein is broken
into segments (exons) scattered among longer
noncoding segments (introns).
Initially, primary RNA transcript is produced for
the entire gene.
– Small nuclear ribonuclearproteins (snRNPs)
associate with proteins to form
spliceosomes.
 Lariat forms, excising introns and splicing
exons to form mature mRNA.
 alternative splicing
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RNA Splicing
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During RNA processing, intron sequences
are cut out of primary transcript before it is
used in polypeptide synthesis.
– remaining sequences are not translated
 remaining exon sequences are spliced
together to form final processed mRNA
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Eukaryotic Genes are Fragmented
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Compartmentalization
of processes (thus,
transport is
important)
replication
DNA Replication
nucleus
Transcription 
Nucleus
mRNA transferred to
cytoplasm
Translation 
Ribosome (in
cytoplasm)
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From a sequence to a protein
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Copyright © The McGraw -Hill Companies, Inc. Permission required for reproduction or dis
play.
An overview of gene expression in eukaryotes
1
Nuclear
membrane
3‘
DNA
RNA polymerase
Nucleus
Primary RNA
transcript
5‘
3‘
5‘
In the cell nucleus, RNA polymerase transcribes RNA from DNA.
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An overview of gene expression in eukaryotes
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An overview of gene expression in eukaryotes
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An overview of gene expression in eukaryotes
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An overview of gene expression in eukaryotes
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An overview of gene expression in eukaryotes
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Differences Between Prokaryotic and
Eukaryotic Gene Expression
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Most eukaryotic genes possess introns
(prokaryotic genes do not.)
Individual bacterial mRNA molecules often
contain transcripts of several genes.
Eukaryotic mRNA molecules must be
completely formed and must pass across the
nuclear membrane before translation.
In prokaryotes, translation begins at the AUG
codon preceded by a special nucleotide
sequence.
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Differences Between Prokaryotic and
Eukaryotic Gene Expression
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Eukaryotic mRNA molecules have introns cut
out and exons joined together before
translation.
Eukaryotic ribosomes are larger than
prokaryotic ribosomes.
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Exceptions to the Central Dogma
Epigenetic marks, such as patterns of
DNA methylation, can be inherited and
provide information other than the DNA
sequence
Nobel Prizes
DNA
retroviruses use reverse transcriptase
to replicate their genome
(David Baltimore and Howard Temin)
mRNA introns (splicing)
(Philip Sharp and Richard Roberts)
RNA editing (deamination of cytosine
to yield uracil in mRNA)
RNA
RNA interference (RNAi) a mechanism
of post-transcriptional gene silencing
utilizing double-stranded RNA
RNAs (ribozymes) can catalyze an
enzymatic reaction
(Thomas Cech and Sidney Altman)
RNA viruses
Prions are heritable proteins responsible
for neurological infectious diseases
(e.g. scrapie and mad cow)
(Stanley Pruisner)
Protein
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How do mutation effect proteins
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Any change in DNA sequence is called a mutation.
Mutations can be caused by errors in replication,
transcription, cell division, or by external agents
The effects of point mutations
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A point mutation is a change in a single base pair
in DNA
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A change in a single nitrogenous base can change
the entire structure of a protein because a change
in a single amino acid can affect the shape of the
protein
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Mutations – the effects of point
mutations
mRNA
Normal
Protein
Stop
Replace G with A
mRNA
Point
mutation Protein
Stop
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Frameshift mutations
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A mutation in which a single base is added or
deleted from DNA is called a frameshift mutation
because it shifts the reading of codons by one
base.
• Structural changes in chromosomes are
called chromosomal mutations.
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Causes of Mutations
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Any agent that can cause a change in DNA is
called a mutagen.
Mutagens include radiation, chemicals, and
even high temperatures.
Forms of radiation, such as X rays, cosmic
rays, ultraviolet light, and nuclear radiation,
are dangerous mutagens because the energy
they contain can damage or break apart DNA.
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Chromosomal Alterations
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When a part of a chromosome is left out, a deletion
occurs.
A B
C D E
F G H
A B C E
F G H
Deletion
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Chromosomal Alterations
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When part of a chromatid breaks off and
attaches to its sister chromatid, an insertion
occurs.
The result is a duplication of genes on the same
chromosome.
A B C D E
F G H
A B C B C D E
F G H
Insertion
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Chromosomal Alterations
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When part of a chromosome breaks off and
reattaches backwards, an inversion occurs.
A B C D E F G H
A D C B E FGH
Inversion
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Chromosomal Alterations
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When part of one chromosome breaks off and
is added to a different chromosome, a
translocation occurs.
AB C D E F GH
WX Y Z
W X AB C DE F GH
Translocation
Y
Z
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