Download 5 end

PROTEIN STRUCTURE
Amino Acid Monomers

Amino acids are the monomers of proteins

There are 20 different types of amino acids


Amino acids differ in their properties due to differing
side chains, called R groups
When you see an ‘R’ on a molecule, it means that
multiple different things could be in the spot that says
‘R.’ There is no element with the symbol R.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 5-UN1
Different things will be in the
R spot depending on which
of the 20 amino acids you
are looking at.
Amino
group
Carboxyl
group
Fig. 5-17
Glycine
(Gly or G)
Nonpolar
Valine
(Val or V)
Alanine
(Ala or A)
Methionine
(Met or M)
Leucine
(Leu or L)
Trypotphan
(Trp or W)
Phenylalanine
(Phe or F)
Isoleucine
(Ile or I)
Proline
(Pro or P)
Polar
Serine
(Ser or S)
Threonine
(Thr or T)
Cysteine Tyrosine
(Cys or C) (Tyr or Y)
Asparagine Glutamine
(Asn or N) (Gln or Q)
Electrically
charged
Acidic
Aspartic acid Glutamic acid
(Glu or E)
(Asp or D)
Basic
Lysine
(Lys or K)
Arginine
(Arg or R)
Histidine
(His or H)
Chart showing the 20 different
amino acids. The part in the
white area is the R group
(different in each amino acid).
The part on the bottom (gray
area) is the same for all amino
acids. If you can recognize
that part, you’ll be able to tell if
something is an amino acid or
not.
Polypeptides




Polypeptides are polymers built from amino acids
A protein consists of one or more polypeptides
Polypeptides range in length from a few to more than
a thousand monomers
Each polypeptide has its amino acids in a different
order. Some polypeptides have more of some kinds of
amino acids, others include different kinds of amino
acids.
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 5-18
Peptide
bond
Amino acids forming a
polypeptide by a dehydration
reaction
(a)
Side chains
Peptide
bond
Backbone
(b)
Amino end
(N-terminus)
Carboxyl end
(C-terminus)
Protein Structure and Function

A functional protein consists of one or more
polypeptides twisted, folded, and coiled into a unique
shape
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Antibody protein


Protein from flu virus
The sequence of amino acids determines a protein’s
three-dimensional structure
A protein’s structure determines its function
Sickle-Cell Disease: A Change in
Protein Structure



A slight change in protein structure can affect a
protein’s structure and ability to function
Sickle-cell disease, an inherited blood disorder, results
from a single wrong amino acid in the protein
hemoglobin
The DNA gives instructions about which amino acids to
put together to make a protein. If there’s a mistake in
the DNA (called a mutation), you can get a protein
with a wrong amino acid; the mutated protein may not
function well
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 5-22c
10 µm
Normal red blood
cells are full of
individual
hemoglobin
molecules, each
carrying oxygen.
10 µm
Fibers of abnormal
hemoglobin deform
red blood cell into
sickle shape.
What Determines Protein Structure?




In addition to the sequence of amino acids, physical
and chemical conditions can affect protein shape
Alterations in pH, salt concentration, temperature, or
other environmental factors can cause a protein to
unravel
This loss of a protein’s original shape is called
denaturation
A denatured protein is biologically inactive (one
reason doctors worry about very high fevers)
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 5-23
Denaturation
Normal protein
Renaturation
Denatured protein
Functions of Proteins: Lots!
Structure:
-Proteins are embedded in cell membranes with
phospholipids
-Proteins direct DNA to fold into chromosomes
before cell division
Image from: http://employees.csbsju.edu/hjakubowski/classes/ch331/dna/chromosome.gif
Proteins control gene expression by
turning genes on and off
http://www.cat.cc.md.us/courses/bio141/lecguide/unit4/genetics/protsyn/regulation/ionoind.html
Images from: http://www.imac.auckland.ac.nz/vaccines/vacc_graph.htm
Proteins fight germs. Antibodies are
proteins.
ANTIBODIES ATTACK & KILL THEM
Proteins help with transport.
Proteins in cell
membranes move
molecules in and out
of cells.
Image from: http://bio.winona.msus.edu/berg/ANIMTNS/FacDiff.htm
Proteins help with transport.
Hemoglobin in
blood helps
transport oxygen
all over the body.
Image from: http://www.cellsalive.com/pics/cover4.gif
Some proteins act as hormones.
Eating carbohydrates
puts glucose in your
bloodstream.
Insulin is a protein hormone that controls blood
glucose.
Image from: http://www.cibike.org/CartoonEating.gif
Insulin function image by Riedell using Glycogen image modified from: http://www.msu.edu/course/lbs/145/smith/s02/graphics/campbell_5.6.gif
People with diabetes can’t make enough
insulin, so glucose stays in their blood
instead of being stored by cells.
Shots can replace the insulin and help to
reduce blood sugar.
Image modified from: http://sonya.lanecurrent.net/Health/Images/meds.gif
Fig. 5-16
Substrate
(sucrose)
Glucose
OH
Fructose
HO
Enzyme
(sucrase)
H2O
PROTEIN SYNTHESIS
The Flow of Genetic Information



Your DNA is a set of instructions that your cells follow
to make proteins. The proteins you have determine
your traits.
Gene expression, the process by which DNA directs
protein synthesis, has two stages: transcription and
translation
Basic process: DNA  mRNA  Protein
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-3a-1
TRANSCRIPTION
DNA
mRNA
(a) Bacterial cell
Fig. 17-3a-2
TRANSCRIPTION
DNA
mRNA
Ribosome
TRANSLATION
Polypeptide
(a) Bacterial cell
Fig. 17-3b-1
Nuclear
envelope
TRANSCRIPTION
DNA
Pre-mRNA
(b) Eukaryotic cell
Fig. 17-3b-2
Nuclear
envelope
TRANSCRIPTION
RNA PROCESSING
mRNA
(b) Eukaryotic cell
DNA
Pre-mRNA
Fig. 17-3b-3
Nuclear
envelope
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
TRANSLATION
Ribosome
Polypeptide
(b) Eukaryotic cell
Summary of Transcription and Translation

Transcription
DNA  mRNA (happens in the nucleus)

Translation
mRNA  polypeptide (happens in the cytoplasm)
- Ribosomes help
The Genetic Code




How does DNA tell us which kinds of amino acids to
put together for each protein?
There are 20 amino acids, but there are only four
nitrogenous bases in DNA
How many bases correspond to an amino acid?
Remember – Proteins are strings of amino acid
monomers put together
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Codons: Triplets of Bases



Every 3 DNA nucleotides represent a triplet, which
your ribosomes read like a word.
Each 3-letter “word” on the DNA codes for a specific
amino acid
Example: AGT on the DNA tells the ribosome to add
the amino acid Serine
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

During transcription, mRNA nucleotides attach to one of the
two DNA strands (called the template strand).
http://www.phschool.com/science/biology_place/biocoach/transcription/tcproc.html



During translation, the mRNA 3-letter words, called codons,
are read in the 5 to 3 direction (the letter at the 5’ end is
at the beginning of the codon word).
Each codon specifies which amino acid should be added to
the polypeptide next
DNA has triplets, which attach to mRNA codons
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-4
DNA
molecule
Gene 2
Gene 1
Gene 3
DNA
template
strand
TRANSCRIPTION
mRNA
Codon
TRANSLATION
Protein
Amino acid
Cracking the Code




All 64 codons were deciphered by the mid-1960s
Of the 64 triplets, 61 code for amino acids; 3 triplets
are “stop” signals to end translation
Just as some words mean the same thing, multiple
codons can give the same amino acid
Codons must be read in the correct reading frame
(correct groupings) in order for the specified
polypeptide to be produced
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Reading frames




For the sequence ‘AUGCCGAC’, you could start at
the beginning and read the codons “AUG, CCG”
If you used a different reading frame, you could
read “UGC, CGA” or “GCC, GAC”
If you start in the wrong reading frame, you’ll
combine the wrong amino acids to make the wrong
protein.
Since AUG is the start codon, ribosomes look for an
AUG and start reading there.
Third mRNA base (3 end of codon)
First mRNA base (5 end of codon)
Fig. 17-5
Second mRNA base
Evolution of the Genetic Code


The genetic code is nearly universal, shared by the
simplest bacteria to the most complex animals
Genes can be transcribed and translated after being
transplanted from one species to another
The same sequence of nitrogenous bases in the DNA will
code for the same amino acids and proteins in almost all
species.
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-6
(a) Tobacco plant expressing
a firefly gene
(b) Pig expressing a
jellyfish gene
http://projecthdesign.com/wp-content/uploads/2007/11/landmineflowers1.jpg
http://www.bbc.co.uk/news/scienceenvironment-14882008
http://en.wikipedia.org/wiki/File:GloFish.jpg
http://www.wired.com/science/plan
etearth/news/2008/01/gm_insects#
Details of Transcription


RNA synthesis is catalyzed by RNA polymerase, which
pries the DNA strands apart and hooks together the RNA
nucleotides
RNA synthesis follows the same base-pairing rules as
DNA, except uracil substitutes for thymine
RNA has U instead of T
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-7b
Nontemplate
strand of DNA
Elongation
RNA
polymerase
3
RNA nucleotides
3 end
5
5
Direction of
transcription
(“downstream”)
Newly made
RNA
Template
strand of DNA
Fig. 17-8
1
Promoter
A eukaryotic promoter
includes a TATA box
Template
5
3
3
5
TATA box
Start point Template
DNA strand
2
Transcription
factors
Several transcription factors must
bind to the DNA before RNA
polymerase II can do so.
5
3
3
5
3
Additional transcription factors bind to
the DNA along with RNA polymerase II,
forming the transcription initiation complex.
RNA polymerase II
Transcription factors
5
3
3
5
5
RNA transcript
Transcription initiation complex


The DNA sequence where RNA polymerase first
attaches is called the promoter; in bacteria, the
sequence signaling the end of transcription is called
the terminator
The stretch of DNA that is transcribed is called a
transcription unit
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
RNA Polymerase Binding and Initiation of
Transcription


Before transcription can start, Transcription factors
attach to the promoter region and help the RNA
polymerase bind onto the DNA in the right place
A promoter called a TATA box is crucial in forming the
initiation complex in eukaryotes
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Elongation of the RNA Strand



As RNA polymerase moves along the DNA, it untwists
the double helix, 10 to 20 bases at a time
Transcription progresses at a rate of 40 nucleotides
per second in eukaryotes
A gene can be transcribed by several RNA
polymerases at a time
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-7a-2
Promoter
Transcription unit
5
3
Start point
RNA polymerase
3
5
DNA
1
Initiation
5
3
Unwound
DNA
3
5
RNA
transcript
Template strand
of DNA
Fig. 17-7b
Nontemplate
strand of DNA
Elongation
RNA
polymerase
3
RNA nucleotides
3 end
5
5
Direction of
transcription
(“downstream”)
Newly made
RNA
Template
strand of DNA
Fig. 17-7a-3
Promoter
Transcription unit
5
3
Start point
RNA polymerase
3
5
DNA
1 Initiation
5
3
3
5
Unwound
DNA
RNA
transcript
Template strand
of DNA
2 Elongation
Rewound
DNA
5
3
3
5
RNA
transcript
3
5
Fig. 17-7a-4
Promoter
Transcription unit
5
3
Start point
RNA polymerase
3
5
DNA
1 Initiation
5
3
3
5
Unwound
DNA
RNA
transcript
Template strand
of DNA
2 Elongation
Rewound
DNA
5
3
3
5
3
5
RNA
transcript
3 Termination
5
3
3
5
5
Completed RNA transcript
3
Animation: http://www.dnalc.org/resources/3d/12-transcription-basic.html
Fig. 17-3b-3
Eukaryotic cells
process RNA


During RNA processing,
both ends of the
primary transcript are
usually altered
Also, usually some
interior parts of the
molecule are cut out,
and the other parts
spliced together
Nuclear
envelope
DNA
TRANSCRIPTION
Pre-mRNA
RNA PROCESSING
mRNA
TRANSLATION
Ribosome
Polypeptide
(b) Eukaryotic cell
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Alteration of mRNA Ends

Each end of a pre-mRNA molecule is modified in a
particular way:
 The
5 end receives a modified nucleotide 5 cap
 The 3 end gets a poly-A tail

Why add these ends?
 They
seem to make it easier to move mRNA out of the
nucleus
 They protect mRNA from hydrolytic (causing hydrolysis)
enzymes
 They help ribosomes attach to the 5 end
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-9
Protein-coding segment
PolyA signal
5
G
3
P P P
5 Cap
AAUAAA
5 UTR Start codon
Stop codon
3 UTR
AAA…AAA
Poly-A tail
Exons, Introns, and RNA Splicing
•
•
•
•
Most eukaryotic genes and their RNA transcripts have
pieces that don’t code for useful proteins.
These noncoding regions are called introns
The other parts are called exons because they are
eventually expressed (translated into amino acid
sequences)
RNA splicing removes the introns and joins the exons
together, creating mRNA with a continuous coding
sequence
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-10
5 Exon Intron
Exon
Exon
Intron
3
Pre-mRNA 5 Cap
Poly-A tail
1
31
30
Coding
segment
104
105
146
Introns cut out and
exons spliced together
mRNA 5 Cap
Poly-A tail
1
146
The Functional and Evolutionary Importance of
Introns



Some genes can code for more than one kind of
protein, depending on which segments are treated as
exons during RNA splicing
These variations are called alternative RNA splicing
Because of alternative RNA splicing, the number of
different proteins an organism can produce is much
greater than its number of genes
http://www.dnalc.org/view/16941-2D-Animation-of-Alternative-RNA-Splicing.html
http://www.dnalc.org/view/16940-Alternative-RNA-Splicing.html
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
After RNA processing, the mRNA is
translated.


A cell translates an mRNA message into protein with
the help of transfer RNA (tRNA)
Molecules of tRNA are not identical:
 Each
carries a specific amino acid on one end
 Each has an anticodon on the other end; the anticodon
base-pairs with a complementary codon on mRNA
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-13
Amino
acids
Polypeptide
tRNA with
amino acid
attached
Ribosome
tRNA
Anticodon
Codons
5
mRNA
3
The Structure and Function of Transfer RNA


A tRNA molecule consists of a single RNA strand that
is only about 80 nucleotides long
Flattened into one plane to reveal its base pairing, a
tRNA molecule looks like a cloverleaf
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-14a
3
Amino acid
attachment site
5
Hydrogen
bonds
Anticodon
(a) Two-dimensional structure


Because of hydrogen bonds, tRNA actually twists and
folds into a three-dimensional molecule
tRNA is roughly L-shaped
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-14b
Amino acid
attachment site
5
3
Hydrogen
bonds
3
Anticodon
(b) Three-dimensional structure
5
Anticodon
(c) Symbol used
in this book

Accurate translation requires two steps:
 First:
a correct match between a tRNA and an amino acid
(tRNA with an amino acid attached is “charged” tRNA)
 Second: a correct match between the tRNA anticodon and
an mRNA codon

Flexible pairing at the third base of a codon is called
wobble and allows some tRNAs to bind to more than
one codon
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Ribosomes


Ribosomes help tRNA anticodons to pair up with
specific mRNA codons in protein synthesis
The two ribosomal subunits (large and small) are made
of proteins and ribosomal RNA (rRNA)
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-16b
P site (Protein
holding site)
E site
(Exit site)
A site (Arrival site)
E P A
mRNA
binding site

Large
subunit
Small
subunit
A ribosome has three binding sites for tRNA:
 The
A site holds the new tRNA that carries the next amino acid
that will be added
 The P site holds the tRNA that carries the growing polypeptide
chain
 The E site is the exit site, where discharged tRNAs leave the
ribosome
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Initiation of Translation




First, a small ribosomal subunit binds with mRNA and a
special initiator tRNA
Then the small subunit moves along the mRNA until it
reaches the start codon (AUG)
Proteins called initiation factors bring in the large subunit
The initiator tRNA sticks to the AUG codon. What is the
anticodon on this tRNA?
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-17
3 U A C 5
5 A U G 3
Initiator
tRNA
Large
ribosomal
subunit
P site
GTP GDP
E
mRNA
5
Start codon
mRNA binding site
3
Small
ribosomal
subunit
5
A
3
Translation initiation complex
Elongation of the Polypeptide Chain




A tRNA recognizes the mRNA codon its anticodon fits
with and binds to the mRNA in the A site
The tRNA in the P site passes the chain of amino acids
to the tRNA in the A site
Both tRNAs move over to the next site
The tRNA that is now in the E site exits and the process
repeats
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-18-1
Amino end
of polypeptide
E
3
mRNA
5
P
A
site site
Fig. 17-18-2
Amino end
of polypeptide
E
3
mRNA
5
P A
site site
GTP
GDP
E
P A
Fig. 17-18-3
Amino end
of polypeptide
E
3
mRNA
5
P A
site site
GTP
GDP
E
P A
E
P A
Fig. 17-18-4
Amino end
of polypeptide
E
3
mRNA
Ribosome ready for
next aminoacyl tRNA
P A
site site
5
GTP
GDP
E
E
P A
P A
GDP
GTP
E
P A
Termination of Translation




Termination occurs when a stop codon in the mRNA
reaches the A site of the ribosome
The A site accepts a protein called a release factor
The release factor adds a water molecule instead of
an amino acid
The polypeptide, the mRNA, and the two ribosomal
subunits come apart and float off into the cytoplasm
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-19-1
Release
factor
3
5
Stop codon
(UAG, UAA, or UGA)
Fig. 17-19-2
Release
factor
Free
polypeptide
3
5
5
Stop codon
(UAG, UAA, or UGA)
3
2 GTP
2 GDP
Fig. 17-19-3
Release
factor
Free
polypeptide
5
3
5
5
Stop codon
(UAG, UAA, or UGA)
3
2 GTP
2 GDP
3
Polyribosomes


A number of ribosomes can translate a single mRNA
simultaneously, forming a polyribosome
Polyribosomes enable a cell to make many copies of a
polypeptide very quickly
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-20
Growing
polypeptides
Completed
polypeptide
Incoming
ribosomal
subunits
Start of
mRNA
(5 end)
(a)
End of
mRNA
(3 end)
Ribosomes
mRNA
(b)
0.1 µm
Targeting Polypeptides to Specific
Locations




Cells have 2 kinds of ribosomes: free ribosomes and bound
ribosomes
Free ribosomes mostly synthesize proteins that are used in the
cytoplasm
Bound ribosomes make proteins of the endomembrane system
and proteins that are secreted from the cell
Ribosomes are identical and can switch from free to bound
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
•
•
•
•
Protein synthesis always begins in the cytoplasm
It finishes in the cytoplasm unless the polypeptide
signals the ribosome to attach to the ER
Polypeptides destined for the ER are marked by a
signal peptide
A signal-recognition particle (SRP) binds to the signal
peptide and brings the signal peptide and its
ribosome to the ER
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-21
Ribosome
mRNA
Signal
peptide
Signal
peptide
removed
Signalrecognition
particle (SRP)
CYTOSOL
ER LUMEN
Translocation
complex
SRP
receptor
protein
ER
membrane
Protein
Types of Mutations



Mutations are changes in the genetic material of a cell
or virus
Point mutations are changes in just one base pair of
a gene
The change of a single nucleotide in a DNA template
strand can lead to the production of an abnormal
protein
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-22
Wild-type hemoglobin DNA
Mutant hemoglobin DNA
C T T
C A T
3
5 3
G T A
5
G A A
3 5
mRNA
5
5
3
mRNA
G A A
Normal hemoglobin
Glu
3 5
G U A
Sickle-cell hemoglobin
Val
3
Types of Point Mutations

Point mutations within a gene can be divided into two
general categories
 Base-pair
substitutions
 Base-pair insertions or deletions
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-23a
Wild type
DNA template 3
strand 5
5
3
mRNA 5
3
Protein
Stop
Amino end
Carboxyl end
A instead of G
5
3
3
5
U instead of C
5
3
Stop
Silent (no effect on amino acid sequence)
Fig. 17-23b
Wild type
DNA template 3
strand 5
5
3
mRNA 5
3
Protein
Stop
Amino end
Carboxyl end
T instead of C
5
3
3
5
A instead of G
3
5
Stop
Missense
Fig. 17-23c
Wild type
DNA template 3
strand 5
5
3
mRNA 5
3
Protein
Stop
Amino end
Carboxyl end
A instead of T
3
5
5
3
U instead of A
5
3
Stop
Nonsense
Fig. 17-23d
Wild type
DNA template 3
strand 5
5
3
mRNA 5
3
Protein
Stop
Amino end
Carboxyl end
Extra A
5
3
3
5
Extra U
5
3
Stop
Frameshift causing immediate nonsense (1 base-pair insertion)
Fig. 17-23e
Wild type
DNA template 3
strand 5
5
3
mRNA 5
3
Protein
Stop
Amino end
Carboxyl end
missing
5
3
3
5
missing
5
3
Frameshift causing extensive missense (1 base-pair deletion)
Fig. 17-23f
Wild type
DNA template 3
strand 5
5
3
mRNA 5
3
Protein
Stop
Amino end
Carboxyl end
missing
5
3
3
5
missing
5
3
Stop
No frameshift, but one amino acid missing (3 base-pair deletion)
Substitutions




A base-pair substitution replaces one nucleotide and
its partner with another pair of nucleotides
Silent mutations have no effect on the amino acid
produced by a codon because of redundancy in the
genetic code
Missense mutations still code for an amino acid, but
not necessarily the right amino acid
Nonsense mutations change an amino acid codon
into a stop codon, nearly always leading to a
nonfunctional protein
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Insertions and Deletions



Insertions and deletions are additions or losses of
nucleotide pairs in a gene
Insertion or deletion of nucleotides may alter the
reading frame, producing a frameshift mutation
These mutations can change the protein much more
than a substitution does, and can be disastrous
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Comparing Gene Expression in Bacteria,
Archaea, and Eukarya



Bacteria have a terminator sequence, archaea and
eukarya do not
Bacteria (and probably archaea) can transcribe and
translate the same gene at the same time
Eukaryotes must do these steps separately, because
transcription happens in the nucleus and translation
happens in the cytoplasm
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-24
RNA polymerase
DNA
mRNA
Polyribosome
RNA
polymerase
Direction of
transcription
0.25 µm
DNA
Polyribosome
Polypeptide
(amino end)
Ribosome
mRNA (5 end)
What Is a Gene? Revisiting the Question


The idea of the gene itself is a unifying concept of life
We have considered a gene as:
A
discrete unit of inheritance
 A region of specific nucleotide sequence in a chromosome
 A DNA sequence that codes for a specific polypeptide
chain
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings
Fig. 17-25
DNA
TRANSCRIPTION
3
RNA
polymerase
5 RNA
transcript
RNA PROCESSING
Exon
RNA transcript
(pre-mRNA)
Intron
Aminoacyl-tRNA
synthetase
NUCLEUS
Amino
acid
CYTOPLASM
AMINO ACID ACTIVATION
tRNA
mRNA
Growing
polypeptide
3
A
Activated
amino acid
P
E
Ribosomal
subunits
5
TRANSLATION
E
A
Codon
Ribosome
Anticodon

In summary, a gene can be defined as a piece of DNA
that can be expressed to produce a functional protein
and is inherited by organisms from their parents
Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings