Download Gene expression

Anu Singh-Cundy • Gary Shin
Discover Biology
SIXTH EDITION
CHAPTER 12
From Gene to Protein
© 2015 W. W. Norton & Company, Inc.
CHAPTER 12
From Gene to Protein
GREEK MYTHS AND ONE-EYED SHEEP
12.1 How Genes Work
Genes contain information for building RNA molecules
Three types of RNA assist in the manufacture of proteins
12.2 Transcription: Information Flow from DNA to RNA
12.3 The Genetic Code
12.4 Translation: Information Flow from mRNA to Protein
12.5 The Effect of Mutations on Protein Synthesis
Mutations can alter one or many bases in a gene’s DNA sequence
Mutations can cause a change in protein function
12.6 How Cells Control Gene Expression
Most genes are controlled at the transcriptional level
Gene expression can be regulated at several levels
BIOLOGY MATTERS: ONE ALLELE MAKES YOU STRONG, ANOTHER HELPS YOU ENDURE
APPLYING WHAT WE LEARNED: FROM GENE EXPRESSION TO CYCLOPS
Greek Myths and One-Eyed Sheep
•
In the Odyssey, the ancient Greek
classic by Homer, the hero Odysseus
outwits a one-eyed giant, a cyclops
named Polyphemus.
•
In vertebrate animals, cyclopia is
genetic disorder in which offspring are
born with a single eye and nose, and a
malformed mouth
•
To develop normally from a singlecelled zygote, an organism must turn on
(express) the right genes at the right
time and in the right place.
•
Even tiny missteps in gene expression
can result in improper embryo
development leading to birth disorders
such as cyclopia.
Gene expression gone horribly wrong:
cyclopia in newborn lamb
Genes Control Genetic Traits
Dystrophin is the longest human gene: it
consists of 2.4 million base pairs. Mutations in
the gene produce various muscle-wasting
disorders.
Child with Duchenne muscular dystrophy
•
•
•
Genetic information is encoded in genes, which control genetic traits.
A slightly different version of a gene (allele) produces a different version of the genetic
trait (produces a particular phenotype of that genetic trait).
Scientists work to understand how gene mutations produce new phenotypes, including
disease phenotypes.
Coding Genes Store Information Needed to
Build RNA and Proteins, Which in Turn Produce
the Phenotypes of Most Genetic Traits
•
•
A gene is any DNA sequence that is copied (transcribed) into RNA.
Proteins, including enzymes, are the key determinants of an individual’s phenotype.
Genes Contain Information for Building
RNA Molecules
RNA (green) transcribed from the Noggin gene
in a fetal mouse. The Noggin gene is expressed
(turned on) in the developing brain and in the
cartilage and bones of all mammals, including
humans. The gene is shut down in other tissues.
RNA Molecules Are Single-Stranded
Polynucleotides
•
Like DNA, RNA is a
polymer of
nucleotides.
•
RNA is singlestranded, whereas
DNA forms a
double-stranded
molecule twisted
into a spiral shape
(double helix).
• DNA and RNA also differ in the type of sugar used (ribose in RNA, deoxyribose in DNA).
• RNA uses the base uracil (U) in place of thymine (T) in DNA molecules.
DNA Stores Information in the Nucleus, RNA Carries
Information from the Nucleus to the Cytoplasm
DNA
RNA
Structure
Double-stranded; two polynucleotide
strands wound into a helix
Single-stranded polynucleotide; may fold back on itself
Sugar
Deoxyribose
Ribose
Nucleotides A, G, C, and T
A, G, C, and U
Function
Stores genetic information
Expresses genetic information—for example, by directing
the manufacture of a specific protein
Stability
Highly stable in most cells
Generally much less stable
Location
Nucleus, chloroplasts, and mitochondria Nucleus, chloroplasts, mitochondria, and cytosol in
in eukaryotes; cytosol in prokaryotes
eukaryotes; cytosol in prokaryotes
Three Types of RNA Assist in the
Manufacture of Proteins
TYPE OF RNA
FUNCTION
Messenger RNA (mRNA)
Specifies the order of amino acids in a protein
using a series of three-base codons, where
different amino acids are specified by particular
codons.
Ribosomal RNA (rRNA)
As a major component of ribosomes, assists in
making the covalent bonds that link amino acids
together to make a protein.
Transfer RNA (tRNA)
Transports the correct amino acid to the ribosome,
using the information encoded in the mRNA;
contains a three-base anticodon that pairs with a
complementary codon revealed in the mRNA.
Information Flows from DNA to RNA to Proteins
•
•
•
A complementary mRNA sequence is made using the information in the DNA sequence of
protein-coding genes during the process of transcription.
During translation, amino acids are covalently linked in the sequence dictated by the base
sequence of the mRNA; translation is carried out by ribosomes in the cytoplasm.
Translation also requires two other types of RNA: rRNA and tRNA.
RNA Polymerase Synthesizes RNA Using One
Strand of the DNA as a Template
• Transcription occurs in the nucleus.
• An enzyme called RNA polymerase synthesizes RNA using one strand of the DNA
as template.
Table 12.3 A Comparison of Gene Transcription and DNA Replication
GENE TRANSCRIPTION
DNA REPLICATION
Key enzyme involved
RNA polymerase
DNA polymerase
Portion of chromosome copied
Small segment
Entire DNA double
Product
Single-stranded RNA molecule,
complementary to one DNA strand
(the template strand)
Double-stranded DNA molecule
Information Flow
from DNA to RNA
•
Transcription begins when RNA
polymerase binds to a segment
of DNA called a gene promoter.
•
Once bound, RNA polymerase
begins to unwind the DNA and
transcribe the template strand
(bottom strand in diagram);
which strand serves as the
template is dictated by the
positioning of the promoter,
which orients the polymerase.
• Transcription stops when RNA
polymerase reaches a special
sequence of bases called a
terminator.
In Eukaryotes,
mRNA Is Chemically
Modified After
Transcription
• Posttranscriptional processing
modifies RNA and prepares it
for export from the nucleus.
• The newly formed mRNA
undergoes RNA splicing, which
removes the introns, and is
then allowed to leave the
nucleus through the nuclear
pore.
Translation: Information Flow from
mRNA to Protein
• Translation is the process of converting a sequence of bases in mRNA to a
sequence of amino acids in a protein.
• Translation occurs at the ribosomes, which are made up of proteins and rRNA.
The Base Sequence
of mRNA Is Read as
a Sequence Codons
• Each unique sequence of three bases
is called a codon.
• When reading the code, the
ribosomes begin at the start codon,
AUG, and end at one of three stop
codons: UAA, UAG, or UGA.
There Are 64 Codons That Make Up the
Information in the Genetic Code
• The genetic code has
several distinct
characteristics:
– It is unambiguous
– It is redundant
– It is virtually universal
After Ribosomes Bind the mRNA, Each Specific Amino Acid Is
Delivered to the Ribosome-mRNA Complex by a tRNA Molecule
Specialized to Deliver a Specific Type of Amino Acid
• An anticodon is a
three-base sequence
that determines
which codons on the
mRNA can be
recognized by the
tRNA.
•
Each codon on the mRNA is recognized by a specific tRNA, and the ribosome adds the
amino acid delivered by this tRNA to the growing amino acid chain.
Translation Begins
When a tRNA Molecule
Recognizes and Pairs
with the AUG of the
Start Codon
• The process continues until a
stop codon is reached and the
mRNA and the completed
amino acid chain both
separate from the ribosome.
Protein Synthesis through
Translation
Initiation
Elongation
The First
Covalent
Bond
Between
Amino Acids:
The
Polypeptide
Chain Begins
Chain
Elongation
Continues
Chain
Termination
A Mutation Is a Change in the Base
Sequence of an Individual’s DNA
•
Mutations can range from a change in a single base pair to the
deletion of one or more whole chromosomes.
•
Mutations in which a single base is altered are point mutations.
•
•
There are three main types of mutations:
1. Insertions
2. Deletions
3. Substitutions
A substitution mutation occurs when one base is substituted for
another in a DNA sequence.
Insertion/Deletion
Mutations Are Usually
More Disruptive Than
Substitution Mutations
•
Insertion or deletion mutations
occur when a base is inserted
into or deleted from a DNA
sequence.
•
Unlike a substitution mutation,
insertion or deletion of one or
two nucleotides causes a
frameshift mutation, which
scrambles the downstream
sequence of amino acids.
•
Frameshift mutations stop
protein synthesis by introducing
accidental stop codons or alter
the identity of many amino acids
in a protein.
Mutations Can Alter Protein Function
•
Even single-base changes can
alter protein function enough to
produce a harmful phenotype
such as a disease.
•
Frameshift mutations alter the
protein so extensively that they
invariably destroy the normal
function of the protein and
produce a severe phenotype.
•
A silent mutation causes no
change in the structure of the
protein, and therefore no
change in the phenotype of the
organism.
•
Rarely, a mutation can be
beneficial and improve the
efficiency or functionality of a
protein.
The replacement of
glutamic acid by valine
changes the shape of
hemoglobin. The
accumulation of large
amounts of the deformed
protein distorts the shape
of red blood cells in
people with sickle-cell
anemia.
How Cells Control Gene Expression
• Gene expression refers
to the transcription and
translation of a gene to
produce a functional
protein that has an effect
on phenotype.
• Different sets of genes are expressed in different cell types.
• Gene expression changes during development and can change in
response to environmental signals and internal signals such as
hormones.
Most Genes Are Controlled at the
Transcriptional Level
• Cells generally control gene expression by regulating the transcription of specific
genes.
• Regulatory DNA is the part of a gene that controls gene transcription with the
help of gene regulatory proteins.
• Gene regulatory proteins, also called transcription factors, interact with signals
from the environment and regulatory DNA to control gene expression.
In Some Bacteria, the Genes for Utilizing Lactose
Are Turned On Only if Lactose Is Available
Operon: single
promoter controlling
transcription of a
cluster of genes
with related functions.
Gene Expression Can Be
Regulated at Several Levels
•
Tight packing of DNA prevents access to its gene
regulatory DNA, making that segment of DNA
transcriptionally inactive.
•
Regulation of transcription enables the cell to
conserve resources when it does not need a particular
gene product.
•
By limiting the life span of many types of mRNA, a cell
prevents the wasteful synthesis of proteins.
•
Regulation of translation keeps mRNA ready to direct
rapid protein synthesis when needed.
•
Proteins can be directly regulated by modification
following translation.
•
Regulation of protein breakdown conserves resources.
APPLYING WHAT WE LEARNED:
FROM GENE EXPRESSION TO CYCLOPS
• Environmental influences, such as ingestion of the corn lily by a pregnant
animal, can cause gene expression to be altered, resulting in abnormalities such
as cyclopia.
BIOLOGY MATTERS: ONE ALLELE MAKES YOU
STRONG, ANOTHER HELPS YOU ENDURE
• The goal of personal genomics is to inspect and catalog an individual’s total genetic
makeup, or genome.
• Personal genomics brings us personalized medicine,
the practice of tailoring health care and disease
prevention to a person’s genotype.
• Commercial tests for athletic potential are available,
based on the R and X alleles of the ACTN3 gene.
• XX genotype is unusually
common in endurance athletes
(24 percent), but rare in strength-sport
athletes, who are more likely
to be RR than all others.
• Knocking out the ACTN3 gene
(XX genotype) lead to
“marathon mice.”
ENDURANCE-SPORT ELITE
ATHLETES (DISTANCE
RUNNERS)
31
GENOTYPE
RR
CONTROL (NONATHLETES)
30
STRENGTH-SPORT ELITE
ATHLETES (SPRINTERS)
50
RX
52
45
45
XX
18
6
24
List of Key Terms: Chapter 12
anticodon (p. 269)
codon (p. 267)
deletion (p. 271)
elongation (transcription, p. 265;
translation, p. 269)
exon (p. 266)
frameshift (p. 271)
gene (p. 262)
gene expression (p 272)
gene promoter (p. 265)
genetic code (p. 267)
initiation (transcription, p. 265;
translation, p. 269)
insertion (p. 271)
intron (p. 266)
messenger RNA (mRNA) (p. 264)
operator (p. 273)
operon (p. 272)
point mutation (p. 271)
regulatory DNA (p. 272)
regulatory protein (p. 272)
repressor (p. 273)
ribosomal RNA (rRNA) (p. 264)
RNA polymerase (p. 265)
RNA splicing (p. 267)
start codon (p. 267)
stop codon (p. 267)
substitution (p. 271)
template strand (p. 265)
termination (transcription, p. 265;
translation, p. 270)
terminator (p. 266)
transcription (p. 264)
transfer RNA (tRNA) (p. 264)
translation (p. 264)
Class Quiz, Part 1
Which of the following is true?
A. Transcription occurs in the cytoplasm and
produces RNA.
B. Transcription occurs in the nucleus and
produces proteins.
C. Translation occurs in the cytoplasm and
produces proteins.
D. Translation occurs in the nucleus and
produces RNA.
Class Quiz, Part 2
Dystrophin is the largest
protein in the human body.
In this concept map, which of
the following fits best in the box
labeled with an “X”?
A. DNA
B. polypeptide
C. phenotype
D. mRNA
E. tRNA
Class Quiz, Part 3
Which of the following is not true about
the genetic code?
A.
B.
C.
D.
Every codon has a corresponding amino acid.
Every codon consists of three bases.
There are 64 possible codons.
A single amino acid can have more than one
codon.
Class Quiz, Part 4
Frameshift mutations
A.
B.
C.
D.
occur only if three bases are deleted.
occur when one base is changed to another.
don’t change the structure of the protein.
can be caused by either the insertion or
deletion of a single base.
Relevant Art from Other
Chapters
All art files from the book are available in
JPEG and PPT formats online and on the
Instructor Resource Disc
Nucleotides and Nucleic Acids
Most Genes Code for Proteins,
Which Generate Phenotypes
• RNA is a single-stranded nucleic
acid similar to DNA.
• Messenger RNA (mRNA) delivers
the genetic information, or
instructions, from DNA to the
ribosomes, where proteins are
made.
• The conversion of a DNA-based
sequence of nucleotides in a gene
to an RNA-based sequence is
called transcription.
• The process by which ribosomes
convert the genetic information in
mRNA into proteins is known as
translation.
Each protein has a unique amino acid sequence, which
gives it a unique function; protein function produces the
phenotype, the particular version of a genetic trait in the
individual organism.
Failure in DNA Repair Generates a
Mutation, a Change in the DNA Sequence
•
A change to the sequence of bases in
an organism’s DNA is called a
mutation.
•
Mutagens are substances or energy
sources that can cause mutations.
•
New alleles arise as a result of
mutations.
•
Most genetic mutations are neutral
or harmful.
•
A mutation may consist of change in
a single base or a large-scale change
involving chromosomal
abnormalities.
12.1 Concept Check, Part 1
1. Do all genes code for mRNA and therefore
for proteins?
ANSWER: No. RNA-only genes are transcribed into
RNA other than mRNA, and these RNAs have
specialized functions other than coding for proteins.
12.1 Concept Check, Part 2
2. Compare the chemical structures of RNA and DNA.
Which is more stable chemically, and how is that stability
consistent with its function?
ANSWER: RNA is single-stranded; it contains ribose
and the bases A, G, C, and U. DNA is double-stranded;
it contains deoxyribose and A, G, C, and T. DNA is
more stable—a property it must have to serve as the
storehouse of genetic information.
12.1 Concept Check, Part 3
3. What is the product of transcription? What
is the product of translation?
ANSWER: The product of transcription is an mRNA
complementary to the DNA sequence of a gene. The
product of translation is a polypeptide (protein chain)
determined from the sequence of the mRNA.
12.2 Concept Check, Part 1
1. The template strand of a gene has the base sequence
TGAGAAGACCAGGGTTGT. What is the sequence of RNA
transcribed from this DNA, assuming RNA polymerase
travels from left to right on this strand?
ANSWER: ACUCUUCUGGUCCCAACA
12.2 Concept Check, Part 2
2. The dystrophin gene has 78 introns. Are these
introns transcribed? Do they code for amino
acids?
ANSWER: All 78 introns are transcribed into a premRNA, but they are subsequently spliced out. Because
they are absent from the fully processed mature mRNA
transcript, introns do not code for amino acids.
12.3 Concept Check, Part 1
1. Why is the start codon, AUG, so important?
ANSWER: The start codon sets the reading frame, that
is, it determines the grouping of the bases in the mRNA
into triplets to be read as codons.
12.3 Concept Check, Part 2
2. What does it mean to say that the genetic
code is redundant?
ANSWER: There are 64 possible codons, but only 20
amino acids. In most cases, a single amino acid is
specified by more than one codon, and this is what is
meant by redundancy. For example, tyrosine is
specified by either UAU or UAC.
12.4 Concept Check, Part 1
1. What is meant by “translation” of mRNA?
ANSWER: Translation converts a sequence of bases in
mRNA to a sequence of amino acids in a protein.
12.4 Concept Check, Part 2
2. Does each of the 64 codons specify a
different amino acid?
ANSWER: No. The three stop codons do not specify
any amino acids, and a single amino acid may be
specified by as many as six different codons.
12.5 Concept Check, Part 1
1. What is a mutation? Are all mutations
harmful?
ANSWER: A mutation is a change in the base
sequence of an organism’s DNA. A mutation may have
no detectable effects or may be harmful; in rare
instances, it may even be advantageous.
12.5 Concept Check, Part 2
2. A single-base addition or deletion in a gene is likely
to alter the protein product more than a single-base
substitution, such as C for T, would. Why?
ANSWER: Single-base addition or deletion shifts the
reading frame, so all the amino acids downstream of
such a mutation are altered. Single-base substitution
alters, at most, a single amino acid.
12.6 Concept Check, Part 1
1. Why are most genes controlled at the level of
transcription?
ANSWER: Transcriptional regulation prevents gene
expression when a gene’s product is not needed by a
cell, enabling the cell to invest its resources elsewhere.
12.6 Concept Check, Part 2
2. If transcriptional control is the most favored method of
gene regulation, why are not all genes controlled at the
level of transcription?
ANSWER: Transcriptional activation is relatively slow;
controlling gene expression at a posttranscriptional
step enables a cell to respond faster to environmental
changes.