Transcript chapter9_Sections 4-6 - (per 3) and wed 4/24 (per 2,6)

Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 9
From DNA to Protein
(Sections 9.4 - 9.7)
Albia Dugger • Miami Dade College
9.4 RNA and the Genetic Code
• Three types of RNA interact to translate DNA’s information
into a protein: messenger RNA (mRNA), transfer RNA (tRNA),
and ribosomal RNA (rRNA)
Genetic Code
• The protein-building information in mRNA consists of a
sequence of three mRNA bases (codon); each is a code for a
particular amino acid
• The four bases A, C, G, and U can be combined into 64
different codons, which constitute the genetic code
• Example: AUG codes for the amino acid methionine (met),
and UGG codes for tryptophan (trp)
Key Terms
• codon
• In mRNA, a nucleotide base triplet that codes for an amino
acid or stop signal during translation
• genetic code
• Complete set of sixty-four mRNA codons
Codons and Amino Acids
• There are only twenty kinds of amino acids found in proteins,
so some amino acids are specified by more than one
codon
• Some codons signal the beginning and end of a proteincoding sequence:
• AUG (methionine) start translation
• UAA, UAG, and UGA are stop codons
• The order of mRNA codons determines the order of amino
acids in the polypeptide that will be translated from it
The Genetic Code
Animation: Genetic Code
DNA, mRNA, and Proteins
DNA, mRNA, and Proteins
a gene
region in DNA
transcription
codon
codon
codon
mRNA
translation
methionine
(met)
tyrosine
(tyr)
serine
(ser)
amino acid
sequence
Fig. 9.8, p. 143
rRNA and tRNA: The Translators
• Ribosomes (containing rRNA and structural proteins) and
tRNAs interact to translate an mRNA into a polypeptide
• One large and one small ribosomal subunit join with mRNA,
and rRNA enzymatically catalyzes the formation of a peptide
bond between amino acids
• Transfer RNAs deliver amino acids to ribosomes in the order
specified by mRNA
Ribosome Structure
• A polypeptide chain threads through the tunnel in the large
subunit as it is being assembled by the ribosome
Ribosome Structure
tunnel
large subunit
small subunit
intact ribosome
Fig. 9.9, p. 143
ANIMATION: Structure of a ribosome
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tRNA Structure
• Each tRNA has two attachment sites:
• A triplet of nucleotides (anticodon) base-pairs with an
mRNA codon
• Another attachment site binds to the amino acid specified
by the codon
• Transfer RNAs with different anticodons carry specific amino
acids to a ribosome during translation of an mRNA
tRNA Structure
tRNA Structure
anticodon
amino acid
attachment site
A
B
Fig. 9.10, p. 143
ANIMATION: Structure of a tRNA
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Key Concepts
• RNA
• Messenger RNA (mRNA) carries DNA’s protein-building
instructions
• Its nucleotide sequence is read three bases at a time
• Sixty-four mRNA base triplets—codons—represent the
genetic code
• Two other types of RNA interact with mRNA during
translation of that code
9.5 Translating the Code:
RNA to Protein
• Translation converts the information carried by an mRNA into
a polypeptide
• Translation occurs in the cytoplasm
• Translation proceeds in three stages: initiation, elongation,
and termination
6 Steps in Translation
(1) Translation
initiates when
ribosome subunits
and an initiator
tRNA (attached to
a start codon)
converge on an
mRNA
• A second tRNA
binds to the
second codon, and
so on…
6 Steps in Translation
(2) The ribosome
catalyzes
formation of a
peptide bond
between the first
two amino acids
6 Steps in Translation
(3) The first tRNA
is released and
the ribosome
moves to the next
codon.
• A third tRNA binds
to the third codon
6 Steps in Translation
(4) A peptide
bond forms
between the
second and third
amino acids
6 Steps in Translation
(5) The second
tRNA is released
and the
ribosome moves
to the next codon
• A fourth tRNA
binds the fourth
codon
6 Steps in Translation
(6) A peptide bond
forms between the
third and fourth
amino acids
• Process repeats
until the ribosome
encounters a stop
codon
6 Steps in Translation
start codon
(AUG)
Ribosome subunits
and an initiator tRNA
converge on an
mRNA. A second tRNA
binds to the second
codon.
1
initiator tRNA
first amino acid
of polypeptide
Fig. 9.11.1, p. 144
6 Steps in Translation
A peptide bond
forms between the
first two amino
acids.
2
peptide bond
Fig. 9.11.2, p. 144
6 Steps in Translation
The first tRNA is
released and the
ribosome moves to the
next codon. A third tRNA
binds to the third codon.
3
Fig. 9.11.3, p. 144
6 Steps in Translation
A peptide bond forms
between the second and
third amino acids.
4
Fig. 9.11.4, p. 144
6 Steps in Translation
The second tRNA is
released and the ribosome
moves to the next codon.
A fourth tRNA binds the
fourth codon.
5
Fig. 9.11.5, p. 144
start codon
(AUG)
initiator tRNA
first amino acid
of polypeptide
6 Steps in Translation
1 Ribosome
subunits and an
initiator tRNA
converge on an
mRNA. A second
tRNA binds to the
second codon.
3 The first tRNA
is released and the
ribosome moves to
the next codon. A
third tRNA binds to
the third codon.
5 The second
tRNA is released
and the ribosome
moves to the next
codon. A fourth
tRNA binds the
fourth codon.
peptide bond
2 A peptide
bond forms
between the
first two amino
acids.
4 A peptide bond
forms between the
second and third
amino acids.
6 A peptide bond
forms between the
third and fourth
amino acids.
The process repeats
until the ribosome
encounters a stop
codon in the mRNA.
Stepped Art
Fig. 9.11, p. 144
ANIMATION: Translation
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Overview of Translation
Overview of Translation
Transcription
polysomes
RNA
transport
ribosome
subunits
tRNA
Convergence of RNAs
mRNA
Translation
polypeptide
mRNA polysomesnewly forming polypeptide
Fig. 9.12, p. 145
Key Concepts
• RNA to Protein: Translation
• Translation is an energy-intensive process by which a
sequence of codons in mRNA is converted to a sequence
of amino acids in a polypeptide chain
• Transfer RNAs deliver amino acids to ribosomes, which
catalyze the formation of peptide bonds between the
amino acids
9.6 Mutated Genes
and Their Protein Products
• If a mutation (change in genetic code) changes the genetic
instructions encoded in the DNA, an altered gene product
may result
• Example: Hemoglobin consists of four polypeptides (globins)
folded around a heme (iron-containing cofactor)
• Various defects in the polypeptides can cause anemia
Hemoglobin
• Hemoglobin consists of
4 polypeptides: 2 alpha
globins (blue) and 2
beta globins (green)
• Oxygen molecules bind
to the iron atom at the
center of each heme
Defects in Hemoglobin
• Frameshift mutations:
• A deletion from DNA of the beta globin gene causes a
type of anemia called beta thalassemia
• An insertion mutation can also alter polypeptides
• deletion
• Mutation in which one or more base pairs are lost
• insertion
• Mutation in which one or more base pairs are added
Frameshift Mutations
• A frameshift garbles the genetic message like adding or
deleting a letter garbles the meaning of a sentence:
The cat ate the rat
T hec ata tet her at
Th eca tat eth era t
Base-Pair Deletion
Defects in Hemoglobin (cont.)
• Other types of mutations do not cause frameshifts:
• In base-pair-substitution, a nucleotide and its partner are
replaced by a different base pair
• Sickle-cell anemia results from a substitution of valine for
glutamic acid
• base-pair substitution
• Type of mutation in which a single base-pair changes
Base-Pair Substitution
Base-Pair
Substitution
B Part of the DNA (blue), mRNA (brown), and amino acid sequence (green) of human
beta globin.
C A base-pair deletion causes the reading frame for the rest of the mRNA to shift, so a
completely different protein product forms. The mutation shown results in a defective
beta globin. The outcome is beta thalassemia, a genetic disorder in which a person has an
abnormally low amount of hemoglobin.
D A base-pair substitution replaces a thymine with an adenine. When the altered mRNA is
translated, valine replaces glutamic acid as the sixth amino acid of the polypeptide.
Hemoglobin with this form of beta globin is called HbS , or sickle hemoglobin.
Stepped Art
Fig. 9.13, p. 146
ANIMATION: Base-pair substitution
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Sickle-Cell Anemia
• Substitution of valine for
glutamic acid causes
HbS protein to clump
• Normally round red
blood cells are distorted
into sickle shapes
What Causes Mutations?
• There are many causes of mutations:
• Transposable elements can cause insertion mutations
• Mistakes occur during DNA replication
• Environmental agents can damage DNA
• Natural or synthetic chemicals can cause mutations
• transposable element
• Segment of DNA that can spontaneously move to a new
location in a chromosome
Environmental Factors in Mutation
• Ionizing radiation (such as x-rays) breaks chromosomes into
pieces that get lost during DNA replication, or forms
destructive free radicals
• Nonionizing radiation (such as UV light) can form thymine
dimers (two adjacent thymine bases covalently bonded to one
another) that kink DNA
• Chemicals in cigarette smoke can cause mispairing during
replication, or stop replication entirely
Key Concepts
• Mutations
• Small-scale, permanent changes in the nucleotide
sequence of DNA may result from replication errors, the
activity of transposable elements, or exposure to
environmental hazards
• Such mutations can change a gene’s product