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Chapter 9
From DNA to
Protein
9.1 The Aptly Acronymed RIPs
• A tiny amount of ricin, a natural protein found in castor-oil
seeds, can kill an adult human – there is no antidote
• Ricin is a ribosome-inactivating protein (RIP) – it inactivates
the organelles which assemble amino acids into proteins
• Other RIPs include shiga toxin, made by Shigella dysenteriae
bacteria, and enterotoxins made by E. coli bacteria, including
the strain O157:H7
Some RIPs
9.2 DNA, RNA, and Gene Expression
• Transcription converts information in a gene to RNA
DNA → transcription → mRNA
• Translation converts information in an mRNA to protein
mRNA → translation → protein
The Nature of Genetic Information
• Each DNA strand consists of a chain of four kinds of
nucleotides: adenine (A), thymine (T), guanine (G), and
cytosine (C)
• The sequence of the bases in the strand is the genetic code
• All of a cell’s RNA and protein products are encoded by DNA
sequences called genes
Converting a Gene to an RNA
• Transcription
• Enzymes use the nucleotide sequence of a gene to
synthesize a complementary strand of RNA
• DNA is transcribed to RNA
• Most RNA is single stranded
• RNA uses uracil in place of thymine
• RNA uses ribose in place of deoxyribose
A DNA Nucleotide
base
(guanine)
3 phosphate groups
A DNA nucleotide:
guanine (G), or
deoxyguanosine triphosphate
sugar
(deoxyribose)
An RNA Nucleotide
base
(guanine)
3 phosphate groups
An RNA nucleotide:
guanine (G), or
guanosine triphosphate
sugar
(ribose)
ANIMATED FIGURE: Gene transcription
details
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adenine A
DNA
deoxyribonucleic acid
RNA
ribonucleic acid
sugar–
phosphate
backbone
guanine G
cytosine C
adenine A
guanine G
cytosine C
nucleotide
base
base pair
thymine T
Nucleotide bases
of DNA
uracil U
DNA has one function:
It permanently stores a
cell’s genetic
information, which is
passed to offspring.
RNAs have various functions.
Some serve as disposable
copies of DNA’s genetic
message; others are catalytic.
Still others have roles in gene
control.
Nucleotide bases
of DNA
Figure 9-3 p151
RNA in Protein Synthesis
• Messenger RNA (mRNA)
• Contains information transcribed from DNA
• Ribosomal RNA (rRNA)
• Main component of ribosomes, where polypeptide chains
are built
• Transfer RNA (tRNA)
• Delivers amino acids to ribosomes
Converting mRNA to Protein
• Translation
• The information carried by mRNA is decoded into a
sequence of amino acids, resulting in a polypeptide chain
that folds into a protein
• mRNA is translated to protein
• rRNA and tRNA translate the sequence of base triplets in
mRNA into a sequence of amino acids
Gene Expression
• A cell’s DNA sequence (genes) contains all the information
needed to make the molecules of life
• Gene expression
• A multistep process including transcription and translation,
by which genetic information encoded by a gene is
converted into a structural or functional part of a cell or
body
Take-Home Message: What is the nature of
genetic information carried by DNA?
• Genetic information occurs in DNA sequences (genes) that
encode instructions for building RNA or protein products
• A cell transcribes the nucleotide sequence of a gene into RNA
• Although RNA is structurally similar to a single strand of DNA,
the two types of molecules differ functionally
• A messenger RNA (mRNA) carries a protein-building code in
its nucleotide sequence; rRNAs and tRNAs interact to
translate the sequence into a protein
9.3 Transcription: DNA to RNA
• RNA polymerase assembles RNA by linking RNA nucleotides
into a chain, in the order dictated by the base sequence of a
gene
• A new RNA strand is complementary in sequence to the DNA
strand from which it was transcribed
DNA Replication and Transcription
• DNA replication and transcription both synthesize new
molecules by base-pairing
• In transcription, a strand of mRNA is assembled on a DNA
template using RNA nucleotides
• Uracil (U) nucleotides pair with A nucleotides
• RNA polymerase adds nucleotides to the transcript
The Process of Transcription
• RNA polymerase and regulatory proteins attach to a
promoter (a specific binding site in DNA close to the start of
a gene)
• RNA polymerase moves over the gene in a 5' to 3' direction,
unwinds the DNA helix, reads the base sequence, and joins
free RNA nucleotides into a complementary strand of mRNA
RNA
polymerase
gene region
RNA
promoter sequence in DNA
1 RNA polymerase binds to a promoter in the
DNA. The binding positions the polymerase near a
gene. In most cases, the base sequence of the gene
occurs on only one of the two DNA strands. Only
the DNA strand complementary to the gene
sequence will be translated into RNA.
DNA winding up
DNA unwinding
2 The polymerase begins to move along the DNA and
unwind it. As it does, it links RNA nucleotides into a
strand of RNA in the order specified by the base
sequence of the DNA. The DNA winds up again after
the polymerase passes. The structure of the “opened”
DNA at the transcription site is called a transcription
bubble, after its appearance.
direction of transcription
3 Zooming in on the gene region, we can see that RNA polymerase
covalently bonds successive nucleotides into an RNA strand. The
base sequence of the new RNA strand is complementary to the base
sequence of its DNA template strand, so it is an RNA copy of the gene.
Stepped Art
Figure 9-4 p152
RNA transcripts
DNA molecule
Figure 9-5 p153
Post-Transcriptional Modifications
• In eukaryotes, RNA is modified before it leaves the nucleus
as a mature mRNA
• Introns
• Nucleotide sequences that are removed from a new RNA
• Exons
• Sequences that stay in the RNA
Alternative Splicing
• Alternative splicing
• Allows one gene to encode different proteins
• Some exons are removed from RNA and others are
spliced together in various combinations
• After splicing, transcripts are finished with a modified guanine
“cap” at the 5' end and a poly-A tail at the 3' end
gene
promoter
exon
intron
exon
intron
exon
DNA
transcription
exon
intron
exon
intron
exon
new transcript
RNA processing
exon
finished RNA
5′
exon
exon
3′
cap
poly-A tail
Figure 9-6 p153
ANIMATED FIGURE: Pre-mRNA transcript
processing
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Take-Home Message:
How is RNA assembled?
• In transcription, RNA polymerase uses the nucleotide
sequence of a gene region in a chromosome as a template to
assemble a strand of RNA
• The new strand of RNA is a copy of the gene from which it
was transcribed
9.4 RNA and the Genetic Code
• Base triplets in an mRNA encode a protein-building message
• Ribosomal RNA (rRNA) and transfer RNA (tRNA) translate
that message into a polypeptide chain
mRNA – The Messenger
• mRNA carries protein-building information to ribosomes and
tRNA for translation
• Codon
• A sequence of three mRNA nucleotides that codes for a
specific amino acid
• The order of codons in mRNA determines the order of
amino acids in a polypeptide chain
Genetic Code
• Genetic code
• Consists of 64 mRNA codons (triplets)
• Twenty kinds of amino acids are found in proteins
• Some amino acids can be coded by more than one codon
• Some codons signal the start or end of a gene
• AUG (methionine) is a start codon
• UAA, UAG, and UGA are stop codons
Figure 9-7a p154
Figure 9-7b p154
From DNA to RNA to Amino Acids
a gene
region in DNA
transcription
codon
codon
codon
mRNA
translation
methionine
(met)
tyrosine
(tyr)
serine
(ser)
amino acid
sequence
rRNA and tRNA – The Translators
• tRNAs deliver amino acids to ribosomes
• tRNA has an anticodon complementary to an mRNA
codon, and a binding site for the amino acid specified by
that codon
• Ribosomes, which link amino acids into polypeptide chains,
consist of two subunits of rRNA and proteins
tRNA Structure
anticodon
anticodon
amino acid
attachment site
Translation: Ribosome and tRNA
Take-Home Message: What roles do mRNA,
tRNA, and rRNA play during translation?
• mRNA carries protein-building information; the bases in
mRNA are “read” in sets of three during protein synthesis;
most base triplets (codons) code for amino acids; the genetic
code consists of all sixty-four codons
• Ribosomes, which consist of two subunits of rRNA and
proteins, assemble amino acids into polypeptide chains
• A tRNA has an anticodon complementary to an mRNA codon,
and it has a binding site for the amino acid specified by that
codon; transfer RNAs deliver amino acids to ribosomes
ANIMATED FIGURE: Structure of a
ribosome
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