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Cecie Starr
Christine Evers
Lisa Starr
www.cengage.com/biology/starr
Chapter 9
From DNA to Protein
(Sections 9.1 - 9.3)
Albia Dugger • Miami Dade College
9.1 Ricin and Your Ribosomes
• Ricin, a natural protein in castor oil beans, is highly toxic: A
dose as small as a few grains of salt can kill an adult
• Ricin inactivates ribosomes – organelles that assemble
amino acids into proteins
• Proteins are critical to all life processes, so cells that cannot
make them die very quickly
Ricin
• One of ricin’s
polypeptide chains
helps the molecule
cross cell membranes
• The other chain
destroys a cell’s
capacity for protein
synthesis
2.2 Nature of Genetic Information
• DNA contains all of the instructions for building a new
individual
• The linear order or sequence of the four bases (A, T, G, C) in
the DNA strand is the genetic information, which occurs in
subsets called genes
• gene
• Part of a DNA base sequence
• Specifies an RNA or protein product
Converting a Gene to RNA
• Transcription converts information in a gene to RNA
• Enzymes use the nucleotide sequence of a gene as a
template to synthesize a strand of RNA (ribonucleic acid)
•
transcription
• Process by which an RNA is assembled from nucleotides
using the base sequence of a gene as a template
Three Types of RNA
• Three types of RNA have roles in protein synthesis:
• Ribosomal RNA (rRNA) is the main component of
ribosomes, the structures upon which polypeptide chains are
built
• Transfer RNA (tRNA) delivers amino acids to ribosomes in
the order specified by a messenger RNA (mRNA)
Key Terms
• messenger RNA (mRNA)
• Type of RNA that carries a protein-building message
• ribosomal RNA (rRNA)
• Type of RNA that becomes part of ribosomes
• transfer RNA (tRNA)
• Type of RNA that delivers amino acids to a ribosome
during translation
RNA Structure
• RNA is a single-stranded chain of four kinds of nucleotides
• Like DNA, a RNA nucleotide has three phosphate groups, a
sugar, and one of four bases, but RNA is slightly different:
• The sugar in RNA is ribose, not deoxyribose
• RNA uses the base uracil instead of thymine
An RNA and a DNA Nucleotide
An RNA Nucleotide
Fig. 9.2a, p. 138
An RNA Nucleotide
base
(guanine)
3 phosphate groups
sugar
(ribose)
An RNA nucleotide: guanine (G),
or guanosine triphosphate (GTP)
A Guanine, one of the four nucleotides in RNA. The others (adenine,
uracil, and cytosine) differ only in their component bases (blue). Three of
the four bases in RNA nucleotides are identical to the bases in DNA
nucleotides.
Fig. 9.2a, p. 138
A DNA Nucleotide
Fig. 9.2b, p. 138
A DNA Nucleotide
base
(guanine)
3 phosphate groups
sugar
(deoxyribose)
A DNA nucleotide: guanine (G), or
deoxyguanosine triphosphate (dGTP)
B Compare the DNA nucleotide guanine. The only difference between
the RNA and DNA versions of guanine (or adenine, or cytosine) is the
hydrogen atom or hydroxyl group at the 2’ carbon of the sugar (shown
in green).
Fig. 9.2b, p. 138
DNA and RNA Compared
DNA and RNA Compared
adenine A
DNA
deoxyribonucleic acid
RNA
ribonucleic acid
adenine A
nucleotide
base
sugar–
phosphate
backbone
guanine G
cytosine C
cytosine C
base pair
thymine T
Nucleotide bases
of DNA
guanine G
A DNA has one function: It
permanently stores a cell’s
genetic information, which
is passed to offspring.
uracil U
B Different types of RNA have
different functions. Some serve
as disposable copies of DNA’s Nucleotide bases
of RNA
genetic message; some are
catalytic; others have roles in
gene control.
Fig. 9.3, p. 139
DNA
Fig. 9.3a, p. 139
adenine A
DNA
DNA
deoxyribonucleic acid
nucleotide
base
guanine G
sugar–
phosphate
backbone
cytosine C
thymine T
Nucleotide bases
of DNA
base pair
A DNA has one function: It permanently
stores a cell’s genetic information,
which is passed to offspring.
Fig. 9.3a, p. 139
RNA
Fig. 9.3b, p. 139
RNA
ribonucleic acid
RNA
adenine A
nucleotide
base
sugar–
phosphate
backbone
guanine G
cytosine C
uracil U
B Different types of RNA have different
functions. Some serve as disposable copies
of DNA’s genetic message; some are
catalytic; others have roles in gene control.
Nucleotide bases
of RNA
Fig. 9.3b, p. 139
Converting mRNA to Protein
• Translation converts information in an mRNA to protein
• mRNA carries a protein-building message encoded in the
sequence of sets of three nucleotide bases
• mRNA is decoded (translated) into a sequence of amino
acids, resulting in a polypeptide chain that folds into a protein
• translation
• Process by which a polypeptide chain is assembled from
amino acids in the order specified by an mRNA
Gene Expression
• Transcription and translation are part of gene expression, a
process by which information encoded by a gene is converted
into a structural or functional part of a cell or a body
• gene expression
• Process by which the information in a gene becomes
converted to an RNA or protein product
Key Concepts
• DNA to RNA to Protein
• The sequence of amino acids in a polypeptide chain
corresponds to a sequence of nucleotide bases in DNA
called a gene
• The conversion of information in DNA to protein occurs in
two steps: transcription and translation
9.3 Transcription
• During transcription, DNA acts as a template upon which a
strand of RNA (transcript) is assembled from RNA nucleotides
• Each new RNA is complementary in sequence to the DNA
template: G pairs with C; A pairs with U (uracil)
• RNA polymerase adds nucleotides to the end of a growing
transcript
3 Steps in Transcription
• Transcription begins with a gene on a chromosome: RNA
polymerase and several regulatory proteins attach to a
specific binding site (promoter) in the DNA
3 Steps in Transcription
• 2. RNA polymerase starts moving along the DNA, in the 3' to
5’ direction over the gene, unwinding the double helix to
“read” the base sequence of the DNA strand
3 Steps in Transcription
•
RNA polymerase bonds free RNA nucleotides into a chain, in
the order dictated by that DNA sequence, making an RNA
copy of the gene
3 Steps in Transcription
RNA
polymerase
gene region
promoter sequence in DNA
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.
1
Fig. 9.4.1, p. 140
3 Steps in Transcription
RNA
DNA winding up
DNA unwinding
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.
2
Fig. 9.4.2, p. 140
3 Steps in Transcription
direction of
transcription
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.
3
Fig. 9.4.3, p. 140
RNA
polymerase
3 Steps in Transcription
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
Fig. 9.4, p. 140
ANIMATION: Gene transcription details
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Finishing Transcription
• When the polymerase reaches the end of the gene, the DNA
and the new RNA strand are released
• Typically, many polymerases transcribe a particular gene
region at the same time, so many new RNA strands can be
produced very quickly
Key Terms
• RNA polymerase
• Enzyme that carries out transcription
• promoter
• In DNA, a sequence to which RNA polymerase binds
Gene Transcription
• Three genes next to one another on the same chromosome
are being transcribed simultaneously
Gene Transcription
RNA transcripts
DNA molecule
Fig. 9.5, p. 141
Post-Transcriptional Modifications
• Eukaryotes modify their RNA inside the nucleus, then ship it
to the cytoplasm
• Introns are nucleotide sequences that are removed from a
new RNA, and exons are sequences that stay in the RNA
• Sometimes, some exons are removed and the remaining
exons are spliced together (alternative splicing) which
enables one gene to encode different proteins
Key Terms
• intron
• Nucleotide sequence that intervenes between exons and
is excised during RNA processing
• exon
• Nucleotide sequence that is not spliced out of RNA during
processing
• alternative splicing
• RNA processing event in which some exons are removed
or joined in various combinations
Post-Transcriptional Modifications
• New transcripts that will become mRNAs are further modified
after splicing
• A modified guanine “cap” is added to the 5’ end, which will
help the mRNA bind to a ribosome
• A tail of 50-300 adenines (poly-A tail) is added to the 3’ end
Post-Transcriptional Modifications
Post-Transcriptional Modifications
gene
promoter
exon
intron
exon
intron
exon
DNA
transcription
exon
intron
exon
intron
exon
new transcript
RNA processing
exon
exon
exon
finished RNA
cap
poly-A tail
Fig. 9.6, p. 141
ANIMATION: Pre-mRNA transcript
processing
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Key Concepts
• DNA to RNA: Transcription
• During transcription, one strand of a DNA double helix
serves as a template for assembling a single,
complementary strand of RNA (a transcript)
• Each transcript is an RNA copy of a gene
ANIMATION: Overview of transcription
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ANIMATION: Transcription - A molecular
view
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ANIMATION: Transcription - Introns and
exons
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ANIMATION: Transcription