Albia Dugger • Miami Dade College
Cecie StarrChristine EversLisa Starr
www.cengage.com/biology/starr
Chapter 9From DNA to Protein
(Sections 9.1 - 9.3)
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
Fig. 9.2a, p. 138
An RNA Nucleotide
Fig. 9.2a, p. 138
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.
An RNA nucleotide: guanine (G), or guanosine triphosphate (GTP)
sugar (ribose)
3 phosphate groups
base (guanine)
An RNA Nucleotide
Fig. 9.2b, p. 138
A DNA Nucleotide
Fig. 9.2b, p. 138
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).
A DNA nucleotide: guanine (G), or deoxyguanosine triphosphate (dGTP)
sugar (deoxyribose)
3 phosphate groups
base (guanine)
A DNA Nucleotide
DNA and RNA Compared
Fig. 9.3, p. 139
A DNA has one function: It permanently stores a cell’s genetic information, which is passed to offspring.
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.
base pair
sugar– phosphate backbone
nucleotide base
RNA ribonucleic acid
DNA deoxyribonucleic acid
Nucleotide bases of DNA
Nucleotide bases of RNA
thymine T
cytosine C cytosine C
guanine G guanine G
adenine A adenine A
uracil U
DNA and RNA Compared
Fig. 9.3a, p. 139
DNA
Fig. 9.3a, p. 139
A DNA has one function: It permanently stores a cell’s genetic information, which is passed to offspring.
base pair
sugar– phosphate backbone
nucleotide base
DNA deoxyribonucleic acid
Nucleotide bases of DNA
thymine T
cytosine C
guanine G
adenine ADNA
Fig. 9.3b, p. 139
RNA
Fig. 9.3b, p. 139
sugar– phosphate backbone
nucleotide base
Nucleotide bases of RNA
cytosine C
guanine G
adenine A
uracil U
RNA ribonucleic acid
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.
RNA
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
Fig. 9.4.1, p. 140
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.
promoter sequence in DNA
gene regionRNA polymerase
1
3 Steps in Transcription
Fig. 9.4.2, p. 140
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 transcriptionsite is called a transcription bubble, after its appearance.
DNA unwindingDNA winding up
RNA
3 Steps in Transcription
Fig. 9.4.3, p. 140
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.
direction of transcription
3 Steps in Transcription
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.
promoter sequence in DNA
gene regionRNA polymerase
1
Stepped ArtFig. 9.4, p. 140
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.
DNA unwindingDNA winding up
RNA
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.
direction of transcription
3 Steps in Transcription
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
Fig. 9.5, p. 141
RNA transcripts DNA molecule
Gene Transcription
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
Fig. 9.6, p. 141
intron
gene
promoter exon intron
DNA
transcription
exon intron exon
exon exon intron exon
exon exon exon
new transcript
finished RNA
cap poly-A tail
RNA processing
Post-Transcriptional Modifications
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
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