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