1. Chapter 9 From DNA to Protein

2. 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

3. Some RIPs
Figure 9.1 A few ribosome-inactivating proteins. The structure of RIPs are
strikingly similar. One of their polypeptide chains (red) helps the molecule
cross a cell’s plasma membrane. The other chain (orange) destroys the cell’s
capacity for protein synthesis.

4. 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

5. 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

6. 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

7. A DNA Nucleotide base (guanine) 3 phosphate groups sugar (deoxyribose)
Figure 9.2 Comparing nucleotides of DNA and RNA.
sugar (deoxyribose)
A DNA nucleotide:
guanine (G), or deoxyguanosine triphosphate

8. An RNA Nucleotide base (guanine) 3 phosphate groups sugar (ribose)
Figure 9.2 Comparing nucleotides of DNA and RNA.
sugar (ribose)
An RNA nucleotide: guanine (G), or
guanosine triphosphate

9. ANIMATED FIGURE: Gene transcription details
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10. deoxyribonucleic acid RNA ribonucleic acid adenine A adenine A
DNA
deoxyribonucleic acid
RNA
ribonucleic acid
adenine A
adenine A
sugar– phosphate backbone
guanine G
guanine G
cytosine C
cytosine C
nucleotide base
Figure 9.3 DNA and RNA compared.
base pair
thymine T
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
Nucleotide bases of DNA
Figure 9-3 p151

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. RNA polymerase binds to a promoter in the 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.
promoter sequence in DNA
gene region
RNA polymerase 1 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 unwinding
DNA 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
Figure 9.4 Animated Transcription. By this process, a strand of RNA is assembled from nucleotides
according to a template: a gene region in DNA. Figure It Out: After the guanine, what is the
next nucleotide that will be added to this growing strand of RNA?
Stepped Art
Figure 9-4 p152

19. RNA transcripts DNA molecule
Figure 9.5 Typically, many RNA polymerases simultaneously transcribe the same gene, producing
a structure often called a “Christmas tree” after its shape. Here, three genes next to one another on
the same chromosome are being transcribed. Figure It Out: Are the polymerases transcribing
this DNA molecule moving from left to right or from right to left?
Figure 9-5 p153

20. 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

21. 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

22. gene promoter exon intron exon intron exon DNA transcription exon
new transcript
RNA processing
Figure 9.6 Animated Post-transcriptional modification of RNA.
Introns are removed and exons spliced together. Messenger RNAs
also get a poly-A tail and modified guanine “cap.”
exon
exon
exon 5′ 3′
finished RNA
cap
poly-A tail
Figure 9-6 p153

23. ANIMATED FIGURE: Pre-mRNA transcript processing
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24. 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

25. 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

26. 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

27. 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

28. three nucleotide bases. In the large chart, the left column lists a
codon’s first base, the top row lists the second, and the right column
lists the third. Sixty-one of the triplets encode amino acids;
the remaining three are signals that stop translation. The amino
acid names that correspond to abbreviations in the chart are
listed above. Figure It Out: Which codons specify the amino
acid lysine (lys)?
Figure 9-7a p154

29. three nucleotide bases. In the large chart, the left column lists a
codon’s first base, the top row lists the second, and the right column
lists the third. Sixty-one of the triplets encode amino acids;
the remaining three are signals that stop translation. The amino
acid names that correspond to abbreviations in the chart are
listed above. Figure It Out: Which codons specify the amino
acid lysine (lys)?
Figure 9-7b p154

30. From DNA to RNA to Amino Acids
a gene
region in DNA
transcription
codon
codon
codon
mRNA
Figure 9.8 Example of the correspondence between DNA,
RNA, and proteins. A DNA strand is transcribed into mRNA,
and the codons of the mRNA specify a chain of amino acids.
translation
methionine (met)
tyrosine (tyr)
serine (ser)
amino acid sequence

31. 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

32. amino acid attachment site
tRNA Structure
anticodon
anticodon
amino acid attachment site
Figure 9.10 tRNA structure.

33. Translation: Ribosome and tRNA
Figure 9.10 tRNA structure.

34. 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

35. ANIMATED FIGURE: Structure of a ribosome
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