1. Discover Biology SIXTH EDITION
Anu Singh-Cundy • Gary Shin
Discover Biology SIXTH EDITION
CHAPTER 12 From Gene to Protein
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© 2015 W. W. Norton & Company, Inc.

2. CHAPTER 12 From Gene to Protein
Greek Myths and One-Eyed Sheep 12.1 How Genes Work Genes contain information for building RNA molecules Three types of RNA assist in the manufacture of proteins 12.2 Transcription: Information Flow from DNA to RNA 12.3 The Genetic Code 12.4 Translation: Information Flow from mRNA to Protein 12.5 The Effect of Mutations on Protein Synthesis Mutations can alter one or many bases in a gene’s DNA sequence Mutations can cause a change in protein function 12.6 How Cells Control Gene Expression Most genes are controlled at the transcriptional level Gene expression can be regulated at several levels BIOLOGY MATTERS: ONE ALLELE MAKES YOU STRONG, ANOTHER HELPS YOU ENDURE APPLYING WHAT WE LEARNED: From Gene Expression to Cyclops

3. Greek Myths and One-Eyed Sheep
In the Odyssey, the ancient Greek classic by Homer, the hero Odysseus outwits a one-eyed giant, a cyclops named Polyphemus.
In vertebrate animals, cyclopia is genetic disorder in which offspring are born with a single eye and nose, and a malformed mouth
To develop normally from a single-celled zygote, an organism must turn on (express) the right genes at the right time and in the right place.
Even tiny missteps in gene expression can result in improper embryo development leading to birth disorders such as cyclopia.
Head of a Lamb with Cyclopia
This lamb suffered severe birth defects, including a single large eye and a fused, malformed brain, because its mother ate a wild plant called corn lily early in pregnancy. Corn lilies make a compound that shuts down the expression of certain genes.
Gene expression gone horribly wrong:
cyclopia in newborn lamb

4. Genes Control Genetic Traits
Dystrophin is the longest human gene: it consists of 2.4 million base pairs. Mutations in the gene produce various muscle-wasting disorders.
(The photo is from page 264, Chapter 12.)
Child with Duchenne muscular dystrophy
Genetic information is encoded in genes, which control genetic traits.
A slightly different version of a gene (allele) produces a different version of the genetic trait (produces a particular phenotype of that genetic trait).
Scientists work to understand how gene mutations produce new phenotypes, including disease phenotypes.

5. Coding Genes Store Information Needed to Build RNA and Proteins, Which in Turn Produce the Phenotypes of Most Genetic Traits
(The RNA-only genes, also known as noncoding genes, store instructions for building any one of a number of ribonucleic acid (RNA) molecules with varied functions that do not include carrying code for the manufacture of a specific protein. In contrast, all protein-coding genes direct the production of messenger RNA (mRNA), which in turn directs the production of a particular protein in the cytoplasm. At the molecular level, we can say that a gene is any DNA sequence that is transcribed into RNA.
Proteins generate the phenotype for most genetic traits. Many proteins are enzymes, which speed up chemical reactions inside cells.
In the early twentieth century, scientists hypothesized that genes were responsible for enzyme production and that metabolic disorders were caused by enzyme deficiencies.)
A gene is any DNA sequence that is copied (transcribed) into RNA.
Proteins, including enzymes, are the key determinants of an individual’s phenotype.

6. Genes Contain Information for Building RNA Molecules
Figure RNA Molecules Carry Genetic Information
A technique called in situ hybridization was used to show where mRNA (green) transcribed from the Noggin gene is located in a fetal mouse. The Noggin gene is expressed in the developing brain and in the cartilage and bones of all mammals, including humans. Serious developmental disorders result if the information in the Noggin gene is corrupted. How is the information in a gene actually “read out” in a cell? How does a change in the DNA sequence produce an altered phenotype?
RNA (green) transcribed from the Noggin gene in a fetal mouse. The Noggin gene is expressed (turned on) in the developing brain and in the cartilage and bones of all mammals, including humans. The gene is shut down in other tissues.

7. RNA Molecules Are Single-Stranded Polynucleotides
Like DNA, RNA is a polymer of nucleotides.
RNA is single-stranded, whereas DNA forms a double-stranded molecule twisted into a spiral shape (double helix).
Figure RNA Is a Single-Stranded Chain of Nucleotides
However, an RNA molecule can assume a variety of three-dimensional shapes by folding over of the single strand, as exemplified by tRNA.
DNA and RNA also differ in the type of sugar used (ribose in RNA, deoxyribose in DNA).
RNA uses the base uracil (U) in place of thymine (T) in DNA molecules.

8. DNA Stores Information in the Nucleus, RNA Carries Information from the Nucleus to the Cytoplasm
DNA
RNA
Structure
Double-stranded; two polynucle­otide strands wound into a helix
Single-stranded polynucleotide; may fold back on itself
Sugar
Deoxyribose
Ribose
Nucleotides
A, G, C, and T
A, G, C, and U
Function
Stores genetic information
Expresses genetic information—for example, by direct­ing the manufacture of a specific protein
Stability
Highly stable in most cells
Generally much less stable
Location
Nucleus, chloroplasts, and mitochondria in eukaryotes; cytosol in prokaryotes
Nucleus, chloroplasts, mitochondria, and cytosol in eukaryotes; cytosol in prokaryotes
(The line drawing is from Figure 12.3.)
(In general, RNA is chemically less stable than DNA, and most RNA molecules in the cell have a limited life. As the permanent store of genetic information, the DNA in the nucleus of eukaryotic cells is much more stable, being destroyed only if the cell itself is destined to die soon.)

9. Three Types of RNA Assist in the Manufacture of Proteins
TYPE OF RNA
FUNCTION
Messenger RNA (mRNA)
Specifies the order of amino acids in a protein using a series of three-base codons, where different amino acids are specified by particular codons.
Ribosomal RNA (rRNA)
As a major component of ribosomes, assists in making the covalent bonds that link amino acids together to make a protein.
Transfer RNA (tRNA)
Transports the correct amino acid to the ribosome, using the information encoded in the mRNA; contains a three-base anticodon that pairs with a complementary codon revealed in the mRNA.

10. Information Flows from DNA to RNA to Proteins
Figure Genetic Information Flows from DNA to RNA to Protein during Transcription and Translation
The transcription of a protein-coding gene produces an mRNA molecule, which is then transported to the cytoplasm, where translation occurs and the protein is made with the help of ribosomes. Different amino acids in the protein being constructed at the ribosome are represented here by different colors and shapes.
A complementary mRNA sequence is made using the information in the DNA sequence of protein-coding genes during the process of transcription.
During translation, amino acids are covalently linked in the sequence dictated by the base sequence of the mRNA; translation is carried out by ribosomes in the cytoplasm.
Translation also requires two other types of RNA: rRNA and tRNA.

11. RNA Polymerase Synthesizes RNA Using One Strand of the DNA as a Template
Figure Genetic Information Flows from DNA to RNA to Protein during Transcription and Translation
The transcription of a protein-coding gene produces an mRNA molecule, which is then transported to the cytoplasm, where translation occurs and the protein is made with the help of ribosomes. Different amino acids in the protein being constructed at the ribosome are represented here by different colors and shapes.
Transcription occurs in the nucleus.
An enzyme called RNA polymerase synthesizes RNA using one strand of the DNA as template.

12. Table 12.3 A Comparison of Gene Transcription and DNA Replication
GENE TRANSCRIPTION
DNA REPLICATION
Key enzyme involved
RNA polymerase
DNA polymerase
Portion of chromosome copied
Small segment
Entire DNA double
Product
Single-stranded RNA molecule, complementary to one DNA strand (the template strand)
Double-stranded DNA molecule

13. Information Flow from DNA to RNA
Transcription begins when RNA polymerase binds to a segment of DNA called a gene promoter.
Once bound, RNA polymerase begins to unwind the DNA and transcribe the template strand (bottom strand in diagram); which strand serves as the template is dictated by the positioning of the promoter, which orients the polymerase.
Transcription stops when RNA polymerase reaches a special sequence of bases called a terminator.
Figure RNA Polymerase Transcribes DNA-Based Information into RNA-Based Information
Although the promoters of different genes vary in size and sequence, all promoters contain several specific sequences of 6–10 bases that enable the RNA polymerase to recognize and bind to them. As an RNA polymerase moves away from the gene promoter and travels down the template strand, another RNA polymerase can bind at the promoter and start synthesizing an mRNA fast on the heels of the previous RNA polymerase. At any given time, therefore, many RNA polymerases can be traveling down a DNA template, each synthesizing its own mRNA strand (at the rate of 60 bases per second in human cells).

14. In Eukaryotes, mRNA Is Chemically Modified After Transcription
Posttranscriptional processing modifies RNA and prepares it for export from the nucleus.
The newly formed mRNA undergoes RNA splicing, which removes the introns, and is then allowed to leave the nucleus through the nuclear pore.
Figure In Eukaryotes, Introns Must Be Removed before an mRNA Leaves the Nucleus
Most eukaryotic genes contain both coding sequences (exons) and noncoding sequences (introns). Before the mRNA transcribed from such genes can be exported to the cytoplasm, enzymes in the nucleus must remove the introns and link the remaining exons.
(Posttranscriptional processing also includes chemical modification of both ends of the mRNA: a chemically unique nucleotide is added to create a cap structure at the end of the mRNA that was transcribed first, and a string of adenines is added at the opposite end to create what is known as the poly-A tail.)

15. Translation: Information Flow from mRNA to Protein
(The line drawing is from Figure 12.3.)
Translation is the process of converting a sequence of bases in mRNA to a sequence of amino acids in a protein.
Translation occurs at the ribosomes, which are made up of proteins and rRNA.

16. The Base Sequence of mRNA Is Read as a Sequence Codons
Each unique sequence of three bases is called a codon.
When reading the code, the ribosomes begin at the start codon, AUG, and end at one of three stop codons: UAA, UAG, or UGA.
Figure The Genetic Code Is Read in Sets of Three Bases, Each Set Constituting a Codon
The start codon (AUG) prevents the scrambling of the protein sequence by establishing the start point. Having a fixed point for the start ensures that the message from a gene is read precisely the same way every time. The start codon specifies the amino acid methionine, which is why most proteins inside a cell have this amino acid at their “starting point,” the portion of the protein that was translated first.

17. There Are 64 Codons That Make Up the Information in the Genetic Code
The genetic code has several distinct characteristics:
It is unambiguous
It is redundant
It is virtually universal
Figure The 64 Possible Codons Specify Amino Acids or Signals That Start or Stop Translation
Most of the 64 codons specify particular amino acids. A few amino acids are specified by only one codon; for example, only one codon (UGG) specifies the amino acid tryptophan. Other amino acids are specified by anywhere from two to six different codons (see arginine, for example).
(Characteristics of the genetic code:
It is unambiguous. Each codon specifies only one amino acid.
It is redundant. Several different codons may have the same “meaning”; that is, they may code for the same amino acid. There are four possible bases at each of the three positions of a codon, so there are a total of 64 codons (4 × 4 × 4 = 64). However, there are only 20 amino acids, so some of these codons specify the same one. For example, six different codons specify the amino acid serine.
It is virtually universal. Nearly all organisms on Earth use the same genetic code, which underscores the common descent of all organisms. A few minor exceptions to the universal genetic code do exist: in certain species, some of the 64 codons are read differently than they are in most other species. In general, however, knowing the genetic code is the key to unlocking gene function in living things.)

18. After Ribosomes Bind the mRNA, Each Specific Amino Acid Is Delivered to the Ribosome-mRNA Complex by a tRNA Molecule Specialized to Deliver a Specific Type of Amino Acid
An anticodon is a three-base sequence that determines which codons on the mRNA can be recognized by the tRNA.
Figure Transfer RNA Delivers Amino Acids Specified by mRNA Codons
A space-filling-model (left) and a diagrammatic version (right) illustrate the general structure of all tRNA molecules. Similar regions in the space-filling model and on the diagram are shown in matching colors. Each tRNA carries a specific amino acid (serine in this example) and has a specific anticodon sequence (UCG in this example) that binds to a complementary three-base sequence (the codon) in the mRNA.
(Some tRNAs can recognize more than one codon because the base at the third position of their anticodon can “wobble,” meaning it can pair with any one of two or three different bases in the codon. For example, one serine-bearing tRNA can pair with either UCU or UCC in the mRNA, while another serine-bearing tRNA pairs with either UCA or UCG. A third tRNA recognizes the two additional serine-specifying codons: AGU and AGC. This flexibility in the pairing between some anticodons and codons means that a cell does not need 61 different tRNAs, one for each of the 61 amino acid–specifying codons. In fact, most organisms have only about 40 different tRNAs, because many tRNA anticodons can recognize and pair with more than one codon in the mRNA.)
Each codon on the mRNA is recognized by a specific tRNA, and the ribosome adds the amino acid delivered by this tRNA to the growing amino acid chain.

19. Translation Begins When a tRNA Molecule Recognizes and Pairs with the AUG of the Start Codon
The process continues until a stop codon is reached and the mRNA and the completed amino acid chain both separate from the ribosome.
Figure In Translation, Information Coded in mRNA Directs the Synthesis of a Protein That Has a Particular Amino Acid Sequence

20. Protein Synthesis through Translation
Figure In Translation, Information Coded in mRNA Directs the Synthesis of a Protein That Has a Particular Amino Acid Sequence
The ribosomal machinery “scans” the mRNA until it finds a start codon, which is the first AUG codon in the mRNA sequence. A tRNA molecule that specializes in carrying methionine recognizes the the start codon and pairs with it through its anticodon. The next codon is GGG, so a tRNA molecule that specializes in carrying glycine recognizes the GGG codon and pairs with it through its anticodon.

21. Initiation
The First Covalent Bond Between Amino Acids: The Polypeptide Chain Begins
Elongation
Figure In Translation, Information Coded in mRNA Directs the Synthesis of a Protein That Has a Particular Amino Acid Sequence
(Elongation proceeds as follows:
1. A tRNA molecule that specializes in carrying glycine recognizes the GGG codon and pairs with it through its anticodon.
2. The ribosome now forms a covalent bond between the first amino acid {methionine} and the second amino acid, glycine. When the bond between the first two amino acids is formed, the first tRNA releases its amino acid (methionine).)

22. Chain Elongation Continues
Figure In Translation, Information Coded in mRNA Directs the Synthesis of a Protein That Has a Particular Amino Acid Sequence
3. The methionine tRNA, now freed from the amino acid it had been carrying, is ejected from the mRNA-ribosome complex, and its place on the ribosomal site is taken by the next tRNA (the one carrying glycine).
4. The ribosome is now ready to read the third codon (UCC) in the mRNA.

23. Chain Termination
Figure In Translation, Information Coded in mRNA Directs the Synthesis of a Protein That Has a Particular Amino Acid Sequence
Finally, a stop codon is reached; this is the termination stage. The amino acid chain cannot be extended further, because none of the tRNAs can recognize and pair with any of the three stop codons. At this point the mRNA molecule and the completed amino acid chain both separate from the ribosome. The new protein folds into its compact, specific three-dimensional shape.

24. A Mutation Is a Change in the Base Sequence of an Individual’s DNA
Mutations can range from a change in a single base pair to the deletion of one or more whole chromosomes.
Mutations in which a single base is altered are point mutations.
There are three main types of mutations:
Insertions
Deletions
Substitutions
A substitution mutation occurs when one base is substituted for another in a DNA sequence.

25. Insertion/Deletion Mutations Are Usually More Disruptive Than Substitution Mutations
Insertion or deletion mutations occur when a base is inserted into or deleted from a DNA sequence.
Unlike a substitution mutation, insertion or deletion of one or two nucleotides causes a frameshift mutation, which scrambles the downstream sequence of amino acids.
Frameshift mutations stop protein synthesis by introducing accidental stop codons or alter the identity of many amino acids in a protein.
Figure A Change in the DNA Sequence Translates into a Change in the Amino Acid Sequence of the Protein
Two kinds of mutations are shown here: a substitution and an insertion. In each case, the mutation and its effects on transcription and translation are shown in red.
Frameshift mutations, whether caused by point mutations or by large insertions or deletions, alter the resulting protein so severely that it fails to function in most cases. The insertion or deletion of one or two bases shifts all “downstream” codons by one or two bases. The entire downstream message is scrambled because the ribosomes assemble a very different sequence of amino acids from that point onward, compared with the protein encoded by the original DNA sequence and its corresponding mRNA.

26. Mutations Can Alter Protein Function
Even single-base changes can alter protein function enough to produce a harmful phenotype such as a disease.
Frameshift mutations alter the protein so extensively that they invariably destroy the normal function of the protein and produce a severe phenotype.
A silent mutation causes no change in the structure of the protein, and therefore no change in the phenotype of the organism.
Rarely, a mutation can be beneficial and improve the efficiency or functionality of a protein.
[The diagram is from a previous edition of Discover Biology. It illustrates how a single base change that alters a single amino acid in the beta polypeptide of hemoglobin A, alters the shape of the protein and leads to a distortion in the shape of red blood cells as large amounts of the malformed protein accumulated in the cytosol in individuals homozygous for the sickle-cell allele; heterozygotes run the risk of sickling only under low oxygen levels.]
The replacement of glutamic acid by valine changes the shape of
hemoglobin. The accumulation of large amounts of the deformed protein distorts the shape of red blood cells in
people with sickle-cell anemia.

27. How Cells Control Gene Expression
Gene expression refers to the transcription and translation of a gene to produce a functional protein that has an effect on phenotype.
(The illustration is from Figure 11.3, Chapter 11.)
Figure Different Cell Types Have the Same Genes
Although all cells within a multicellular organism have the same genes, these cells can differ greatly in structure and function because different genes are active in different types of cells.
In gene expression, a gene is transcribed and its mRNA is translated into a functional protein that has an effect on phenotype.  
Different sets of genes are expressed in different cell types.
Gene expression changes during development and can change in response to environmental signals and internal signals such as hormones.

28. Most Genes Are Controlled at the Transcriptional Level
(Cells receive signals that determine gene expression. Some of these signals are sent from one cell to another, as when one cell releases a signaling molecule that alters gene expression in another cell. Cells also receive signals from the organism’s internal environment (for example, blood sugar level in humans) and external environment (for example, sunlight in plants). Overall, cells process information from a variety of signals, and that information affects which genes are expressed.)
Cells generally control gene expression by regulating the transcription of specific genes.
Regulatory DNA is the part of a gene that controls gene transcription with the help of gene regulatory proteins.
Gene regulatory proteins, also called transcription factors, interact with signals from the environment and regulatory DNA to control gene expression.

29. In Some Bacteria, the Genes for Utilizing Lactose Are Turned On Only if Lactose Is Available
Figure Repressor Proteins Turn Genes Off
In the bacterium E. coli, a repressor protein interacts with an operator to control the transcription of a group of genes that encode the enzymes needed to take up and metabolize lactose. (a) When lactose is absent, the lac repressor protein binds to the operator, which blocks transcription by preventing RNA polymerase from attaching to the promoter. (b) When lactose is present, the sugar binds to the repressor protein, preventing it from docking with the operator. With the repressor gone, RNA polymerase can bind to the promoter and transcribe the three genes that are part of the operon.
(E. coli can feed on a variety of different types of sugars, including the disaccharide lactose. To avoid wasting metabolic resources, the proteins needed to absorb and break down these carbohydrates are “made to order”: the genes encoding the proteins are expressed only when the bacterium detects the specific carbohydrate in its environment. At all other times, these genes are shut down.)
Operon: single promoter controlling transcription of a cluster of genes
with related functions.

30. Gene Expression Can Be Regulated at Several Levels
Tight packing of DNA prevents access to its gene regulatory DNA, making that segment of DNA transcriptionally inactive.
Regulation of transcription enables the cell to conserve resources when it does not need a particular gene product.
By limiting the life span of many types of mRNA, a cell prevents the wasteful synthesis of proteins.
Regulation of translation keeps mRNA ready to direct rapid protein synthesis when needed.
Proteins can be directly regulated by modification following translation.
Regulation of protein breakdown conserves resources.
Figure Steps at Which Gene Expression Can Be Regulated in Eukaryotes
Each point in the pathway from gene to protein provides an opportunity for cells to regulate the production or activity of proteins.
(1. Tight packing of DNA prevents it from being expressed (control point 1). During interphase, parts of the chromosome are “unpacked” down to the beads-on-a-string state. This unpacking gives gene regulatory proteins and RNA polymerase access to gene promoters and other regulatory DNA sequences, making transcription possible. In contrast, the tightly packed regions of the chromosome are transcriptionally inactive because their gene regulatory DNA is not accessible.
2. Regulation of transcription conserves resources (control point 2). Regulation of transcription is an efficient control mechanism because it enables the cell to conserve resources when it does not need the gene product. Transcriptional activation of gene expression is, however, relatively slow in eukaryotes, taking a minimum of 15–30 minutes at best.
3. Regulation of mRNA breakdown prevents wasteful synthesis of proteins (control point 3). Most mRNA molecules are broken down within a few minutes to hours after they are made; a few persist for days or weeks in the cytoplasm. By limiting the life span of many types of mRNA, a cell prevents the wasteful synthesis of proteins that are needed only temporarily. If circumstances change and the protein is needed, its mRNA can be quickly stabilized, and translation and accumulation of the protein may proceed rapidly. Time is saved because transcription does not have to be activated, so protein levels often start rising just a few minutes after the need for the protein is sensed.
4. Regulation of translation keeps mRNA ready for rapid protein synthesis when needed (control point 4). Specific RNA-binding proteins can attach to their target mRNA molecules and block translation. For example, some immune cells in the body make large amounts of mRNA for certain signaling proteins, called cytokines, but these mRNAs are not immediately translated. If these cells detect an invading bacterium, the translation block is immediately lifted. Cytokines are translated within minutes and poured into the bloodstream, where they act like an early-warning system that prepares other components of the immune system to defend the body.
5. Proteins can be directly regulated by modification following translation (control point 5). Many proteins must be chemically modified before they can exert their effect on a phenotype. Some of the blood-clotting proteins, for example, are synthesized as inactive precursors, and a segment of the protein must be cleaved off before clotting can occur. This type of control prevents premature activity of a protein, which could be disruptive, even dangerous.
6. Regulation of protein breakdown conserves resources and prevents damage (control point 6). The activity of a protein provides one final opportunity to control the pathway from gene to protein. Most proteins in the cell have a limited life span, but a few, such as collagen and crystallin, last us through our lifetime. Proteins that are no longer needed, or are damaged, are taken apart and their amino acids recycled into new proteins.)

31. APPLYING WHAT WE LEARNED: From Gene Expression to Cyclops
Cyclopamine blocks the SMO receptor, which turns on genes crucial for the cell divisions that produce the two symmetrical halves of the brain and face during embryonic develoment.
The SMO receptor protein sends out signals that turn on genes needed for the cell divisions that cause the embryonic brain to divide into left and right halves—leading to left and right eyes.
The SMO receptor turns on the right genes only if the SHH (sonic hedgehog) signal protein binds to it.
Cyclopamine prevents the binding of SHH to the SMO receptor, so the SMO receptor fails to turn on the genes needed for the brain and face to develop normally into two symmetrical halves.
Alcohol and other drugs also seem to inhibit this hedgehog signaling pathway, leading to rare cases of cyclopia in human babies. Other causes of cyclopia include diabetes during pregnancy and genetic mutations in the hedgehog gene pathway (including other genes besides shh).
Sonic hedgehog is overexpressed in some types of cancer including certain cancers of the brain and the most common form of skin cancer, basal cell carcinoma. To slow cell division in such cancers, pharmaceutical companies have developed commercial drugs from cyclopamine that suppress the hedgehog pathway and slow cell division.
Environmental influences, such as ingestion of the corn lily by a pregnant animal, can cause gene expression to be altered, resulting in abnormalities such as cyclopia.

32. BIOLOGY MATTERS: ONE ALLELE MAKES YOU STRONG, ANOTHER HELPS YOU ENDURE
The goal of personal genomics is to inspect and catalog an individual’s total genetic makeup, or genome.
Personal genomics brings us personalized medicine,
the practice of tailoring health care and disease
prevention to a person’s genotype.
Commercial tests for athletic potential are available,
based on the R and X alleles of the ACTN3 gene.
XX genotype is unusually
common in endurance athletes
(24 percent), but rare in strength-sport
athletes, who are more likely
to be RR than all others.
Knocking out the ACTN3 gene
(XX genotype) lead to
“marathon mice.”
Alpha-actinin-3, the protein encoded by the ACTN3 gene, is made only in skeletal muscles. It anchors the contractile proteins so that muscle fibers can generate power. Australian scientists found two alleles of the ACTN3 gene seemingly linked to athletic ability. The R allele codes for a functional alpha-actinin-3 protein; the X allele leads to the production of a shortened, nonfunctional version of the protein. The X allele of ACTN3 is the result of a base substitution that changes an amino acid codon into a stop codon—a mutation that halts protein synthesis prematurely.
Success in sports depends on many things, including psychological attributes such as personal drive, and top-level performance always requires extensive training and conditioning. The physical component of athletic success is likely influenced by many genes, not just one or two genes such as ACTN3. In one study, as many as 92 different genes were potentially associated with athletic ability and health-related fitness. Therefore, the predictive power of the ACTN3 alleles is limited. Studies found that as many as 31 percent of the elite distance runners lacked the X allele, and 45 percent had only one copy.
Given the complexities in interpreting the genetic tests, would you want to know your allelic makeup? Would you want your children tested for the ACTN3 gene, and if so, how would you use that information? Researchers justify studies of genes influencing physical performance by pointing out their relevance in conditions such as muscular dystrophy and other muscle diseases. Opponents say genetic tests of this sort will lead to abuse by overambitious athletes and pushy parents of would-be sports stars. Does the potential for medical benefit outweigh the risk for abuse?
GENOTYPE
CONTROL (NONATHLETES)
STRENGTH-SPORT ELITE ATHLETES (SPRINTERS)
ENDURANCE-SPORT ELITE ATHLETES (DISTANCE RUNNERS) RR 30 50 31 RX 52 45 XX 18 6 24

33. List of Key Terms: Chapter 12
anticodon (p. 269) codon (p. 267) deletion (p. 271) elongation (transcription, p. 265; translation, p. 269) exon (p. 266) frameshift (p. 271) gene (p. 262) gene expression (p 272) gene promoter (p. 265) genetic code (p. 267) initiation (transcription, p. 265; insertion (p. 271) intron (p. 266) messenger RNA (mRNA) (p. 264) operator (p. 273) operon (p. 272)
point mutation (p. 271)
regulatory DNA (p. 272)
regulatory protein (p. 272)
repressor (p. 273)
ribosomal RNA (rRNA) (p. 264)
RNA polymerase (p. 265)
RNA splicing (p. 267)
start codon (p. 267)
stop codon (p. 267)
substitution (p. 271)
template strand (p. 265)
termination (transcription, p. 265;
translation, p. 270)
terminator (p. 266)
transcription (p. 264)
transfer RNA (tRNA) (p. 264)
translation (p. 264)
Extreme Protein Clumps
Protein garbage is destructive to tissues, especially in complex organs like the brain. The failure to remove damaged proteins contributes to the death of brain cells in Alzheimer’s disease and “mad cow” disease (bovine spongiform encephalopathy, BSE). In patients with BSE, the brain develops protein clumps and large holes where cells have died.

34. Class Quiz, Part 1 Which of the following is true?
Transcription occurs in the cytoplasm and produces RNA.
Transcription occurs in the nucleus and produces proteins.
Translation occurs in the cytoplasm and produces proteins.
Translation occurs in the nucleus and produces RNA.
The correct answer is C. Mouse click brings up arrow pointing to the correct answer.

35. Class Quiz, Part 2
Dystrophin is the largest protein in the human body.
In this concept map, which of the following fits best in the box labeled with an “X”?
DNA
polypeptide
phenotype
mRNA
tRNA
The correct answer is D. Mouse click brings up arrow pointing to the correct answer.

36. Class Quiz, Part 3
Which of the following is not true about the genetic code?
Every codon has a corresponding amino acid.
B. Every codon consists of three bases.
C. There are 64 possible codons.
A single amino acid can have more than one
codon.
The correct answer is A. Mouse click brings up arrow pointing to the correct answer.
Every codon doesn’t specify an amino acid; three codons have no amino acids and are STOP codons.

37. Class Quiz, Part 4 Frameshift mutations
occur only if three bases are deleted.
occur when one base is changed to another.
don’t change the structure of the protein.
can be caused by either the insertion or deletion of a single base.
The correct answer is D. Mouse click brings up arrow pointing to the correct answer.

38. Relevant Art from Other Chapters
All art files from the book are available in JPEG and PPT formats online and on the Instructor Resource Disc

39. Nucleotides and Nucleic Acids
Figure Nucleotides Are the Building Blocks of Nucleic Acids
Each nucleotide consists of a five-carbon sugar linked to a nitrogenous base and one or more phosphate groups. The bases adenine, guanine, cytosine, and thymine, when linked to the sugar deoxyribose, form the building blocks of DNA. The bases adenine, guanine, cytosine, and uracil, when linked to the sugar ribose, form the building blocks of RNA.

40. Most Genes Code for Proteins, Which Generate Phenotypes
RNA is a single-stranded nucleic acid similar to DNA.
Messenger RNA (mRNA) delivers the genetic information, or instructions, from DNA to the ribosomes, where proteins are made.
The conversion of a DNA-based sequence of nucleotides in a gene to an RNA-based sequence is called transcription.
The process by which ribosomes convert the genetic information in mRNA into proteins is known as translation.
Figure From Gene to Phenotype at the Molecular Level
The readout of information coded in DNA produces the phenotype, such as the activity of the lactase enzyme in human intestines. The gene promoter controls whether, and to what degree, a gene is transcribed.
Each protein has a unique amino acid sequence, which gives it a unique function; protein function produces the phenotype, the particular version of a genetic trait in the individual organism.

41. Failure in DNA Repair Generates a Mutation, a Change in the DNA Sequence
A change to the sequence of bases in an organism’s DNA is called a mutation.
Mutagens are substances or energy sources that can cause mutations.
New alleles arise as a result of mutations.
Most genetic mutations are neutral or harmful.
A mutation may consist of change in a single base or a large-scale change involving chromosomal abnormalities.
Figure Mistakes in DNA Replication May Lead to Mutations
DNA repair enzymes usually detect and fix mismatch errors such as the one shown here before the cell’s DNA is replicated again. If the mismatch is not repaired before the next round of DNA replication, half of the daughter helices made by this DNA will have a C-G base pair in place of the original A-T base pair. Such a change in the DNA sequence constitutes a mutation.
Changes in chromosome number or chromosome organization generally affect large numbers of genes, and such changes also amount to DNA mutations because they have the effect of adding, deleting, or rearranging nucleotide sequences.

42. 12.1 Concept Check, Part 1
1. Do all genes code for mRNA and therefore for proteins?
ANSWER: No. RNA-only genes are transcribed into RNA other than mRNA, and these RNAs have specialized functions other than coding for proteins.

43. 12.1 Concept Check, Part 2
2. Compare the chemical structures of RNA and DNA. Which is more stable chemically, and how is that stability consistent with its function?
ANSWER: RNA is single-stranded; it contains ribose and the bases A, G, C, and U. DNA is double-stranded; it contains deoxyribose and A, G, C, and T. DNA is more stable—a property it must have to serve as the storehouse of genetic information.

44. 12.1 Concept Check, Part 3
3. What is the product of transcription? What is the product of translation?
ANSWER: The product of transcription is an mRNA complementary to the DNA sequence of a gene. The product of translation is a polypeptide (protein chain) determined from the sequence of the mRNA.

45. 12.2 Concept Check, Part 1
1. The template strand of a gene has the base sequence TGAGAAGACCAGGGTTGT. What is the sequence of RNA transcribed from this DNA, assuming RNA polymerase travels from left to right on this strand?
ANSWER: ACUCUUCUGGUCCCAACA

46. 12.2 Concept Check, Part 2
2. The dystrophin gene has 78 introns. Are these introns transcribed? Do they code for amino acids?
ANSWER: All 78 introns are transcribed into a pre-mRNA, but they are subsequently spliced out. Because they are absent from the fully processed mature mRNA transcript, introns do not code for amino acids.

47. 12.3 Concept Check, Part 1
1. Why is the start codon, AUG, so important?
ANSWER: The start codon sets the reading frame, that is, it determines the grouping of the bases in the mRNA into triplets to be read as codons.

48. 12.3 Concept Check, Part 2
2. What does it mean to say that the genetic code is redundant?
ANSWER: There are 64 possible codons, but only 20 amino acids. In most cases, a single amino acid is specified by more than one codon, and this is what is meant by redundancy. For example, tyrosine is specified by either UAU or UAC.

49. 12.4 Concept Check, Part 1 1. What is meant by “translation” of mRNA?
ANSWER: Translation converts a sequence of bases in mRNA to a sequence of amino acids in a protein.

50. 12.4 Concept Check, Part 2
2. Does each of the 64 codons specify a different amino acid?
ANSWER: No. The three stop codons do not specify any amino acids, and a single amino acid may be specified by as many as six different codons.

51. 12.5 Concept Check, Part 1
1. What is a mutation? Are all mutations harmful?
ANSWER: A mutation is a change in the base sequence of an organism’s DNA. A mutation may have no detectable effects or may be harmful; in rare instances, it may even be advantageous.

52. 12.5 Concept Check, Part 2
2. A single-base addition or deletion in a gene is likely to alter the protein product more than a single-base substitution, such as C for T, would. Why?
ANSWER: Single-base addition or deletion shifts the reading frame, so all the amino acids downstream of such a mutation are altered. Single-base substitution alters, at most, a single amino acid.

53. 12.6 Concept Check, Part 1
1. Why are most genes controlled at the level of transcription?
ANSWER: Transcriptional regulation prevents gene expression when a gene’s product is not needed by a cell, enabling the cell to invest its resources elsewhere.

54. 12.6 Concept Check, Part 2
2. If transcriptional control is the most favored method of gene regulation, why are not all genes controlled at the level of transcription?
ANSWER: Transcriptional activation is relatively slow; controlling gene expression at a posttranscriptional step enables a cell to respond faster to environmental changes.