CHAPTER 3

Question 1

a gene participates in three major activities:

1. A gene is a ______ of information. That is, it holds the information for making one of the key molecules of life, an _____. The sequence of bases in the RNA depends directly on the sequence of bases in the gene. Most of these RNAs, in turn, serve as templates for making other critical cellular molecules, proteins. The production of RNAs and proteins from a DNA blueprint is called _______.

Answer 1

repository ; RNA ; gene
expression

Question 2

a gene participates in three major activities:

2. A gene can be ______. This duplication is very _____, so the genetic information can be passed essentially unchanged from generation to generation.

Answer 2

replicated ; faithful

Question 3

a gene participates in three major activities:

3. A gene can accept ______ or _____. This allows organisms to evolve. Sometimes, these changes involve recombination, exchange of DNA
   between chromosomes or sites within a chromosome. A subset of recombination events involve
   pieces of DNA (transposable elements) that move from one place to another in the genome.

Answer 3

occasional changes, or mutations ;

Question 4

(OVERVIEW OF GENE EXPRESSION)

As we have seen, producing a protein from information in a DNA gene is a two-step process.

 The first step is synthesis of an RNA that is complementary to one of the strands of DNA called _______

 In the second step, called ________, the information in the RNA is used to make a polypeptide. Such an informational RNA is called a _______ to denote the fact that it carries information —like a message —from a gene to the cell ’s protein factories

Answer 4

transcription ; translation ; messenger RNA (mRNA)

Question 5

Like DNA and RNA, ______ are polymers —long, chain-like molecules.

Answer 5

proteins

Question 6

The monomers, or links, in the protein chain are called ________

Answer 6

amino acids.

Question 7

DNA and protein have this informational relationship: Three nucleotides in the DNA gene stand for _________

Answer 7

one amino acid in a protein.

Question 8

The codeword (or _____) for methionine in this mRNA is the triplet _____; similarly, the codons for serine, asparagine, and alanine are _____, _____, and ____ respectively

Answer 8

codon ; AUG ; AGU, AAC, and GCG,

Question 9

The bottom DNA strand is complementary to the mRNA, which serve as the template for making the mRNA. Thus, we call the bottom strand the ________, or the ________

Answer 9

template strand ; transcribed strand

Question 10

The top strand is the _________ or the __________.

Answer 10

nontemplate strand ; non-transcribed strand

Question 11

The top strand has essentially the same coding properties as the corresponding mRNA, many geneticists call it the _______ and the opposite strand would therefore be the ________.

Answer 11

coding strand ; anti coding strand

Question 12

Since the top strand has the same sense as the mRNA, this same system of nomenclature refers to this top strand as the ________, and to the bottom strand as the _________.

Answer 12

sense strand ; antisense strand

Question 13

“coding strand ” and “sense strand” are opposite, so to avoid confusion, the terms _________ and ________ is used.

Answer 13

“template strand” ; “nontemplate strand”

Question 14

(PROTEIN STRUCTURE)

_______, like nucleic acids, are chain-like polymers of small subunits.

Answer 14

Proteins

Question 15

In DNA and RNA, the links in the chain are ________.

Answer 15

nucleotides

Question 16

The chain links of proteins are __________

Answer 16

amino acids

Question 17

DNA contains only four different _______

Answer 17

nucleotides

Question 18

Proteins contain ___ different amino acids

Answer 18

20

Question 19

Each amino acid has:

 an _____ (NH3 +)

 a ______ (COO2)

 a ______ (H),

 and ______ where the difference between 2 amino acids is their side chain (it is the arrangement as well the distinct side chain that makes protein unique)

Answer 19

amino group ; carboxyl group ; hydrogen atom ; side chain

Question 20

The amino acids join together in proteins via ________ which gives rise to the name ________ for a chain of amino acids.

Answer 20

peptide bonds ; polypeptide

Question 21

A polypeptide chain has ______, just as the DNA chain does.

Answer 21

polarity

Question 22

The dipeptide (two amino acids linked together) has a free amino group at its left end which is the _______ or ______. It also has a free carboxyl group at its right end, which is the ________ or _________

Answer 22

amino terminus, or N-terminus ; carboxyl terminus, or C-terminus.

Question 23

The linear order of amino acids constitutes a protein’s _________.

Answer 23

primary structure

Question 24

The way these amino acids interact with their neighbors gives a protein its ________

Answer 24

secondary structure.

Question 25

The ______ is a common form of secondary structure. It results from hydrogen bonding among near-neighbor amino acids.

Answer 25

a-helix

Question 26

___________ is another common secondary structure found in proteins which involves extended protein chains, packed side by side, that interact by _______. The packing of the chains next to each other creates the sheet apperance — _____ is a protein very rich in B-pleated sheets

Answer 26

B-pleated sheet ; hydrogen bonding ; silk

Question 27

third example of secondary structure is _____ — connects the a-helices and B- pleated sheet elements in a protein.

Answer 27

turn

Question 28

The total three-dimensional shape of a polypeptide is its ________

Answer 28

tertiary structure

Question 29

Elements of secondary structure are apparent, especially the several a-helices of the molecule. Note the overall roughly spherical shape of myoglobin. Most polypeptides take this form, which we call _____.

Answer 29

globular

Question 30

ribbon model depicts the tertiary structure of an enzyme known as ________ or ____________

Answer 30

GAMT or the guanidinoacetate methyl transferase.

Question 31

three types of secondary structure:

• a- helices, represented by ________;
• b- pleated sheets, represented by _____ laid side by side;
• and ______ between the structural elements, represented by strings.

Answer 31

• helical ribbons
• flat arrows
• turns

Question 32

Both ______ and ______ are composed of a single, more or less globular, structure, but other proteins can contain more than one compact structural region where each of these regions is called a ______

Answer 32

myoglobin and GAMT ; domain

Question 33

______ or the proteins that white blood cells make to repel invaders provide a good example of domains.

Answer 33

Antibodies

Question 34

 Each of the four polypeptides in the IgG- type antibody contains __________

 domains contain common structural- functional motifs such as finger-shaped motif called a _____ that is involved in DNA binding.

Answer 34

• globular domains.
• zinc finger

Question 35

The highest level of protein structure — _________ — which is the way two or more individual polypeptides fit together in a complex protein

Answer 35

quaternary structure

Question 36

The forces that hold a protein in its proper shape are _______, but most are noncovalent.

Answer 36

covalent bonds

Question 37

The principal covalent bonds within and between polypeptides are __________ between cysteines.

Answer 37

disulfide (S–S) bonds

Question 38

The noncovalent bonds are primarily _______ and ________

Answer 38

hydrophobic and hydrogen bonds

Question 39

 __________ cluster together in the interior of a polypeptide, or at the interface between polypeptides, so they can avoid contact with water (hydrophobic, meaning water-fearing).

Answer 39

Hydrophobic amino acids

Question 40

 ______________ play a major role in tertiary and quaternary structures of proteins.

Answer 40

Hydrophobic interactions

Question 41

(PROTEIN FUNCTION)

Some proteins provide the structure that helps give cells _____ and ______

Answer 41

ntegrity and shape

Question 42

Other proteins serve as _______ to carry signals from one cell to another.

Answer 42

hormones

Question 43

________ can also bind and carry substances.

Answer 43

Proteins

Question 44

The __________ carries oxygen from the lungs to remote areas of the body.

Answer 44

protein hemoglobin

Question 45

________ stores oxygen in muscle tissue until it is used.

Answer 45

Myoglobin

Question 46

_______ also control the activities of genes.

Answer 46

Proteins

Question 47

_________ serve as enzymes that catalyze the hundreds of chemical reactions necessary for life.

Answer 47

Proteins

Question 48

(THE RELATIONSHIP BETWEEN GENES AND PROTEINS)

1902, ________ noticed that a human disease, _______ behaved as if it were caused by a single recessive gene

Answer 48

Archibald Garrod ; alcaptonuria

Question 49

Patients with alcaptonuria excrete copious amounts of _________, which has the startling effect of coloring their urine black

Answer 49

homogentisic acid

Question 50

Garrod reasoned that the abnormal buildup of this compound resulted from a ____________

Answer 50

defective metabolic pathway.

Question 51

Somehow, a blockage somewhere in the _______ was causing the intermediate, homogentisic acid, to accumulate to abnormally high levels, much as a dam causes water to accumulate behind it.

Answer 51

pathway

Question 52

Several years later, Garrod proposed that the problem came from a defect in the pathway that degrades the amino acid ___________

Answer 52

phenylalanine

Question 53

By that time, metabolic pathways had been studied for years and were known to be controlled by enzymes — ___________. Thus, it seemed that alcaptonuria patients carried a defective _______

Answer 53

one enzyme catalyzing each step ; enzyme

Question 54

Because the disease was inherited in a simple Mendelian fashion, Garrod concluded that a gene must control the enzyme ’s production so when that gene is defective, it gives rise to a __________

Answer 54

defective enzyme

Question 55

1940s, ______ and _______, carried the argument further with their studies of a common bread mold, the _________

Answer 55

George Beadle and E.L Tatum ; Neurospora crassa

Question 56

They performed their experiments as follows:

 First, they bombarded the ______ (spore-forming parts) of Neurospora with ______ to cause mutations.

 Then, they collected the spores from the irradiated mold and germinated them separately to give pure strains of ______.

 They screened many thousands of strains to find a _______.

 The mutants revealed themselves by their inability to grow on minimal medium composed only of ______, _____, ____ and _______

 Wild-type Neurospora grows readily on such a medium; the mutants had to be fed something extra —a _______, for example —to survive.

Answer 56

• peritheca ; x-rays
• mold
• few mutants
• sugar, salts, inorganic nitrogen, and the vitamin biotin.
• vitamin

Question 57

 Next, Beadle and Tatum performed biochemical and genetic analyses on their mutants. By carefully adding ______, one at a time, to the mutant cultures, they pinpointed the biochemical defect.

 For example, the last step in the synthesis of the vitamin pantothenate involves putting together the two halves of the molecule: pantoate and b-alanine where one “pantothenateless” mutant would grow on pantothenate, but not on the two halves of the vitamin. This demonstrated that the last step (step 3) in the biochemical pathway leading to pantothenate was blocked, so the _______________.

 The genetic analysis was just as straightforward. Neurospora is an ascomycete, in which nuclei of two different mating types fuse and undergo meiosis to give eight haploid ascospores, borne in a fruiting body called an _____.

 Beadle and Tatum collected the spores, germinated them separately, and checked the phenotypes of the resulting molds and they found that four of the eight spores gave rise to mutant molds, demonstrating that the mutant phenotype was controlled by a _______

 The event happened over and over again, leading these investigators to the conclusion that each enzyme in a biochemical pathway is controlled by one gene.

 __________ - a subsequent work has shown that many enzymes contain more than one polypeptide chain and that each polypeptide is usually encoded in one gene.

Answer 57

• substances
• enzyme that carries out that step must have been defective
• ascus
• single gene.
• one-gene/one-polypeptide hypothesis

Question 58

(DISCOVERY OF MESSENGER RNA)

______ carry information from gene to ribosome.

Answer 58

mRNA

Question 59

1958, Crick proposed that ______ serves as an intermediate carrier of genetic information where he based his hypothesis in part on the fact that the DNA resides in the nucleus of eukaryotic cells, whereas ______ are made in the cytoplasm which means that something must carry the information from one place to the other

Answer 59

RNA ; proteins

Question 60

Crick noted that ribosomes contain RNA and suggested that this _________ is the information bearer.

Answer 60

ribosomal RNA (rRNA)

Question 61

________ proposed an alternative hypothesis calling for nonspecialized ribosomes that translate unstable RNAs called _________ — are independent RNAs that bring genetic information from the genes to the ribosomes.

Answer 61

François Jacob and colleagues ; messengers

Question 62

1961, ____, ______, and _____, published their proof of the messenger hypothesis.

Answer 62

Jacob, Sydney Brenner and Mathew Meselson

Question 63

The premise of the experiments was this: When phage ____ infects E. coli, it subverts its host from making bacterial proteins to making _____

If Crick’s hypothesis were correct, this switch to phage protein synthesis should be accompanied by the production of new ______ equipped with phage- specific RNAs

Answer 63

T2 ; phage proteins

ribosomes

Question 64

 To distinguish new ribosomes from old, these investigators labeled the ribosomes in uninfected cells with heavy isotopes of ________ and ________.

 This made “_____” ribosomes heavy. Then they infected these cells with phage T2 and simultaneously transferred them to medium containing light nitrogen (14 N) and carbon (12 C).

 Any “_____ ” ribosomes made after phage infection would therefore be light and would separate from the old, heavy ribosomes during ________

Answer 64

• nitrogen (15 N) and carbon (13 C)
• old
• new ; density gradient centrifugation.

Question 65

________ also labeled the infected cells with 32P to tag any phage RNA as it was made.

Answer 65

Brenner and colleagues

Question 66

 Then they asked this question: Was the radio actively labeled phage RNA associated with new or old ribosomes?

 It was shown that the phage RNA was found on _______ whose rRNA was made before infection even began.

 Clearly, this old rRNA could not carry phage genetic information; by extension, it was very unlikely that it could carry host genetic information, either. Thus, the ______ are constant.

Answer 66

• old ribosomes
• ribosomes

Question 67

_______, unlike host rRNA, has a base composition similar to that of host DNA. This lends further weight to the hypothesis that _____, not rRNA, is the informational molecule.

Answer 67

Host mRNA ; mRNA

Question 68

(TRANSCRIPTION)

Transcription follows the same base pairing rules as DNA replication: _______ in the DNA pair with ________ respectively, in the RNA product.

Answer 68

T, G, C, and A ; A, C, G, and U,

Question 69

Highly directed chemical reactions such as transcription do not happen at significant rates by themselves—they are enzyme-catalyzed and the enzyme that directs transcription is called ________.

Answer 69

RNA polymerase

Question 70

THREE PHASES OF TRANSCRIPTION

Answer 70

1. Initiation
2. Elongation
3. Termination

Question 71

(iNITIATION)

 First, the enzyme recognizes a region called a _______, which lies just “upstream” of the gene. The polymerase binds tightly to the promoter and causes localized melting, or separation, of the two DNA strands within the promoter. At least ______ are melted.

 Next, the polymerase starts building the RNA chain. The substrates, or building blocks, it uses for _____, ______, _____ and ______.

Answer 71

• promoter ; 12 bp
• ATP, GTP, CTP, and UTP.

Question 72

 The first, or initiating, substrate is usually a _______.

 After the first nucleotide is in place, the polymerase joins a second nucleotide to the
first, forming the initial _______ in the RNA chain. Several nucleotides may be joined before the polymerase leaves the promoter and elongation begins.

Answer 72

• purine nucleotide
• phosphodiester bond

Question 73

(ELONGATION)

 During the elongation phase of transcription, ________ directs the sequential binding of ribonucleotides to the growing RNA chain in the 5’ → 3’ direction (from the 5’-end toward the 3’-end of the RNA).

 As it does so, it moves along the _______, and the “_____” of melted DNA moves with it. This melted region exposes the bases of the template DNA one by one so they can pair with the bases of the incoming ribonucleotides.

 As soon as the transcription machinery passes, the two DNA strands wind around each other again, re-forming the _______

Answer 73

• RNA polymerase
• DNA template ; bubble
• double helix

Question 74

This points to two fundamental differences between transcription and DNA replication:

 (a) ________makes only one RNA strand during transcription, which means that it copies only one DNA strand in a given gene. (However, the opposite strand may be transcribed in another gene.) Transcription is therefore said to be _________. This contrasts with semiconservative DNA replication, in which both DNA strands are copied.

 (b) In transcription, DNA melting is limited and transient. Only enough strand separation occurs to allow the polymerase to “read ” the DNA template strand. However, during replication, the two parental DNA strands ___________.

Answer 74

(a) RNA polymerase ; asymmetrical

(b) separate permanently

Question 75

(TERMINATION)

 Just as promoters serve as initiation signals for transcription, other regions at the ends of genes, called ________, signal termination.

 These work in conjunction with RNA polymerase to loosen the association between _______ and _______

 The result is that the RNA dissociates from the RNA polymerase and DNA, thereby _________

Answer 75

• terminators
• RNA product and DNA template.
• stopping transcription

Question 76

RNA sequences are usually written ______, left to right. This feels natural to a molecular biologist because RNA is made in a 5’-to-3’ direction.

Answer 76

5’ to 3’

Question 77

mRNA is also translated _________. Thus, because ribosomes read the message 5’ to 3’, it is appropriate to write it 5’ to 3’ so that we can read it like a sentence.

Answer 77

5’ to 3’

Question 78

Genes are also usually written so that their transcription proceeds in a __________ direction.

Answer 78

left-to-right

Question 79

This “______” of transcription from one end to the other gives rise to the term _______, which refers to the DNA close to the start of transcription (near the left end when the gene is written conventionally).

Answer 79

flow ; upstream

Question 80

Thus, we can describe most promoters as lying just _______ of their respective genes.

Answer 80

upstream

Question 81

By the same convention, we say that genes generally lie _______ of their promoters.

Answer 81

downstream

Question 82

Genes are also conventionally written with their _________ on top.

Answer 82

nontemplate strands

Question 83

(TRANSLATION)

TWO SUBSTANCES THAT PLAY KEY ROLES IN TRANSLATION:

Answer 83

Ribosomes
tRNA

Question 84

protein-synthesizing machines.

Answer 84

Ribosomes

Question 85

Ribosomes 2 subunits

Answer 85

50s and 30s

Question 86

refer to the sedimentation coefficients of the two subunits.

Answer 86

50S and 30S

Question 87

These __________ are a measure of the speed with which the particles sediment through a solution when spun in an ultracentrifuge.

Answer 87

coefficients

Question 88

The ___________, with a larger sedimentation coefficient, migrates more rapidly to the bottom of the centrifuge tube under the influence of a centrifugal force.

Answer 88

50S subunit

Question 89

The ________ are functions of the mass and shape of the particles.

Answer 89

coefficients

Question 90

______ sediment more rapidly than light ones; ______ migrate faster than extended or flattened ones

Answer 90

Heavy particles ; spherical particles

Question 91

The 50S subunit is actually about _____ as massive as the 30S.

Answer 91

twice

Question 92

Together, the 50S and 30S subunits compose a ______ where the numbers do not add up that is because the sedimentation coefficients are not proportional to the particle mass; in fact, they are roughly proportional to the two- thirds power of the particle mass.

Answer 92

70S ribosome

Question 93

Each ribosomal subunit contains ____ and ______.

 The _____includes one molecule of ribosomal RNA (rRNA) with a sedimentation coefficient of ____, plus ___ ribosomal proteins.

 The ____ is composed of 2 rRNAs (_______) and ___ proteins.

Answer 93

• RNA and protein
• 30S subunit ; 16S ; 21
• 50S subunit ; 23S + 5S ; 34

Question 94

________ are gene products thus, a ribosome is produced by dozen of genes.

Answer 94

Ribosomal proteins

Question 95

_____ participate in protein synthesis but do not code for proteins.

Answer 95

rRNAs

Question 96

_______ is the only step in expression of the genes for rRNAs, aside from some trimming of the transcripts.

Answer 96

Transcription

Question 97

the adapter molecule

Answer 97

tRNA

Question 98

The adapter molecule in translation is indeed a small RNA that recognizes both RNA and amino acids; it is called __________

Answer 98

transfer RNA (tRNA).

Question 99

the molecule has two “business ends.”

 One end (the ____ of the model) attaches to an amino acid. Because this is a tRNA specific for ______, only _______ will attach. An enzyme called ______ catalyzes this reaction. The generic name for such enzymes is _________

 The other end (the _____ of the model) contains a 3-bp sequence that pairs with a complementary 3-bp sequence in an mRNA. Such a triplet in mRNA is called a _____; naturally enough, its complement in a tRNA is called an ______.

Answer 99

• top ; phenylalanine (tRNA Phe) ; phenylalanine-tRNA synthetase ; phenylalanine ; aminoacyl-tRNA synthetase.
• bottom; codon ; anticodon

Question 100

 ______ contains 3 letter words.

 Actually, three of the possible codons (____, _____ and ____) code for termination; that is, they tell the ribosome to stop.

 All of the other codons ________. This means that most amino acids have more than one codon; the genetic code is therefore said to be degenerate.

Answer 100

• Genetic code
• UAG, UAA, and UGA)
• specify amino acids

Question 101

(INITIATION OF PROTEIN SYNTHESIS)

three codons terminate _______

Answer 101

translation.

Question 102

three termination codons interact with protein factors, whereas the initiation codon interacts with a special aminoacyl tRNA. In eukaryotes this is methionyl-tRNA (a tRNA with methionine attached); in bacteria it is a derivative called ________ which is just methionyl tRNA with a formyl group attached to the amino group of methionine.

Answer 102

N formylmethionyl-tRNA

Question 103

 We find _____ codons not only at the beginning of mRNAs, but also in the middle of messages.

 When they are at the beginning, AUGs serve as ________, but when they are in the middle, they simply _______

Answer 103

• AUG
• initiation codons ; code for methionine

Question 104

 Bacterial messages have a special sequence, called a ________, named for its discoverers, just upstream of the initiating AUG.

 The Shine – Dalgarno sequence attracts _________ to the nearby AUG so translation can begin.

 Eukaryotes, by contrast, do not have Shine – Dalgarno sequences. Instead, their mRNAs have a special methylated nucleotide called a _____ at their 5’ends.

 A cap-binding protein known as _____ binds to the cap and then helps attract ribosomes.

Answer 104

• Shine–Dalgarno sequence
• ribosomes
• cap
• eIF4E

Question 105

(TRANSLATION ELONGATION)

At the end of the initiation phase of translation, the initiating aminoacyl-tRNA is bound to a site on the ribosome called the _____

Answer 105

P site.

Question 106

For elongation to occur, the ribosome needs to add amino acids one at a time to the __________

Answer 106

initiating amino acid

Question 107

Elongation begins with the binding of the second aminoacyl-tRNA to another site on the ribosome called the ______

Answer 107

A site

Question 108

This process requires an elongation factor called ______, where EF stands for “elongation factor, ” and energy provided by GTP.

Answer 108

EF-Tu

Question 109

Next, a peptide bond must form between the two amino acids. The large ribosomal subunit contains an enzyme known as ________, which forms a peptide bond between the amino acid or peptide in the P site (formylmethionine [fMet] in this case) and the amino acid part of the aminoacyl tRNA in the A site.

Answer 109

peptidyl transferase

Question 110

The result is a ______ in the A site. The dipeptide is composed of fMet plus the second amino acid, which is still bound to its tRNA. The large ribosomal RNA contains the peptidyl transferase active center.

Answer 110

dipeptidyl tRNA

Question 111

The third step in elongation, _________, involves the movement of the mRNA one codon’s length through the ribosome. This maneuver transfers the dipeptidyl-tRNA from the A site to the P site and moves the deacetylated tRNA from the P site to another site, the _____, which provides an exit from the ribosome. Translocation requires another elongation factor called ______ and _______.

Answer 111

translocation ; E site ; EF-G and GTP

Question 112

(TERMINATION OF TRANSLATION AND mRNA STRUCTURE)

Three different codons (____, ____ and _____) cause termination of

Answer 112

UAG, UAA, and UGA

Question 113

Protein factors called _______ recognize these termination codons (or stop codons) and cause translation to stop, with release of the polypeptide chain.

Answer 113

release factors

Question 114

The initiation codon at one end, and the termination codon at the other end of a coding region of a gene identify an ________. It is called “open ” because it contains no _______ to interrupt the translation of the corresponding mRNA.

Answer 114

open reading frame (ORF) ; internal termination codons

Question 115

The “_______ ” part of the name refers to the way the ribosome can read the mRNA in three different ways, or “frames,” depending on where it _____.

Answer 115

reading frame ; starts

Question 116

This _____ (shorter than any gene you would expect to find) contains a start codon (___) and a stop codon (____). (Remember that these DNA codons will be transcribed to mRNA with the corresponding codons AUG and UAG.) In between (and including these codons) we have a short open reading frame that can be translated to yield a tetrapeptide (a peptide containing four amino acids): fMet-Gly-Tyr-Arg.

Answer 116

minigene ; ATG ; TAG

Question 117

_______ may also have more than one open reading frame, but the largest is usually the one that is used.

Answer 117

natural mRNA

Question 118

_____ begins with the first G and _______ begins 9 bp downstream at the start codon (AUG). Thus, the mRNA produced from this gene has a 9-bp leader, which is also called the _____ or ______

Answer 118

Transcription ; translation ; 5’ -untranslated region, or 5’-UTR.

Question 119

Similarly, a _____ is present at the end of the mRNA between the stop codon and the transcription termination site. The trailer is also called the ____ or _____. In a eukaryotic gene, the transcription termination site would probably be farther downstream, but the mRNA would be cleaved downstream of the translation stop codon and a string of A ’s [poly(A)] would be added to the 3’-end of the mRNA. In that case, the trailer would be the stretch of RNA between the stop codon and the poly(A)

Answer 119

trailer ; 3’-untranslated region, or 3’-UTR ;

Question 120

(REPLICATION)

second characteristic of genes is that they _________.

Answer 120

replicate faithfully

Question 121

The Watson–Crick model for DNA replication assumes that as new strands of DNA are made, they follow the usual base-pairing rules of ______ and ______

Answer 121

A with T and G with C.

Question 122

The model also presupposes that the two parental strands _____ and that each then serves as a ______ for a new progeny strand.

Answer 122

separate ; template

Question 123

This is called ________ because each daughter double helix has one parental strand and one new strand.

Answer 123

semiconservative replication

Question 124

In other words, one of the parental strands is _______ in each daughter double helix.

Answer 124

“conserved ”

Question 125

Another potential mechanism is ________, in which the two parental strands stay together and somehow produce another daughter helix with two completely new strands.

Answer 125

conservative replication

Question 126

Another possibility is ________, in which the DNA becomes fragmented so that new and old DNA regions coexist in the same strand after replication.

Answer 126

dispersive replication

Question 127

_______ and ______proved that DNA really does replicate by a semiconservative mechanism.

Answer 127

Matthew Meselson and Franklin Stahl

Question 128

A third characteristic of genes is that they accumulate changes, or _______

Answer 128

mutations

Question 129

By this process, life itself can change, because _____ is essential for evolution.

Answer 129

mutation

Question 130

If a nucleotide in a gene changes, it is likely that a corresponding change will occur in an ______ in that gene’s protein product. Sometimes, because of the degeneracy of the genetic code, a nucleotide change will not affect the protein.

Answer 130

amino acid

Question 131

For example, changing the codon ____ to _____ is a mutation, but it would probably not be detected because both AAA and AAG code for the same amino acid: lysine.

Such innocuous alterations are called _______

Answer 131

AAA to AAG ; silent mutations

Question 132

changed ______ in a gene results in an altered _______ in the protein.

Answer 132

nucleotide ; amino acid

Question 133

This may be harmless if the amino acid change is ______ (e.g., a leucine changed to an isoleucine). But if the new amino acid is much different from the old one, the change frequently impairs or destroys the function of the protein.

Answer 133

conservative

Question 134

An excellent example of a disease caused by a defective gene is ________

Answer 134

sickle cell disease

Question 135

The shape of normal cells is a ______; that is, the disc is concave viewed from both the top and bottom. However, when these people exercise, or otherwise deplete the oxygen in their blood, their red blood cells change dramatically to a _____ or ______

Answer 135

biconcave disc ; sickle, or crescent, shape

Question 136

The _______ cannot fit through tiny capillaries, so they clog and rupture them, starving parts of the body for blood and causing internal bleeding and pain.

Answer 136

sickle cells

Question 137

_______ are so fragile that they burst, leaving the patient anemic. Without medical attention, patients undergoing a sickling crisis are in mortal danger.

Answer 137

Sickle cells

Question 138

(What causes this sickling of red blood cells?)

The problem is in _______, the red, oxygen- carrying protein in the red blood cells.

Answer 138

hemoglobin

Question 139

Normal hemoglobin remains soluble under ordinary physiological conditions, but the hemoglobin in sickle cells precipitates when the blood oxygen level falls, forming _____, _____ that distort the blood cells into the sickle shape.

Answer 139

long, fibrous aggregates

Question 140

(What is the difference between normal hemoglobin (HbA) and sickle cell hemoglobin (HbS)?)

Vernon Ingram answered this question in 1957 by determining the amino acid sequences of parts of the two proteins using a process that was invented by Frederick Sanger and is known as ______.

Answer 140

protein sequencing

Question 141

Ingram focused on the ______ of the two proteins. _________ is one of the two different polypeptide chains found in the tetrameric (four- chain) hemoglobin protein.

Answer 141

b-globin

Question 142

First, Ingram cut the two polypeptides into pieces with an enzyme that breaks selected peptide bonds. These pieces, called ______, can be separated by a two-dimensional method called _______.

Answer 142

peptides ; fingerprinting

Question 143

The peptides are separated in the first dimension by _________

Answer 143

paper electrophoresis

Question 144

Then the paper is turned 90 degrees and the peptides are subjected to _______ to separate them still farther in the second dimension. The peptides usually appear as ____ on the paper.

Answer 144

paper chromatography ; spots

Question 145

Different proteins, because of their different amino acid compositions, give different patterns of spots. These patterns are aptly named _______

Answer 145

fingerprints

Question 146

When Ingram compared the fingerprints of ____ and _____, he found that all the spots matched except for one where his spot had a different mobility in the HbS fingerprint than in the normal HbA fingerprint, which indicated that it had an altered

Answer 146

HbA and HbS

Question 147

(What change in the b-globin gene caused the change Ingram detected in its protein product?)

The two codons for glutamate (Glu) are ____ and ____; two of the four codons for valine (Val) are ____ and _____

Answer 147

GAA and GAG ; GUA and GUG

Question 148

If the glutamate codon in the HbA gene is GAG, a single base change to GTG would alter the mRNA to GUG, and the amino acid inserted into HbS would be ____ instead of _____. A similar argument can be made for a GAA→GTA change.

Answer 148

valine ; glutamate

Question 149

(Why has this deleterious mutation spread so successfully through the population?)

Although the _________ can be lethal, heterozygotes have little if any difficulty because their normal allele makes enough product to keep their blood cells from sickling.

Answer 149

homozygous condition