Chapter 17 Flashcards Preview

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Flashcards in Chapter 17 Deck (122):
1

The information of DNA is in the form of

specific sequences of nucleotides

2

The DNA inherited by an organism leads to

specific traits by dictating the synthesis of proteins

3

Proteins are the links between

genotype and phenotype

4

Gene expression, the process by which DNA directs protein synthesis, includes two stages:

transcription and translation

5

RNA is

the bridge between genes and the proteins for which they code

6

Transcription is

the synthesis of RNA using information in DNA.

Transcription happens in the nucleus of eukaryotes.

7

Transcription produces

messenger RNA (mRNA)

8

Translation is

the synthesis of a polypeptide, using information in the mRNA

9

Ribosomes are

the sites of translation

10

Every kind of cell has

ribosomes and DNA

11

In prokaryotes, translation of mRNA can begin before

transcription has finished

12

In a eukaryotic cell, the nuclear envelope

separates transcription from translation

13

Eukaryotic RNA transcripts are modified through

RNA processing to yield the finished mRNA

14

A primary transcript is the

initial RNA transcript from any gene prior to processing

15

The central dogma is the concept that

cells are governed by a cellular chain of command:

DNA---> RNA ---> Protein

16

There are 20 amino acids, but

there are only four nucleotide bases in DNA

17

The flow of information from gene to protein is based on a

triplet code: a series of nonoverlapping, three-nucleotide words

18

The words of a gene are transcribed into

complementary nonoverlapping tree-nucleotide words of mRNA.

These words are then translated into a chain of amino acids, forming a polypeptide

19

During transcription,

one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript

20

The template strand is always the same strand for

a given gene

21

During translation,

the mRNA base triplets, called codons, are read in the 5' to 3' direction

22

Codons along an mRNA molecule are read by

translation machinery in the 5' to 3' direction

23

Each codon specifies the amino acid (one of 20) to be placed at the

corresponding position along a polypeptide

24

All 64 codons were deciphered by the

mid-1960s

25

Of the 64 triplets,

61 code for amino acids; 3 triplets are "stop signals to end translation (stop codons)

26

The genetic code is redundant (more than one codon may specify a particular amino acid) but

not ambiguous; no codon specifies more than one amino acid

27

Codons must be read in the correct reading frame (correct groupings) in order for

the specified polypeptide to be produced

28

AUG

start codon
always codes for placement of amino acid called methionine

29

The genetic code is

nearly universal, shared by the simplest bacteria to the most complex animals

30

Genes can be transcribed and translated after

being transplanted from one species to another

31

Transcription is the

DNA-directed synthesis of RNA

32

Transcription is the

first stage of gene expression

33

RNA synthesis is catalyzed by

RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides

34

The RNA is complementary to the

DNA template strand

35

RNA synthesis follows the same

base-pairing rules as DNA, except that uracil substitutes for thymine

36

The DNA sequence where RNA polymerase attaches is called the

promoter,
in bacteria, the sequence signaling the end of transcription is called the terminator

37

The stretch of DNA that is transcribed is called a

transcription unit

38

The three stages of transcription

1. Initiation
2. Elongation
3. Termination

39

Promoters signal the transcriptional start point and usually

extend several dozen nucleotide pairs upstream of the start point

40

Transcription factors mediate the binding of

RNA polymerase and the initiation of transcription

41

The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a

transcription initiation complex

42

A promoter called a TATA box is

crucial in forming the initiation complex in eukaryotes

43

As RNA polymerase moves along the DNA,

it untwists the double helix, 10 to 20 bases at a time

44

Transcription progresses at a rate of

40 nucleotides per second in eukaryotes

45

A gene can be transcribed simultaneously by

several RNA polymerases

46

Nucleotides are added to the

3' end of the growing RNA molecule

47

The mechanisms of termination are different in

bacteria and eukaryotes

48

In bacteria,

the polymerase stops transcription at the end of the terminator and the mRNA can be translated without further modification

49

In eukaryotes,

RNA polymerase II transcribes the polyadenylation signal sequence; the RNA transcript is released 10-35 nucleotides past this polyadenylation sequence

50

Eukaryotic cells modify

RNA after transcription

51

Enzymes in the eukaryotic nucleus modify

pre-mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm

52

During RNA processing,

both ends of the primary transcript are usually altered.

Also, usually some interior parts of the molecule are cut out, and the other parts spliced together.

53

Each end of a pre-mRNA molecule is modified in

a particular way
-The 5' end and receives a modified nucleotide 5' cap
-The 3' end gets a poly-A tail

54

These modifications to the end of a pre-MRNA molecule share several functions

-They seem to facilitate the export of mRNA to the cytoplasm
-They protect mRNA from hydrolytic enzymes
-They help ribosomes attach to the 5' end

55

Most eukaryotic genes and their RNA transcripts have

long noncoding stretches of nucleotides that lie between coding regions

56

These noncoding regions are called

intervening sequences, or introns

57

The other regions are called

exons because they are eventually expressed, usually translated into amino acid sequences

58

RNA splicing removes

introns and joins exons, creating an mRNA molecule with a continuous coding sequence

59

In some cases,

RNA splicing is carried out by spliceosomes

60

Spliceosomes consist of

a variety of proteins and several small nuclear rinonucleoproteins (snRNPs) that recognize the splice sites

61

Ribozymes are

catalytic RNA molecules that function as enzymes and can splice RNA

62

The discovery of ribozymes rendered

obsolete the belief that all biological catalysts are proteins

63

There properties of RNA enable it to function as an enzyme:

-it can form a three-dimensional structure because of its ability to base-pair with itself
-some bases in RNA contain functional groups that may participate in catalysis
-RNA may hydrogen-bond with other nucleic acid molecules

64

Some introns contain sequences that may regulate

gene expression

65

Some genes can encode more than

one kind of polypeptide, depending on which segments are treated as exons during splicing.

This is called alternative RNA splicing

66

Consequently, the number of different proteins an organism can produce is

much greater than its number of genes

67

Proteins often have a modular architecture consisting of discrete regions called

domains

68

In many cases,

different exons code for the different domains in a protein

69

Exon shuffling may result in

the evolution of new proteins

70

Translation is the

RNA-directed synthesis of a polypeptide

71

Genetic information flows from

mRNA to protein through the process of translation

72

A cell translates an mRNA message into protein with the help of

transfer RNA (tRNA)

73

tRNAs transfer

amino acids to the growing polypeptide in a ribosome

74

Translation is a complex process in terms of its

biochemistry and mechanics

75

Molecules of tRNA are not identical

-Each carries a specific amino acid on one end
-each has an antiocodon on the other end; the anticodon base-pairs with a complementary codon on mRNA

76

A tRNA molecule consists of

a single RNA strand that is only about 80 nucleotides long

77

Flattened into one plane to reveal its base pairing,

a tRNA molecule looks like a cloverleaf

78

Because of hydrogen bonds, tRNA actually

twists and folds into a three-dimensional molecule

79

tRNA is roughly

L-shaped

80

Accurate translation requires two steps

-First: a correct match between a tRNA and an amino acid, done by the enzyme aminoacyl-tRNA synthetase
-Second: a correct match between the tRNA anticodon and an mRNA codon

81

Flexible pairing at the third base of a codon is called

wobble and allows some tRNAs to bind to more than one codon

82

Ribosomes facilitate

specific coupling of tRNA anticodons with mRNA codons in protein synthesis

83

The two ribosomal subunits (large and small) are

made of proteins and ribosomal RNA (rRNA)

84

Bacterial and eukaryotic ribosomes are somewhat similar but have significant differences:

some antibiotic drugs specifically target bacterial ribosomes without harming eukaryotic ribosome

85

A ribosome has three binding sites for tRNA:

-the P site
-the A site
-the E site

86

The P site

holds the tRNA that carries the growing polypeptide chain

87

The A site

holds the tRNA that carries the next amino acid to be added to the chain

88

The E site

is the exit site, where discharged tRNAs leave the ribosome

89

The three stages of translation

1. Initiation
2. Elongation
3. Termination

All three stages require protein "factors" that aid in the translation process

90

The initiation stage of translation brings together

mRNA, a tRNA with the first amino acid, and the two ribosomal subunits

91

First, a small ribosomal subunit binds with

mRNA and a special initiator tRNA

(initiation stage in translation??)

92

Then the small subunit moves

along the mRNA until it reaches the start codon (AUG)

(initiation stage in translation??)

93

Proteins called initiation factors bring in the

large subunit that completes the translation initiation complex

(initiation stage in translation)

94

During the elongation stage,

amino acids are added one by one to the preceding amino acid at the C-terminus of the growing chain

95

Each addition involves proteins called

elongation factors and occurs in three steps: codon recognition, peptide bond formation, and translocation

(elongation stage in translation)

96

Translation proceeds along the

mRNA in a 5' to 3' direction

97

Termination occurs when

a stop codon in the mRNA reaches the A site of the ribosome

(termination stage in translation)

98

The A site accepts a protein called a

release factor

(termination stage in translation)

99

The release factor causes the

addition of a water molecule instead of an amino acid.
This reaction releases the polypeptide, and the translation assembly then comes apart.

(termination stage in translation)

100

A number of ribosomes can translate a single mRNA simultaneously forming a

polyribosome (or polysome)

101

Polyribosomes enable a cell to

make many copies of a polypeptide very quickly

102

Often translation is not sufficient to make a

functional protein

103

Polypeptide chains are modified after translation or

targeted to specific sites in the cell

104

During and after synthesis,

a polypeptide chain spontaneously coils and folds into its three-dimensional shape

105

Proteins may also require

post-translational modifications before doing their job

106

Some polypeptides are activated by

enzymes that cleave them

107

Other polypeptides come together to form

the subunits of a protein

108

Two populations of ribosomes are evident in cells:

-free ribosomes (in the cytosol)
-bound ribosomes (attached to the ER)

109

Free ribosomes mostly synthesize

proteins that function in the cytosol

110

Bound ribosomes make proteins of the

endomembrane system and proteins that are secreted from the cell

111

Ribosomes are identical and

can switch from free to bound

112

Polypeptide synthesis always begins in

the cytosol

113

Synthesis finished in the

cytosol unless the polypeptide signals the ribosome to attach to the ER

114

Polypeptides destined for the ER for for secretion are marked by a

signal peptide

115

A signal-recognition particle (SRP) binds to

the signal peptide

116

The signal-recognition particle (SRP) brings teh

signal peptide and its ribosome to the ER

117

Messenger RNA (mRNA) function

carries information specifying amino acid sequences of proteins from DNA to ribosomes

118

Transfer RNA (tRNA) function

serves as adapter molecule in protein synthesis; translates mRNA codons into amino acids

119

Ribosomal RNA (rRNA) function

plays catalytic (ribozyme) roles and structural roles in ribosomes

120

Primar transcript function

serves as a precursor to mRNA, rRNA, or tRNA, before being processed by splicing or cleavage

121

Small nuclear RNA (snRNA) function

plays structural and catalytic roles in spliceosomes

122

SRP RNA function

is a component of the signal-recognition particle (SRP)