Chapter 17 Flashcards
1
Q
The information of DNA is in the form of
A
specific sequences of nucleotides
2
Q
The DNA inherited by an organism leads to
A
specific traits by dictating the synthesis of proteins
3
Q
Proteins are the links between
A
genotype and phenotype
4
Q
Gene expression, the process by which DNA directs protein synthesis, includes two stages:
A
transcription and translation
5
Q
RNA is
A
the bridge between genes and the proteins for which they code
6
Q
Transcription is
A
the synthesis of RNA using information in DNA.
Transcription happens in the nucleus of eukaryotes.
7
Q
Transcription produces
A
messenger RNA (mRNA)
8
Q
Translation is
A
the synthesis of a polypeptide, using information in the mRNA
9
Q
Ribosomes are
A
the sites of translation
10
Q
Every kind of cell has
A
ribosomes and DNA
11
Q
In prokaryotes, translation of mRNA can begin before
A
transcription has finished
12
Q
In a eukaryotic cell, the nuclear envelope
A
separates transcription from translation
13
Q
Eukaryotic RNA transcripts are modified through
A
RNA processing to yield the finished mRNA
14
Q
A primary transcript is the
A
initial RNA transcript from any gene prior to processing
15
Q
The central dogma is the concept that
A
cells are governed by a cellular chain of command:
DNA—> RNA —> Protein
16
Q
There are 20 amino acids, but
A
there are only four nucleotide bases in DNA
17
Q
The flow of information from gene to protein is based on a
A
triplet code: a series of nonoverlapping, three-nucleotide words
18
Q
The words of a gene are transcribed into
A
complementary nonoverlapping tree-nucleotide words of mRNA.
These words are then translated into a chain of amino acids, forming a polypeptide
19
Q
During transcription,
A
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
Q
The template strand is always the same strand for
A
a given gene
21
Q
During translation,
A
the mRNA base triplets, called codons, are read in the 5’ to 3’ direction
22
Q
Codons along an mRNA molecule are read by
A
translation machinery in the 5’ to 3’ direction
23
Q
Each codon specifies the amino acid (one of 20) to be placed at the
A
corresponding position along a polypeptide
24
Q
All 64 codons were deciphered by the
A
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)
