Chapter 17

Question 1

gene expression

Answer 1

In other words, proteins are the
link between genotype and phenotype. Gene expression is the process by which
DNA directs the synthesis of proteins (or, in some cases, just RNAs).

Question 2

one gene one polypep hyp

Answer 2

. For example, hemoglobin—the
oxygen-transporting protein of vertebrate red blood cells—
contains two kinds of polypeptides (see Figure 5.18), and
thus two genes code for this protein, one for each type of
polypeptide. Beadle and Tatum’s idea was therefore restated
as the one gene–one polypeptide hypothesis. E

Question 3

why is calling it the one gene one polypep hyp not rly correct

Answer 3

Even this description is not entirely accurate, though. First, in many cases, a
eukaryotic gene can code for a set of closely related polypeptides via a process called alternative splicing, which you will
learn about later in this chapter. Second, quite a few genes
code for RNA molecules that have important functions incells even though they are never translated into protein

Question 4

RNA is chemically similar to DNA except that

Answer 4

t it contains ribose
instead of deoxyribose as its sugar and has the nitrogenous
base uracil rather than thymine

Question 5

transcription

Answer 5

Transcription is the synthesis of RNA using information
in the DNA. The two nucleic acids are written in different
forms of the same language, and the information is simply
transcribed, or “rewritten,” from DNA to RNA. J

Question 6

mrna

Answer 6

For a protein-coding gene, the resulting
RNA molecule is a faithful transcript of the gene’s proteinbuilding instructions. This type of RNA molecule is called
messenger RNA (mRNA) because it carries a genetic message from the DNA to the protein-synthesizing machinery of
the cell.

Question 7

translation

Answer 7

Translation is the synthesis of a polypeptide using the
information in the mRNA. During this stage, there is a change
in language: The cell must translate the nucleotide sequence of a polypep

Question 8

ribosomes

Answer 8

The sites of translation are ribosomes, molecular complexes that facilitate the orderly linking of amino
acids into polypeptide chains.

Question 9

triplet code

Answer 9

Experiments have verified that the flow of information from
gene to protein is based on a triplet code: The genetic
instructions for a polypeptide chain are written in the DNA
as a series of nonoverlapping, three-nucleotide words

Question 10

template strand

Answer 10

This strand is called the

template strand because it provides the pattern, or template, for the sequence of nucleotides in an RNA transcript.

Question 11

coding strand

Answer 11

These codons are complementary to the template strand and
thus identical in sequence to the mRNA, except that they
have a T wherever there is a U in the mRNA. For this reason,
the nontemplate DNA strand is often called the coding
strand; by convention, the sequence of the coding strand
is used when a gene’s sequence is reported

Question 12

Because diverse forms of life share a

common genetic code due to their shared ancestry,

Answer 12

one species can be
programmed to produce proteins characteristic of a second species by
introducing DNA from the second species into the first (jellyfish pig)

Question 13

rna pol

Answer 13

. An enzyme called an
RNA polymerase pries the two strands of DNA apart and
joins together RNA nucleotides complementary to the DNA
template strand, thus elongating the RNA polynucleotide

Question 14

terminator

Answer 14

; in bacteria, the
sequence that signals the end of transcription is called the
terminator. (The termination mechanism is different in
eukaryotes; we’ll describe it later.)

Question 15

rna procesing

Answer 15

Enzymes in the eukaryotic nucleus modify pre-mRNA in
specific ways before the genetic message is dispatched to the
cytoplasm. During this RNA processing, both ends of the
primary transcript are altered. Also, in most cases, certain
interior sections of the RNA molecule are cut out and the
remaining parts spliced together. These modifications
produce an mRNA molecule ready for translatio

Question 16

rna splicing

Answer 16

A remarkable stage of RNA processing in the eukaryotic nucleus
is RNA splicing (Figure 17.12), where large portions of
the RNA molecules are removed and the remaining portions are reconnected

Question 17

alternative rna splicing

Answer 17

One important consequence of the presence of introns in
genes is that a single gene can encode more than one kind of
polypeptide. Many genes are known to give rise to two or
more different polypeptides, depending on which segments
are treated as exons during RNA processing; this is called
alternative RNA splicing

Question 18

domains

Answer 18

Because of alternative splicing, the number of
different protein products an organism produces can be much
greater than its number of genes.
Proteins often have a modular architecture consisting of
discrete structural and functional regions called domains.

Question 19

transfer rna

Answer 19

. The message is a series
of codons along an mRNA molecule, and the translator is
called a transfer RNA (tRNA). The function of a tRNA is
to transfer an amino acid from the cytoplasmic pool of
amino acids to a growing polypeptide in a ribosome.

Question 20

how does a cell mke nd keep aas

Answer 20

A cell
keeps its cytoplasm stocked with all 20 amino acids, either
by synthesizing them from other compounds or by taking
them up from the surrounding solution

Question 21

aminoacyl-tRNA synthetases

Answer 21

The correct matching up of tRNA and amino acid is carried
out by a family of related enzymes that are aptly named
aminoacyl-tRNA synthetases (Figure 17.17). The active
site of each type of aminoacyl-tRNA synthetase fits only a
specific combination of amino acid and tRNA. There are
20 different synthetases, one for each amino acid. A

Question 22

wobble

Answer 22

The flexible base pairing
at this codon position is called wobble. Wobble explains
why the synonymous codons for a given amino acid most
often differ in their third nucleotide base. Accordingly,
a tRNA with the anticodon 3¿-UCU-5¿ can base-pair with
either the mRNA codon 5¿-AGA-3¿ or 5¿-AGG-3¿, both of
which code for arginine (see Figure 17.6).

Question 23

rrnas

Answer 23

A ribosome
consists of a large subunit and a small subunit, each made
up of proteins and one or more ribosomal RNAs (rRNAs).

Question 24

n
addition to a binding site for mRNA, each ribosome has three
binding sites for tRNA (say each and what they do)

Answer 24

The P site (peptidyltRNA binding site) holds the tRNA carrying the growing
polypeptide chain, while the A site (aminoacyl-tRNA binding site) holds the tRNA carrying the next amino acid to be
added to the chain. Discharged tRNAs leave the ribosome
from the E site (exit site).

Question 25

The ribosome holds the tRNA and

mRNA in close proximity and

Answer 25

positions the new amino acid
so that it can be added to the carboxyl end of the growing
polypeptide. It then catalyzes the formation of the peptide
bond. As the polypeptide becomes longer, it passes through
an exit tunnel in the ribosome’s large subunit. When the polypeptide is complete, it is released through the exit tunnel.

Question 26

what comes after the primary factors creating the transl. initiation?

Answer 26

This is followed by the
attachment of a large ribosomal subunit,
completing the translation initiation complex. Proteins called initiation factors are
required to bring all these components
together

Question 27

signal peptide and SRP

Answer 27

The polypeptides of proteins destined for the endomembrane system or for secretion are marked by a signal peptide,
which targets the protein to the ER (Figure 17.22). The signal
peptide, a sequence of about 20 amino acids at or near the leading end (N-terminus) of the polypeptide, is recognized as it
emerges from the ribosome by a protein-RNA complex called
a signal-recognition particle (SRP)

Question 28

what happens with polypep synth in er

Answer 28

Polypeptide synthesis continues there, and the growing polypeptide snakes across the membrane into the ER lumen via a
protein pore. The rest of the completed polypeptide, if it is to
be secreted from the cell, is released into solution within the ER
lumen.

Question 29

mutations

Answer 29

ow that you have explored the process of gene expression,
you are ready to understand the effects of changes to the genetic
information of a cell. These changes, called mutations, are
responsible for the huge diversity of genes found among organisms because mutations are the ultimate source of new genes.

Question 30

pt mutations

Answer 30

s. Here we examine small-scale mutations
of one or a few nucleotide pairs, including point mutations,
changes in a single nucleotide pair of a gene

Question 31

nuc-pair substitution

Answer 31

A nucleotide-pair substitution is the replacement of

one nucleotide and its partner with another pair of nucleotides (

Question 32

y. Occasionally, such
a mutation leads to an improved protein or one with novel
capabilities, but much more often

Answer 32

such mutations are neutral
or detrimental, leading to a useless or less active protein that
impairs cellular function

Question 33

Substitution mutations are usually

Answer 33

y missense mutations;
that is, the altered codon still codes for an amino acid and
thus makes sense, although not necessarily the right sense.

Question 34

nonsense mutations

Answer 34

But a point mutation can also change a codon for an amino
acid into a stop codon. This is called a nonsense mutation,
and it causes translation to be terminated prematurely; the
resulting polypeptide will be shorter than the polypeptide
encoded by the normal gene. Most nonsense mutations lead
to nonfunctional proteins

Question 35

insertions and deletions

Answer 35

Insertions and deletions are additions or losses of nucleotide pairs in a gene (Figure 17.27b). These mutations have
a disastrous effect on the resulting protein more often than substitutions do

Question 36

frameshift mutation- d and what does it cause

Answer 36

Insertion or deletion of nucleotides may alter
the reading frame of the genetic message, the triplet grouping of
nucleotides on the mRNA that is read during translation. Such
a mutation, called a frameshift mutation, occurs whenever
the number of nucleotides inserted or deleted is not a multiple
of three. All nucleotides downstream of the deletion or insertion will be improperly grouped into codons; the result will be extensive missense mutations, usually ending sooner or later
in a nonsense mutation that leads to premature termination.

Question 37

In many cases, the error

will be corrected by what and otherwise what happens

Answer 37

In many cases, the error
will be corrected by DNA proofreading and repair systems
(see Concept 16.2). Otherwise, the incorrect base will be used
as a template in the next round of replication, resulting in a
mutation. Such mutations are called spontaneous mutations

Question 38

mutagens

Answer 38

A number of physical and chemical agents, called mutagens,
interact with DNA in ways that cause mutations. In the 1920s,
Hermann Muller discovered that X-rays caused genetic changes
in fruit flies, and he used X-rays to make Drosophila mutants for
his genetic studies. But he also recognized an alarming implication of his discovery: X-rays and other forms of high-energy
radiation pose hazards to the genetic material of people as well
as laboratory organisms

Question 39

In

this chapter, you have learned in molecular terms how a typical gene is expressed—

Answer 39

by transcription into RNA and then
translation into a polypeptide that forms a protein of specific
structure and function. Proteins, in turn, bring about an
organism’s observable phenotype.