Chapter 17  Gene Expression: From Gene to Protein
Student Guided Notes
Overview: The Flow of Genetic Information

The information content of genes is in the form of specific sequences of nucleotides along the DNA
strands.

The DNA of an organism leads to specific traits by dictating the synthesis of _____________________
and of ________________ molecules involved in protein synthesis (gene expression.)
○ Proteins are the link between __________________________ and _________________________.
Concept 17.1 Genes specify proteins via transcription and translation
The study of metabolic defects provided evidence that genes specify proteins.

In 1902, __________________________________________ was the first to suggest that genes dictate
___________________________ through enzymes that catalyze specific chemical reactions in the cell.
o He speculated that ________________________________________, a hereditary disease, is
caused by the absence of an enzyme that breaks down a specific substrate, and research several
decades later supported Garrod’s hypothesis.

Progress in linking genes and enzymes rested on the growing understanding that cells synthesize and
degrade most organic molecules in a series of steps, a metabolic pathway.

_______________________________ and ___________________________________ were able to
establish the link between __________________________________________ by exploring the
metabolism of a bread mold, _________________________________________.
o One type of mutant required only the addition of the amino acid arginine to the minimal growth
medium.
○ Beadle and Tatum concluded that this mutant was defective in the biochemical pathway that
synthesizes ____________________________________.

Later research refined this _________________________________________________ hypothesis,
eventually revised as the one gene–one protein hypothesis and then the ______________________
____________________________________ hypothesis.

This hypothesis was not entirely accurate and was eventually revised to reflect other discoveries:
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o The newest definition of a gene (find this at the end of the chapter!): _____________________
_________________________________________________________________________________
_________________________________________________________________________________
________________________________________________________________________________.
_______________________ and ____________________ are the two main processes linking gene to protein.

Genes provide the instructions for making specific proteins.

Getting from DNA to protein requires two major stages: transcription and translation.

During transcription, a DNA strand provides a ________________________________ for the
synthesis of a ____________________________________________________________________.
o Transcription of a protein-coding gene produces a _______________________________________
______________________________ molecule.

Translation is the synthesis of a ________________________________________________, using the
information in ____________________________.
o The sites of translation are the _________________________________________, complex particles
that facilitate the orderly assembly of amino acids into polypeptide chains.

The basic mechanics of transcription and translation are similar in eukaryotes and bacteria.

Bacteria lack nuclei, and their DNA is not separated from ribosomes and other protein-synthesizing
equipment.
○ This allows the coupling of _______________________________ and _______________________.

In a eukaryotic cell, transcription occurs in the _______________________________, and translation
occurs at ______________________________________ in the cytoplasm.

The molecular chain of command in a cell has a directional flow of genetic information:

_____________________  ___________________  __________________________

Francis Crick dubbed this concept the ____________________________________ in 1956.
In the genetic code, nucleotide triplets specify amino acids.

Triplets of nucleotide bases are the smallest units of uniform length that can code for all the amino acids.
○ With a triplet code, three consecutive bases specify an __________________________________,
creating _____________________ possible code words.
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
During transcription, one DNA strand, the template strand, provides a template for ______________
___________________________________________________________________________________.

The mRNA base triplets are called _______________________.
○ These codons are complementary to the template strand and thus identical in sequence to the
__________________ except that they have ______ instead of __________.

During translation, the sequence of codons along an mRNA molecule is translated into a sequence of
_______________________________________ making up the polypeptide chain.
○ Each codon specifies which one of the 20 amino acids will be incorporated at the corresponding
position along a polypeptide.
o The task of matching each codon to its amino acid counterpart began in the early 1960s.
o ________________________________ determined the first match: UUU codes for the amino acid
__________________________________________.
o ____________________ of 64 triplets code for amino acids.
o The codon AUG not only codes for the amino acid _____________________________ but
also indicates the “__________________” or initiation of ______________________________.
o Three codons do not indicate amino acids but are “__________________” signals marking the
termination of translation.
○ The starting point establishes the ___________________________________________; subsequent
codons are read in groups of three nucleotides.
The genetic code must have evolved very early in the history of life.

The genetic code is nearly universal, _____________________________________________________
___________________________________________________________________________________.
Concept 17.2 Transcription is the DNA-directed synthesis of RNA: a closer look

_____________________________________________ separates the DNA strands at the appropriate
point and joins RNA nucleotides complementary to the DNA template strand.
○ Like DNA polymerases, RNA polymerases can assemble a polynucleotide only in its 53
direction (therefore the template strand is ____________________.)

Specific sequences of nucleotides along the DNA mark where gene transcription begins and ends.
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○ RNA polymerase attaches and initiates transcription at the __________________________. The
promoter has a sequence of A’s and T’s and is called the ___________________________. This
helps identify where the promoter is located.
○ The sequence that signals ______________________________________ is called the terminator.
Transcription can be separated into three stages: ______________________________,
__________________________________, and ___________________________________ of the RNA chain.

The promoter (where RNA polymerase binds) determines where ______________________________
and which strand of DNA is the _________________________________.

In eukaryotes, proteins called ___________________________________________ mediate the binding
of RNA polymerase and the initiation of transcription.

The complex of transcription factors and RNA polymerase bound to a promoter is called a
__________________________________________________________.
○ As RNA polymerase moves along the DNA, it untwists the double helix, ______ to _______
nucleotides at a time and adds nucleotides to the __________ end of the growing strand.

Many polymerase molecules simultaneously transcribing a single gene increases the amount of mRNA
transcribed from it and helps the cell make the encoded protein in large amounts.

The mechanism of termination differs between bacteria and eukaryotes.
o In bacteria, transcription stops ______________________________________________________
________________________________________________________________________________.
o In eukaryotes, RNA polymerase transcribes the signal for about _____________________________
past the _________________________________.
Concept 17.3 Eukaryotic cells modify RNA after transcription

During RNA processing, both ends of the primary transcript are altered.
o At the 5 end of the pre-mRNA molecule, a modified form of ______________________________
is added, called the ______________.
o At the 3 end, an enzyme adds _________ to __________________________________________,
called the _________________________________________.

These modifications have several important functions.
○ They facilitate the export of mRNA from the _______________________________.
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○ They help protect mRNA from _______________________________________.
○ They help the ________________________________________ to the _______ end of the mRNA.

The most remarkable stage of RNA processing occurs during the removal of a large portion of the RNA
molecule in a cut-and-paste job of ____________________________________________.
o Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides.
o ______________________________________ segments of nucleotides called
_________________________________ lie between coding regions.
o The coding regions, called ____________________________, are eventually expressed, usually
by being translated into __________________________________________________________.
o RNA splicing removes ____________________________ and joins _________________________
to create an mRNA molecule with a continuous coding sequence.

The signal for RNA splicing is a short nucleotide sequence at each end of an intron.

Particles called ______________________________________________________________
(______________________) recognize the splice sites.

snRNPs are located in the cell nucleus and are composed of RNA and protein
molecules.

The RNA in a snRNP particle is called a ___________________________________
________________________________ (_________________).

Several different snRNPs join with a variety of proteins to form a larger assembly called a
________________________________________________.

The spliceosome interacts with certain sites along an intron, releasing the introns and joining
together the two exons that flanked the introns.
Ribozymes are RNA molecules that function as ______________________________.

The idea of a catalytic role for snRNA arose from the discovery of ribozymes, ___________________
___________________________________________________________________________________.

In some organisms, RNA splicing occurs without proteins or additional RNA molecules: The intron
RNA functions as a _________________________ and catalyzes its own _______________________.

The discovery of ribozymes rendered obsolete the idea that ___________________________________
________________________________________________________.
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Introns may play a regulatory role in the cell.

Specific functions have not been identified for most introns, but some contain sequences that regulate
___________________________________________________, and many affect gene products.

Many genes give rise to two or more different polypeptides, depending on which segments are treated as
exons during RNA processing; this is called _______________________________________________.

Results from the ____________________________________ Project suggest that alternative splicing
explains why humans can get along with a ________________________________________________.
Concept 17.4 Translation is the RNA-directed synthesis of a polypeptide: a closer look

In the process of translation, a cell translates a genetic message to build a polypeptide.
The ________________ molecule is a translator.

The interpreter is transfer RNA (tRNA), which transfers ____________________________________
___________________________________________________________________________________.
○ A cell has all _____ amino acids available in its cytoplasm, either by synthesizing them from scratch
or by ___________________________________________________________________________.
○ Each tRNA molecule bears a ________________________________________________ at one end.
○ At the other end of the tRNA is a nucleotide triplet called an _______________________________,
which base-pairs with a _____________________________________________________________.

Each amino acid is joined to the correct tRNA by ___________________________________________.

The second recognition process involves a correct match between the ___________________________
and an ___________________________________.

If each anticodon had to be a perfect match to each codon, we would expect to find 61 types of tRNA,
but the actual number is about _________, because _________________________________________
___________________________________________________________________________________.
o Such versatility is possible because the rules for base pairing between the ________________ base
of the codon and the anticodon are relaxed. This flexible base pairing is called _________________.
The ribosome is the site of translation.
A ribosome consists of a ___________________ and a ______________________ subunit, each made up
of _____________________________ and __________________________________________________.

Each ribosome has a binding site for mRNA and ____________ binding sites for tRNA molecules.
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○ The P site (peptidyl tRNA-binding site) holds the tRNA carrying ___________________________
________________________________________________________________________________.
○ The A site (aminoacyl tRNA-binding site) holds the tRNA carrying _________________________
________________________________________________________________________________.
○ Discharged tRNAs leave the ribosome at the __________________________________________.
○ The ribosome catalyzes the formation of the __________________________ bond.
The process of translation builds a polypeptide.

Translation can be divided into three stages: ______________________, ________________________,
and _____________________________________.
o Initiation brings together ___________________________________________________________
________________________________________________________________________________.

A small ribosomal subunit binds with mRNA and a special initiator tRNA, which carries
___________________________________.

The union of mRNA, initiator tRNA, and a small ribosomal subunit is followed by the
attachment of a ___________________________________________ subunit, forming the
______________________________________________________.
o In the _______________________________________ stage of translation, amino acids are added
one by one to the previous amino acid at the C-terminus of the growing chain.
o Termination occurs when _________________________________________________________
________________________________________________________________________________.
o A ___________________________________________________ binds to the stop codon and
causes hydrolysis of the bond between the polypeptide and its tRNA in the P site, and the
polypeptide is released. So a molecule of water is added instead of an amino acid.

A single mRNA may be used to make many copies of a polypeptide simultaneously as multiple
ribosomes, ______________________________________ (or _______________________________),
trail along the same mRNA.
Folding and modification of a protein follows translation.

During and after synthesis, a polypeptide spontaneously coils and folds to its
____________________________________ shape.
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
In addition, proteins may require ________________________________________________________
before doing their particular job.

These modifications may require additions such as ____________________, ___________________, or
_____________________________________________ to amino acids.
○ In other cases, a polypeptide may be cleaved in two or more pieces OR two or more polypeptides
may join to form a protein with __________________________________________.
Signal peptides target some eukaryotic polypeptides to specific destinations in the cell.

Two populations of ribosomes, ____________________ and ________________________, are active
participants in protein synthesis.
○ Free ribosomes are suspended in the ___________________________ and synthesize proteins that
_______________________________________________________________________________.
○ Bound ribosomes are attached to the ___________________________________________________
or to the ____________________________________________.
o Bound ribosomes make proteins of the endomembrane system as well as proteins secreted from
the cell.

Translation in all ribosomes begins ___________________________________, but a polypeptide
destined for the endomembrane system or for export has a specific _____________________________
region at or near the leading end.
○ The signal peptide consists of a sequence of about __________ amino acids.
o A ______________________________________________________________ binds to the signal
peptide and attaches it and its ribosome to a receptor protein in the ________ membrane.
o Protein synthesis resumes, with the growing polypeptide snaking across the membrane into the ER
lumen via a protein pore.
Concept 17.5 Mutations of one or a few nucleotides can affect protein structure and function

Mutations are ______________________________________________________________________
__________________________________________________________________________________.

__________________________________ are the ultimate source of new genes (and genetic diversity!)
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
Mutations include large-scale mutations, as well as ____________________________________, which
are chemical changes in just ________________ nucleotide pair of a gene. (NOTE: this is also called a
substitution, see below.)

If a point mutation occurs in a _______________________________ or in a cell that produces gametes,
it may be transmitted to future generations.
○ For example, _________________________________________ disease is caused by a mutation of
a single nucleotide pair in the gene that encodes the Beta-globin polypeptide of hemoglobin.
A nucleotide-pair substitution is the replacement of one nucleotide and its partner with another pair of
nucleotides.
○ In a silent mutation, a change in a nucleotide pair _________________________________________
__________________________________________________________________________________.

_____________________________________________________ change one amino acid for another
with little effect on protein function.
○ In some cases, the mutation switches one amino acid for another with similar properties.

Nonsense mutations change an amino acid codon into a __________________________________,
causing premature _____________________________________ of translation and nearly always
leading to a nonfunctional protein.

While some mutations have little to no effect, other nucleotide-pair substitutions can cause a major
change in a protein.
o Changes in amino acids at crucial sites, especially _______________________ sites, are likely to
affect function.
Insertions and deletions are additions or losses of nucleotide pairs in a gene.

_________________________________ and ________________________________ are more likely
than substitutions to have a disastrous effect on the resulting protein than ________________________.

Unless insertion or deletion mutations occur in multiples of 3, they cause a ______________________
________________________________________.
○ All the nucleotides downstream of the deletion or insertion will be __________________________
___________________________________________________ because the reading frame is shifted.
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Mutations can occur during DNA replication, DNA repair, or DNA recombination.

Errors during DNA replication or recombination can lead to nucleotide-pair substitutions, insertions, or
deletions.

Mutagens are _______________________________________________________________________
___________________________________________________________________________________.

Physical agents include high-energy radiation like _________________________ and
_____________________________________ light.

Chemical mutagens cause mutations in different ways.
What is a gene? We revisit the question.

In spite of the differences in gene expression between living things, the gene itself is a unifying concept
among all forms of life.

Recall from above: The newest definition of a gene: A gene is a region of DNA that can be
expressed to produce a final functional product that is either a polypeptide or an RNA molecule.
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