Download notes 12B

Chapter
12b
Gene Activity: How Genes Work
Modified chapter 14
Chapter Outline
14.1 The Function of Genes
1. Sir Archibald Garrod (early 1900s) introduced the phrase inborn error of
_______________.
a. Garrod proposed that inherited defects could be caused by the lack of a particular
enzyme.
b. Knowing that _______________ are proteins, Garrod suggested a link between
genes and proteins.
A. Genes Specify Enzymes
1. George Beadle and Edward Tatum (1940) X-rayed spores of the _______________
_______________ Neurospora crassa.
2. They observed that some resulting cultures lacked a _______________ enzyme for
growth on _______________ medium.
3. They found that a single gene was _______________, which resulted in the lack of a
single enzyme.
4. They proposed the one gene–one enzyme hypothesis: one gene _______________ the
synthesis of one enzyme.
B. Genes Specify a Polypeptide
1. Linus Pauling and Harvey Itano (1949) compared _______________ in red blood
cells of persons with sickle-cell disease and normal _______________.
2. They discovered that the chemical properties of a protein chain of sickle-cell
hemoglobin _______________ from that of normal hemoglobin.
3. Vernon Ingram subsequently showed that the _______________ difference in the
protein chain of sickle-cell hemoglobin is the substitution of a nonpolar amino acid
valine for the negatively charged amino acid _______________.
4. Pauling and Itano formulated the one gene–one polypeptide hypothesis: each gene
specifies one polypeptide of a _______________, a molecule that may contain one or
more different polypeptides.
C. From DNA to RNA to Protein
1. A gene is a sequence of _______________ nucleotide bases that codes for a sequence
of nucleotides in an RNA molecule.
2. DNA is restricted to nucleus; protein synthesis occurs at _______________ in the
cytoplasm.
3. Ribonucleic acid (_______________) is found in both regions of the cell.
D. Types of RNA
1. Like DNA, RNA is a polymer of nucleotides.
2. Unlike DNA, RNA is single-stranded, contains the sugar _______________, and the
base _______________ instead of thymine (in addition to cytosine, guanine, and
adenine).
3. There are three major classes of RNA.
a. Messenger RNA (_______________) takes a message from DNA in the nucleus
to ribosomes in the _______________.
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b. Ribosomal RNA (rRNA) and proteins make up _______________ where
proteins are synthesized.
c. Transfer RNA (_______________) transfers a particular amino acid to a
ribosome.
E. Gene Expression
1. DNA undergoes _______________ to mRNA, which is _______________ to a
protein.
2. DNA is a template for _______________ formation during _______________.
3. Transcription is the first step in gene _______________; it is the process whereby a
DNA strand serves as a template for the formation of _______________.
4. During translation, an mRNA _______________ vdirects the sequence of amino
acids in a polypeptide.
14.2 The Genetic Code
1. The central dogma of molecular biology states that the sequence of nucleotides in
DNA _______________ the order of _______________ in a polypeptide.
2. The _______________ is a _______________ code, comprised of _______________
-base code words (e.g., AUG).
3. A _______________ consists of 3 nucleotide bases of DNA.
4. Four nucleotides based on 3-unit _______________ allows up to 64 different amino
acids to the specified.
A. Finding the Genetic Code
1. Marshall Nirenberg and J. Heinrich Matthei (1961) found that an enzyme that could
be used to construct synthetic RNA in a cell-free system; they showed the
_______________ UUU coded for phenylalanine.
2. By translating just three nucleotides at a time, they _______________ an amino acid
to each of the RNA _______________, and discovered important properties of the
genetic code.
3. The code is _______________: there are 64 triplets to code for 20 naturally occurring
amino acids; this protects against potentially harmful mutations.
4. The genetic code is _______________; each triplet _______________ specifies one
and only one amino acid.
5. The code has start and stop _______________: there are one start codon and three
stop codons.
B. The Code Is Universal
1. The few exceptions to _______________ of the genetic code suggests the code dates
back to the very first organisms and that all organisms are related.
2. Once the code was established, changes would be _______________.
14.3 First Step: Transcription
A. Messenger RNA is Formed
1. A segment of the _______________ unwinds and unzips.
2. Transcription begins when RNA polymerase attaches to a promoter on
_______________. A promoter is a region of DNA which defines the start of the
gene, the direction of transcription, and the strand to be _______________.
3. As RNA polymerase (an enzyme that speeds formation of RNA from a
_______________ template) moves along the tem_______________ strand of the
DNA, complementary RNA nucleotides are paired with DNA nucleotides of the
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coding strand. The strand of _______________ not being transcribed is called the
noncoding strand.
4. _______________ adds nucleotides to the 3'-end of the polymer under construction.
Thus, RNA synthesis is in the _______________ direction.
5. The RNA/DNA association is not as stable as the DNA double helix; therefore, only
the newest portion of the RNA molecule associated with _______________ is bound
to DNA; the rest dangles off to the side.
6. Elongation of _______________ continues until RNA polymerase comes to a stop
sequence.
7. The stop sequence causes _______________ to stop transcribing DNA and to release
the mRNA transcript.
8. Many _______________ molecules work to produce mRNA from the same DNA
region at the same time.
9. Cells produce thousands of copies of the same _______________ molecule and
many copies of the same protein in a shorter period of time than if a single copy of
RNA were used to direct _______________ synthesis.
B. RNA Molecules Are Processed
1. Newly formed primary _______________ transcript is processed before leaving the
nucleus.
2. Primary mRNA transcript is the immediate product of transcription; it contains
_______________ and _______________.
3. The ends of the mRNA molecule are altered: a _______________ is put on the 5' end
and a poly-A tail is put on the 3' end.
a. The “cap” is a modified _______________ (G) where a ribosome attaches to
begin translation.
b. The “poly-A tail” consists of a 150–200 _______________ (A) nucleotide chain
that facilitates transport of mRNA out of the nucleus and inhibits enzymatic
degradation of _______________.
4. Portions of the primary mRNA transcript, called _______________, are removed.
a. An _______________ is a portion of the DNA code in the primary mRNA
transcript eventually expressed in the final polypeptide product.
b. An _______________ is a non-coding segment of DNA removed by spliceosomes
before the mRNA leaves the nucleus.
5. _______________ are RNAs with an enzymatic function restricted to removing
introns from themselves.
a. _______________ could have served as both genetic material and as the first
enzymes in early life forms.
6. Spliceosomes are complexes that contains several kinds of ribonucleoproteins.
a. Spliceosomes cut the primary _______________ transcript and then rejoin
adjacent _______________.
C. Function of Introns
1. In humans, _______________ comprise _______________ of the average proteincoding gene.
2. Possibly _______________ divide a gene into regions that can be joined in different
combinations for different products.
3. _______________ may function to determine which genes are to be expressed and
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how they should be spliced.
14.4 Second Step: Translation
1. Translation takes place in the _______________ of eukaryotic cells.
2. Translation is the second step by which gene expression leads to protein synthesis.
3. One language (_______________) is translated into another language (protein).
A. The Role of Transfer RNA
1. transfer RNA (_______________) molecules transfer amino acids to the ribosomes.
2. The _______________ is a single-stranded ribonucleic acid that doubles back on
itself to create regions where complementary bases are hydrogen-bonded to one
another.
3. The amino acid binds to the 3’ end; the opposite end of the molecule contains an
_______________ that binds to the mRNA codon in a _______________ fashion.
4. There is at least one _______________ molecule for each of the 20 amino acids
found in proteins.
5. There are fewer _______________ than codons because some tRNAs pair with more
than one codon; if an anticodon contains a U in the third position, it will pair with
either an A or G–this is called the _______________ hypothesis.
6. The _______________ are amino acid-activating enzymes that recognize which
amino acid should join which _______________ molecule, and covalently joins
them. This requires _______________.
7. An amino acid–tRNA complex forms, which then travels to a ribosome to
“_______________” its amino acid during protein synthesis.
B. The Role of Ribosomal RNA
1. Ribosomal RNA (_______________) is produced from a DNA template in the
nucleolus of the _______________.
2. The rRNA is packaged with a variety of proteins into ribosomal _______________,
one larger than the other.
3. Subunits move _______________ through nuclear envelope pores into the cytoplasm
where they combine when _______________ begins.
4. _______________ can float free in cytosol or attach to endoplasmic reticulum.
5. Prokaryotic cells contain about 10,000 _______________; eukaryotic cells contain
many times more.
6. Ribosomes have a binding site for _______________ and binding sites for two tRNA
molecules.
7. They facilitate complementary base pairing between tRNA _______________ and
mRNA _______________; rRNA acts as an enzyme (ribozyme) that joins amino
acids together by means of a peptide bond.
8. A ribosome moves down the mRNA molecule, new tRNAs arrive, the
_______________ join, and a polypeptide forms.
9. Translation terminates once the polypeptide is formed; the ribosome then dissociates
into its two subunits.
10.
Polyribosomes are clusters of several _______________ synthesizing the same
protein.
11.
To get from a polypeptide to a function protein requires _______________
bending and _______________; chaperone molecules assure that the final protein
develops the correct shape.
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12.
Some proteins contain more than one polypeptide; they must be
_______________ to achieve the final three-dimensional _______________.
C. Translation Requires Three Steps
1. During translation, mRNA _______________ base-pair with tRNA anticodons
carrying specific _______________.
2. Codon order determines the order of tRNA molecules and the sequence of
_______________ polypeptides.
3. Protein synthesis involves _______________, _______________, and
_______________.
4. Enzymes are required for all three steps; energy (_______________) is needed for the
first two steps.
5. Chain Initiation
a. A small ribosomal subunit attaches to mRNA in the vicinity of the start
_______________ (AUG).
b. First or initiator tRNA pairs with this _______________; then the large ribosomal
subunit joins to the small subunit.
c. Each ribosome contains three binding sites: the P (for _______________) site, the
A (for _______________) site, and the E (for _______________) site.
d. The initiator tRNA binds to the P site although it carries one _______________,
methionine.
e. The A site is for the next _______________ carrying the next amino acid.
f. The E site is to discharge _______________ from the ribosome.
g. Initiation factor proteins are required to bring together the necessary
_______________ components: the small ribosomal subunit, mRNA, initiator
tRNA, and the large ribosomal _______________.
6. Chain Elongation
a. The tRNA with attached polypeptide is at the P site; a tRNA-_______________
complex arrives at the A site.
b. Proteins called elongation factors facilitate _______________ base pairing
between the tRNA _______________ and the mRNA _______________.
c. The polypeptide is transferred and attached by a _______________ bond to the
newly arrived _______________ in the A site.
d. This reaction is catalyzed by a ribozyme, which is part of the larger
_______________.
e. The tRNA molecule in the P site is now _______________.
f. Translocation occurs with mRNA, along with peptide-bearing tRNA, moving to
the P site and the spent _______________ moves from the P site to the E site and
exits the ribosome.
g. As the ribosome moves _______________ three nucleotides, there is a new codon
now located at the empty A site.
h. The complete cycle is rapidly _______________, about 15 times per second in
Escherichia coli.
7. Chain Termination
a. Termination of polypeptide synthesis occurs at a stop _______________ that does
not code for amino acid.
b. The polypeptide is enzymatically cleaved from the last _______________ by a
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release factor.
c. The tRNA and polypeptide leave the _______________, which dissociates into its
two subunits.
8. Definition of a Gene and a Genetic Mutation
a. Originally a _______________ was defined as a locus on the chromosome.
b. The one gene-one polypeptide concept connected inborn errors of metabolism
with a sequence of _______________ bases.
c. A gene could also be defined as a sequence of _______________ bases coding for
a single polypeptide or a single RNA.
d. These concepts can allow us to define a _______________ as a permanent
change in the sequence of DNA bases.
e. Current definitions: a protein-coding gene is one that is _______________ into
mRNA, while a noncoding _______________ is one that is transcribed into any
other type of RNA.
D. Protein Synthesis and the Eukaryotic Cell
1. The first few _______________ of a polypeptide act as a signal peptide that indicates
where the polypeptide _______________ in the cell or if it is to be secreted by the
cell.
2. After the polypeptide enters the lumen of the ER, it is folded and further processed by
addition of _______________, phosphates, or _______________.
3. Transport _______________ carry the proteins between _______________ and to the
plasma membrane.
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