Chapter 17 Study Guide
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Vocabulary:
Wood Roots:
anti- = opposite
exo- = out, outside
intro- = within
muta- = change
-gen = producing
poly- = many
trans- = across
-script = write
Key Terms:
5' cap: The 5' end of a pre-mRNA molecule modified by the addition of a cap of guanine nucleotide.
A site: One of a ribosome’s three binding sites for tRNA during translation. The A site holds the tRNA
carrying the next amino acid to be added to the polypeptide chain. (A stands for aminoacyl tRNA.)
Anticodon: A specialized base triplet at one end of a tRNA molecule that recognizes a particular
complementary codon on an mRNA molecule.
Base-pair substitution: A type of point mutation; the replacement of one nucleotide and its partner
in the complementary DNA strand by another pair of nucleotides.
Codon: A three-nucleotide sequence of DNA or mRNA that specifies a particular amino acid or
termination signal; the basic unit of the genetic code.
Deletion: A mutational loss of one or more nucleotide pairs from a gene.
E site: One of a ribosome’s three binding sites for tRNA during translation. The E site is the place
where discharged tRNAs leave the ribosome. (E stands for exit.)
Exon: A coding region of a eukaryotic gene.
Frameshift mutation: A mutation occurring when the number of nucleotides inserted or deleted is
not a multiple of three, resulting in the improper grouping of the following nucleotides into codons.
Insertion: A mutation involving the addition of one or more nucleotide pairs to a gene.
Intron: A noncoding, intervening sequence within a eukaryotic gene.
Messenger RNA (mRNA): A type of RNA, synthesized from DNA, that attaches to ribosomes in the
cytoplasm and specifies the primary structure of a protein.
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Missense mutation: The most common type of mutation, a base-pair substitution in which the new
codon makes sense in that it still codes for an amino acid.
Mutation: A rare change in the DNA of a gene, ultimately creating genetic diversity.
Nonsense mutation: A mutation that changes an amino acid codon to one of the three stop codons,
resulting in a shorter and usually nonfunctional protein.
P site: One of a ribosome’s three binding sites for tRNA during translation. The P site holds the tRNA
carrying the growing polypeptide chain. (P stands for peptidyl tRNA.)
Point mutation: A change in a gene at a single nucleotide pair
Poly-A tail: The modified end of the 3’ end of an mRNA molecule consisting of the addition of some
50 to 250 adenine nucleotides.
Primary transcript: An initial RNA transcript; also called pre-mRNA when transcribed from a
protein-coding gene.
Promoter: A specific nucleotide sequence in DNA that binds RNA polymerase and indicates where to
start transcribing RNA.
Reading frame: The way a cell’s mRNA-translating machinery groups the mRNA nucleotides into
codons.
Ribosomal RNA (rRNA): The most abundant type of RNA, which together with proteins forms the
structure of ribosomes.
Ribosome: A cell organelle constructed in the nucleolus and functioning as the site of protein
synthesis in the cytoplasm; consists of rRNA and protein molecules, which make up two subunits.
RNA polymerase : An enzyme that links together the growing chain of ribonucleotides during
transcription.
RNA processing: Modification of RNA before it leaves the nucleus, a process unique to eukaryotes.
RNA splicing: The removal of noncoding portions (introns) of the RNA molecule after initial
synthesis.
TATA box: A promoter DNA sequence crucial in forming the transcription initiation complex.
Template strand: The DNA strand that provides the template for ordering the sequence of
nucleotides in an RNA transcript.
Transcription factor: A regulatory protein that binds to DNA and stimulates transcription of specific
genes.
Transcription initiation complex: The completed assembly of transcription factors and RNA
polymerase bound to the promoter
Transcription unit: A region of a DNA molecule that is transcribed into an RNA molecule.
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Transfer RNA (tRNA): An RNA molecule that functions as an interpreter between nucleic acid and
protein language by picking up specific amino acids and recognizing the appropriate codons in the
mRNA.
Translation: The synthesis of a polypeptide using the genetic information encoded in an mRNA
molecule.
Triplet code: A set of three-nucleotide-long words that specify the amino acids for polypeptide
chains.
Chapter 17 Important Information:
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DNA provides inherited genetic information that can be reliably copied and passed between
generations
o This information is encoded in the sequence of DNA nucleotides
o Individual segments of DNA called genes provide the specific sequences that
ultimately instruct the cell through gene expression to manufacture particular RNA
or polypeptides
o The strand of DNA used for a given gene is called the template strand
 The template strand’s complementary strand is called the non-template
strand
 Which strand is the template strand is dependent on each individual gene
o Base pairs are bracketed in groups of 3 (triplet code) which are called codons
 Codons must be kept in the same reading frame for proper gene expression
Gene expression has 2 major steps
1. Transcription: Using a DNA template to provide the pattern to make a messenger,
mRNA transcript
2. Translation: Using the mRNA transcripts to assemble amino acids in the correct
order to make a polypeptide
Universal Genetic Code
o All organisms share the same genetic code of 64 possible unique codons from the 4
nucleotide bases
o 61 of the 64 code for amino acids, 3 are stop codons, one codon (AUG) is the start
codon as well as coding for an amino acid
o No codon codes for more than one amino acids but, for a given amino acid, there can
be more than one codon that codes for it
Transcription:
 The promoter region of a gene is a reliable sequence along the DNA template strand that
signals where transcription will begin
o A common promoter region in eukaryotes is the TATA Box
 RNA polymerase is the enzyme responsible for adding RNA nucleotides to the mRNA
transcript
o RNA polymerase binds to the promoter region along with several proteins
collectively called transcription factors so together they are called the transcription
initiation complex
o The section of DNA transcribed is the transcription unit which includes the eventual
coding region along with many other additional nucleotides upstream and
downstream from the coding region of the gene
 Transcription proceeds in a 3 step process
1. Initiation: Matching of the transcription initiation complex to the promoter
region and separation of the DNA double strands to allow access
2. Elongation: Adding of RNA nucleotides using the base pairing rules with U
substituting for T on the mRNA transcript
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a. RNA nucleotides are added to the 3’ end of the transcript
Termination: The mRNA transcript will be released from the DNA template
strands at a pre-determined sequence
RNA processing: Eukaryotes make a pre-mRNA transcript during transcription that needs to
be edited before translation (Note: Prokaryotes do not have RNA processing and the
transcribed mRNA is used immediately)
o The 5’ end of the pre-mRNA receives a 5’ cap (G nucleotide with 3 phosphate
groups) and the 3’ end receives a long series of A nucleotides called the poly-A tail
 Both structures serve to protect the mRNA transcript as it moves from the
nucleus to the cytoplasm and to help recognition and binding of the mRNA
to the ribosomes
o Certain regions in the coding segment called introns are removed from the
transcript, leaving segments called exons that will be expressed during translation
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Translation:
 Translation occurs at the ribosomes, bound or free
o Ribosomes are made of 2 protein subunits (a large and small) + ribosomal or rRNA
molecules
o The initiation of translation occurs when mRNA transcripts attach to the small
subunit at the mRNA binding site
o Amino acids are present in the cytoplasm and enzymes called aminoacyl-tRNA
synthases catalyze the attachment of each amino acid to transfer, tRNA molecules
 Each tRNA has an amino acid attachment site on one end and a region called
the anticodon, which is a triplet of bases complementary to the codon, that
matches the amino acid that tRNA holds
o tRNA’s are matched to the codons along the mRNA transcript
 Initiation of translation begins when the first tRNA (which holds
methionine, Met) binds to the start codon AUG on the mRNA
 This first tRNA docks at a specific place on the ribosome called the P site
 Once it has docked, the large subunit joins to the small subunit
 Energy is needed in the form of GTP molecules
 The next tRNA will dock at a site called the A site using its anticodon to
match with the next codon on the mRNA
 The first amino acid (Met) will release from its tRNA and move over to the
tRNA at the A site, forming a peptide bond with the next amino acid in the
sequence
 The start tRNA is now empty and will move to the E or exit site of
the ribosome and eventually released back out into the cytoplasm
to find another Met molecule
 The tRNA holding the now 2 amino acid long chain will move over to the P
site, leaving the A site free for the next tRNA bearing the next amino acid in
the chain to join
 GTP molecules are needed to provide the energy for this process
 The polypeptide chain continues to grow until one of the 3 stop codons is
reached on the mRNA
 Proteins called release factors bind to the stop codons and this
signals the breaking apart of the ribosome subunits, release of the
mRNA and completed polypeptide
 The polypeptide chain will undergo modification to form the
secondary, tertiary or possibly quaternary structure needed for a
functional final protein
Mutations:
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A mutation is a change in the DNA nucleotide sequence
o Mutations can be spontaneous mistakes or caused by other agents called mutagens
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A point mutation is a change in a single base pair
o The base may be substituted, an extra base inserted or one deleted
o More than one base may be affected however in mutations other than point;
sometimes whole codons are inserted or deleted
If a base substitution alters the codon in a way that allows it to still code for the same amino
acid as the wild type, the mutation is silent and has no effect on the protein
o This is possible because of the redundancy in the genetic code
If the base substitution codes for a different codon than the wild type version, it is a
missense mutation
o Depending on what amino acid is altered and the location in the polypeptide chain
of that amino acid, the result to the protein is variable
If the base substitution creates a premature stop codon, the protein is more likely to be
negatively affected and non-functional, again, dependent on where the mutation occurs in
the sequence
o This called a nonsense mutation
Insertions and deletions of single bases have a far greater potential effect on the protein than
substitutions as they will cause a reading frame shift and all remaining codons will be
incorrect
o Insertions or deletions of whole codons however may not have a great affect
because there will be no reading frame shift, just an extra or missing amino acid
which can vary in affect
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