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Chapter 17 Notes
From Gene to Protein
Concept 17.1
The study of metabolic defects provided
evidence that genes specify proteins
- (1909) Garrod suggests that genes
dictate phenotypes that catalyze
specific chemical reactions in the cell
- ex. alkaptonuria
Concept 17.1
One Gene- One Enzyme
- mutations that affect eye color in
Drosophila block pigment synthesis at a
specific step by preventing production
of the enzyme that catalyzes that step
-b/c each mutant was defective in a
single gene, the function of a gene is to
dictate the production of an enzyme
Concept 17.1
One Gene- One Polypeptide
- after researchers discovered that not
all proteins are enzymes, they revised
their hypothesis
- many proteins are made from two or
more polypeptide chains, and each
chain is specified by its own gene
Concept 17.1
Transcription and Translation are the two
main processes linking genes to
proteins
- transcription is the synthesis of RNA
under the direction of DNA
- translation is the actual synthesis of
a polypeptide, which occurs under the
direction of mRNA
Concept 17.1
Concept 17.1
Nucleotide triplets specify amino acids
- there are only 4 nucleotides to code
for the 20 amino acids
- triplet code: the genetic instructions
for a polypeptide chain are written in
the DNA as a series of three nucleotide
words
Concept 17.1
Concept 17.1
During transcription, the gene determines
the sequence of base triplets along an
mRNA molecule
- template strand: the one of the two
possible DNA strands that is transcribed
Concept 17.1
- the mRNA strand is complementary,
not identical, since the RNA bases are
assembled on the template according to
the base-pairing rules
- the mRNA base triplets are called
codons
Concept 17.1
- the codon AUG has a dual function: it
codes for the amino acid Methionine
(Met) and it functions as a “start”
signal, or initiation codon
- information is extracted by reading
symbols in the correct reading frame
- ex. the big red dog ate the cat
- ex. heb igr edd oga tet hec at
Concept 17.2
Messenger RNA (mRNA), a carrier of
information from DNA to the ribosome,
is transcribed from the template strand
of a gene
- RNA polymerase: connects the RNA
nucleotides as they base-pair along the
DNA template
Concept 17.2
- like DNA replication, RNA
polymerases can only add nucleotides
to the 3’ end of a polymer. RNA
elongates in the 5’  3’ direction.
- promoter region: DNA sequence
where the RNA polymerase attaches to
begin transcription
Concept 17.2
- terminator region: sequence that
signals the end of transcription
- special proteins, called transcription
factors, mediate the initiation of
transcription
Concept 17.2
Concept 17.2
Concept 17.3
After transcription, RNA processing occurs
- primary RNA transcript becomes mRNA
- the 5’ cap is added to the front of the
mRNA (acts to help in the translation
process)
- the poly(A) tail is added to the 3’ end
Concept 17.3
- RNA splicing: the introns, the
noncoding segments, are removed and
the exons are fused together
Concept 17.3
Concept 17.4
In translation, the cell interprets the
genetic message and builds a protein
accordingly
- transfer RNA (tRNA): transfers
amino acids from the cytoplasm’s amino
acid pool to a ribosome
- contains an anticodon that is
complementary to the mRNA codon
Concept 17.4
Concept 17.4
Ribosomes are made of 2 subunits that
are constructed of proteins and
ribosomal RNA (rRNA)
- contains a binding site for mRNA
- contains 3 binding sites for tRNA
- P site: holds the tRNA carrying the
growing peptide chain
Concept 17.4
- A site: holds the tRNA carrying the
next amino acid that is to be added
to the chain
- E site: location for tRNA to leave
the ribosome
Concept 17.4
Concept 17.6
Mutations can affect protein structure and
function
- mutations: changes in the genetic
material of a cell
- point mutations: chemical changes
in just one base pair of a gene
Concept 17.6
Concept 17.6
Types of point mutations
- substitutions: the replacement of
one nucleotide with another nucleotide
- missense mutations: the altered
mutations still codes for the amino acid
and the protein functions
- nonsense mutations: change an
amino acid to a stop signal;
Concept 17.6
Frameshift mutations
- insertions: additions of nucleotide
pairs in a gene
- deletions: losses of nucleotide pairs
in a gene
- frameshift mutation: all nucleotides
that are downstream are affected