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Chapter 17
AP Biology
From Gene to Protein
Central Dogma of Molecular
Biology
DNA  RNA Protein
What is RNA?
• Contains the bases A, C, G, and U instead of T
• single-stranded (often folds onto itself)
• Three types of RNA: messenger RNA (mRNA),
transfer RNA (tRNA) and ribosomal RNA
(rRNA)
Protein Synthesis
1. Transcription - DNA message is transcribed
into mRNA and sent to ribosomes.
2. Translation - mRNA is translated into a Protein
by a ribosome.
DNA  RNA  Protein
Overview –
Eukaryotic Cell
Notice that
transcription and
translation occur
in different
places in a
eukaryotic cell.
Overview – Prokaryotic Cell
The Triplet Code
Note that the
bases of mRNA are
complementary to
the template
strand of DNA
The bases of mRNA
are read in groups
of three – each
group is called a
codon
From mRNA to Amino Acids
• The mRNA base triplets are called codons
• mRNA is written in the 5’ to 3’ direction
• The codons code for each of the 20 amino
acids
• The genetic code is redundant
– More than one codon codes for each of the 20
amino acids
The Genetic
Code
The three bases of
an mRNA codon
code for the 20
amino acids that
are the subunits of
proteins
The amino acids
are designated by
three letters
Universal Nature of the Genetic Code
Transcription
Promoter
• The DNA sequence at which RNA polymerase
attaches is called the promoter
• RNA polymerase adds RNA nucleotides to a
growing RNA strand in the 5’  3’ direction
Transcription Unit
• The entire stretch of DNA that is transcribed
into an RNA molecule
• A transcription unit may code for a
polypeptide or an RNA (like tRNA or rRNA)
– RNA Polymerase II makes mRNA
Terminator
• The DNA sequence that signals the end of
transcription is called the terminator
Stages of
Transcription
1. Initiation
2. Elongation
3. Termination
Initiation of Transcription at a
Eukaryotic Promoter
RNA Processing
• In eukaryotes, transcription results in premRNA
– Pre-mRNA undergoes processing to yield final
mRNA which leaves the nucleus and goes to the
ribosomes
• In prokaryotes transcription results in mRNA
– Processing of mRNA does not occur in prokaryotes
– Transcription and translation can occur
simultaneously
Processing of Pre-mRNA
• Addition of a 5’ cap
– A modified form of a guanine nucleotide
• Addition of a poly-A tail
– 50-250 adenine (A) nucleotides are added
– This is referred to as the poly-A tail
• RNA splicing
– Editing of the initial strand of mRNA (a cut and
paste job)
Addition of 5’ cap and poly-A tail
5’ cap and poly-A tail
• Both facilitate export of mRNA from the
nucleus
• Both help protect mRNA from degradation by
enzymes
• Both facilitate the attachment of mRNA to the
ribosome
RNA Splicing
• Only takes place in eukaryotic cells
• Large sections are spliced out; these are called
introns
• The sections that remain are called exons
– “exons EXIT the nucleus”
• These exons are spliced together by a
spliceosome to form the mRNA that leaves the
nucleus and travels to the ribosomes
RNA Splicing
snRNPs and
Spliceosomes
Small nuclear RNAs
(snRNA) and
small nuclear
ribonucleoproteins
(snRNPs) serve a
catalytic function
in the spliceosome
When RNA acts as
a catalyst, it is
called a ribozyme
Alternative RNA Splicing
• Different regulatory proteins in different cells
splice the pre-mRNA in different ways
• This is called alternative gene splicing
• This allows for different combinations of exons
• This results in more than one polypeptide per
gene
• This explains why we have fewer genes in our
genome than what was expected
Transcription Animation
• https://www.youtube.com/watch?v=SMtWvD
bfHLo
17.4 Translation
• tRNA functions in transferring amino acids
from the cytoplasm to a ribosome
• rRNA complexes with proteins to form the two
subunits that make up ribosomes
tRNA
• Each type of tRNA is specific for a particular
amino acid
• At one end tRNA loosely binds the amino acid
and at the other end it has a nucleotide triplet
called the anticodon
• The anticodon allows it to pair specifically
with a complementary codon on mRNA
Structure of tRNA
Basic Concept of
Translation
Codons are the
triplet nucleotides
on mRNA
Anticodons are the
triplet nucleotides
on tRNA
Wobble
• If one tRNA variety existed for each codon, there
would need to be 61 tRNAs, there are only about
45, some can bind to more than codon
• The rule for base pairing is not as strict between
the third base of a codon and an anticodon, this
relaxation of base-pairing rules is called wobble
• Ex: a tRNA with the anticodon 3’-UCU-5’ can
base pair with either the mRNA codon 5’-AGA-3’
or 5’-AGG-3’, both code for Arg
Three Stages of Translation
• Initiation
– Begins with the start codon AUG (always!)
• Elongation
– Codon recognition
– Peptide bond formation (between 2 a.a.)
• Termination
– A stop codon is reached and translation stops
Initiation
Elongation
Termination
Translation Animation
• https://www.youtube.com/watch?v=TfYf_rPW
UdY
Folding of the Polypeptide
• Following release from the ribosome, the
polypeptide then folds to its specific
conformation
• Chaperonins are the proteins that help with
this folding process
• The first 20 amino acids of the polypeptide
serve as a signal peptide and act as a cellular
zip code, directing the polypeptide to its final
destination
Targeting Proteins to the ER
17.5 Point Mutations
• Mutations are alterations in the genetic material of the
cell caused by mutagens
• Point mutations are alterations of just 1 base pair in a
gene
– Base-pair substitution
• Silent mutations – have no effect on the encoded protein
• Missense mutations – change one amino acid to another;
might still code for the correct amino acid
• Nonsense mutations – change a regular amino acid codon
into a stop codon
– Insertions & deletions
• Frameshift mutation – mRNA is read incorrectly
A Point Mutation Disorder
Base-Pair Substitution - Silent
Base-Pair Substitution - Missense
Base-Pair Substitution - Nonsense
Frameshift Mutation- Insertion
Frameshift Mutation - Deletion