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Protein Synthesis
Transcription and Translation
AP Biology
Unit 2
Flow of Genetic Information
• All living organisms use
DNA to synthesize RNA to
make proteins
• Same two-step process:
Transcription 
Translation
• Some antibiotics inhibit
protein synthesis in bacteria.
– Ex. Neomycin (the antibiotic
in Neosporin) interferes with
the process of translation)
Genes and Chromosomes
• DNA is organized into
chromosomes
– Humans have 46 chromosomes in
each cell.
• Genes are “coding” regions of
DNA
– Each gene is the code for how to
make a specific protein.
• Human chromosomes are made up
of
– DNA
– Histone proteins that DNA is
wound around
Structure of DNA
• The carbons in the 5C sugar
each have a number
– Start to the right of the oxygen
and go around clockwise
• This gives the nucleotide 2
distinct ends
–5’ end (closer to carbon 5)
–3’ end (closer to carbon 3)
phosphate
5
P
O
4
3
Sugar
1
2
Base
A way to remember it:
Human Nucleotide 
Phosphate
5
C
4
C
3C
C
C
2
Base
1
Nucleic Acid Structure
• DNA is double
stranded
• Hydrogen bonds
between bases
– A pairs with T
– C pairs with G
• The strands are antiparallel
–One strand runs 5’-3’
–The other runs 3’-5’
Image taken without permission from http://bcs.whfreeman.com/thelifewire
Question…
• Why can’t the DNA strands be parallel
(both running 5’-3’)?
– This wouldn’t allow the bases to be near each
other to hydrogen bond.
Transcription
• DNA is transcribed into 3 kinds of RNA
– mRNA = messenger RNA (the RNA code used
to make protein)
– tRNA = transfer RNA (participates in
translation)
– rRNA = ribosomal RNA (part of ribosomes)
• RNA Polymerase is the enzyme that
transcribes the DNA into RNA
Initiation
• How transcription starts
• RNA Polymerase recognizes a
promoter sequence on the DNA
• RNA Polymerase binds to the
promoter
• DNA is unwound to start
transcription
– What kind of bonds are being
broken to unwind/separate the
strands of DNA?
– Hydrogen bonds
Promoter Sequences
• In prokaryotes, RNA Polymerase must find these
sequences:
5’
3’
5’
3’
• + 1 is the first base in the RNA (where the actual
transcription of DNA starts from)
Eukaryotic Promoter Sequences
• In eukaryotes, the RNA polymerase must
find the following sequences:
• Eukaryotic genes can also have enhancer
sequences to help RNA polymerase bind
– We’ll talk about these a little later– don’t
worry about them right now 
What do you think this diagram shows
about transcription?
Bases
changed
to…
Promoters
• In order for RNA Polymerase to recognize
it, the promoter sequences
– Must be the correct sequence of bases (small
changes OK)
– Must be correctly spaced apart
• If these conditions aren’t met, RNA
Polymerase can’t bind to the DNA and no
transcription occurs.
Elongation
• How the RNA strand is built
• RNA Polymerase matches the
appropriate (complementary)
nucleotides to the DNA
template strand
– Template strand = the actual
strand RNA Polymerase uses to
build RNA
– Coding (Nontemplate) strand =
not used for building RNA, but
has the same sequence as the
RNA.
Building the RNA
• The RNA Polymer grows in a 5’-3’ direction
• RNA Polymerase only adds new nucleotides on
to the 3’ end.
• Considering this, in what direction must the
template strand of DNA be running?
– 3’-5’ (since it is building its complement)
P P P
5’
O
P P P
O
3’
P P P
5’
5’
5’
P
P
O
P
O
3’
3’
3’
Question …
• In terms of the sequence, how will the RNA
differ from the sequence of the coding
strand in the DNA?
– T’s are replaced with U’s
Termination
• How transcription of RNA ends
• RNA Polymerase recognizes a
termination signal on the DNA
template
– Usually a long string of A’s or a
series of A’s and T’s
• RNA Polymerase falls off the
DNA template
• Stability of mRNA is minutes
 hours (depends on type of
cell and RNA)
Question…
• How do the specific chemical properties of
the termination sequence cause termination
to occur?
– There are only 2 hydrogen bonds between A
and T/U
– With a string of A’s and U’s, there are much
fewer bonds to hold the DNA template and
RNA together  they separate  transcription
ends
Translation
• Using the mRNA code to create the
appropriate protein.
• Occurs in the cytoplasm/on the rough ER
• Sequence of 3 nucleotides codes for a
particular amino acid = codon
• 64 different codons
Question…
• Why can’t 1 or 2 nucleotides code for an
amino acid?
– Not enough combinations to code for all 20
amino acids
– With 1 nucleotide  only 4 possibilities (A, C,
G, U)
– With 2 nucleotides  only 4 x 4 = 16
possibilities (AA, AU, AC, AG, CC, …)
The codon table
tRNA
• tRNA brings the correct
amino acid to match with the
mRNA codon
• Each tRNA holds a specific
amino acid and has a
particular anticodon.
• Aminoacyl tRNA
synthetases are enzymes that
attach the correct amino
acids to the tRNA
Amino acid
attached
here
Question…
• For the anticodon
shown in the diagram,
what would the
complementary codon
on the mRNA be?
– 5’ UUC 3’
• Which amino acid is
attached to this tRNA?
– Phe
Ribosomes
• Made up of 2 subunits
• Composed of rRNA and protein
• Not specific to any particular
protein– can be used to translate
any RNA into protein
• Workbench for translation –
holds mRNAs and tRNAs in the
correct positions to assemble
protein.
Ribosomes
• 3 sites on the ribosome
– A site = where tRNA
first binds to mRNA
– P site = where the amino
acid is added on to the
polypeptide chain
– E site = exit site
Translation
• Begins with the Start codon = AUG
– Codes for methionine (Met)
– Not the same thing as +1
Translation
• Ribosome moves
along mRNA in a
5’->3’ direction,
catalyzing the
translation of the
mRNA into protein
– breaks bond between
tRNA and amino acid
– creates a new peptide
bond to link it to
polypeptide chain
Question…
• How does the mRNA know if it is correctly
matched to the tRNA?
– Hydrogen bonding between the bases is correct
Stopping Translation
• Ribosome is released when a stop codon is
reached
– UAA, UAG, UGA = stop codons (don’t code for any
tRNA anticodons)
– A release factor binds to the mRNA instead
– Ribosome breaks apart, mRNA and protein are
released
Summary of Protein Synthesis
• In Eukaryotes
• How is it
different in
prokaryotes?
Why is this important?
1. Changes in the DNA sequence will lead to changes in
the transcribed _________.
2. This results in a different codon which may code for a
different ______________.
3. A different ___________ means a different R group.
4. A different R group may have different chemical
properties.
5. These different chemical properties may lead to a
different protein ____________.
6. A different protein structure may affect its _________!
7. See how this is all starting to connect! Exciting!!! 
Why is this important?
1. Changes in the DNA sequence will lead to changes in
the transcribed RNA.
2. This results in a different codon which may code for a
different amino acid.
3. A different amino acid means a different R group.
4. A different R group may have different chemical
properties.
5. These different chemical properties may lead to a
different protein structure.
6. A different protein structure may affect its function!
7. See how this is all starting to connect! Exciting!!! 
microRNAs and RNAi
• Small, single stranded RNA molecules
(miRNAs and siRNAs)
– microRNA = miRNA
– Small interfering RNA = siRNA
• Bind to complementary sequences in
mRNA molecules
• Can control the expression of (translation
of) specific RNA molecules
Question…
• How will microRNAs disrupt translation?
– Block translation by creating a physical road block
– RNA-RNA binding also marks the mRNA for
degradation
• Called RNAi = RNA interference