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Protein Synthesis
Chapter 12
What’s the Point?
 How
do we go from a code in our
DNA (a gene sequence, like
GAATTC) to how we look?
 “Read”
the DNA code, and
transcribe it into a different
format so it can be translated
into a protein.
A. Comparison of DNA & RNA
B. Transcription
Process by which a DNA sequence (gene)
is converted to an RNA sequence.
 Occurs in the nucleus of eukaryotic
cells & cytoplasm of prokaryotic cells.
 Is regulated by operons (bacterial cells)
or transcription factors (multicellular
organisms).
 Involves 3 processes: initiation,
elongation & termination
1. Initiation

RNA polymerase attaches to a promoter on
DNA strand.
Helicase unzips a short section of DNA.
 Free RNA nucleotides move in & H-bond to
complementary bases on DNA template
strand.

2. Elongation
RNA polymerase links RNA nucleotides
together in a 5’ to 3’ direction.
 Growing RNA strand peels away from DNA
template.

3. Termination


RNA polymerase detaches when it reaches a
terminator.
Completed RNA
molecule is released
from DNA template.
Usually, several copies of RNA are made
at a time.
1
2
3
Determine the base sequence of RNA
transcribed from the following DNA
template strand.
DNA template C A G T A A G C C
RNA strand
G T C A U U C G G
Three types of RNA are transcribed.
 mRNA
(messenger RNA) - encodes
genetic information from DNA & carries
it into the cytoplasm.
5’
3’
codon
Each three consecutive mRNA bases
forms a genetic code word (codon)
that codes for a particular amino acid.
 rRNA
(ribosomal RNA) - associates
with proteins to form ribosomes.
large subunit
small subunit
Subunits are separate in the cytoplasm,
but join during protein synthesis
(translation).
 tRNA
(transfer RNA) - transports
specific amino acids to ribosome during
protein synthesis (translation).
Anticodon - specific
sequence of 3
nucleotides;
complementary to
an mRNA codon.
Amino acid
accepting end
Anticodon sequence determines the
specific amino acid that binds to tRNA.
Eukaryotic mRNA must be processed
before it exits nucleus & enters
cytoplasm.
 nucleotide
cap
is added
 “poly A tail” is
added
 introns are
removed
C. Translation
Process by which an mRNA sequence is
translated into an amino acid
sequence (polypeptide/protein).
 Occurs in the cytoplasm of
eukaryotic & prokaryotic cells.
 Requires: mRNA, tRNAs, amino
acids & ribosomes.
 Involves 3 processes: initiation,
elongation & termination
1. Initiation
 Small
ribosomal subunit binds to “start
codon” [AUG] on mRNA molecule.
 AUG codon attracts initiator tRNA.
2. Elongation
 Large
ribosomal subunit binds to small
subunit.
 A second tRNA anticodon binds to the
next mRNA codon.
A
peptide bond forms between the two
amino acids.
 Initiator
tRNA is released.
 Ribosome moves down mRNA by 1 codon.
 A third tRNA anticodon binds to the
next mRNA codon.
A
peptide bond forms between 2nd &
3rd amino acids.
 tRNAs
continue to add amino acids;
polypeptide lengthens.
3. Termination
 Occurs
when ribosome reaches an
mRNA stop codon (UGA, UAG or UAA).
Stop codons
do NOT
specify an
amino acid.
 Last
tRNA is released,
ribosomal subunits
separate & new
polypeptide/protein is
released.
Usually, several copies of the
polypeptide/protein are made at a time.
6
5
4
3
2
1
Some polypeptides must be altered
before they can function.
Determine the amino acid sequence
a ribosome would translate from
the following mRNA strand.
mRNA C A U G G C U C A A U G A
Met
Ala
Gln
STOP
Review: Genetic information flows in cell
from DNA  RNA  protein.
Each gene on DNA codes for production
of a specific polypeptide/amino acid.
Play this as a review:
https://www.youtube.com/watch?v
=9kOGOY7vthk&feature=youtube
_gdata_player
D. Mutation
A physical change in the nucleotide
sequence of DNA.
 May not affect phenotype (silent
mutation).
 Can affect somatic cells (somatic
mutation) or sex cells (germinal
mutation).
 Can
form spontaneously or be
induced by a mutagen.
1. Point mutation - replacement of one
DNA nucleotide with another.
 missense mutation - point mutation
that changes a codon so that a
different amino acid is specified.
Ex. sickle cell anemia
 nonsense
mutation - point mutation
that changes an amino acid-specifying
codon into a stop codon.
2. Frameshift mutation - the insertion
or deletion of DNA nucleotides; results
in disruption of the reading frame.
Ex. cystic fibrosis
3. Expanding repeat - the # of copies
of a 3 nucleotide sequence increases
over several generations.
Ex. myotonic dystrophy