PROTEIN SYNTHESIS
The “Central Dogma”
• Flow of genetic information in a cell
• How do we move information from DNA to proteins?
DNA
replication
RNA
protein
trait
Comparing DNA and RNA
Comparing DNA and RNA
DNA
RNA
Shape
Double helix
2 strands
Single strand
Sugar
Deoxyribose
Ribose
Bases
A, T, C, and G
A, U, C and G
Location
Only in the
nucleus
Allowed to travel
from nucleus to
cytoplasm
Types of RNA
• Messenger RNA (mRNA)
• Sequence of nucleotides that determines the primary
sequence of the polypeptide
• RNA (rRNA)
• Major component of ribosomes
• Transfer RNA (tRNA)
• Carries the amino acids to the mRNA
a
a
Protein Synthesis: From gene to protein
nucleus
a
cytoplasm
a
a
transcription
DNA
a
a
translation
mRNA
a
a
a
protein
a
a
a
a
a
a
ribosome
trait
Protein Synthesis:
• 1. Transcription – the synthesis of RNA using a DNA
template
• Occurs in the nucleus of eukaryotic cells
• 2. Translation – the synthesis of a polypeptide using the
genetic information encoded in an mRNA
molecule
• Occurs on the ribosome
Which gene is read on the DNA?
• Specific sequences of nucleotides along the DNA mark
where transcription of a gene begins and ends.
• A promoter is a specific nucleotide sequence in DNA that
binds RNA polymerase, positioning it to start transcribing
RNA at the appropriate place
Initiation of Transcription
• Certain sections of a promoter are especially important for
binding RNA polymerase.
• In prokaryotes, the RNA polymerase itself
specifically recognizes and binds to the promoter
• In eukaryotes, a collection of proteins called
transcription factors mediate the binding of RNA
polymerase and the initiation of transcription.
Transcription: DNA to mRNA
• Takes place in the nucleus
• A section of DNA is
unzipped
• RNA polymerase lays
down nucleotides 5’ to 3’
direction.
• The mRNA then leaves the
nucleus through the
nuclear pores and enters
the cytoplasm
RNA Processing in Eukaryotes
• mRNA must be modified before it leaves the nucleus
1) Introns are cut out of the mRNA and exons are
spliced back together
2) A 5’ GTP cap and a
3) A poly-A tail are added to the mRNA
Final mRNA processing…
• Need to protect mRNA on its trip from nucleus to cytoplasm
(enzymes in cytoplasm attack mRNA)
• protect the ends of the molecule
• add 5 GTP cap
• add poly-A tail
• longer tail, mRNA lasts longer
3'
A
mRNA
5'
G
P
P
P
The Code
• You don’t need to
memorize the
codons
(except for AUG)

Start codon



AUG
methionine
Stop codons

UGA, UAA, UAG
mRNA codes for proteins in triplets
DNA
TACGCACATTTACGTACGCGG
codon
mRNA
AUGCGUGUAAAUGCAUGCGCC
?
proteinMet Arg Val Asn Ala Cys Ala
Ribosomes: Site of Protein Synthesis
• Facilitate coupling of tRNA anticodon to mRNA
codon
• Structure
• ribosomal RNA (rRNA) & proteins
• 2 subunits
• large
• small
E P A
Translation
• A ribosome has three binding sites for tRNA:
• The P site holds the tRNA that carries the growing
polypeptide chain
• The A site holds the tRNA that carries the next amino
acid to be added to the chain
• The E site is the exit site, where discharged tRNAs
leave the ribosome
Transfer RNA
• Found in cytoplasm
• Carries amino acids to ribosome
• Contains an “anticodon” of nitrogen bases
• Anticodons use complementary bond with codons
Translation: mRNA to Protein
• In the cytoplasm ribosomes attach to the mRNA
• Ribosome covers 3 codons at a time
• Initiation
- The tRNA carrying an amino acid comes
into P-site and bonds by base pairing its
anticodon with the mRNA start codon
(what is the start codon?)
• Elongation
– The second tRNA then comes into A-site
and bonds to codon of mRNA
(what type of bond forms between adjacent amino acids?)
• Termination –
ribosome continues reading mRNA until
a STOP codon is reached
(doesn’t code for anything)
Protein Synthesis in Prokaryotes & Eukaryotes
Prokaryote vs. Eukaryote Differences
• Prokaryotes
• Eukaryotes
• DNA in cytoplasm
• DNA in nucleus
• circular chromosome
• linear chromosomes
• naked DNA
• DNA wound on
• no introns
histone proteins
• introns and exons
• TATA box promoter
• Transcription factors
present
• No splicing
• Promoter &
terminator sequence
• Smaller ribosomes