Transcription
and
Translation
Chapter 14

Central Dogma
• DNA mRNA
Protein
• DNA DNA –
Replication – nucleus
• DNA mRNA –
Transcription – Nucleus
• mRNA Protein –
Translation – Ribosome in
the cytoplasm

Central Dogma

RNA vs. DNA

DNA vs. RNA
DNA
• Nucleus only
• Double Stranded
• ATCG
• Deoxyribose Sugar
RNA
• Nucleus and cytoplasm
(ribosome)
• Single Stranded
• AUCG
• Ribose sugar
• 3 main types
mRNA - messenger
tRNA - transfer
rRNA - ribosomal

Triplet Code: Codons
Universal code: Strong evidence for evolution
3 Nucleotides = 1 amino acid Codon Chart pg. 273
6

Transcription
14_08TranscriptionIntro_A.swf
• DNA  mRNA
• Copying of DNA’s
message to mRNA
• Occurs in the
nucleus
• Pre-mRNA is
processed into
mRNA and then
leaves the nucleus
for the cytoplasm
(ribosome)

Detailed Transcription
• Initiation: RNA polymerase binds to DNA at
promoter region.
– Promoter is before the gene that is to be
transcribed (eukaryotes need transcription factors
to help RNA polymerase bind to the TATA box)
– Determines which strand of DNA to use
• Elongation - adds nucleotides to mRNA strand
based on DNA strand in a 5’ 3’ direction (adding
only to the 3’ end).
• Termination – RNA polymerase “falls off” the DNA
strand when the termination sequence (terminator) is
reached.
– AAUAAA in eukaryotes, this is now pre-mRNA

Transcription
Transcription Initiation
Eukaryotes Transcription Process

RNA Processing
• Pre-mRNA RNA
• 5’ cap – guanine and phosphate cap on the 5’
end of mRNA
• 3’ poly-A tail – 50 to 250 Adenines are added
to the 3’ end of the mRNA
• Both the 5’ cap and 3’ poly-A tail facilitate the
export of mRNA from the nucleus
• Both protect the mRNA from degradation by
hydrolytic enzymes in the cytoplasm
• Both help ribosomes attach to the 5’ ends of
the mRNA strand

RNA processing

Alternative RNA splicing
• Introns – non-coding sections of mRNA
– Don’t leave the nucleus – only on pre-mRNA
• Exons – coding sections (expressed) of mRNA
– Exit the nucleus – final mRNA
• Keeps the exons  gets rid of the introns
• snRNPs – cut the introns
– Ribozyme – RNA that functions like an enzyme
• Spliceosomes – join remaining exons together
to form final mRNA

Translation
14_18TranslationIntro_A.swf
• mRNA  protein
• Process of mRNA
converting to a protein
• Occurs in the cytoplasm
– ribosome
14

tRNA
• Translator of mRNA’s message is tRNA –
transfer RNA
– 80 nucleotides long
– Hairpin shape – L shaped
• One end contains an anticodon which
pairs with the codon on the mRNA
– Codons determine which amino acid is coded
for by the DNA
• The other end contains an amino acid
attachment site
– Aminoacyl-tRNA synthetase attaches the
correct amino acid to the tRNA 15

tRNA
16

Ribosomes
• Pair codons on mRNA with anticodons on
tRNA to form polypeptides
• Made of large and small subunits
– rRNA – ribosomal RNA
– Made in the nucleolus
• Contain multiple binding sites
– mRNA binding site
– P site – peptidyl – tRNA site
– A site – aminoacyl – tRNA site
– E site – exit site
17

Ribosomes
18

Making a protein
• Initiation
– Small subunit binds to mRNA
– Start codon AUG – methionine at P site
• Elongation
– A site recognizes codon and pairs with correct tRNA
– Peptide bond forms between the carboxyl end of the
polypeptide at the P site and amino acid at the A site
– Amino acid in the A site translocates to the P site
• Termination
– Stop codon is reached at the A site
• UAA, UAG, UGA
– Release factors free the polypeptide from the
ribosome
19

Making a Protein
ProteinSynthesis.mpg
Figure 17.13
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
Polypeptide
Amino
acids
tRNA with
amino acid
attached
Ribosome
tRNA
Anticodon
mRNA
Gly
A A A
U G G U U U G G C
Codons
5 3

Proteins
• Fold spontaneously into
primary, secondary, and
tertiary structures.
• Chaperone proteins assist
in folding.
• Some polypeptides
become quaternary with
multiple subunits
• Signal peptide – directs
proteins through the
endomembrane system 21

Targeting Polypeptides to
locations
22

Prokaryotes vs. Eukaryotes
Prokaryotes
• No nucleus
– Tanscription and
translation same location
• Smaller ribosomes
Eukaryotes
• Locations:
– Free cytosol Ribosomes: Stay in the
cell somewhere (free floating,
mitochondria, etc)
– Rough ER Ribosomes: SRP sends
ribosome to go to ER, becomes
protein in endomembrane system, or
cell membrane protein or is secreted
from cell (all in vesicles)
• Nucleus
– Pre-mRNA  mRNA
– transcription
• Larger ribosomes
– translation
23

Mutations
• Point Mutations – one base
altered
– Base-pair substitution
• Silent mutation – no
effect
• Missense mutation
– changes an amino
acid
• Nonsense mutation
– creates a stop
codon
– Insertion – extra base
– Deletion – removal of a
base
• Frameshift
mutations –
nonfunctional
proteins 24