Download Protein Synthesis (B7)

What will you learn?
Protein Synthesis
PLO
B7
B8
It is expected that students will…
demonstrate an understanding of the process
of protein synthesis
explain how mutations in DNA affect protein
synthesis
What is the connection between
DNA and protein?
• Remember: DNA can’t leave the nucleus but
protein synthesis occurs in the cytoplasm …
How?
• protein message must be carried from the
nucleus  cytoplasm  ribosomes
RNA Structure:
•polymer of nucleotides
•Ribose sugar unit
•single stranded
•linear (not helical)
•A, C, G, U - uses uracil
(U) rather than thymine (T)
p. 510
• made in nucleus, but functions outside of the
nucleus
Function:
• aids in protein synthesis
• uses DNA strand as the template to make its
strand using complimentary base pairing
3 Types of RNA
all help in the protein synthesis
• messenger RNA (mRNA)
– carries transcribed message from DNA to
ribosome (nucleus to cytoplasm)
• Transfer RNA (tRNA)
– carries the amino acid that will base pair with
mRNA to make polypeptide
• ribosomal RNA (rRNA)
– makes up the ribosome (along with proteins)
Overview of gene expression
Two Steps of Protein Synthesis:
Transcription & Translation
Step 1:Transcription
• DNA code (gene) has
instructions to make a
particular protein
• Instructions are copied
into mRNA in the
nucleus
DNA
mRNA
A
U
T
A
C
G
G
C
Steps of Transcription
A. Helicase opens the section of DNA (the gene)
– Begins at INITIATION site
– Continues until TERMINATION site
B. complementary RNA base pairs attach to form the
mRNA strand (RNA polymerase forms the RNA
sugar-phosphate backbone and checks for
mistakes)
C. complementary mRNA detaches & leaves the
nucleus through the nuclear pore and goes to
the cytoplasm
Transcription Animation
• http://www.johnkyrk.com/DNAtranscription.h
tml
Step 2: Translation
 The mRNA code is made up of groups of three nucleotide
bases known as codons. Each codon codes for a specific
amino acid.
Eg. AGC = Serine
(see mRNA codon table handout)
Eg. UGC = Cysteine
CAU?
CUU?
GGC?
AUG?
UGA?
AAA?
How many different aa’s are there?
mRNA
Codons
CODONS – mRNA base triplets
Translation (in cytoplasm only)
• Occurs at ribosome (a large and a small
subunit made up of rRNA and proteins)
– rRNA is made in the nucleolus
• tRNA is found free-floating in the cytoplasm
tRNA structure
tRNA structure:
» tRNA molecule is a small piece of RNA that
has a specific aa attached to one end
» at the other end, an anticodon (3 bases that
will complement mRNA codon)
Please note that there is more than one codon for each
amino acid:
mRNA codons:
U C U
G C C
tRNA anti-codons:
A G A
C G G
SERINE
ALANINE
Why do you think that aa is?
3 STEPS in Translation
A. Initiation (means “start”)
–
–
ribosomal subunits come together and mRNA
attaches
an INTIATOR (start) codon (always AUG) turns on
the process
B. Elongation (what is getting bigger?)
– tRNA anticodon (with specific aa) matches up with the
mRNA codon
– Each tRNA leaves to find another aa as mRNA over one
codon & another tRNA brings the next aa
– aa’s continually peptide bond building a polypeptide
C. Termination
– When ribosomal unit reaches any
‘Terminator’ codon, the message is to
STOP translation
•
There are 3 stop codons (UAA, UAG, UGA)
–
None of these have matching tRNA anticodon
– Ribosome subunits split in 2 again
What does translation look like?
still translation but not a step
Polyribosome – often several ribosomes work
together on one protein
• Each ribosome makes a section of the protein
• This is done for very large proteins. This
enables these large proteins to be made
faster.
• Also called a polysome
Polyribosome
Central Dogma of Biology
Replication
DNA
Translation
RNA
Transcription
Nucleus
Enzymes
Protein
Structural
Components
of the Cell
Cytoplasm (ribosomes)