Protein Synthesis
Section 4.4 - 4.5 Molecular Cell Biology
Lodish, etc 2000
I.
Proteins
A.
Definition - A linear polymer of amino acids linked together in a specific
order or sequence. Formed in the process known as translation.
B.
Translation
1.
A ribosome mediated production of a protein whose amino acid
sequence is specified by the mRNA.
2.
Translation further requires energy, tRNA and ribosomes made up
of proteins and rRNA.
C.
RNA
1.
Three types
a.
rRNA=ribosomal RNA
b.
tRNA=transfer RNA
c.
MRNA=messenger RNA
2.
3.
Components
a.
Bases
U
Uracil
A
Adenine
G
Guanine
C
Cytosine
b.
Ribose sugar
c.
Phosphate ion
Base sequence of mRNA determines the primary structure of the
protein
a.
Genetic code - also referred to as the triplet code
The set of rules whereby nucleotide triplets in DNA and
RNA specify amino acids in proteins
1
- 3 nucleic acids make 1 amino acid/codon
- 64 different combinations with the 4
different bases allowed.
- make 20 different amino acids
- 61 codons specify amino acids
- AUG or GUG - start codon
- 3 stop or nonsense codons
- UAA, UGA, UAG
b.
D.
Reading frame
The sequence of nucleotide triplets (codons) beginning
with a specific translation start codon (AUG=met) in a
mRNA and ending with a stop codon.
mRNA
1. Carries the genetic information copied from DNA in the form of a
series of three base code words (codons).
2
E.
tRNA
1.
2.
3.
4.
5.
Transfer RNA is the key to deciphering the code words in mRNA.
Carries specific amino acids to the mRNA to synthesize the
protein.
Can only carry 1 amino acid at any particular time
Cell may have many tRNAs for each amino acid.
Codons - Anticodons
- Codons are the triplet code of bases that designated the amino
acid.
- Anticodons are the triplet complementary code of the tRNA that
the codons attach to. Some tRNAs have unconventional bases that
can pair with >1 codon by "wobble pairing"
F.
II.
rRNA
1.
Ribosomal RNA associates with a set of proteins to form the
complex structure known as a ribosome.
2.
Ribosomes are a complex structure made up of two subunits that
consist of:
Several rRNAs
50+ proteins
3.
Subunits
a.
Each ribosome is made up of two subunits of different
densities.
b.
Prokaryotes - 30S and 50S makes a 70S ribosome.
c.
Eukaryotes - 40S and 60S makes a 80S ribosome.
Protein Formation - Translation
A.
Review of RNA formation - applies to all three types of RNA
1.
Transcription
2.
5' capping
3.
3' poly(A) tail addition
4.
splicing
5.
transport out of nucleus
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B.
C.
Components of Translation
1.
mRNA
2.
tRNA
3.
rRNA - both subunits of the ribosome
4.
GTP and ATP
5.
amino acids
6.
various initiation, elongation and termination factors.
Stepwise formation of proteins
1.
Initiation – Prokaryotes
a.
b.
c.
d.
e.
30S ribosome subunit is activated with the addition of GTP
and initiation factors.
Met attached to tRNA is brought to the 30S subunit.
At the same time the mRNA is attached to the ribosome
subunit at the start codon position
The 50S subunit is combined to the 30S subunit to form the
ribosome and the initiation complex.
Eukaryotic initiation - substitute 30S for 40S and 50S for
60S.
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2.
Elongation
a.
3.
The ribosome translocates along the mRNA
has two sites - A and P site.
A site - amino acid attachment
P site - peptide bonding site
b.
tRNA with amino acids are attached to the A site with the
help of elongation factors. If the tRNA and the mRNA
match then the amino acid is attached. If they do not, the
tRNA is ejected.
c.
The ribosome translocates along the mRNA and moves the
tRNA from the A to P site where a polypeptide bond is
formed with the help of an enzyme and 3 ATP.
Termination
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a.
b.
D.
Ribosome translocates to a stop codon.
Release factors attach to the A site of the Ribosome
- release factors have a similar shape of a tRNA
- There are 3 different release factors
(RF1, RF2, and RF3), and each one attaches for a
different stop codon.
c.
Release factors separate the ribosome into its subunits and
thus stops elongation and protein synthesis.
Regulation of Protein Synthesis
1.
1 typical eukaryotic protein takes 30-60 seconds to be synthesized.
2.
There are 2 factors that effect synthesis
a.
Multiple ribosomes working together.
b.
Rapid recycling of ribosome after termination
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