Download 05. Protein synthesis

PROTEIN SYNTHESIS
Protein Synthesis
Process where DNA encodes for the
production of amino acids and
proteins.
Protein synthesis starts in the
nucleus, where DNA is held and ends
in the cytosol, where proteins are
made.
It is divided into two main parts:
transcription and translation
Transcription
Process where DNA is used as a template for the
production of mRNA (mRNA is transcribed from
DNA)
Transcription has three phases: Initiation,
elongation and termination
1. Initiation
RNA polymerase binds to DNA at the promoter
region (high in A and T, takes less energy to break
these bonds)
As in DNA replication, the DNA is "unzipped" by
the enzyme helicase, leaving the single nucleotide
chain open to be copied.
When DNA is unwound it exposes the template
strand
The part of DNA not being used to make mRNA is
called the coding strand
2. Elongation
RNA polymerase reads the DNA
strand and synthesizes a single
strand of mRNA in the 5’ to 3’
direction
Uracil complements adenine (instead
of thymine)
As DNA is transcribed, it goes back
into double-helical form
3. Termination
When RNA polymerase reaches the
termination sequence at the end of the
gene, RNA synthesis stops
mRNA and RNA polymerase are released
mRNA is not yet ready to leave the cell.
First it has a 5’ cap added to protect it and
initiate translation.
Also, a 3’ end is added containing a polyA tail (200 – 300 adenine base pairs).
This single strand of mRNA then
leaves the nucleus through nuclear
pores, and goes into the cytoplasm
where it joins with ribosomes, where
protein synthesis occurs by the
formation of peptide bonds and
polypeptide chains.
Translation
The process of converting the mRNA
codon sequences into an amino acid
polypeptide chain.
It also has 3 stages: initiation, elongation
and termination
The making of an amino acid takes place
on the ribosomes. Ribosomes consist of
two subunits (large and small). The two
subunits bind to mRNA and clamp the
mRNA between them.
The ribosome moves along the mRNA in
the 5’ to 3’ direction, adding a new amino
acid each time it reads a codon.
1. Initiation
A ribosome attaches to the
mRNA and starts to code at the
start codon (AUG).
2. Elongation
tRNA brings the corresponding amino
acid to each codon as the ribosome
moves down the mRNA strand (See
handouts, you should know these
handouts for the test)
3. Termination
Reading of the final mRNA codon
(aka the STOP codon, UAG, UAA,
UGA), which ends the synthesis of
the peptide chain and releases it.
http://www.accessexcellence.o
rg/RC/VL/GG/protein_synthesi
s.php
Protein Synthesis Video
http://www.wisconline.com/objects/index_tj.asp?objID
=AP1302
Questions on Handout
P. 241 Questions 1, 2, 3, 4, 9, 10, 11,
13
Readings P. 237 – 241
P. 242-244
P. 250-253