Download protein synthesis - Science with Mrs Beggs

PROTEIN SYNTHESIS
-Transcription & Translation –
‘Patterns of Life’ pg 121-133
Introduction
• DNA contains the code to make protein
• DNA is in the nucleus
• Protein is made in the ribosome
• The information must get from the nucleus
to the ribosome - mRNA
Nuclear envelope
• Continuous with
pores
• rough endoplasmic
reticulum and
ribosomes.
Key Pieces of Equipment
• DNA
• RNA – 3 types
» Messenger
» Transfer
» Ribosomal
(mRNA)
(tRNA)
(rRNA)
• RNA polymerase (protein)
RNA vs DNA
•
•
•
•
Single stranded
Thyamine (T) is replaced by Uracil(U)
A lot smaller
No oxygen molecule
RNA
• Linear polymer
• Bases: Purines (adenine & guanine)
Pyrimidines (cytosine & uracil)
• Most are single stranded, therefore does
not form a double helix. Although can fold
and twist into itself.
Messenger RNA
• Codon a sequence of three nucleotides
that code for a single amino acid
• mRNA carries the code from the nucleus
to the ribosome
Transfer RNA
• Transfer RNA (tRNA): decodes the
information
• tRNA has an anti-codon which matches a
specific codon of mRNA
• Each tRNA attaches to a specific amino
acid that compliments its anti-codon
• There are 20 different tRNA types (one for
each type of amino acid)
Ribosomal RNA
• Ribosomal RNA (rRNA): makes up
approximately 50% of a ribosome.
• The assembly place of protein synthesis.
Ribosome
• 2 subunits – heavy and light
The Genetic Code
• 20 different amino acids used to produce
proteins.
• The code for a specific amino acid is
controlled by the 4 different nucleotides
(T,C,A,G) in the DNA.
• This code goes in sequences of three
(Triplets)
• A triplet codes for a single amino acid
• WHY?
• These triplets produce codons in mRNA
• A codon codes for a specific amino acid
• Some combinations act as STOP (UAA,
UGA, UAG) and START (AUG) signals.
• One codon is the start codon - AUG
Genetic Code cont.
Protein Synthesis-the process
• Two parts to protein synthesis:
– Translation of mRNA from DNA
» Occors in the nucleus
– Transcription of mRNA into a polypeptide chain
» Occurs at the ribosome
Transcription
1. One section of DNA unwinds. One side acts as
a template for the mRNA to be synthesised.
• The RNA polymerase reads the DNA from 3’ end
of the DNA.
• RNA polymerase then binds complementary
bases to form the mRNA.
• Codes on the DNA act as
punctuation, indicating
beginning and end of
protein and transcription.
• Once termination
sequence reached,
transcription ceases. The
mRNA is then completed
and it travels out of the
nucleus to the ribosome
Translation
• mRNA becomes bound to a ribosome.
• The mRNA’s strand is placed in the ‘P’ site (the
first position on a ribosome)
• Next to the ‘P’ site is
the ‘A’ site.
• Both sites hold one
tRNA molecule.
• Once the start codon
is placed into the ‘P’
site, translation
occurs.
• Next a tRNA fits into the ‘P’ site based upon the
codon on the mRNA lined up on the ‘P’ site.
• Another tRNA fits into the ‘A’ site also based on
the codon on the mRNA at the ‘A’ site.
• When both sites have a tRNA present, the amino
acids on the tRNA bond together to form the first
link of a protein chain.
•
i.
ii.
Once the two amino
acids have linked
together:
The tRNA in the P
site is released
The mRNA moves
codon so the tRNA
in the ‘A’ site is
shifted into the ‘P’
site.
iii.
iv.
v.
The ‘A’ site is now clear
so a new tRNA molecule
moves in corresponding
to the mRNA
This amino acid links to
the protein chain.
This process continues
until a stop codon is read
at the ‘A’ site.
Termination
• Once a stop codon is read at the ‘A’ site, the
ribosome has finished synthesising the protein.
• A new tRNA molecule is not added to the protein
chain.
• The two ribosome units separate.
• Protein chain released from ribosome.
Protein synthesis animation
• http://highered.mcgrawhill.com/sites/0072437316/student_view0/
chapter15/animations.html#