Download Protein Synthesis

From Gene to Protein;
how are genes/ DNA
expressed?
RNA is a very flexible polymer. This means it can take
on multiple shapes depending on the bonds in it.
There are 3 types of RNA (each one has a unique
shape as each one has a unique function):
1) mRNA- messenger (linear, like a line)
2) rRNA- ribosomal (large and like a glob)
3) tRNA- transfer (like a t)
It is a two-step process:
• 1-Transcription occurs:
synthesis of mRNA under
the direction of DNA.
Occurs in the nucleus.
• 2-Translation occurs:
actual decoding of the
mRNA into a polypeptide.
Occurs in the ribosomes in
the cytoplasm or on
ribosomes on the rough ER
We need DNA’s code (A,T,G,C combination) so
we know what amino acids to connect so we
can build a proper protein in the cell
• So, first we copy the
DNA into mRNA
(Transcription)
• Then, decode those DNA
instructions (now in the
form of mRNA) to
construct correct amino
acids into a protein.
(Translation)
The DNA’s information
(genetic instructions)
to make specific amino
acids that will form the
polypeptide chain are
‘written’ in the DNA as
a series of
3-nucleotide ‘words’
called Codons.
Example: DNA: TAC
mRNA: AUG
The DNA sequence is broken down into groups of 3
nucleotides, each corresponding to a mRNA codon.
• RNA polymerase: Copies DNA into RNA.
RNA polymerase lays down a 5’ (phosphate)
structure first. Since RNA is a single
stranded molecule, we only need to copy
one side of the DNA. The side we use is the
3’ side. (NO lagging strand!!!)
• Tell a partner WHY you thing we use the 3’
side of the DNA, be ready to share.
• The 3’ side of DNA is called the antisense
strand. The 5’ (uncopied) side is called the
sense strand.
• Tell your partner why you think we give
them those names! Be ready to share.
Don’t tell them the answer to the second question until you watch the animation.
• RNA polymerase looks for a start
sequence on the DNA, it is called the
initiation sequence, or promoter. The
stop sequence, or terminator is the end of
the transcribed strand.
• Any one of the 3 types of RNA can be made
during transcription.
Click on picture for animation
Before pressing play the second time, label all of the things.
• We must edit our
mRNA in a process
called “RNA processing”
• Introns: non-protein
making regions of the
pre-mRNA are removed
and caps and tails are
added
• The edited, linear mRNA is now exported from
the nucleus to the ribosome in the cytoplasm.
• Why is this process similar to a architect
copying expensive blue prints before sending
them to the job site? What do the blue prints,
the copy, and the architect each represent?
• Think of an additional analogy that can be
compared to this process. Be ready to share.
Picture is hyperlink to video
Now we need the other 2 types
of RNA, the tRNA and rRNA.
Let’s start with the tRNA.
tRNA is shaped like a “t” and
has three jobs:
1. carry an inactive amino acid
into the factory.
2. Provide the opposite codon to
activate the amino acids.
(this is called the “anticodon”
Ex: mRNA= AUG/ tRNA =
UAC
3. Hold the amino acids as they
bond together to form the
growing protein.
rRNA is the same as saying a”ribosome”.
• rRNA or a ribosome is the factory where
we can build the protein following DNA’s
instructions (mRNA codons)
• Just like a real factory, there are different
things that occur within the rRNA.
• A site: Holds the tRNA
carrying the amino acid to
be activated and the
matching anticodon
(matches the codon on
mRNA)
• P site: Holds the tRNA
carrying the growing
polypeptide chain
• E site: Exit of tRNA once
that tRNA is done
We can say there is a “recipe” for
making a protein… it is
Codon + anticodon + inactive amino
acid = human protein
Also, please memorize the start codon
and what amino acid it makes and
the stop codons (all 3)
Click on the picture to watch a video of
the process. Look though all the
pictures and write down any questions
you have for lecture.