Download DNA, RNA, Protein Synthesis

RNA, & PROTEIN
SYNTHESIS
7th Grade, Week 4, Day 1
Monday, July 15, 2013
The Central Dogma
RNA vs. DNA
Ribonucleic Acid
• RNA is required for translation of genetic
information stored in DNA into protein products
• Transcribed from DNA
• Contains ribose sugar instead of deoxyribose
• Single stranded instead of double stranded
• Uracil instead of
thymine
Types of RNA
• Precursor mRNA (pre-mRNA): an immature form of
messenger RNA that contains introns and exons
• Messenger RNA (mRNA): contains only exons that form
the code for the sequence of amino acids that makes up a
protein
• Transfer RNA (tRNA): decodes the message contained in
mRNA and allows for the synthesis of proteins
• Ribosomal RNA (rRNA): forms part of the structure of
ribosome
Transcription
• Making RNA from DNA
• Occurs in the nucleus
• RNA Polymerase
synthesizes pre-mRNA
from DNA in the 5’  3’
direction
• Happens in much the same
way that DNA polymerase
synthesizes new strands of
DNA during replication
Transcription
• Template (antisense) strand is transcribed, while the other
(sense) strand remains inactive
• Uracil bonds with Adenosine in place of Thymine
• Initial pre-mRNA made contains both exons and introns
3 Steps of Transcription
• Initiation: occurs when the RNA polymerase binds to
promoter and forms the transcription bubble
• Elongation: the RNA chain is lengthened by the addition
of bases in the 5’ to 3’ direction
• Termination: RNA polymerase runs into a termination
region.
Splicing
• In order for the pre-mRNA
to leave the nucleus and
travel to the ribosomes in
the cytoplasm for
translation, it must be
made into mature mRNA
• This happens by removing
the introns so that only
exons are left
• An enzyme complex called
a spliceosome performs
this task
Alternative Splicing
• Not all exons are always
left – sometimes they get
spliced out as well
• Therefore, the same premRNA can be spliced
differently to get different
gene products (proteins)
and create diversity
RNA Processing
• Addition of a poly-adenosine
(poly-A) tail and a 5’ cap
before the mRNA can exit the
nucleus and move into the
cytoplasm
• The cap: is a modified
guanine (G)
• protects the RNA from
being degraded by
enzymes that degrade
RNA from the 5′ end;
• serves as an assembly
point for the proteins
needed to recruit the small
subunit of the ribosome to
begin translation.
Proteins
• DNA acts like a blueprint that determines the structure of
every protein made in your body
• Every protein is made up of amino acids
• There are 20 amino acids:
• Essential: must be supplied in the diet
• Non – essential: synthesized de novo
• We obtain most of our amino acids by digesting proteins
taken in with our food.
• The digestive process breaks the protein chains down
into individual amino acid molecules which are then
absorbed by the blood and transported to the individual
body cells.
Proteins
• During protein synthesis, the separate amino acids are
reassembled into new chains. Each kind of protein has its
own particular sequence of amino acids, which differs
from the sequence in every other kind of protein.
• Just the way the order of letters in a word give it its own
specific form and meaning, it is the order of the amino
acids in the chain that determines the protein's structure
and function.
• The code for ordering the amino acids of a protein is
written as a sequence of bases in the DNA in the nucleus.
The Genetic Code
• Triplet code: codons are
made of 3 nucleotide
bases which are nonoverlapping
• The system is redundant –
amino acids are encoded
by more than one codon
• Practice – Translate:
5’ – AUG ACU AAU GCU
UAA – 3’
Translation
• mRNA is translated into
amino acids using the
genetic code, which are
then assembled into a
protein
• This process takes place
in the cytoplasm on
ribosomes
• Both mRNA and tRNA are
necessary for this process
Steps in Translation
• Ribosomes bind mature
mRNA
• There are about 32 different
tRNA molecules
• Each tRNA molecule has an
anticodon that is
complementary to a codon on
mRNA coding for a certain
amino acid (so most amino
acids have more than one
tRNA that will code for them)
• The tRNA will then retrieve
that amino acid and bring it to
the ribosome for protein
assembly
Ribosomes
• Made up of rRNA
• Composed of two subunits
– one large and one small
• Ribosomes can be free in
the cytosol or membrane
bound to the endoplasmic
reticulum (ER) called the
"rough ER“
• Where the process of
protein assembly is carried
out
Amino acid
tRNA molecule
A transfer RNA molecule arrives.
It brings an amino acid to the first three bases
(codon) on the mRNA.
anticodon
The three unpaired bases (anticodon) on the
tRNA link up with the codon.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule comes into place,
bringing a second amino acid.
Its anticodon links up with the second codon on
the mRNA.
UAC
A U G G G C U U AAA G C A G U G C A C G U U
Peptide bond
A peptide bond forms between the two amino
acids.
A U G G G C U U AAA G C A G U G C A C G U U
The first tRNA molecule releases its amino acid and
moves off into the cytoplasm.
A U G G G C U U AAA G C A G U G C A C G U U
The ribosome moves along the mRNA to the next
codon.
A U G G G C U U AAA G C A G U G C A C G U U
Another tRNA molecule brings the
next amino acid into place.
A U G G G C U U AAA G C A G U G C A C G U U
A peptide bond joins the second and third
amino acids to form a polypeptide chain.
A U G G G C U U AAA G C A G U G C A C G U U
The process continues.
The polypeptide chain gets longer.
This continues until a termination (stop)
codon is reached.
The polypeptide is then complete.
A U G G G C U U AAA G C A G U G C A C G U U
Rules of Translation
• The start codon is AUG which codes for methionine
• What does this mean about the first amino acid in every protein?
• Chain elongation continues on the ribosome as it reads
the mRNA strand
• Translation continues until a stop codon is reached
• The stop codons, unlike the start codon, do not encode amino
acids
• Completed protein is then folded with help from
chaperones
But what happens when this process
goes wrong?
Mutations
• Frame shift mutation: adding or deleting one base causes
a change in the reading frame…why?
• Missense mutation: a base change that results in
substituting one amino acid for another
• Nonsense mutation: a base change that results in
substituting a stop codon in place of an amino acid
• This results in early termination of the protein
• Silent mutation: a base change that results in no change
in the encoded amino acid (or stop codon)
• Why could this happen?
• Are these still dangerous? Why or why not?