Transcript The Code of Life: Topic 3

The Code of Life: Topic 3
Gene expression (protein synthesis)
What's in your genes?
• Genes are sequences of
nucleotides along DNA strands.
• Genes (100s-1000s of
nucleotides long) code for
polypeptides.
• Your genotype is the actual
sequence of DNA that you
inherited from your parents.
• The expression of those genes
results in your phenotype, or
how you look.
The big question…
How do we go from this
DNA
to this?
An organism
The Central Dogma
DNA
RNA
Polypeptide
Protein
Universal genetic code of life
RNA
• There are 3 major
structural differences
between RNA and DNA.
• There are 3 types of
RNA involved in gene
expression (protein
synthesis):
– mRNA
– rRNA
– tRNA
Types of RNA
mRNA
rRNA
tRNA
Gene expression (protein synthesis)
Overview
Transcription: copying DNA's message
• DNA is stored safely in the
nucleus, but proteins are
made in the cytoplasm.
• RNA carries the
instructions for proteins out
of the nucleus to the
cytoplasm.
• The first step is
transcription.
Stages of
transcription
• Three stages:
– Initiation
– Elongation
– Termination
• Important notes:
– RNA polymerase
does all the work
– The end product is
called a primary RNA
transcript
– The primary
transcript is then
modified before
leaving the nucleus.
Practice transcribing…
If your DNA strand says this…
TACAGGTCAGACTTC
What will your complementary RNA strand say?
AUGUCCAGUCUGAAG
Before the RNA transcript can leave the nucleus, it has to
be processed.
Step 1: Cap and tail
5’ cap (GTP):
3’ poly-A tail:
•Provides stability to the mRNA •Provides stability to the mRNA
•Point of attachment for a small •Appears to control the
subunit of the ribosome during movement of mRNA across the
translation.
nuclear membrane.
Step 2: RNA splicing
1. removes introns
2. joins exons, creating an mRNA molecule with a
continuous coding sequence.
After transcription comes translation
(protein synthesis)
Transcription
Translation
Review: Types of RNA
mRNA
rRNA
tRNA
Translation
• During translation, the
code carried in mRNA is
"translated" into amino
acids.
• There are 4 nucleotides
and 20 amino acids.
• Clearly, the nucleotides
must be combined
somehow in 20 different
ways.
• mRNA is "read" as
codons, combinations of
3 nucleotides.
• One codon, AUG, always
signals that start of a
gene sequence.
• Three codons (UAA,
UAG, and UGA) are stop
signals, ending the
formation of a
polypeptide.
Take a moment to look at your chart. What
observations can you make about this code?
Translating practice
• What amino acid does each of the
following codons correspond to?
AAA Lysine
• Try these with the
GCA Alanine
wheel:
UGU Cysteine
CCC Proline
CAG Glutamine
AGU Serine
UCA
GUC
Serine
Valine
tRNA: a closer look
Translation: Initiation
• The 5' cap of mRNA attaches to a a small ribosome subunit.
• The initiator tRNA has the anticodon for the start codon (AUG)
on mRNA.
• The initiator tRNA always carries the amino acid methionine
(MET).
• After the initiator tRNA hydrogen bonds to the mRNA, a large
ribosomal subunit also attaches.
Translation: Elongation
• Amino acids are added on
sequentially when the
appropriate tRNA matches
with the next mRNA codon.
• Each new tRNA bonds its
anticodon to the
complementary codon on the
mRNA.
• The amino acid from the old
tRNA gets passed to the new
amino acid on the new tRNA.
They form a peptide bond.
Translation: Termination
• Once the stop codon of a sequence is
reached, the whole complex comes apart
and there is now a new polypeptide.
Summary
Imagine the first line is the message in DNA and the
rest of the lines are the same message carried in RNA.
What is going on? Pinpoint SPECIFIC errors.
How do those errors affect the message?
DNA message
thesunwashotbuttheoldmandidnotgethishat
mRNA message
the sun was hot but the old man did not get his hat
versions
(as codons)
the sun was hot but the ole man did not get his hat
the sun was hot but the old man did not get his cat
the sun was hot but the old ma. did not get his hat
thd esu nwa sho tbu tth eol dma ndi dno tge thi sha t
Mutations
• Mutation - any change in the DNA sequence
• Causes of mutations:
– Errors in DNA replication (permanent error; 1 in every
100,000,000 bases)
– Mutagens
• UV light
• Radiation
• Chemicals
• There are 2 major categories of mutations:
– Point mutations
– Frameshift mutations
mRNA transcribed from
normal DNA
A base pair substitution is
when the wrong nucleotide is
substituted for the correct one.
These mutations may be:
silent (no effect, usually last
nucleotide in a codon)
missense (results in a different
amino acid, effect depends on the
properties of the new amino)
nonsense (premature stop codon)
mRNA transcribed from mutated DNA
Point mutations are base
pair substitutions.
mRNA transcribed from normal DNA
mRNA transcribed from mutated DNA
• Frameshift mutation
– Insertion
– Deletion
• In this case the entire
sequence after the
insertion or deletion is
shifted by a whole
nucleotide.
• After the mutation, none
of the amino acids are
correct**.
• Result: non-functioning
polypeptide.
** RARE
case of three
nucleotides
being
deleted.
http://www-mic.ucdavis.edu/sklab/genetic%20recomb.htm