Transcript Transcription and Translation

Transcription and
Translation
From DNA to RNA to protein
Overview
• Genes in DNA contain
information to make
proteins.
• The cell makes mRNA
copies of genes that are
needed.
• The mRNA is read at the
ribosomes in the rough
ER.
• Protein is produced.
Key Players
• mRNA carries the
information from a gene
in DNA.
• Ribosomes, made of
rRNA, consist of
subunits and carry out
an enzyme-like role.
• tRNA carries specific
amino acids to the
ribosome.
Transcription
RNA polymerase is the enzyme responsible for making
mRNA copies of genes. DNA unzips at the site of the gene
that is needed.
Transcription
RNA polymerase matches bases in the sense strand with
RNA bases, building a strand of mRNA that carries the
information encoded in the DNA.
Transcription
Encoded in DNA is a signal telling RNA polymerase where
to stop. Transcription ends at that point.
Transcription
The completed mRNA molecule then moves from the
nucleus to the rough ER for translation.
Translation
• Initiation begins with a
tRNA bearing
methionine (met)
attaching to one of the
ribosomal units. The
codon for methionine is
a universal “start” codon
for “reading” the mRNA
strand.
Translation
• The ribosomal unit binds
to mRNA where the
code for met is located
(AUG). The anticodon
(UAC) of the tRNA
matches the “start”
codon on mRNA (AUG).
Translation
• The larger ribosomal
subunit now binds to the
smaller unit, forming a
ribosomal complex. The
tRNA binds to the first
active site on the
ribosome. Translation
may now begin.
Translation
• The second codon in
mRNA (GUU) matches
the anticodon of a tRNA
carrying the amino acid
valine (CAA). The
second tRNA binds to
the second active site on
the large subunit.
Translation
• A catalytic site on the
larger subunit binds the
two amino acids
together using
dehydration synthesis,
forming a peptide bond
between them.
Translation
• The first tRNA now
detaches and goes of to
find another met in the
cytoplasm. The mRNA
chain shifts over one
codon, placing the
second codon (CAU)
over the second active
site.
Translation
• A tRNA with an
anticodon (GUA)
matching the exposed
codon (CAU) moves
onto the ribosome. This
tRNA carries histidine
(his).
Translation
• A new peptide bond
forms between val and
his on the catalytic site.
The tRNA that carried
val will detach and find
another val in the
cytoplasm. The mRNA
strand will then shift over
one more codon.
Translation
• The process continues
until the ribosome finds
a “stop” codon. The
subunits detach from
one another, the mRNA
is released, and the
polypeptide chain moves
down the ER for further
processing. The initial
met is removed and the
chain is folded into its
final shape.
Summary
Transcription Animation
• http://learn.genetics.utah.edu/units/basics/tra
nscribe/
Mutations
• Any mistakes in the DNA code can result
in a “broken” (non-functional) protein.
• A mutation affecting only a few somatic
cells (body cells) might not have any
effect, unless the mutation turns the cell
cancerous.
• A mutation affecting a sex cell can be
passed on to the offspring.
Types of Mutations
• Point mutation: base substitution that may
or may not code for a different amino acid.
• Insertion mutation: one or more bases is
inserted into the DNA strand.
• Deletion: one or more bases is deleted
from the DNA strand.
Effects of mutations
• Original: THE FAT CAT SAT
• Point mutation: THA FAT CAT SAT
• Insertion: THE FTT ATC ATS AT
• Deletion: THE FTC ATS AT
Hemoglobin mutations
DNA
mRNA
Amino Acid
Properties of
AA
Effect on protein
Disease
Original
codon 6
CTC
GAG
Glutamic Acid
Hydrophilic
Normal
None
Mutation 1
CTT
GAA
Glutamic Acid
Hydrophilic
Neutral
None
Mutation 2
GTC
CAG
Glutamine
Hydrophilic
Neutral
None
Mutation 3
CAC
GUG
Valine
Hydrophobic
Loses water
solubility
Sickle Cell
Anemia