Transcript From DNA to Proteins

From DNA to Proteins
Chapter 13
Life’s Book of Instructions
• DNA is like a book of instructions
– The instructions are written in the alphabet of A,C,G,
and T
– All species use the same nucleotide subunits but the
sequence differs from species to species.
• The instructions for each protein is written in a
segment of DNA called a gene.
– Genes are linear stretches of DNA that codes for the
assemble of amino acids into a polypeptide chain
The Path from Genes to Proteins
• Two steps
– Transcription: Molecules of RNA are
produced on the DNA template in the nucleus
– Translation: RNA molecules are shipped from
the nucleus to the cytoplasmand are used as
a template for polypeptide assembly.
Central Dogma
• The central dogma is a description of how
information flows in a cell to make proteins
Transcription
DNA
RNA
Translation
Protein
RNA
• Three types of RNA are needed for protein
synthesis
– Messenger RNA (mRNA) carries the blueprint
for protein assembly to the ribosome.
– Ribosomal RNA (rRNA) combines with
proteins to form ribosomes upon which
polypeptides are assembled.
– Transfer RNA (tRNA) brings the correct amino
acid to the ribosome.
How RNA is assembled
• Transcription begins
when RNA Polymerase
sits down on the promoter
region just upstream of
the gene.
• The RNA Polymerase will
unwind the DNA strand
and begin adding
ribonucleotides to the
template in the order
specified by the DNA
template.
Promoter region
RNA polymerase
RNA Polymerase
transcribed DNA winds up
again
DNA to be
transcribed
unwinds
Newly forming RNA
transcript
The DNA template at the
assembly site
• RNA Polymerase unwinds the DNA
• Ribonucleotides (brown) are brought in and
temporally bound to the DNA template inside the
polymerase
Transcription of RNA inside the
RNA polymerase is determined by
the DNA template.
growing RNA transcript
3’
5’
3’
5’
direction of transcription
transcribed DNA winds up
again
DNA to be
transcribed
unwinds
Newly forming RNA
transcript
The DNA template at
the assembly site
• As the RNA polymerase moves down the DNA strand it
opens up the DNA to allow the RNA to be built.
• As the DNA exits the RNA polymerase the two strands of
DNA are joined back together and the RNA is disjoined
from the DNA.
• The new RNA strand is pushed out a different opening
from the double stranded DNA.
• As the RNA polymerase comes to the end of the gene,
the polymerase falls off and the RNA detaches and floats
away.
Maturation of mRNA
1 Introns must be cut out and exons must
spliced together.
1 Introns are nonsense pieces of DNA that interrupt the
message.
2 Exons are the informative portions of the message
2 A cap is added to the 5’ end
1 Helps direct the mRNA to the Ribosome
3 A poly-A tail is added to the 3’ end
1 Protects the 3’ end from endonucleases that will
chewing up the message
unit of transcription in a DNA strand
exon
intron
exon
intron
exon
3’
5’
transcription into pre-mRNA
poly-A
tail
cap
5’
3’
(snipped out)
(snipped out)
5’
3’
mature mRNA transcript
Fig. 14.9, p. 229
The Genetic Code
• The message is
read in triplets
called Codons
– Cells have 64 kinds
of codons
– Only 20 amino acids
Fig. 14.11, p. 230
Translation
Ribosome
• The mRNA in the
cytoplasm joins with a
Ribosome which is
made out of rRNA
• The mRNA runs
through the ribosome
tRNA inTranslation
codon in mRNA
• tRNA shuttle the
appropriate amino
acids to the ribosome
and mRNA
• The tRNA contains an
anticodon that
matches up to the
codons on mRNA
anticodon
amino
acid
Translation
• In the Ribosome the
mRNA runs through
the top
• There are two sites
that allow tRNA’s with
their accompanying
amino acids to bind
Binding site for mRNA
P
(first
binding
site for
tRNA)
A
(second
binding
site for
tRNA)
Translation
• Only tRNA’s that have
the appropriate
anticodons which
match to the mRNA
codons can bind to
the mRNA in the
ribosome
Translation
• As the two tRNA’s
bind to the codons
their amino acids
become in close
enough proximity to
form a peptide bond.
Peptide bond forms between adjacent
Amino acids
• Once the amino acids
have joined together
the tRNA in the first
position breaks loose
both from the condon
and its amino acid. It
then floats away to
find another amino
acid.
Translation
• Once the first tRNA is
gone, the Ribosome
moves the mRNA one
codon further through.
• The tRNA that was in the
second postion is now
dragged into the first
position.
• This leaves room for
another tRNA with its
amino acid to come into
the ribosome
Binding site for mRNA
P
(first
binding
site for
tRNA)
A
(second
binding
site for
tRNA)
Fig. 14.14a, p. 232
Fig. 14.14b, p. 233
Mutations
• Two types of mutations
– Base pair substitution mutations
– One base is copied incorrectly during DNA replication
– A protein may incorporate the wrong amino acid
– Frame-shift mutations
– One or more base pairs become inserted into DNA or are
deleted from it.
– This almost always makes the protein nonfunctional and
is often lethal.
Base pair
substitution
original
base triplet
in a DNA
strand
a base
substitution
within the
triplet (red)
As DNA is replicated, proofreading
enzymes detect the mistake and
make a substitution for it:
POSSIBLE OUTCOMES:
OR
One DNA molecule
carries the original,
unmutated sequence
VALINE
PROLINE
The other DNA
molecule carries
a gene mutation
THREONINE
VALINE
Fig. 14.15, p. 234
LEUCINE
HISTIDINE
GLUTAMATE
mRNA transcribed from the DNA
PART OF PARENTAL DNA TEMPLATE
ARGININE
GLYCINE
TYROSINE
TRYPTOPHAN
ASPARAGINE
resulting amino acid sequence
altered message in mRNA
A BASE INSERTION (RED) IN DNA
ARGININE
GLYCINE
LEUCINE
LEUCINE
GLUTAMATE
the altered amino acid sequence
Fig. 14.16, p. 234
TRANSCRIPTION
Pre mRNA
Transcript
Processing
Unwinding of gene regions of a DNA molecule
mRNA
rRNA
tRNA
protein
subunits
Mature mRNA
transcripts
TRANSLATION
Synthesis of a
polypetide chain at
binding sites for
mRNA and tRNA
on the surface of
an intact ribosome
ribosomal
subunits
mature
tRNA
Convergence
of RNAs
Cytoplasmic
pools of
amino acids,
tRNAs, and
ribosomal
subunits
FINAL PROTEIN
Destined for use in
cell or for trasport
Fig. 14.18, p. 237
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