Transcript Document

8.4 Transcription
• Proteins are made in the ribosomes
outside the nucleus.
• DNA is copied (replicated) in the
nucleus but cannot leave the
nucleus.
THEREFORE…….
• A message must be sent to the
ribosomes in the cytoplasm telling
them what proteins to make.
• This message is carried by a nucleic
acid called messenger (mRNA ).
8.4 Transcription
RNA Structure
1.
2.
3.
4.
RNA differs from DNA in 4 ways:
RNA is a single strand
RNA has the sugar ribose
Adenine bonds with Uracil (NOT thymine).
Can leave the nucleus
8.4 Transcription
KEY CONCEPT
Transcription converts a gene into a
single-stranded RNA molecule.
8.4 Transcription
RNA carries DNA’s instructions.
• The central
dogma states
that information
flows in one
direction from
DNA to RNA to
proteins.
8.4 Transcription
• The central dogma
includes three
processes.
1. Replication
2. Transcription
3. Translation
• RNA is a link between
DNA and proteins.
replication
transcription
translation
8.4 Transcription
Transcription copies DNA to
make a strand of RNA.
DNA
8.4 Transcription
Transcription occurs in two major steps.
1. DNA unzips & unwinds at the start of a gene.
transcription complex
start site
nucleotides
8.4 Transcription
•
Transcription occurs in two major steps.
2. Nucleotides pair with their complementary bases
on one strand of the DNA.
– RNA polymerase bonds the nucleotides together.
– The DNA helix winds again as the gene is transcribed.
DNA
RNA polymerase
moves along the DNA
8.4 Transcription
The RNA strand detaches from
the DNA once the gene is
transcribed. Then it leaves the
nucleus and heads to a ribosome.
RNA
8.4 Transcription
Transcription makes three types of RNA.
1. Messenger RNA (mRNA) - carries the
message that will be translated to form a
protein.
2. Ribosomal RNA (rRNA) - forms part of
ribosomes where proteins are made.
3. Transfer RNA (tRNA) - brings amino
acids from the cytoplasm to a ribosome.
8.4 Transcription
KEY CONCEPT
Translation converts an mRNA message into a
polypeptide, or protein.
8.4 Transcription
Amino acids are coded by mRNA base sequences.
• Translation
converts mRNA
messages into
polypeptides.
• A codon is a
sequence of three
nucleotides that
codes for an
amino acid.
codon for
methionine (Met)
codon for
leucine (Leu)
8.4 Transcription
The language of amino acids is based on codons
1 codon =
3 mRNA nucleotides
1 codon =
1 amino acid
A UA U A U G C C C G C
How many codons are in this sequence of mRNA?
4
How many Amino Acids does this gene code for?
4
8.4 Transcription
• The genetic code matches each codon to its amino
acid or function.
• three stop
codons
• one start
codon, codes
for methionine
13.1 Ecologists Study Relationships
How to read a codon table
Use in protein synthesis for translating the
mRNA code into amino acid sequence
8.4 Transcription
You need a sequence of mRNA
• Where does this come from?
From the transcription
of DNA --> mRNA
Example:
DNA sequence: TAC GGA CAT AAC ACC TGC ATC
mRNA sequence: AUG CCU GUA UUG UGG ACG UAG
8.4 Transcription
Transcription
• mRNA sequence leaves the nucleus and travels to
the cytoplasm to a free floating ribosome or to
the rough ER.
• It will attach to the ribosome and begin the
second step of protein synthesis, translation.
8.4 Transcription
Translation
• mRNA is read as a series of codons (three letters)
within the ribosome.
• tRNA molecules have an anticodon sequence of
letters that are complements to the mRNA
ex: mRNA CGA UCC (codon)
tRNA GCU AGG (anticodon)
8.4 Transcription
So now we get to the codon
table!
• Locate the first letter
of your codon using
the left side of the
table.
• Ex. AUG
• look for the A
8.4 Transcription
• Now move to the
second letter of your
codon which is ‘U’
• Look at the top of the
table where you see the
title ‘2nd letter’
• Find the letter ‘U’ and
follow it down until it
intersects with the letter
‘A’ from the left side.
• You should see four
amino acids (isoleucine,
isoleucine, isoleucine,
and (start) methionine.
8.4 Transcription
• Down to the last letter
of the codon!
• Look to the right hand
side for the third letter.
Find the letter ‘G’ which
will intersect with the
box that had our four
choices.
• Move your finger from
the ‘G’ on the left over
to the left and you
should land on …..
Methionine (start)
• Yes you did it!!!
• Now try another codon
8.4 Transcription
Try the codon CAC
Don’t peek until you
come up with
your answer!
Did you get the
amino acid
‘histidine’?
8.4 Transcription
Using this
chart, you can
determine
which amino
acid the codon
“codes” for!
Which amino
acid is
encoded in the
codon CAC?
8.4 Transcription
Notice there is one
start codon AUG.
Transcription begins
at that codon!
8.4 Transcription
Notice there are three
stop codons.
Transcription stops
when these codons
are encountered.
8.4 Transcription
What do these codons have to
do with proteins?
• Each codon represents
an amino acid that will
eventually form a
protein that is used
within a cell.
• Proteins are made up of
hundreds of amino
acids in a specific
sequence.
• When they get “out of
order’ a mutation
occurs.
Long string of
amino acids
will form
8.4 Transcription
• A change in the order in which codons are read
changes the resulting protein.
• Regardless of the organism, codons code for
the same amino acid.
8.4 Transcription
Amino acids are linked to become a protein.
• An anticodon is a set of three nucleotides
that is complementary to an mRNA codon.
• An anticodon is carried by a tRNA.
8.4 Transcription
• Ribosomes consist of two subunits.
– The large subunit has three binding sites for
tRNA.
– The small subunit binds to mRNA.
8.4 Transcription
• For translation to begin, tRNA binds to a start codon
and signals the ribosome to assemble.
– A complementary tRNA molecule binds to the
exposed codon, bringing its amino acid close to the
first amino acid.
8.4 Transcription
8.4 Transcription
• The
ribosome
helps form a
peptide bond
between the
amino acids.
8.4 Transcription
– The now empty tRNA molecule exits the ribosome.
• Once the stop
codon is reached,
the ribosome
releases the
protein and
disassembles.
8.4 Transcription
From gene to protein
aa
aa
transcription
DNA
translation
aa
protein
mRNA
aa
aa
aa
aa
ribosome
A C CA U GU C G A U C A GU A GC A U GGC A
nucleus
tRNA
cytoplasm
aa
trait
8.4 Transcription
cytoplasm
aa
aa
aa
protein
aa
aa
transcription
translation
aa
aa
aa
aa
aa
aa
nucleus
trait
8.4 Transcription
From gene to protein
protein
transcription
translation