Download DNA Transcription

DNA Transcription &
Translation
*How do genes work?
We’ve seen how DNA is replicated, but
still haven’t learned exactly how
genes work!
The first step in understanding how
genes work is to know how to
“decode” the DNA.
Decoding DNA
• The decoder: RNA
(Ribonucleic acid)
• Long stranded like DNA,
except:
– Ribose instead of
Deoxyribose
– Uracil instead of Thymine
– Single stranded instead of
double stranded
Types of RNA
• The main job of all RNA types is protein
synthesis (creation)
• 3 types: (*You will learn what each does later)
Messenger RNA
mRNA
Transfer RNA
tRNA
Ribosomal RNA
rRNA
Protein Creation
• DNA holds the instructions for making
proteins
– Proteins do the cell’s work
• Two steps required for protein synthesis
(creation)
1. TRANSCRIPTION
2. TRANSLATION
Transcription
• TRANSCRIPTION is the process by
which a DNA gene is “rewritten” into
RNA
Transcription
• A gene is only a segment of DNA
– Only the gene is transcribed, not all of
the DNA (like replication)
DNA
Transcription
mRNA
Transcription
3 phases:
1. Initiation
•
(“the beginning”)
Enzymes recognize appropriate gene in
DNA
2. Elongation
•
(“the middle”)
Enzymes copy gene into mRNA
3. Termination
•
(“the end”)
Enzymes unbind from DNA
Why re-write the instructions?
• DNA is like the
• RNA is like an
“master plan” for
inexpensive,
the construction of
disposable copy of
a house
the master plan
• Too important to
bring to the work
site and risk being
damaged or lost
• Used at the work
site (the
ribosomes)
Transcription
INITIATION
• RNA POLYMERASE (RP)
recognizes a promoter on
the DNA sequence
– PROMOTER: base
sequence that signals
the start of a gene
• DNA is split by RP and
one strand is used as a
template
Animation #1
Animation #2
Transcription
• ELONGATION:
– RP makes an mRNA copy of the gene
Transcription
TERMINATION
• RP reaches TERMINATOR SEQUENCE
– Signals it to unbind and stop transcription
Transcription
• After transcription is done you end
with one mRNA copy of a gene
• Some of the mRNA must be “spliced
out” (removed) before we translate it
mRNA Processing
• Pieces of the
mRNA that
don’t contain
genes (called
INTRONS) are
removed by a
SPLICEOSOME
mRNA Processing
• Pieces of DNA
that do code
for genes
(called EXONS)
are left and
pieced together
The Genetic Code
• The mRNA strand is then used to make
proteins in the process of TRANSLATION
The Genetic Code
• The “language” of
mRNA is known as
the genetic code
• The 4 “letters”
(A-G-C-U) are used
to write “words”
that correspond to
different amino
acids
The Genetic Code
• “Words”are written
using 3 letters
• Every 3 letters = 1
codon
• Every 1 codon = 1
amino acid
The Genetic Code
• Because there are
four different
bases, there are 64
(43) different
codon combinations
• Certain codons
code for the same
amino acid
Translation
• Shortly after mRNA is
transcribed, translation
begins.
• The sequence of bases in
mRNA serves as
instructions for the
order in which amino
acids should be joined
Translation
• Translation: The
decoding of an
mRNA message and
creation of a
polypeptide chain
(protein)
• RIBOSOMES are
the factories that
assemble the
proteins
Translation
INITIATION
• mRNA in the
cytoplasm attaches
to a ribosome
ELONGATION
• The ribosome
“reads” the codons
and attaches the
proper amino acids
Start
codon
Stop
codon
Transfer RNA
• Transfer RNA brings the
correct amino acids for
the ribosome to use
• Has an ANTI – CODON on
one end, and an amino acid
on the other
• Anti-codon has a sequence
complementary to an
mRNA codon
Translation
TERMINATION
• The ribosome
continues along until it
reaches a stop codon
• The amino acid chain is
then released and
allowed to fold into a
protein
• Protein then performs
its job within the cell
Translation
Translation can occur at
many different spots along
the mRNA strand, creating
many proteins at one time.
Let’s Try it ourselves
• Transcribe and translate the
following DNA sequence:
TAC GCA TGG AAT
AUG CGU ACC UUA
Translation Animations
Translation
Transcribe and Translate a Gene
Why do fireflies glow?
Let’s try it ourselves
• Transcribe and translate the
following DNA sequence:
TAC GCA TGG AAT
AUG CGU ACC UUA
Met – Arg – Thr – Leu
Mutations
• Every now and then cells make
mistakes.
– Inserting incorrect bases
– Deleting bases
– Adding extra bases
• Mistakes that cause a change in
genetic information are called
mutations.
Point Mutations
• Point mutations are mutations that
affect one nucleotide.
• 3 different types:
– Substitution
– Insertion
– Deletion
Substitution
• Occurs when the wrong base is added
to a growing nucleotide chain
Original DNA: TAC GCA TGG AAT
Original mRNA: AUG CGU ACC UUA
Original Protein: Met – Arg – Thr - Leu
Substitution
Original DNA: TAC GTA TGG AAT
Original mRNA: AUG CAU ACC UUA
Original Protein: Met – His – Thr - Leu
Insertion
• Occurs when an extra base is added
to a growing nucleotide chain
Original DNA: TAC GCA TGG AAT
Original mRNA: AUG CGU ACC UUA
Original Protein: Met – Arg – Thr - Leu
Insertion
Original DNA: TAT CGC ATG GAA T
Original mRNA: AUA GCG UAC CUU A
Original Protein: Ile – Ala – Tyr – Leu
Deletion
• When a base is deleted from a
growing nucleotide chain
THE FAT CAT ATE THE RAT
TEF ATC ATA TET HER AT
Frameshift Mutations
• Mutations that cause a shift in the
reading frame
– Insertion
– Deletion
TACGCATGGAAT
TATCGTATGGAAT
• More than one amino acid gets
changed downstream
Silent Mutations
• DNA changes that do not cause a
change in the amino acid chain
TAC GCA TGG AAT
Mutated DNA: TAC GCC TGG AAT
mRNA: AUG CGG ACC UUA
Protein:
Met – Arg – Thr - Leu
Original DNA: