Transcription and Translation
 Class notes
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DNA
 Deoxyribonucleic acid
 DNA is a polynucleotide; made up of many nucleotides
 Phosphate, deoxyribose (pentose sugar), nitrogen-containing
base
PURINES
Adenine (A)
Guanine (G)
PYRIMIDINES
Thymine (T)
Cytosine (C)
DNA STRUCTURE
 Double helix
 Determined by James Watson and Francis Crick in the early
1950s.
Complementary Base Pairing
DNA REPLICATION
 When cells divide, each new cell needs an exact copy of
DNA.
 Original strand acts as a template
 Semiconservative: each new double helix has one old
conserved strand and one new strand
Enzyme
DNA
helicase
“unzips”
the DNA
Enzyme DNA
ligase seals any
breaks in the
sugar-phosphate
backbone
PROTEIN SYNTHESIS
 The process in which a cell makes a protein based on the
message contained within its DNA
 Problem!
 DNA is only found in the nucleus
 Proteins are only made outside the nucleus –in the cytoplasm
 So, how do the messages within the DNA get to the
ribosomes outside the nucleus?
 RNA is used to carry these messages
RNA
 Ribonucleic Acid
 Found in cytoplasm
 Like DNA, is a
polynucleotide
RNA
DNA
Sugar
Ribose
Deoxyribose
Bases
Adenine, Guanine,
Cytosine, Uracil
Adenine, Guanine,
Cytosine, Thymine
Strands
Single stranded
Double stranded
Helix
No
Yes
RNA
 Job is to carry messages from the DNA in the nucleus to the
ribosomes in the cytoplasm.
 There are three types of RNA:
 Messenger RNA (mRNA): takes a message from DNA to
ribosomes.
 Transfer RNA (tRNA): transfers amino acids to the ribosomes.
 Ribosomal RNA (rRNA): makes up the ribosomes, where
proteins are synthesized.
PROTEIN SYNTHESIS
 Occurs in two steps:
1. Transcription: DNA is transcribed into RNA (mRNA) in the
nucleus.
2. Translation: the mRNA, with the help of the ribosome,
forms a chain of amino acids (eventually forming a protein)
based on the information contained on the mRNA. In the
cytoplasm.
DNA
RNA
P
R
O
T
E
I
N
STEP ONE: TRANSCRIPTION
1. DNA unzips: Enzyme RNA polymerase binds to the
promoter site, splits apart base pairs and unwinds the DNA
double helix
Promoter=region of DNA where
RNA initiates transcription
 Determines which strand of DNA
will serve as the template
STEP ONE: TRANSCRIPTION
2. Bases pair up: Free nucleotides in the cell find their
complementary bases along the new strands.
STEP ONE: TRANSCRIPTION
3. New backbone formed: The sugar-phosphate backbone is
assembled to complete the RNA strand and separates from the
DNA strand.
TRANSCRIPTION ANIMATION
 http://www.stolaf.edu/people/giannini/flashanimat/molge
netics/transcription.swf
TRANSCRIPTION
 What RNA strand will be made from the following DNA
sequence?
TACGCATGACTAGCAAGTCTAACT
AUGCGUACUGAUCGUUCAGAUUGA
Step 1 ½: Processing of mRNA
 A primary mRNA molecule must be processed in order for it
to be useful. The unnecessary information needs to be
removed.
 An mRNA sequence that does NOT code for a protein is
called an intron.
 A sequence that is useful in making a protein is called an
exon.
Step 1 ½: Processing of mRNA
1. One end of mRNA receives a cap composed of an altered
guanine nucleotide. Other end receives a poly-A tail, a series of
adenosine nucleotides.
2. Introns are removed, exons are joined to form a mature
mRNA molecule.
The mRNA can now enter
the cytoplasm.
STEP 2: TRANSLATION
 Now that our mature mRNA molecule has been made, it’s
time for its message to be made into a protein sequence.
 How does the mRNA sequence translate into an amino acid
sequence?
 Problem
 There are 20 different amino acids
 There are 4 RNA bases
A
T
phe
C
G
ile
leu
val
met
pro
ser
ala
thr
his
tyr
asn
gln
asp
lys
cys
glu
arg
trp
gly
STEP TWO: TRANSLATION
 So how do you go about determining what protein your cells
are going to make?
 Divide the mRNA sequence into codons
 3-base unit
 There are 64 mRNA codons
 61 codons correspond to a particular amino acid
 3 are stop codons which signal polypeptide termination
 AUG is methionine and is the start codon; signals polypeptide
initiation.
AUG|CGU|ACU|GAU|CGU|UCA|GAU|UGA
STEP TWO: TRANSLATION
 Since each 3-base combination “codes” for an amino acid, you
can figure out what amino acid matches up with each codon:
AUG|CGU|ACU|GAU|CGU|UCA|GAU|UGA
?
STEP TWO: TRANSLATION
 Since each 3-base combination “codes” for an amino acid, you
can figure out what amino acid matches up with each codon:
AUG|CGU|ACU|GAU|CGU|UCA|GAU|UGA
met
arg
thr
asp
arg
ser
asp
STOP
STEP TWO: TRANSLATION
 Transfer RNA (tRNA)
 Bring amino acids to the ribosomes
 Single-stranded nucleic acid that doubles back on itself to create
regions where complementary base pairing results.
 At least one tRNA molecule for each amino acid
STEP TWO: TRANSLATION
 Amino acid on one end, opposite end has an anticodon
 Anticodon= group of 3 bases that is complementary to a
specific codon of mRNA.
 tRNA acts as the translator between mRNA and protein by
bringing the specific amino acid coded for by the mRNA
codon.
STEP TWO: TRANSLATION
 Example:
Codon (mRNA)
Anticodon (tRNA)
Amino Acid
GCA
CGU
Alanine
 Lysine would be carried by a tRNA molecule with the
anticodon CGU
STEP TWO: TRANSLATION
 Ribosomes
 Cytoplasm and on rough ER
 Composed of proteins and rRNA
 Binding site for mRNA and 2 tRNA at a time
 Ribosomal RNA (rRNA)
 Produced in nucleolus within the nucleus
 Forms two ribosomal subunits (one large, one small) in
cytoplasm
3 STEPS OF TRANSLATION
1. Chain initiation: small ribosomal
subunit, mRNA, tRNA with methionine
and large ribosomal subunit come
together.
3 STEPS OF TRANSLATION
2. Chain elongation: a second tRNA with amino acid enters the
ribosome and pairs with the next codon.
 The amino acids are attached by peptide bonds
 The ribosome then moves one codon forward and a new
tRNA with amino acid can enter the ribosome
 If the new tRNA has the wrong anticodon, it is rejected
3 STEPS OF TRANSLATION
2. Chain elongation: a second tRNA with amino acid enters the
ribosome and pairs with the next codon.
 The amino acids are attached by peptide bonds
 The ribosome then moves one codon forward and a new
tRNA with amino acid can enter the ribosome
 If the new tRNA has the wrong anticodon, it is rejected
3 STEPS OF TRANSLATION
3. Chain termination: Occurs when the ribosome reaches the
stop codon (UAA, UAG, UGA).
 A release factor binds to the site.
 The ribosome separates into two subunits and the
polypeptide (protein) is released.
PROTEIN SYNTHESIS ANALOGY
Construction Analogy
Blueprints
Protein Synthesis
Supervisor
Trucks
Finished Building
Concrete and Lumber
Master Plans
Building Site
Architect’s Vault
Nucleus, DNA, Cytoplasm, Ribosome, mRNA, tRNA, Amino Acids, Protein
PROTEIN SYNTHESIS ANALOGY
Construction Analogy
Blueprints
Protein Synthesis
mRNA
Supervisor
Trucks
Finished Building
Ribosome
tRNA
Protein
Concrete and Lumber
Master Plans
Building Site
Amino Acids
DNA
Cytoplasm
Architect’s Vault
Nucleus
Nucleus, DNA, Cytoplasm, Ribosome, mRNA, tRNA, Amino Acids, Protein
WHY IS THIS IMPORTANT?
 Proteins are important for the structure and function of cells.
 It is important for proteins to work properly.
 A mutated gene can cause a protein to have the wrong
sequence of amino acids which can cause a genetic disorder
or the risk of cancer
RECAP
DNA is transcribed into mRNA in the nucleus.
2. The mRNA leaves the nucleus and enters the cytoplasm.
3. The protein is translated from the mRNA sequence using
tRNA and amino acids.
1.
RECAP
 Every cell must contain the genetic information and the
DNA is therefore duplicated before a cell divides
(replication).
 When proteins are needed, the corresponding genes are
transcribed into RNA (transcription).
 The RNA is first processed so that non-coding parts are
removed (processing) and is then transported out of the
nucleus.
 Outside the nucleus, the proteins are built based upon the
code in the RNA (translation).