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Transcription &
Translation
Protein Synthesis
Biology 12
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Genes direct the production of proteins that
determine the phenotypical characteristics of organisms.
Genes also direct the production of other physiologically
essential proteins such as antibodies and hormones.
Proteins drive cellular processes such as metabolism;
determining physical characteristics and producing
genetic disorders by their absence or presence in an
altered form.
Metabolism is a term that is used to describe all chemical reactions
involved in maintaining the living state of the cells and the organism.
Metabolism can be conveniently divided into two categories:
Catabolism - the breakdown of molecules to obtain energy
Anabolism – the synthesis of all compounds needed by cells )
The Central Dogma
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An organism’s genome is housed within the
nucleus. Proteins are synthesized outside the
nucleus, in the cytoplasm, on ribosomes.
Since information for protein synthesis is
specified by DNA (called the one gene-one
polypeptide hypothesis), and DNA is not able
to exist outside the nucleus, a problem exists
as to how the blueprint of life is brought to the
ribosomes.
The Connection Between
Genes and Proteins
Nucleic acids carry information in their nucleotide sequence.
Proteins carry information in their amino acid sequence.
To get from DNA (in nucleic acid language) to protein
(in amino acid language) requires two steps.
1. Transcription- a DNA strand provides a template for
the synthesis of a complementary RNA strand.
This molecule is called mRNA (messenger RNA).
DNA is too valuable to be allowed to exit the nucleus. This
could lead to the death of the cell and possible the
Death of the organism.
- use of mRNA provides protection for the
Genetic information contained in DNA.
- more protein can be made simultaneously
because many mRNA copies of a gene can be
made than if one strand of DNA left the nucleus.
- each mRNA can be translated many times.
mRNA delivers the encoded genetic
material to the ribosomes.
The ribosomes translate the message
into polypeptide chains, which are
processed into proteins.
This entire sequence is described as
the Central Dogma of Molecular
Genetics, first stated by Francis Crick
in 1958.
Central Dogma
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In nucleus
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Produced in nucleus
Travels to cytoplasm
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Produced in cytoplasm
Transcription vs Translation
Transcription involves the copying of the
information in DNA into mRNA.
(copy from one medium to another- think
of a medical or legal stenographer)
Translation involves ribosomes using the
Messenger RNA as a blueprint to synthesize
a protein composed of amino acids.
(converting into a different language, think English
to French)
Definition: Transcription
Transcription
Nucleus
Location
DNA
Template
(What is read)
To change DNA into a form that
can make a protein
Purpose
Messenger RNA
(mRNA)
Outcome
(End result)
Definition: Translation
Translation
Location
Cytoplasm (by ribosome)
Template
(What is read)
mRNA
Purpose
Amino acids assembled in particular
order to make a protein
Outcome
(End result)
Protein (polypeptide)
Central Dogma
RNA
RNA: How is it difference
from DNA?
- contains a ribose sugar
- contains the base Uracil
(not thymine)
- single stranded
- found in both nuclues and
cytoplasm
Purine Bases (double ring)
Adenine & Guanine
Pyrimidine Bases (single ring)
Cytosine & Uracil
Base Pairs:
(purine always pairs with pyrimidine)
Adenine + Uracil
Guanine + Cytosine
Image: www.biologycorner.com/bio1/DNA.html
Types of RNA
Genetic information copied from
DNA is transferred to 3 types
of RNA:
Messenger RNA: mRNA
Copy of information in DNA that is
brought to the ribosome and
translated into protein by tRNA &
rRNA
Varies in length , the longer the gene the
longer the mRNA>
Transfer RNA: tRNA
Brings the amino acid to the ribosome
that mRNA coded for.
Ribosomal RNA: rRNA
Most of the RNA in cells is associated
with structures known as ribosomes,
the protein factories of the cells.
Provides the construction site for the
assembly of polypeptides.
It is the site of translation where
genetic information brought by mRNA
is translated into actual proteins.
Transcription & Translation
Transcription occurs in 3 steps:
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Initiation, Elongation and Termination
Initiation:
RNA polymerase binds to the DNA at
a specific site known as a promotor.
DNA:
RNA:
A T G C A A
U A C G U U
The RNA transcript is known as
elongation.
After the RNA polymerase passes the end
of the gene, it stops transcribing which is termination.
Transcription : ‘to copy’
Initiation:
 RNA polymerase binds to DNA at ‘promoter’
 untwists the double helix 10 to 20 bases at a
time
Elongation:
 RNA polymerase builds mRNA
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From DNA 3’ end
Uses complimentary base pairing
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Remember: thymine (T) is replaced by uracil (U)
Termination:
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RNA polymerase reaches end of gene.
Stops transcribing
Double helix reforms as mRNA molecule peels
away.
End Result:
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mRNA breaks away from DNA
mRNA exits nucleus
If there is a high demand for a
protein, the cell can have several RNA
polymerases transcribing the gene at the same
time to produce several mRNA’s.
Translation: ‘new language’
Initiation:
 Ribosome binds at a specific sequence on
the mRNA.
 The ribosome moves along the mRNA three
nucleotides at a time. This is called a codon.
Each set of three (a codon) codes for an
amino acid. Why?
There are only 4 bases but 20 amino acids.
41 = 4 (1 base=1 acid) 42 = 16 43 = 64
The codon AUG not only codes for the amino acid
Methionine, but it also indicates the start of a
translation.
Some amino acids are coded for by two or more
codons but a given codon ALWAYS only codes
for one amino acid.
GAA and GAG both code for glutamic acid,
but never mean any other amino acid.
Elongation:
Ribosome moves along mRNA
 From mRNA 5’ end
 3 nucleotides of mRNA = codon = amino acid
 The “interpreter” tRNA delivers the proper complimentary base
to the ribosome. Anticodons are blocks of 3 tDNA bases that
actually attach to the correct protein.
 The anticodon( tRNA) binds by complimentary base pairing to the
nucleotides of the codon.
 Example: if the codon on a mRNA is UUU,
a tRNA with an AAA anticodon will bind to it.
The ribosome links adjacent amino acids with a peptide bond, causing
the amino acid to let go of it tRNA.
The finished protein has a sequence of amino acids that have been
determined by the mRNA base sequence which has been translated
by the tRNA.
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The ribosome then adds each amino acid
and the polypeptide chain is elongated.
Elongation occurs until a stop signal occurs.
Termination:
 Ribosome reaches stop codon
 Stops translating
End Result:
 Ribosome falls off mRNA
 Protein (polypeptide chain) is released
Start and Stop Codons
Start Codon:
 Begins translation
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AUG (universal start codon)
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ALSO Codes for methionine (Met)
Sometimes GUG or UUG
Stop Codon:
 Ends translation
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UGA, UAA, UAG
The
Whole
Picture
Next amino
acid to be
added to
polypeptide
Growing
polypeptide
tRNA
mRNA
Example
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DNA template:
3’ TAC ACA CGG AAT GGG TAA AAA ACT 5’
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Complimentary DNA
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mRNA codon
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Read from DNA template (start reading at 3’)
tRNA anticodon
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Read from DNA template (start reading at 3’)
Read from mRNA
Amino Acids (protein)
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Read from mRNA
Task A:
#2 – Central Dogma
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DNA makes RNA (mRNA)
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through transcription
RNA makes proteins
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through translation
#4 – RNA types
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mRNA
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Messenger RNA
End product of
transcription
Takes message from
DNA into cytoplasm
Used by ribosome to
make protein
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tRNA
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Transfer RNA
Delivers amino acid
to ribosome
rRNA
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Ribosomal RNA
Helps form and
maintain ribosomes
#5 – DNA vs. RNA
DNA
RNA
 Sugar – deoxyribose  Sugar – ribose
 Double stranded
 Single Stranded
 Base pair – thymine
 Base pair – uracil
 Stays in nucleus
 Can leave nucleus
 Can replicate itself
 Cannot replicate
itself
 Longer strands
 Shorter strands
#6 – Transcription/Translation
Transcription
 Purpose:
 To make mRNA
from DNA
 Location:
 Nucleus
Translation
 Purpose:
 To make a
specific protein
from mRNA
 Location:
 Cytoplasm
(ribosome)
#9 – Stop vs. Start Codon
Start Codon
 mRNA code
 Tells ribosome to
begin translation
 Example:
 AUG
Also codes for
methionine
And: UUG, GUG
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Stop Codon
 mRNA code
 Stops translation of
that specific amino
acid chain
 Examples:
 UAA, UAG, UGA
#10 – Transcribe to mRNA
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DNA:
GGA TCA GGT CCA GGC AAT
TTA GCA TGC CCC AA
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*mRNA*:
CCU AGU CCA GGU CCG UUA
AAU CGU ACG GGG UU
#11 – Translate to Amino Acids
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mRNA sequence divided into codons:
GGC AUG GGA CAU UAU UUU GCC
CGU UGU GGU GGG GCG UGA
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*Protein translation*:
Gly Met(start) Gly His Tyr Phe Ala
Arg Cys Gly Gly Ala (stop)
Task B:
#2 – Transcribe to mRNA
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DNA:
TAC TAC GGT AGG TAT A
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*mRNA*:
AUG AUG CCA UCC AUA U
Task C:
#3 – Anticodons
#4 – Change in 3rd Base May
Not Result in Error
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Why not?
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Amino acids have more than one codon
Example: proline
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Codons CCU, CCC, CCA, and CCG
CC - always codes for proline
Third base/nucleotide does not matter
#6 – Translate to Amino Acids
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mRNA:
GGC CCA UAG AUG CCA CCG GGA
AAA GAC UGA GCC CCG
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*Protein translation*:
Met (start) Pro Pro Gly Lys Asp (stop)