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DNA, RNA, Protein synthesis-from genes to proteins
NUCLEIC ACIDS: Structure and Function
 DNA: Where the genetic information is stored, blueprint
for making proteins. Can be copied and passed from
generation to generation
 RNA: Always involved in protein synthesis. It is produced
every time proteins are needed
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Cytosine
Nitrogenous base
Macromolecules (polymers!)
Monomers (units): nucleotides
Adenine
Guanine
Phosphate group
Ribose
(RNA)
5-carbon sugar
Deoxyribose
(DNA)
Thymine
DNA
DNA
Uracil
Uracil
RNA
 “Accepted” as the heritable material
in 1952
 TWO strands with helical
disposition. One strand is the code, the
other helps protecting the coding strand
 backbone: sugar and phosphate
sequence. Both N-base and phosphate
connect to the sugar
Nitrogen bases (N-bases)
1) Hydrogen bonds between them attach
one strand to the other.
2) arranged in a specific “format”, always
is
3 H bonds: Cytosine
C
G
Guanine
2 H bonds: Adenine
A
T
Thymine
N-bases store the genetic information!
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DNA, RNA, Protein synthesis-from genes to proteins
DNA Replication
 When a cell reproduces, a complete copy of the DNA must pass from one
generation to the next.
 Occurs (in the nucleus) during Interphase,
in preparation for cell division
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(1) An enzyme breaks H bonds between
nitrogenous bases
(2) Strands are separated and both are
copied (two directional copy!)
(3) Enzyme sequentially
attaches new nucleotides with
dehydration reactions
(4) Replication bubbles
combine and two new
strands complementary to
the original ones are formed
DNA Replication: Is done by the enzyme DNA polymerase
(1) Breaking of H bonds by enzymes
(2) Strands are separated
(3) Enzymes copy in both directions
New nucleotides are
added by the enzyme
(4) Combination of the different replication sites
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DNA, RNA, Protein synthesis-from genes to proteins
Mutations
• Are any change in the nucleotide sequence of DNA
 Some mutations result in
changes in the final protein
4 types of mutations
1) Missense: One nucleotide changes. Only one amino
acid is changed
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2) Nonsense: One nucleotide changes. Produces a STOP
codon and shorter protein as a result
3) Frameshift: By either inserting or deleting 1 or 2 letters
the entire sequence is changed after the mutation, which
adds a bunch of wrong amino acids to the protein
4) Silent mutations: One nucleotide changes, but the
amino acid that is being coded for is the SAME!
Mutation happen, but the new codon codifies for the same
amino acid  DNA changes but protein is the same
Hemoglobin: 145 amino acids
(aa) in four chains. A missense
mutation (one nucleotide)
changes one aa
• hemoglobin shape changes
when oxygen levels are low
•Irregular shape causes
hemoglobin to clump, blocking
the blood vessels
• no oxygen delivering 
weakness, brain damage,
rheumatism
In which cells is a mutation
potentially more dangerous?
Sex cells!
Cells used for reproduction
THE FLOW OF GENETIC INFORMATION
FROM DNA TO RNA TO PROTEIN
 Set of ideas describing how the cell uses the
information stored in the DNA
Makes a copy of DNA
in the form of RNA
Synthesizes proteins
using mRNA as template
Structural Carrier, Enzymes,
Hormones, antibodies
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DNA, RNA, Protein synthesis-from genes to proteins
RNA
• ONE strand
• backbone: sugar and phosphate sequence
• bases: arranged
in a specific
“format”, always
2
Cytosine
C
G
Guanine
Adenine
A
U
Uracil
Three types of RNA: all copies of parts of DNA
strands
1) mRNA: “messenger” RNA, carries the
genetic information from the cell nucleus to the
cytoplasm
2) tRNA: “transfer”RNA, transfers or carries
amino acids to the ribosomes, and aids in the
pairing of amino acids
3) rRNA: “ribosomal” RNA, covered in protein
to form a ribosome, where the protein synthesis
(translation) takes place
Transcription
DNA
• DNA to RNA
• Takes place in the nucleus
tRNA
rRNA (ribosomes)
transcription
mRNA
Proteins
translation
Why a copy?
(1) Every chromosome ranges between hundreds to thousands of genes, so it is
impractical to move out the entire chromosome when only one gene is actually needed
(2) The nucleus would have to be destroyed every time a chromosome has to move out!
(3) Also, in this way the original “library” remains safe, no risks of getting the DNA
damaged or destroyed
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DNA, RNA, Protein synthesis-from genes to proteins
transcription
DNA
• DNA to RNA
• Takes place in the nucleus
tRNA
rRNA (ribosomes)
mRNA
Proteins
translation
RNA polymerase
•The two strands of DNA are
separated and the bases of the
coding strand exposed
2
Nucleotides
Available in the
nucleus
•The coding strand is read
in one direction (Only the
“coding strand is used in
this case)
•Dehydration synthesis
reactions  bonding of
nucleotides and construction of
a single strand of RNA
mRNA, carries the code or “message”
on how to make a protein
Controlling Transcription…
Which strand of DNA must be used for transcription?
When to stop making RNA?
Where must transcription (RNA generation) begin?
rr
rr
Promoter
Sequence recognized by the RNA polymerase
The “start transcribing”
signal is a nucleotide sequence
called a promoter.
The first phase of transcription
is initiation, in which:
RNA polymerase attaches
to the promoter
RNA synthesis begins
rr
Gene
rr
Termination region
Marks the end of transcription
 During the third phase of transcription,
called termination:
 RNA polymerase reaches a sequence
of DNA bases called a terminator
 RNA Polymerase detaches from the
RNA
 The DNA strands rejoin
Regulatory Regions (rr): Specific proteins called transcription factors attach to these
parts making the promoter region more o less “visible” to the RNA polymerase
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DNA, RNA, Protein synthesis-from genes to proteins
Translation
tRNA
rRNA (ribosomes)
transcription
DNA
• RNA language to protein language
• Using a mRNA template to synthesize proteins
mRNA
How does the cell machinery know which
amino acid goes with each part of RNA?
Proteins
translation
2 tRNA is the translator!
mRNA is the dictionary!
 The code in the mRNA is
used to direct the sequential
assembly of amino acids in the
cytoplasm
tRNA
Each sequence of 3
nitrogenous bases (called
codon) in the mRNA defines
an amino acid
 Translates the code to the amino
acid language and transports the
aminoacids
 The tRNA has bases (named
anticodon) that match the codon
of the mRNA with the correct
amino acid
Codon
mRNA
(3 bases code in mRNA)
UGU = Cystine
Translation: the players…
 Translation requires:
 Messenger RNA (mRNA), bring the code on how to make the protein
 Transfer RNA (tRNA), works as the “translator” from Nucleic acid language to
protein language
 Amino acids to build the
Next aminoacid
protein
to be added
 Ribosomes as the “factory”
where the entire process takes
place
Growing
polypeptide
(protein)
Ribosome
tRNA
Anti codon
mRNA
Codons
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DNA, RNA, Protein synthesis-from genes to proteins
Where to begin and end translation?
 The “signal” is present in the mRNA!
STOP!
 3 specific codons are used to indicate STOP!
 Termination of the translation process
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Codon
(3 bases code in mRNA)
UGU = Cystine
START!
 Only one codon is
the initiator
 Start of the
translation
process
The process of translation (protein synthesis)
 A large ribosomal subunit binds,
creating a functional ribosome
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Initiation
2
3
 mRNA molecule binds to a
small ribosomal subunit
 initiator tRNA binds to the
start codon
 A new tRNA
with an amino
acid approximates
to the ribosome
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When a stop codon is
found in the code
translation stops
 tRNA, ribosomes,
and enzymes can be
reused
 mRNA is broken
down
An enzyme
Elongation
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4
 The first
tRNA detaches
from the amino
acid and leaves
ribosome
 The mRNA
moves
catalyzes
peptide
formation
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DNA, RNA, Protein synthesis-from genes to proteins
How fast can cells synthesize proteins?
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Average protein synthesis time
• Prokaryotes (~6 sec)
– ~300 amino acids
– Translation rate (~50 aa/sec)
• Eukaryotes (~2 min)
– ~500 amino acids
– Translation rate (5 aa/sec)
Fedorov and Baldwin (1997) J. Biol. Chem. 272:32715
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