Nucleic Acids
RNA and Transcription
The Genetic Code
Protein Synthesis
Mutations
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Types of RNA
• Messenger RNA (mRNA)
Carries genetic information for protein
synthesis from DNA in nucleus to the
ribosomes
• Transfer RNA (tRNA)
Small molecules of 20 types that recognize
and transfer amino acids for protein synthesis
• Ribosomal RNA (rRNA)
Makes up 2/3 of ribosomes (1/3 protein) where
protein synthesis takes place
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Transcription
• Genetic information contained
in nucleus
• One strand of DNA is copied
• Complementary bases build mRNA
DNA(1 strand)
mRNA
• In mRNA uracil (U) complements A (DNA)
• New mRNA moves out of nucleus to
ribosomes in cytoplasm
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Genetic Code
• Needed to build a particular protein
• The sequence of amino acids is coded by
the mRNA
• Each triplet of bases along mRNA codes
for an amino acid
• The triplet are called codons
• Codons are known for all 20 amino acids
• Some codons signal the “start” and “end”
of a polypeptide chain
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Codons and Amino Acids
Suppose that a section of a mRNA has the
following series of bases.
CCU–AGC –GGA –CUU
Use a codon reference to determine the order of
amino acids
CCU = Proline
AGC = Serine
GGA = Glycine
CUU = Leucine
The mRNA codes for the peptide chain of
Pro –Ser –Gly –Leu
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Learning Check G1
What is the order of amino acids coded for by
a section of RNA with the base sequence GCC
–GUA –GAC ?
GGC = Glycine
GAC = Aspartic acid
CUC = Leucine
GUA =Valine
GCC = Alanine
CGC = Arginine
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Solution G1
GGC = Glycine GAC = Aspartic acid
CUC = Leucine GUA =Valine
GCC = Alanine CGC = Arginine
GCC –GUA –GAC
Ala – Val – Asp
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Activation of tRNA
Each tRNA binds to its specific amino acid
Pro
Pro
GGG
GGG
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Anticodons on tRNA
• A three-base sequence on each tRNA
• Complements a triplet on mRNA
Pro
anticodon
GGG
CCC
codon on mRNA
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Initiation and Elongation
• mRNA attaches to a ribosome
• tRNA with anticodon UAC binds to first codon
(AUG) to initiate synthesis
• The second codon picks up a tRNA with the
proper anticodon.
• A peptide bond forms between the amino
acids at the first and second codons.
• The first tRNA detaches and the ribosome
shifts to the next codon on the mRNA
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Peptide Formation
Peptide starts to form
Met
Met
Ser
UAC AGA
• •• • ••
AUG UCU CUC
Ser
Leu
AGA GAG
• • • •• •
UCU CUC UUU
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Termination
• Protein grows as tRNAs bring amino acids to
the codons on the mRNA
• When all amino acids for a protein are linked
the next codon is “stop”
• There is no tRNA for the “stop” codon
• Protein synthesis ends
• Protein released from ribosome
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Learning Check G2
A.
B.
C.
D.
Match the following processes in protein
synthesis with the statements:
(1) Activation
(2) Initiation
(3) Elongation
(4) Termination
Ribosome moves along mRNA to add new amino
acids to a growing peptide chain
Completed peptide chain released
A tRNA attaches to its specific amino acid
tRNA binds to the AUG codon of the mRNA on
the ribosome
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Solution G2
A.
B.
C.
D.
Match the following processes in protein
synthesis with the statements:
(1) Activation
(2) Initiation
(3) Elongation
(4) Termination
(3) Ribosome moves along mRNA to add new
amino acids to a growing peptide chain
(4) Completed peptide chain released
(1) A tRNA attaches to its specific amino acid
(2) tRNA binds to the AUG codon of the mRNA
on the ribosome
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Learning Check G3
The following section of DNA is used to build
a mRNA for a protein.
GAA-CCC-TTT
A. What is the corresponding base sequence
on mRNA?
B. What are the anticodons for the tRNAs?
C. What is the amino acid order in the
peptide?
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Solution G3
GAA-CCC-TTT
A. What is the corresponding base sequence
on mRNA?
CUU-GGG-AAA
B. What are the anticodons for the tRNAs?
GAA CCC UUU
C. What is the amino acid order in the
peptide?
Leu-Gly-Lys
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Mutations
• Caused by mutagens such as radiation and
chemicals
• Alter the nucleotide sequence of DNA
• Produce an incorrect series of codons in the
mRNA from the altered DNA
• A different codon binds with a different tRNA
• The protein has one or more wrong amino
acids in the peptide sequence
• Can result in a defective protein or enzyme
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Examples of Genetic Diseases
Galactosemia
Cystic fibrosis
Downs syndrome
Muscular dystrophy
Huntington’s disease
Sickle-cell anemia
Hemophilia
Tay-Sachs disease
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