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 Ser Leu
UAC AGA AGA GAG
• •• • •• • • • •• •
AUG UCU CUC 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
Match the following processes in protein
synthesis with the statements:
(1) Activation (2) Initiation
(3) Elongation (4) Termination
A. Ribosome moves along mRNA to add new amino
acids to a growing peptide chain
B. Completed peptide chain released
C. A tRNA attaches to its specific amino acid
D. tRNA binds to the AUG codon of the mRNA on
the ribosome 13
Solution G2
Match the following processes in protein
synthesis with the statements:
(1) Activation (2) Initiation
(3) Elongation (4) Termination
A. (3) Ribosome moves along mRNA to add new
amino acids to a growing peptide chain
B. (4) Completed peptide chain released
C. (1) A tRNA attaches to its specific amino acid
D. (2) tRNA binds to the AUG codon of the mRNA
on the ribosome 14
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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