1. Nucleic Acids RNA and Transcription The Genetic Code Protein Synthesis
Mutations

2. Types of RNA Messenger RNA (mRNA) Transfer RNA (tRNA)
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

3. 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

4. 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

5. 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

6. 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

7. Solution G1 GGC = Glycine GAC = Aspartic acid
CUC = Leucine GUA =Valine
GCC = Alanine CGC = Arginine
GCC –GUA –GAC
Ala – Val – Asp

8. Activation of tRNA Each tRNA binds to its specific amino acid Pro Pro
GGG
GGG

9. Anticodons on tRNA A three-base sequence on each tRNA
Complements a triplet on mRNA
anticodon
codon on mRNA
Pro
GGG
CCC

10. 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

11. Peptide Formation Peptide starts to form UAC AGA AGA GAG
• • • • • • • • • • • •
AUG UCU CUC UCU CUC UUU
Met
Met
Ser
Ser
Leu

12. 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

13. 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

14. 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

15. 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?

16. 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

17. 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

18. Examples of Genetic Diseases
Galactosemia
Cystic fibrosis
Downs syndrome
Muscular dystrophy
Huntington’s disease
Sickle-cell anemia
Hemophilia
Tay-Sachs disease