GENETIC CODE

1. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
21.6 The Genetic Code and Protein
Synthesis
The function of the different
types of RNA in the cell is
to facilitate the task of
synthesizing proteins.
An activated tRNA with
anticodon AGU bonds to
serine at the acceptor stem.
Learning Goal Use the genetic
code to write the amino acid
sequence for a segment of mRNA.

2. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Protein Synthesis, Transcription
The RNA in a cell facilitates the synthesis of proteins in the following steps:
1. Genetic information encoded in DNA is transcribed and processed into
mature mRNA molecules. These molecules move out of the nucleus to
the ribosomes in the cytosol.

3. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Protein Synthesis, Transcription
2. mRNA leaves the nucleus and attaches to a ribosome, where the
genetic information in the mRNA is translated into a sequence of
amino acids in a protein.
Core Chemistry Skill Writing the Amino Acid for an mRNA Codon

4. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Genetic Code
The genetic code consists of sets of three nucleotides
(triplets) in mRNA called codons that specify the amino
acids and their sequence in the protein.
• There is a different codon for all 20 amino acids
needed to build a protein.
The genetic code
• has stop signals, UGA, UAA, and UAG, that signal the
termination of protein synthesis.
• has a start codon, AUG, that signals the start of
protein synthesis.

5. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Codons and Amino Acids
To determine the amino acid sequence coded in the following
section of an mRNA,
5′ CCU AGC GGA CUU 3′
1. refer to the genetic code for the identity of the amino acids
CCU = Proline AGC = Serine
GGA = Glycine CUU = Leucine
2. mRNA section codes for the amino acid sequence are
written in the following order:
Pro—Ser—Gly—Leu

6. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
The Genetic Code: mRNA Codons

7. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Protein Synthesis, Activation
3. Molecules of tRNA pick up specific amino acids according to their
anticodons. tRNA activation occurs when the correct amino acid is
attached to each tRNA.

8. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Protein Synthesis, Initiation and Chain
Elongation
4. A start codon binds the first tRNA carrying the amino acid methionine
to the mRNA. As the ribosome moves along the mRNA, a new amino
acid forms a peptide bond to the growing protein chain.

9. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Protein Synthesis, Termination
5. Eventually, a ribosome encounters a stop codon with no
corresponding tRNAs, which signals the termination of polypeptide
synthesis and its release from the ribosome.
tRNA reenters the pool of free tRNA, ready to be recharged with a
new amino acid.

10. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Complementary Sequences
Table 21.8 summarizes the nucleotide and amino acid
sequences in protein synthesis.

11. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Study Check
Write the amino acids coded for by a section of mRNA with
the following base sequence:
5ʹ GCC GUA GAC 3ʹ
Some possible codons to use are the following:
GGC = Glycine GAC = Aspartic acid
CUC = Leucine GUA = Valine
GCC = Alanine CGC = Arginine

12. General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Solution
Write the amino acids coded for by a section of mRNA with
the following base sequence:
5ʹ GCC GUA GAC 3ʹ
Ala—Val—Asp