1. 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. Protein Synthesis, Transcription
The RNA in a cell facilitates the synthesis of proteins in the following steps:
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. 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. 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. 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. The Genetic Code: mRNA Codons

7. 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. 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. 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. Complementary Sequences
Table 21.8 summarizes the nucleotide and amino acid sequences in protein synthesis.

11. 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. 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