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ribosomal subunit ribosomal subunit

"2 Large ribosomal subunit completes initiation complex

"2 Large ribosomal subunit completes initiation complex

B. elongation

^ A tRNA binds to the ribosome at the A site an enzyme forms a peptide bond an enzyme forms a peptide bond

"2 The ribosome advances one codon along the mRNA; second tRNA moves to P site first tRNA released

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"2 The ribosome advances one codon along the mRNA; second tRNA moves to P site

another tRNA moves into A site

A new amino acid is added to the growing polypeptide lengthening

Figure 13.6 Translation. A. During initiation, the components necessary for translation bind together. These include the large and small ribosomal subunits, messenger RNA, and the initiator transfer RNA carrying the amino acid methionine. B. During elongation, the ribosome moves along the messenger RNA. Transfer RNA molecules bring amino acids to the ribosome, where a polypeptide chain is constructed. C. When the ribosome reaches a stop codon, translation terminates. The large and small ribosomal subunits, messenger RNA, transfer RNA, and polypeptide separate from each other.

all cells. This mutation has now become a permanent feature of that plant's lineage. Germ-line mutations are important for the genetic improvement of horticultural and agronomic plants. They are responsible for variability in all traits, including flower color and fragrance, fruit taste and texture, grain yield and quality, and disease and stress tolerance.

While we tend to think of mutations as "bad," they are an essential feature of DNA. All the genetic variability in nature has arisen as a result of mutation. The accumulation of mutations is the mechanism that drives evolution of species (see Chapters 15 and 16). That variability was essential for the first plants, which were in the ocean, to become established on land; it continues to be necessary for plants to survive and compete in an ever-changing environment.

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