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3. There are 64 different possible codons in the genetic code, and the same genetic code is
employed, for the most part, by every living creature.
B. Translating the RNA Message
1. In translation, organelles called ribosomes use the mRNA transcript to direct the
synthesis of a protein.
2. Translation occurs in the cytoplasm in conjunction with ribosomes, which are made up
of proteins and ribosomal RNA (rRNA).
3. Ribosomes hold the mRNA in position.
4. A third type of RNA, called transfer RNA (tRNA), brings amino acids to the ribosome.
5. On one end of the tRNA is an anticodon sequence, which is a sequence of three
nucleotides complementary to an mRNA codon.
6. On another end of the tRNA molecule is a binding site for the amino acid that
corresponds to the anticodon.
7. Special activating enzymes match amino acids in the cytoplasm with their tRNAs.
E. Making the Polypeptide
1. After mRNA attaches to a ribosome, codons are positioned in each of three sites on the
ribosome: the A, P, and E sites.
2. The mRNA passes through the ribosome three-nucleotides at a time, exposing each
codon at the A site, where tRNA molecules can bind.
3. tRNAs whose anticodons match the exposed mRNA codon bring their amino acids in
and bind at the A site on the ribosome.
4. As the ribosome proceeds along the mRNA, the old tRNA is moved to the P site, where
peptide bonds form between the incoming amino acid and the growing polypeptide
chain; at the same time, a new codon is exposed at the A site, and a new tRNA binds
with its corresponding amino acid.
5. As the ribosome proceeds again, the tRNA at the P site moves to the E site, where it is
released.
6. As the process continues, more amino acids are brought in and added to the chain.
7. When a “stop” codon is encountered, the process is finished, the ribosome complex falls
apart, and the completed protein is released into the cell.
12.4 Gene Expression (p. 211; Figs. 12.9–12.12)
A. Architecture of the Gene
1. Prokaryotic DNA is made up of a continuous sequence of genes with no interruptions.
2. Eukaryotic DNA is constructed differently because it possesses gene sequences that
code for amino acids, called exons, plus intervening, extraneous sequences of
nucleotides, called introns.
3. In eukaryotes, transcription first produces a primary RNA transcript; a 5´ cap and a 3´
poly-A tail are added to the primary transcript to protect it from degradation.
4. Enzyme complexes then remove the introns and join the remaining exons to form the
mature mRNA.
5. Alternative splicing allows the exons to be joined in different orders, producing many
different possible mRNAs.
B. Protein Synthesis
1. In prokaryotes, there is no nucleus and no barrier between transcription and translation;
mRNA can be translated as it is being transcribed.
2. In eukaryotes, transcription and RNA processing take place in the nucleus, and then the
mRNA travels to the cytoplasm for translation.
Regulating Gene Expression (p. 214)
12.5 Transcriptional Control in Prokaryotes (p. 214; Figs. 12.13–12.15)
A. Cells must have the ability to regulate which genes will be expressed and how often
expression occurs.
B. How Prokaryotes Turn Genes Off and On
1. Prokaryotes use regulatory proteins to control when genes are transcribed.