Concept 10.1 Genetics Shows That Genes Code for Proteins
- Studies of human genetic diseases such as alkaptonuria linked genes to proteins. Review Figure 10.1
- Hemoglobin abnormalities demonstrate that mutations can alter the sequence of amino acids in proteins. Review Figure 10.2
- Genes are expressed via transcription and translation. During transcription, the information in a gene is copied into a complementary RNA sequence. During translation, this RNA sequence is used to create the amino acid sequence of a polypeptide. Review Figure 10.3 and ACTIVITY 10.1
- The product of transcription is messenger RNA (mRNA). Transfer RNA (tRNA) molecules translate the genetic information in the mRNA into a corresponding sequence of amino acids.
- Ribosomal RNA (rRNA) helps provide structure to the ribosome and acts as a ribozyme that catalyzes peptide bond formation between amino acids during protein synthesis.
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Concept 10.2 DNA Expression Begins with Its Transcription to RNA
- In a given gene, only one of the two strands of DNA (the template strand) acts as a template for transcription. RNA polymerase is the catalyst for transcription.
- RNA transcription from DNA proceeds in three steps: initiation, elongation, and termination. Initiation requires a promoter to which RNA polymerase binds. Elongation of the RNA molecule proceeds by the addition of nucleotides to the 3′ end of the molecule. Review Figure 10.5 and ANIMATED TUTORIAL 10.1
- After transcription, eukaryotic pre-mRNA is spliced to remove introns. Review Figure 10.6, Figure 10.9 and ANIMATED TUTORIAL 10.2
- Eukaryotic mRNA is also modified by the addition of a 5′ cap and, at the 3′ end, a poly A tail.
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Concept 10.3 The Genetic Code in RNA Is Translated into the Amino Acid Sequences of Proteins
- Experiments involving synthetic mRNAs and protein synthesis in the test tube established the genetic code. Review Figure 10.10 and ANIMATED TUTORIAL 10.3
- The genetic code consists of triplets of mRNA nucleotide bases (codons) that correspond to 20 specific amino acids and to start codons and stop codons.
- The genetic code is redundant (an amino acid may be represented by more than one codon) but not ambiguous (no single codon represents more than one amino acid). Review Figure 10.11 and ACTIVITY 10.2
- Mutations in the coding regions of genes can be silent, missense, nonsense, or frame-shift mutations. Review Figure 10.12 and ANIMATED TUTORIAL 10.4
- Review ANIMATED TUTORIAL 10.5
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Concept 10.4 Translation of the Genetic Code Is Mediated by tRNAs and Ribosomes
- Transfer RNA (tRNA) mediates between mRNA and amino acids during translation at the ribosome.
- Each tRNA species has an amino acid attachment site and an anticodon that is complementary to a specific mRNA codon. Review Figure 10.13
- A specific synthase enzyme charges each tRNA with its specific amino acid.
- Three sites on the large subunit of the ribosome interact with tRNA anticodons. The A site is where the charged tRNA anticodon binds to the mRNA codon. The P site is where the tRNA adds its amino acid to the growing polypeptide chain. The E site is where the tRNA is released. Review Figure 10.14
- Translation occurs in three steps: initiation, elongation, and termination. Review Figure 10.15, Figure 10.16, and Figure 10.17
- In a polyribosome, or polysome, more than one ribosome moves along a strand of mRNA at one time. Review Figure 10.18
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Concept 10.5 Proteins Are Modified after Translation
- Signal sequences are short sequences of amino acids that direct polypeptides to their cellular destinations.
- These destinations include the nucleus and other organelles, which proteins enter after being recognized and bound by surface receptors.
- If a ribosome begins translating a polypeptide with an N-terminus RER signal sequence, it pauses and then resumes translation after attachment to a receptor in the RER membrane. Review Figure 10.19
- Posttranslational modifications of polypeptides include proteolysis, in which a polypeptide is cut into smaller fragments; glycosylation, in which sugars are added; and phosphorylation, in which phosphate groups are added. Review Figure 10.21
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