13.1 How Does a Cell Access Genetic Information?
1. A gene’s information must be transcribed into RNA before it is translated into a sequence of amino acids. RNA is a single-stranded nucleic acid similar to DNA, but it contains uracil and ribose rather than thymine and deoxyribose.
2. In bacteria, operons coordinate expression of grouped genes whose encoded proteins participate in the same metabolic pathway. In multicellular organisms, transcription factors regulate which genes are transcribed and when in a particular cell type. These factors have certain common regions called motifs.
13.2 RNA Orchestrates Protein Synthesis
3. Transcription begins when transcription factors help RNA polymerase bind to a promoter on the DNA template strand, and then builds an RNA molecule. After transcription, introns are cut out of RNA and the remaining exons spliced together.
4. Several types of RNA participate in translation. Messenger RNA (mRNA) carries a protein-encoding gene’s information. Ribosomal RNA (rRNA) associates with certain proteins to form ribosomes, which support and help catalyze protein synthesis. Transfer RNA (tRNA) has an anticodon sequence complementary to a particular mRNA codon on one end and a particular amino acid at the other end.
13.3 How Does a Cell Build a Protein Using Genetic Information?
5. Each group of three consecutive mRNA bases is a codon that specifies a particular amino acid. The correspondence between codons and amino acids constitutes the genetic code. Of the 64 different codons, 61 specify amino acids and three signal the end of translation. Degenerate codons encode the same amino acid. The genetic code is nonoverlapping, triplet, and identical in all species. Scientists deciphered the code by exposing synthetic RNA molecules to the contents of E. coli cells and noting which amino acids formed peptide chains. Other experiments confirmed the triplet nature of the code.
6. Translation requires tRNA, ribosomes, energy storage molecules, enzymes, and protein factors. An initiation complex forms when mRNA, a small ribosomal subunit, and a tRNA usually carrying methionine join. A large ribosomal subunit joins the small one. Next, a second tRNA binds by its anticodon to the next mRNA codon, and its amino acid bonds with the methionine the first tRNA brought in. tRNAs continue to add amino acids, elongating a polypeptide. The ribosome moves down the mRNA as the chain grows.
7. When the ribosome reaches a “stop” codon, it separates into its subunits and is released, and the new polypeptide breaks free. Chaperone proteins help fold the polypeptide, and it may be shortened or combined with others.
13.4 Mutation - Genetic Misinformation
8. A mutation adds, deletes, alters, or moves nucleotides. A phenotype that a mutation alters is mutant. A gene can mutate spontaneously, particularly if it contains regions of repetitive DNA sequences. Mutagens are chemicals or radiation that increase the mutation rate.
9. A germinal mutation originates in meiosis and affects all cells of the progeny. A somatic mutation originates in mitosis and affects a subset of cells. A point mutation alters a single DNA base, and may be missense (substituting one amino acid for another) or nonsense (substituting a “stop” codon for an amino acid–coding codon). Altering the number of bases in a gene (a frameshift mutation) may disrupt the reading frame, altering the amino acid sequence of the gene’s product. Expanding triplet repeat mutations cause some inherited illnesses. Transposable elements move, possibly disrupting a gene’s function.
10. Some mutations are silent. A mutation in the third position of a degenerate codon can substitute the same amino acid. A mutation in the second codon position can replace an amino acid with a similarly shaped one. A mutation in a nonvital part of a protein may not affect function.