Chap 7

Return to Essential Concepts

  • The flow of genetic information in all living cells is DNA —> RNA —> protein. The conversion of the genetic instructions in DNA into RNAs and proteins is termed gene expression.
  • To express the genetic information carried in DNA, the nucleotide sequence of a gene is first transcribed into RNA. Transcription is catalyzed by the enzyme RNA polymerase. Nucleotide sequences in the DNA molecule indicate to the RNA polymerase where to start and stop transcribing.
  • RNA differs in several respects from DNA. It contains the sugar ribose instead of deoxyribose and the base uracil (U) instead of thymine (T). RNAs in cells are synthesized as single-stranded molecules, which often fold up into precise three-dimensional shapes.
  • Cells make several different functional types of RNAs, including messenger RNA (mRNA), which carried the instructions for making proteins; ribosomal RNA (rRNA), which is a component of ribosomes; and transfer RNA (tRNA), which acts as an adaptor molecule in protein synthesis.
  • Transcription begins at DNA sites called promoters. To initiate transcription, eukaryotic RNA polymerases require the assembly of a complex of general transcription factors at the promoter, whereas bacterial RNA polymerase requires only an additional subunit, called sigma factor.
  • In eukaryotic DNA, most genes are composed of a number of smaller coding genes (exons) interspersed with noncoding regions (introns). When a eukaryotic gene is transcribed from DNA into RNA, both the exons and introns are copied.
  • Introns are removed from the RNA transcripts in the nucleus by a process of RNA splicing. In a reaction catalyzed by small ribonucleo-protein complexes known as snRNPs, the introns are excised from the RNA and the exons are joined together.
  • Eukaryotic mRNAs go through several additional RNA processing steps before they leave the nucleus, including RNA capping and poly-adenylation. These reactions, along with splicing take place as the RNA is being transcribed. The mature mRNA is then transported into the cytoplasm.
  • Translation of the nucleotide sequence of mRNA into a protein takes place in the cytoplasm on large ribonucleoprotein assemblies called ribosomes. As the mRNA is threaded through a ribosome, its message is translated into protein.
  • The nucleotide sequence in mRNA is read in sets of three nucleotides (codons), each codon corresponding to one amino acid.
  • The correspondence between amino acids and codons is specified by the genetic code. The possible combinations of the 4 different nucleotides in RNA give 64 different codons in the genetic code. Most amino acids are specified by more than one codon.
  • tRNA acts as an adaptor molecule in protein synthesis. Enzymes called aminoacyl-tRNA syntetases link amino acids to their appropriate tRNAs. Each tRNA contains a sequence of three nucleotides, the anticodon, which matches a codon in mRNA by complementary base-pairing between codon and anticodon.
  • Protein synthesis begins when a ribosome assembles at an initiation codon (AUG) in mRNA, a process that is regulated by proteins called translation initiation factors. The completed protein chain is released from the ribosome at a stop codon (UAA, UAG, or UGA) is reached.
  • The stepwise linking of amino acids into a polypeptide chain is catalyzed by an rRNA molecule in the large ribosomal subunit. Thus, the ribosome is an example of a ribozyme, an RNA molecule that can catalyze a chemical reaction.
  • The degradation of proteins in the cell is carefully controlled. Some proteins are degraded in the cytosol by large protein complexes called proteasomes.
  • From our knowledge of present-day organisms and the molecules they contain, it seems likely that living systems began with the evolution of RNA molecules that could catalyze their own replication.
  • It has been proposed that, as cells evolved, the DNA double helix replaced RNA as a more stable molecule for storing genetic information, and proteins replaced RNAs as a major catalytic and structural component. However, important reactions such as peptide bond formation are still catalyzed by RNA; these are thought to provide a glimpse into an ancient, RNA-based world.

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