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3.5Summary

  • Protein genes are DNA regions that determine the amino acid sequences of proteins.
  • rRNA and tRNA are categorized as non-coding RNA without protein information, and function as RNA. This RNA is also transcribed from rRNA and tRNA genes on DNA.
  • The information unit of protein genes is a three-base sequence on a DNA strand, which corresponds to one amino acid.
  • Gene function (or gene expression) refers to the process by which RNA is synthesized based on genetic information and a protein is then synthesized using the RNA information.
  • In RNA synthesis, the base sequence of a gene is read using one of the DNA double strands as a template, thereby synthesizing an RNA strand with a sequence complementary to the DNA strand.
  • RNA synthesis is known as transcription because DNA sequence information is copied to the RNA sequence.
  • An enzyme that synthesizes RNA is called an RNA polymerase.
  • A DNA region to which RNA polymerase binds is called a promoter.
  • The roles of promoters are to recruit RNA polymerase and determine the initiation point of transcription and the DNA strand to be used as the template.
  • A sequence involving a gene with protein information is transcribed to an mRNA sequence. mRNA types are as numerous as gene types, and both correspond to the number of protein types.
  • In prokaryotes, transcription and translation are coupled.
  • mRNA in eukaryotes is first transcribed in the form of precursors called pre-mRNA, which undergo modifications in the nucleus (such as capping, poly-A addition and splicing) to become complete mRNA. This is then transferred to the cytoplasm, where it is used for protein synthesis.
  • A three-base set corresponding to one amino acid on an mRNA strand transcribed from DNA is called a gene codon.
  • The first AUG on mRNA encodes methionine, and is also the initiation codon for protein synthesis.
  • Protein synthesis occurs on granules called ribosomes.
  • A three-base set of mRNA (i.e., a codon) and a three-base set of anticodon aminoacyl-tRNA (an amino acid bound with tRNA) form pairs on a ribosome, through which amino acids are arranged by tRNA in accordance with the order of the mRNA codes.
  • A reaction is continuously repeated in which an amino acid and tRNA are dissociated and amino acids are then connected together. As a result, amino acids are linked following the order of the mRNA codes, thus forming proteins.
  • There are three types of code (known as termination codons) that do not correspond to any amino acids. Protein synthesis stops at one of the termination codons on mRNA.

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