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Biology I for Non-Majors

Learn about the process of DNA transcription and translation, including initiation, elongation, and termination. Understand the steps of mRNA processing and the role of introns and exons. Discover how translation machinery, including ribosomes and tRNAs, work together to synthesize proteins. Explore the genetic code, mutations, and their impact on genetic variation and diseases.

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Biology I for Non-Majors

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  1. Biology I for Non-Majors DNA Transcription and Translation

  2. DNA is Transcribed to RNA • Initiation • Beginning of transcription when the enzyme RNA polymerase binds to a region of a gene called the promoter • Signals the DNA to unwind so the enzyme can ‘‘read’’ the bases in one of the DNA strands • Elongation • Addition of nucleotides to the mRNA strand • Termination • Ending of transcription, and occurs when RNA polymerase crosses a stop (termination) sequence in the gene

  3. Pre-RNA and mRNA • After transcription, eukaryotic pre-mRNAs must undergo several processing steps before they can be translated • mRNA processing involves several steps • A cap is added to the 5′ end of the growing transcript by a phosphate linkage • An enzyme called poly-A polymerase adds a string of approximately 200 A residues, called the poly-A tail to the pre-mRNA • Eukaryotic genes are composed of exons and introns • Intronsare removed from the pre-mRNA during processing • The process of removing introns and reconnecting exons is called splicing

  4. Translation • The process of translation, or protein synthesis, involves the decoding of an mRNA message into a polypeptide product • Amino acids are covalently strung together by interlinking peptide bonds • Polypeptides are formed when the amino group of one amino acid forms an amide (i.e., peptide) bond with the carboxyl group of another amino acid • In addition to the mRNA template, many molecules and macromolecules contribute to the process of translation • An mRNA template • Ribosomes • tRNAs • various enzymatic factors

  5. Translation Machinery • Ribosomes • A complex macromolecule composed of structural and catalytic rRNAs, and many distinct polypeptides • tRNAs • Structural RNA molecules that were transcribed from genes by RNA polymerase III • Actually “translate” the language of RNA into the language of proteins • Aminoacyl tRNA synthetases • Each tRNA molecule is linked to its correct amino acid by a group of enzymes called aminoacyl tRNA synthetases • At least one type of aminoacyl tRNA synthetase exists for each of the 20 amino acids

  6. Genetic Code • Amino acids were encoded by nucleotide triplets • The genetic code was degenerate • A given amino acid could be encoded by more than one nucleotide triplet • Of the 64 possible mRNA codons: • 3 specify the termination of protein synthesis • Also known as nonsense or stop codons • 61 specify the addition of amino acids to the polypeptide chain • The genetic code is universal

  7. Translation • Initiation • Initiation complex • Interacts with the start codon • Elongation • 3 compartments: • A (aminoacyl) site binds incoming charged aminoacyl tRNAs • P (peptidyl) site binds charged tRNAs carrying amino acids that have formed peptide bonds with the growing polypeptide chain but have not yet dissociated from their corresponding tRNA • E (exit) site releases dissociated tRNAs so that they can be recharged with free amino acids

  8. Translation • Termination • Occurs when a nonsense codon (UAA, UAG, or UGA) is encountered

  9. Central Dogma of Life

  10. Mutations • Mutations are changes in DNA sequence • Either due to mistakes when the DNA is copied or as the result of environmental factors such as UV light and cigarette smoke • Contribute to genetic variation within species • Can also be inherited, particularly if they occur in a germ cell (reproductive egg or sperm) • Mutations that have a positive effect are more likely to be continually passed on • Mutations can also disrupt normal gene activity and cause diseases, like cancer

  11. Major Types of Mutations • Induced mutations are those that result from an exposure to chemicals, UV rays, x-rays, or some other environmental agent • Spontaneous mutations occur without any exposure to any environmental agent • A result of natural reactions taking place within the body • Silent mutations have no impact on an organism • Point mutations affect a single base pair • Transition substitutions refers to a purine or pyrimidine being replaced by the same • Insertions add a base • Deletions remove a base

  12. Quick Review • Outline the basic steps of DNA transcription into RNA • What is the key difference between pre-RNA and mRNA? • List the components of translation • What is the genetic code and its key features? • Outline the basics of translation of RNA into proteins. • What is the central dogma of life? • What is a mutation? • How do mutations occur? • List the major types of DNA mutations

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