Microbiology chapter 7

• DNA: Blueprint of Life
• DNA


  • Incredible diversity of life dictated by information within DNA
• Nucleotides


  Each containing a nucleobase: Adenine (A), Thymine (T), Cytosine (C), Guanine (G)

  
• 3 nucleotides encode a specific amino acid
• Amino acids make up protein
• Sequence of amino acids determines structure and function of protein
• Types of proteins


  • Structural proteins
  • Enzymes
• Genome


  Complete set of genetic information, including chromosomes and (technically) plasmids
• All cells have a DNA genome, while viruses may have an RNA genome
• Gene


  Functional unit that encodes a gene product, usually a protein
• Genomics


  Study of nucleotide sequence of DNA

  
• Tasks cells must accomplish to multiply

  • DNA replication
  • Gene expression: Transcription and Translation

  
• Central dogma of molecular biology

  Information flow from DNA to RNA to protein

  
• DNA


  • Forms double-stranded helix
  • Carbon atoms of the pentose sugar are numbered
  • Nucleotides joined between 5'PO4 and 3'OH
  • Forms sugar-phosphate backbone
  • Single DNA strand has a 5' end and 3' end

  
• Base-pairing


  • Adenine (A) to thymine (T) (two hydrogen bonds)
  • Cytosine (C) to guanine (G) (three hydrogen bonds)

  
• Strands are anti-parallel; oriented in opposite directions
• RNA (ribonucleic acid)


  • Ribose instead of deoxyribose
  • Uracil in place of thymine
  • Usually shorter single strand
  • Synthesized from DNA template strand

  
• Types of RNA


  • Messenger RNA (mRNA)
  • Ribosomal RNA (rRNA)
  • Transfer RNA (tRNA)

  
• Cells regulate expression of certain genes through rapid degradation of mRNA transcripts

  
• DNA replication


  1. Bidirectional from origin of replication

  2. Two replication forks meet at terminating site when process complete

  
• Semiconservative DNA replication


  DNA contains one original, one newly synthesized strand

  
• Initiation of DNA replication

  1. DNA gyrase and helicases bind to origin of replication

  2. Break and unwind DNA helix

  3. Expose single-stranded region that can act as template

  4. Primases synthesize short regions of RNA called primers

  5. Requires coordinated action of many enzymes and other components

  6. Many of the enzymes form "assembly lines" called replisomes

  
• DNA polymerases


  • Synthesize in 5' to 3' direction
  • Can only add nucleotides, not initiate
  • Require primers at origin of replication

  
• DNA replication


  1. Helicases "unzip" DNA strands

  2. Leading strand synthesized continuously

  3. Lagging strand synthesized discontinuously

  4. Production of Okazaki fragments

  5. DNA ligase forms covalent bond between adjacent nucleotides

  
• Components of DNA replication in bacteria

  • DNA gyrase
  • DNA ligase
  • DNA polymerases
  • Helicases
  • Okazaki fragment
  • Origin of replication
  • Primase
  • Primer
  • Replisome

  
• Transcription


  1. RNA polymerase synthesizes single-stranded RNA from DNA template

  2. RNA polymerase binds to sequence called a promoter

  3. Synthesizes in 5' to 3' direction

  4. Can initiate without primer

  5. Transcription stops at sequence called a terminator

  
• Promoter


  Sequence that RNA polymerase binds to initiate transcription

  
• RNA sequence is complementary, antiparallel to DNA template strand

  
• Prokaryotic mRNA transcripts


  • Monocistronic (one gene)
  • Polycistronic (multiple genes)

  
• Sigma (σ) factor


  Component of RNA polymerase that recognizes the promoter regions

  
• Initiation of Transcription


  1. Sigma (σ) factor on RNA polymerase recognizes promoter

  2. Synthesis of sigma (σ) factors controls transcription of sets of genes

  3. Eukaryotic cells, archaea use transcription factors to recognize promoters

  
• Once RNA polymerase has moved beyond promoter, another RNA polymerase can bind

  
• Components of Transcription in Bacteria

  • (-) strand of DNA
  • (+) strand of DNA
  • Promoter
  • RNA polymerase
  • Sigma (σ) factor
  • Terminator

  
• Translation


  Process of decoding information in mRNA
• Components of Translation


  • mRNA, ribosomes (contain rRNA), tRNAs, and accessory proteins
  • mRNA is temporary copy of genetic information

  
• Genetic code


  More than one codon can specify a specific amino acid; code is degenerate
• The Genetic Code


  • Codons and their corresponding amino acids
• Ribosomes


  Serve as translation "machines" that align and form peptide bonds between amino acids
• Transfer RNAs (tRNAs)


  Deliver correct amino acid, each has specific anticodon sequence that base-pairs with codon

  
• Initiation of Translation


  1. Part of ribosome binds to mRNA sequence called the ribosome-binding site

  2. First AUG after that site serves as start codon, where complete ribosome assembles

  3. Initiating tRNA brings altered form of methionine and occupies P-site