Rna

• The Replication of DNA

  1. Occurs during the S phase of the cell cycle

  2. Ensures proper duplication of genetic material

  3. Allows distribution of chromosome copies to daughter cells
• Human cells replicate DNA at a rate of 3,000 nucleotides per minute
• DNA replication models


  • Semiconservative
  • Conservative
  • Dispersive
• Semiconservative model


  • Each DNA strand serves as a template
  • New DNA molecules consist of one old and one new strand
• Conservative model


  • Entire double-stranded DNA serves as a template
  • Produces an entirely new DNA molecule while conserving the original copy
• Dispersive model


  • DNA strands separate and break down into fragments
• Semiconservative nature of DNA replication

  • Proposed by Watson and Crick
  • Each new DNA double helix consists of one complete "old" strand and one complete "new" strand
  • Original DNA partially conserved
• Meselson & Stahl experiment


  1. Used Escherichia coli bacteria

  2. Cells initially grown in heavy nitrogen isotope (N-15)

  3. Later generations grown in lighter nitrogen isotope (N-14)
• Prokaryotic models are usually used to describe the basic processes in the replication of DNA
• Studies usually involve E. coli as their model organism
• Molecules needed for DNA replication

  • DNA template
  • Deoxyribonucleotides
  • Enzymes and other proteins
• Origin of replication


  • Specific genome sequence where DNA replication begins
  • Identified by John Cairns in E. coli's circular chromosome
• Prokaryotic cells


  • Consist of 100-200 nucleotide pairs
  • E. coli's oriC is 245 bp long and well characterized
  • Replication starts with the breaking of hydrogen bonds, with A-T regions breaking more easily due to fewer hydrogen bonds
• Eukaryotic cells


  • Have multiple origins of replication to allow rapid replication despite large, linear chromosomes
• Initiation of Replication


  1. Initiator Proteins bind to the origin, unwinding a small DNA segment to start replication

  2. DNA Helicase breaks hydrogen bonds between DNA strands, allowing strand separation

  3. Single-Strand-Binding Proteins (SSBPs) stabilize unwound DNA, preventing re-annealing

  4. DNA Gyrase (Bacteria) / Topoisomerase II (Humans) relieve supercoiling and strain at the replication fork
• Types of RNAs


  • Messenger RNA (mRNA)
  • Transfer RNAs (tRNA)
  • Ribosomal RNAs (rRNAs)
  • Small nuclear RNAs (snRNA)
  • Micro RNAs (miRNA)
• Messenger RNA (mRNA)


  • Intermediate molecule during gene expression
  • Contains a copy of the information in DNA that must be translated into proteins
• Transfer RNAs (tRNA)


  Serve as adaptor molecules during protein synthesis
• Ribosomal RNAs (rRNAs)


  Structural and functional components of the ribosomes
• Small nuclear RNAs (snRNA)


  Function during the post-processing of the mRNA after transcription
• Micro RNAs (miRNA)


  Small RNAs that are essential in the regulation of gene expression
• Transcription


  Process by which an RNA molecule is synthesized using a DNA template
• Gene


  Sequence of DNA nucleotides that code for a specific protein
• Translation


  Process that converts mRNA information into amino acid sequences in proteins
• George Beadle and Edward Tatum's Nobel-prize winning study elaborated on the genetic control of enzymatic pathways
• Genetic code


  • Read in nucleotide triplets or codons
  • Each codon codes for one amino acid in the resulting polypeptide
  • Has one start signal (AUG) and three stop signals (UAA, UAG, UGA)
  • Nonoverlapping - a nucleotide is part of only one codon
  • Comma-less or comma-free - no punctuation among the codons
  • Degenerate - an amino acid can be coded by more than one codon
  • Unambiguous - one codon codes for only one amino acid
  • Polarity - fixed direction of 5' -> 3'
  • Near universal - same correspondence between codons and amino acids in all living organisms