Gen Bio 2

• DNA Replication


  Cells need to make a copy of DNA before dividing so each daughter cell has a complete copy of genetic information
• DNA Replication


  • It is semi-conservative
  • One strand from each of the initial two strands end up in a daughter strand
  • Each strand serves as a template for a new strand
  • New strand is formed by complementary base-pairing of the correct nucleotide plus formation of a phosphodiester bond
  • Synthesis begins at replication origins
• Replication origins


  • About 100 nucleotides long, rich in A-T which are easier to pull apart because they have 2 rather than 3 hydrogen bonds
  • 1 in bacteria
  • 1000 in humans
• Replication: 1st Step


  1. Helicase enzyme unwinds part of DNA helix, stabilized by single-stranded binding proteins

  2. DNA gyrase prevents tangling upstream from the replication fork
• Replication: 2nd Step


  RNA Primase adds small section of RNA (RNA primer) to the 3' end of template DNA
• Replication: 3rd Step


  DNA polymerase III adds new complementary bases to build daughter DNA strand
• Replication: 4th Step


  DNA polymerase I replaces RNA primer with DNA
• Lagging strand


  Okazaki fragments joined by ligase
• Leading strand


  Continuous synthesis
• Initiator proteins


  • Bind at replication origins and recruit DNA replication machinery proteins
• DNA Polymerase


  Responsible for catalyzing synthesis of new strands
• Replication forks


  • Involve a leading and a lagging strand
  • DNA is directional, two strands are antiparallel
  • DNA polymerase can only synthesize from 5' to 3' direction adding new nucleotides to the 3' end
  • Lagging strand must be synthesized by first spooling out some template strand and then synthesizing in reverse
• Error-correction machinery


  • Mutations occur 1 in 10^9 nucleotides copied
  • DNA polymerase proofreads to make sure correct nucleotide is added, if not it excises and goes back to add the correct one
  • Mismatch repair machinery fixes incorrectly added nucleotides not found by DNA polymerase, detects nicks in newly created strand
• Damage to DNA continuously occurs
• Homologous recombination


  Uses similar sequences in nearby strands in order to fill in excised damage
• DNA


  The basis of heredity
• Transcription


  Messenger RNA or mRNA, is the RNA "copies" of genes ultimately used to synthesize proteins, although some RNA are the final product themselves
• RNA


  • Has ribose rather than deoxyribose sugars, uracil instead of thymine, single-stranded, and typically folds into unique shapes, less chemically stable
• 3 Types of RNA made from DNA

  • mRNA "messenger"
  • tRNA "transfer"
  • rRNA "ribosomal"
• Other kinds of RNA


  • Ribosomal RNA, rRNA - is RNA that becomes part of the ribosome
  • Transfer RNA, tRNA - is used to bring correct amino acids to the ribosome during protein synthesis
  • Micro RNAs (mRNAs) - are important in regulating gene-expression
• Messenger RNA (mRNA)


  Carries copies of instructions for assembling amino acids into proteins
• Ribosomes


  Made up of proteins and ribosomal RNA (rRNA)
• Transfer RNA (tRNA)


  Transfers each amino acid to the ribosome during protein construction
• Transcription


  1. Involves the synthesis of mRNA from DNA using RNA polymerase

  2. RNA molecules are produced by copying part of a nucleotide sequence of DNA into a complementary sequence in RNA

  3. RNA polymerase must unpair and unwind DNA as it is reading it

  4. Much less accurate than replication, errors of 1 in 10^4

  5. Protein synthesis can tolerate more errors

  6. Multiple RNAs can be sequences from the same gene at the same time
• Transcription in Bacteria


  1. RNA polymerase binds to specific regions of DNA called promoters, specific nucleotide sequence

  2. Promoter orient polymerase in a specific direction

  3. RNA polymerase binds to the promoter with the help of an accessory protein, called a sigma factor

  4. RNA transcript is synthesized by ribonucleotide triphosphate additions

  5. Synthesis stops at a terminator sequence, typically of a poly A-T stretches of DNA
• Transcription Steps


  1. RNA polymerase binds to the promoter site (TATA box) (start) on the DNA

  2. RNA polymerase adds RNA nucleotides complimentary to the DNA strand

  3. mRNA building is complete when the RNA polymerase reaches a Termination (stop) site on the DNA

  4. This strand of mRNA is EDITED before leaving the nucleus & carrying the code into the cytoplasm
• DNA never leaves the nucleus
• Not all the DNA is transcribed into mRNA
• mRNA editing


  The introns are cut out of RNA molecules, the exons are the spliced together to form mRNA
• Translation


  Information transmission, the decoding of an mRNA message into a polypeptide chain (protein), takes place on ribosomes in the cytoplasm
• Translation Steps


  1. Messenger RNA is transcribed in the nucleus, and then enters the cytoplasm where it attaches to a ribosome (to begin translation)

  2. As each codon (group of 3 nucleotides) of the mRNA molecule moves through the ribosome, the proper amino acid is brought into the ribosome by tRNA

  3. In the ribosome, amino acids are transferred to the growing polypeptide chain by the action of the tRNA (elongation)

  4. When the "STOP" codon is reached the mRNA uncouples from the ribosome
• Translation: Decoding the Message

  • 1. mRNA leave nucleus and enters ribosome
  • 2. mRNA codons read & tRNA brings matching amino acid to the ribosome
  • 3. The tRNA anticodon is complimentary to the mRNA codon
  • 4. Amino acids are strung together like beads on a necklace
  • 5. Amino Acids are held together by peptide bonds
  • 6. 1000 or more Amino Acids = protein
• Each tRNA molecule carries only one kind of amino acid, as determined by the anti-codon
• Many proteins can be made at once from the same RNA transcript by having multiple ribosomes bound to it -> polyribosomes
• James Watson and Francis Crick (1953) - DNA has a "double-helix" structure, A = T (2 hydrogen bonds), C = G (3 hydrogen bonds)
• Okazaki Fragments -> lagging strands (joined by ligase)
• Transcription = DNA -> mRNA
• Translation = RNA -> proteins
• Exon – coding region of mRNA, Introns – non-coding region
• Heredity


  Passing of traits from parent to offspring