BIOL 111 Chapter 16

End Exam 3 Content

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Nucleic Acids

Heredity thought to be passed on through Proteins

• more complex molecules store information
• DNA wound around proteins to hold them together
• most cell activities rely on protein-related reactions

Griffith's Experiment (1928)

• Living S cells (control) → kills mouse
• Living R cells (control) → healthy mouse
• Boiled S cells (control) → healthy mouse
• Mixture of Boiled S cells and living R cells → kills mouse

Conclusion: "Heritable substance" that changed cells

Avery, McCarty, and MacLeod's experiment (1944)

• DNA from S cells and mixed with R cells → kills mouse

Chargaff (1950)

• # adenines = # thymines
• # guanines = # cytosines

Hershey and Chase (1952)

• Used Bacteriophages to inject ³²P-labeled DNA and ³⁵S-labeled protein
• Let phages infect bacteria and then remove them from the mixture. DNA was passed into the bacteria, and it can reprogram cells

Wilkins and Franklin (1950s)

• X-ray diffraction: pass X-rays through DNA fibers to reveal helical structure

Watson and Crick (1953)

• "radically different structure"
  • 2 chains of deoxyribonucleic acids arranged 5' → 3' anti-parallel (in opposite directions)
  • Double-helix around central axis
  • Bases located on inside of helix
• "novel features"
  • purine must bond with pyrimidine, otherwise the structure would be too wide (purine + purine) or too narrow (pyrimidine + pyrimidine)
  • hydrogen bonds: 2 bonds in A–T, 3 bonds in G–C (easy to pull apart)
• other points:
  • Only possible with Deoxyribose sugar; impossible with Ribose due to extra Oxygen
    (RNA is normally found in one strand and is easily broken down)
  • Structure suggests replication mechanism

Replication of DNA

Proposed by Watson and Crick

Figure 16.17: Overview of DNA Replication

1. origin of replication: bubbles form where proteins can attach and begin the process
   • eukaryotes have multiple origins of replication along a linear strand of DNA; prokaryotes have a single that continues around their circular DNA
   • forms replication forks that expand laterally (left and right)
2. Important: Always moves 5' → 3'.
   Think 5 > 3 (5 is greater than 3)
3. RNA used as primer to begin DNA synthesis
4. Each new strand pieced together at same time using parent strand as template:
   • Leading Strand is formed continuously (on 3' end of origin of replication)
   • Lagging Strand formed in pieces–Okazaki Fragments (on 5' end of origin of replication)
5. Primer replaced with DNA
6. Gaps between (what used to be) primers and DNA are closed

Result is two daughter strands, each containing 1/2 parent + 1/2 new

Semi-Conservative Model

Parent strand split into two template strands, then daughter strand formed on parent strand

Meselson and Stahl

Tagged strands of DNA using two isotopes: ¹⁵N and ¹⁴N

1. Cultured Bacteria in ¹⁵N solution for some time
2. Transferred to ¹⁴N solution for some time
3. culture centrifuged for 20 mins to separate different densities of DNA

Proteins involved

Replication

Figure 16.14: Adding a nucleotide to a DNA strand

Helicase

unwinds and separates DNA

Single-strand binding protein

keeps DNA unwound and separate

Topoisomerase

prevents stress by allowing DNA to unwind

Primase

attaches complementary RNA primer to parental DNA

DNA Polymerase III

attaches nucleotides (not RNA) to parent strand

requires a template and a primer

synthesizes only from 5' to 3''; ratchets itself along the DNA

Leading & Lagging strands

DNA Polymerase I

replaces RNA Primer with DNA nucleotides

Ligase

binds fragments together

Proofreading/Repair

Figure 16.18: Repairing a damaged nucleotide

DNA Polymerase (I|III)

Fixes its own mistakes: "delete key"

Nuclease

cuts out damaged/incorrect DNA

DNA Polymerase I

fills gap with correct nucleotides

Ligase

reattaches the sugar-phosphate backbones

Telomeres and Telomerase

Won Nobel Prize in Physiology or Medicine in 2009

Telomeres: repetitive DNA at ends of chromosomes

• Act as a buffer to account for shortening DNA strands
• DNA Polymerase I can only replace primer with nucleotides if nucleotides are already present at the 5' end before primer. At ends of linear DNA, primer has no preceding nucleotides.
• allow for limited cell divisions without eroding genes

Telomerase: enzyme that lengthens telomeres; active in gametes and cancer cells, not in somatic (non-gamete) cells (where division continues indefinitely)