chapter 13 ~ Bio101

  • X-rays and Molecular Structure

    • X-rays can be used to determine molecular structures, including those of DNA.

    • The technique provides a pattern of dots rather than a direct image of the molecule.

    • Mathematical and computational methods are required to interpret the patterns.

  • Rosalind Franklin’s Contributions

    • Her specialty was X-ray crystallography, used to elucidate DNA's structure.

    • Discovered that DNA is a double helix, along with several other findings.

    • Mapped dimensions of DNA: 2 nanometers wide, helical turning at 3.4 nanometers intervals.

    • Overlooked in textbooks for years; Watson and Crick later used her data without credit in their DNA model, winning the Nobel Prize.

  • Objectives in DNA Research

    • Understand key experiments that led to the conclusion that DNA is genetic material.

    • Review structure of DNA, leading to a deeper understanding of its replication process.

  • Griffith’s Experiment

    • Worked on vaccines for pneumonia during early 1900s; utilized mice for experimentation.

    • Found pathogenic (S cells) bacteria killed mice when injected: control experiment.

    • Harmless bacteria (R cells) did not kill mice: immune system recognized them.

    • Heated S cells (pathogenic but dead) were injected into mice, which survived.

    • Combined dead S cells with live R cells led to dead mice, indicating genetic transformation.

    • Discovered the concept of transformation: genetic information can be absorbed from the environment, changing phenotype.

  • Hershey-Chase Experiment

    • Used viruses composed of DNA and protein to confirm DNA as genetic material.

    • Experiment used radioactive labeling to trace DNA (phosphorus) versus protein (sulfur).

    • Conclusion: DNA enters bacterial cells, determining heredity.

  • Chargaff’s Rules

    • Analyzed nucleotide bases in various species; observed base composition differences.

    • A = T and C = G ratios consistently found.

  • DNA Structure

    • Comprised of nucleotides (phosphate+ sugar + base).

    • Nucleotide bonds: phosphodiester bonds between the sugar and phosphate of adjacent nucleotides.

    • DNA strands are antiparallel: 5’ to 3’ directionality.

  • Base Pairing

    • Adenine (A) pairs with Thymine (T) – 2 hydrogen bonds.

    • Cytosine (C) pairs with Guanine (G) – 3 hydrogen bonds.

  • Enzymes in DNA Replication

    • DNA Polymerase III: main enzyme for adding nucleotides to growing DNA strand.

    • Can only extend from a 3' end; work occurs from 5' to 3'.

    • Leading Strand: continuously synthesized in direction of replication fork.

    • Lagging Strand: synthesized in short fragments (Okazaki fragments) away from the fork.

    • Primase: synthesizes RNA primers needed for DNA polymerase to start.

    • DNA Ligase: joins Okazaki fragments, sealing nicks in the sugar-phosphate backbone.

    • Topoisomerase: relieves tension ahead of the replication fork by making cuts and allowing strands to unwind.

  • Proofreading and Repair

    • DNA polymerase can proofread and correct errors during replication.

    • Mismatch repair enzymes find and correct mismatched bases after replication.

    • Nucleotide Excision Repair: removes damaged nucleotides and replaces them with correct ones.

  • Telomeres and Telomerase

    • Linear chromosomes shorten with replication, leading to loss of vital DNA over time.

    • Telomeres: repetitive non-coding DNA sequences at ends of chromosomes to prevent loss.

    • Telomerase: enzyme that extends telomeres; active in germline cells and some cancer cells, allowing uncontrolled replication.

  • Conclusion

    • Understanding DNA structure and replication is fundamental in genetics.

    • Enzymes play vital roles in maintaining DNA integrity and facilitating replication processes to ensure stable inheritance of genetic material.