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Biology LibreTexts

Unit 5: DNA

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    • 5.1: Transformation in Bacteria
      Bacteria have no sexual reproduction in the sense that eukaryotes do. The have no alternation of diploid and haploid generations, no gametes, and no meiosis. However, the essence of sex is genetic recombination, and bacteria do have three mechanisms to accomplish that: transformation, conjugation and transduction.
    • 5.2: The Hershey - Chase Experiments
      Hershey and Chase found that when bacteriophages containing 32P (radioactive), were allowed to infect nonradioactive bacteria, all the infected cells became radioactive and, in fact, much of the radioactivity was passed on to the next generation of bacteriophages. However, when the bacteria were infected with bacteriophages labeled with 35S, and then the virus coats removed (by whirling them in an electric blender), practically no radioactivity could be detected in the infected cells.
    • 5.3: The Double Helix of DNA
      This structure of DNA was worked out by Francis Crick and James D. Watson in 1953. It revealed how DNA - the molecule that Avery had shown was the physical substance of the genes. It could be replicated and so passed on from generation to generation. For this epochal work, they shared a Nobel Prize in 1962.
    • 5.4: Base Pairing in DNA and RNA
      The rules of base pairing tell us that if we can "read" the sequence of nucleotides on one strand of DNA, we can immediately deduce the complementary sequence on the other strand. The rules of base pairing explain the phenomenon that whatever the amount of adenine (A) in the DNA of an organism, the amount of thymine (T) is the same (called Chargaff's rule). Similarly, whatever the amount of guanine (G), the amount of cytosine (C) is the same.
    • 5.5: DNA Replication
      Before a cell can divide, it must duplicate all its DNA. In eukaryotes, this occurs during S phase of the cell cycle. When the replication is complete, two DNA molecules — identical to each other and identical to the original — have been produced. Each strand of the original molecule has remained intact as it served as the template for the synthesis of a complementary strand. This mode of replication is described as semi-conservative: one-half of each new molecule of DNA is old; one-half new.
    • 5.6: The Meselson - Stahl Experiment
      While Watson and Crick had suggested that this was the way the DNA would turn out to be replicated, proof of the model came from the experiments of M. S. Meselson and F. W. Stahl. They grew E. coli is a medium using ammonium ions as the source of nitrogen for DNA (as well as protein) synthesis. 14N is the common isotope of nitrogen, but they could also use ammonium ions that were enriched for a rare heavy isotope of nitrogen, 15N.
    • 5.7: Restriction Enzymes
      Restriction enzymes are DNA-cutting enzymes found in bacteria (and harvested from them for use). Because they cut within the molecule, they are often called restriction endonucleases.
    • 5.8: DNA Sequencing by the Dideoxy Method
      The most popular chemical method for sequencing the nucleotides sequence in a sample of DNA this the dideoxy method which gets its name from the critical role played by synthetic nucleotides that lack the -OH at the 3′ carbon atom. A dideoxynucleotide (dideoxythymidine triphosphate) can be added to the growing DNA strand, but stop chain elongation because there is no 3′ -OH for the next nucleotide to be attached to. For this reason, the dideoxy method is also called the chain termination method.
    • 5.9: Genome Sizes
      The genome of an organism is the complete set of genes specifying how its phenotype will develop (under a certain set of environmental conditions). In this sense, then, diploid organisms (like ourselves) contain two genomes, one inherited from our mother, the other from our father.
    • 5.10: The Human Genome Projects
    • 5.11: The Human and Chimpanzee Genomes
      Now that the genomes of both the human and the chimpanzee have been determined, it is possible to make more direct comparisons between the two species. Their genomes are 98.8% identical (between any two humans — picked at random — the figure is closer to 99.5%).
    • 5.12: Pyrosequencing
      All of the sequenced genomes listed in Genome Sizes were determined using the dideoxy method invented by Frederick Sanger and described elsehwere. However, now a great effort is being expended to find ways to sequence DNA more rapidly (and more cheaply).  Several new methods are being developed and many are already commercially available. Its method is called pyrosequencing or sequencing by synthesis.
    • 5.13: DNA Repair
      DNA in the living cell is subject to many chemical alterations (a fact often forgotten in the excitement of being able to do DNA sequencing on dried and/or frozen specimens). If the genetic information encoded in the DNA is to remain uncorrupted, any chemical changes must be corrected.  A failure to repair DNA produces a mutation.
    • 5.14: Harlequin Chromosomes
      Incredible though it may seem, each single human chromosome that you observe under 440x magnification of your laboratory microscope contains a single molecule of DNA. For some chromosomes, this molecule — if stretched out — would extend 5 cm (2 inches).  Each chromosome contains but a single molecule of DNA and the replication of a chromosome is semi-conservative. The information encoded in each strand of DNA remains intact and serves as the template for the assembly of a complementary strand.
    • 5.15: Metagenomics - Exploring the Microbial World