Discovering the structure of DNA

  Created April 2020 by Bree Latner, Offline version here
  Video by Paul Anderson, also on his website Bozeman Science.

       Many famous experiments lead to the discovery of DNA as the genetic material of a cell. Fredrick Griffith, used various strains of a bacteria, Streptococcus pneumoniae, to infect mice and determine that something in a cell could transform harmless strains of the bacteria into virulent strains. He called this thing, a . A few years later, Oswald Avery, Maclyn McCarty, and Colin MacLeod expanded on Griffith’s work by breaking the bacteria down into its basic molecules, RNA, proteins, and DNA. They then treated samples with that would breakdown those molecules to see which one was responsible for transforming the harmless bacteria into virulent bacteria. They determined that after breaking down RNA and protein, transformation still occurred. Once they broke down the DNA, transformation no longer occurred. This lead to the conclusion that was the genetic material of cells. While their experiment was conclusive that DNA was the genetic material of the cell, it was doubt-ed because of the of DNA’s chemical composition. DNA is composed of only 4 repeating molecules called nucleo-tides, while proteins are composed of 20 different repeating molecules called amino acids. Alfred Hershey and Martha Chase’s experiment with was the final determining factor that DNA was the genetic material of the cell. In their experiment they used labeled sulfur which is found in proteins, and labeled phosphorus, which is found in DNA to grow bacteriophages. These bacteriophages were then used to infect bacteria. They found that only the labeled was detected in the bacteria. This shows that DNA, not proteins are the genetic material of cells. Now that DNA was known to be the genetic material, several scientists provided data that allowed James Watson and Francis Crick to decipher the structure of DNA. They found that DNA was a double helix, using the x-ray crystallography images from . The image showed them that the backbones of DNA are on the exterior with the bases in the interior. They also used the data of to determine that the bases paired together in a particular manner, A pairs with T and G pairs with C. DNA inside of cells is organized into . In eukaryotic cells, like in plants and animals, the long DNA strand is wrapped around histone proteins which coil together to form the chromosome. Eukaryotic chromosomes are linear. We often see them represented as “X-shaped” because most images show it in metaphase of cell division where it is in its replicated form. In eukaryotes these chromosomes are stored in the of the cell. In prokaryotic cells, the chromosomes are looped. This looped chromosome is found in a region of the cell called the . Prokaryotes also have tiny loops of DNA in their cytoplasm called plasmids. Plasmids can be ex-changed between bacteria. We now know that is what Fredrick Griffith observed in his experiment transforming the harmless bacteria into virulent bacteria. A chromosome is organized into . The genes of the chromosome contain the genetic code to build proteins. In prokaryotes the genes are one right after the other. In eukaryotes the genes are often separated by DNA that does not code for proteins. This DNA was long thought to be junk DNA but now we are coming to understand that it is not junk DNA, but does indeed have a function. The structure of DNA and RNA are similar but have some key differences. RNA is a helix of nucleotides and DNA is a double helix of nucleotides. The bases in DNA are paired togeth-er in the interior of the molecule. DNA and RNA are both composed of the same basic units, nucleotides, which are made of three parts, a sugar, phosphate, and base. The sugar found in RNA is and the sugar found in DNA is deoxyribose. The backbone of the DNA and RNA are alternating sugars, and phosphates. On the inside of DNA and RNA are the nitrogenous bases. The bases found in RNA are adenine, guanine, cytosine, and . In DNA adenine, guanine, cytosine, and thymine are found. The structure of DNA leads us to an understanding of DNA replication. For new cells to contain a copy of DNA, replication must occur. To replicate DNA the double helix must be separated. Then, the molecule uses the old stand to build the complementary new strand. At the end of replication there are two identical copies of DNA. The two strands of DNA are replicated in a slightly different manner because you can only add new nucleotides on the 3’ end, so one strand is replicated continuously following the unzipping, it is called the . The other strand is replicated in the opposite direction of unzipping, in a discontinuous manner; jumping backward to replicated the newly unzipped DNA. This is called the . The structure of DNA not only leads to the understanding of how it is replicated during the cell cycle but also how it functions to encode the genetic information that is used to build a unique or-ganism. This process, called the , was discovered by Francis Crick. In the Central Dogma, DNA is transcribed into , which is translated into a protein. Proteins build the physical traits of the organism or phenotype. The collective phenotypes together make a unique organism.
    occurs in the nucleus. DNA is stored there where it is protected. A molecule called RNA polymer-ase copies a gene into a message called mRNA. That message can leave the nucleus where its code is used to produce a protein. Once mRNA is in the cytoplasm, it binds to a ribosome where it will be into a protein. The ribosome reads the message in mRNA and matches its sequences to tRNAs. Each tRNA brings an amino acid. The ribosomes connect the amino acids into a chain called a protein. Proteins build many cellular structures that determine the physical characteristics of them. Proteins are also enzymes that perform the biochemical reactions of cells. This means proteins determine the structure and function of cells. All of the physical traits or of an organism make that organism unique. These extended phenotypes are the variety that natural selection selects from. Understanding that all life shares the same genetic material, DNA, has lead to genetic engineering. DNA from one organism can be transferred to other organisms. Plasmid DNA from bacteria can be transferred from one bacteria to another, like in the Fredrick Griffith experiment. Genes from or-ganisms like human insulin can be added to a plasmid and then transferred into other bacteria, through . The gene product insulin is then produced by the bacteria.