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DNA: The Molecular Basis of Inheritance (Determining Structure of DNA…
DNA: The Molecular Basis of Inheritance
Nucleus
Structure
Membrane bound (double/envelope): Inner and outer membrane
Studded with pore complex allowing larger molecules to transport out of the nucleus (i.e. Ribosomes, mRNA)
Continuous with the rough ER
Within nucleus: DNA within chromosomes
Nucleolus: Synthesis of ribosomes
Nuclear Lamina: a netlike array of protein filaments that lines the inner surface of the nuclear envelope that helps maintain the shape of the nucleus
DNA Units
Chromosomes: structures that carry the genetic information
Chromatin: complex of DNA and proteins that makes up eukaryotic chromosomes. When the cell is not dividing, chromatin exists in its dispersed form.
DNA Synthesis
protein synthesis is directed by mRNA, mRNA is then transported to the cytoplasm via the nuclear pors. nRNA reaches cytoplasm, ribosomes translates mRNA into genetic message for the structure of polypeptides.
Ribosomes
Made of ribosomal RNA and proteins that carry out protein syntehsis
Free and Bound Ribosomes: free are suspended in cytosol while bound are in rough ER and Nucleus
Free: function within cytosol (i.e. enzymes for first steps of sugar breakdown
Bound: make proteins that are destined for insertion into membranes for packaging within lysosome or for export from the cell
Genetic Material of the Cell:
Proteins? This is because of the structural heterogeneity, functional heterogeneity, versatile yet specific
Nucleic Acids? This is because of the largely unknown structure and uniform chemical property
DNA Historical Perspective
Fredrich Miescher (1869)
ID Chemical composition of cell nucleus: acidic, nitrogen, phosphorus
Coined the term nucleic acid
Archibald Gerrod (1909)
Molecule of interitance: Protein
People with inherited inborn errors in metabolism lacked specific proteins/enzymes
Important figures in identifying DNA as genetic Material:
Frederick Griffith (1928)
Working with Streptococcus pneumonia: bacteria that causes pneumonia in mammals. In experiment, living S cells result in mouses being dead. Living R cells allowed for the mouse to be healthy. Heat-Killed S cells allowed for mouse to be healthy. Mixture of heat-killed S cell and living R cells allowed for the mouse to die due to the living S cells found in blood sample. Concluded that R bacteria were "transformed" by a substance from the S cell
There is a transforming material that can cause changes in genotypes and phenotypes
Avery, McCarty, Macleod (1944)
They were curious as to what the "transforming material in the S cell was. They removed the Lipids and Carbohydrates and added Heat Killed S cells. They mixture was then separated into three equal vials and the samples contained protein, DNA, RNA. If they remove the three individually, what will it do to the S cell. Ultimately if you remove RNA and protein, the transformation will still be occurred with the R cells added. With no DNA there were no S cells appearing. Those who were spectacle proposed that there was something with DNA that could effect it.
Proteinases is added which digests off protein, leaving behind DNA and RNA
deoxyribonuclease: digests DNA in which protein and RNA will be left in the sample
Ribonuclease: digests RNA in which protein and DNA is left
DNA is transforming material
Hersey and Chase (1952)
They worked with Bacteriophages (phages) which are viruses that infect bacteria and consist of DNA/ RNA and protein because they are so simple, it is easy to radioactively tag them. They labeled proteins and DNA separately. For protein they labeled radioactive sulfur and for DNA they labelled phosphorus. They were incubated to inject the genetic material then disrupt them to pause the process. If you label protein, the radioactivity stayed outside of the cell thus concluding that DNA is the transmitter of information and is the genetic material
Confirmed that DNA is genetic material
Erwin Chargaff
Analyzed DNA composition in organisms
Observed that DNA composition varies from species to species. In any other species, the four bases are found in characteristic ratio
Cargaff's Rule: %T=%A and %G and %C
Determining Structure of DNA
Known:
DNA is a polymer of nucleotides consisting of nitrogenous base, deoxyribose, and a phosphate group
Base bare of AGTC
Arrangement of DNA polymer
Unknown
The three dimensional structure
The Players
Linus Pauling (CA)
Maurice Wilkins and Rosalind Franklin (London)
X-ray crystallography to study the structure of DNA
James Watson and Francis Crick
Watson saw Rosalind Franklin's DNA diffraction pattern and deduced that the DNA is a helix, the width of the helix, and the spacing of the bases along the helix
Options: sugarphosphate chains were in the inside and nitrogen bases on outside but then what woul dneutralize the negative charges in the interior
Nitrogen bases on the inside, sugar-phosphate chains on the outisde
Triple helix
Double helix
Ultimately it was that nitrogen bases on the inside and sugar phosphate chains were on the outside with a double helix
Correct model was created with a double helix with nitrogen bases on the inside, sugar phosphate chains as the backbone
From X- Ray diffractiom data they foudn out that the doule helix makes one full turn every 3.4 nm (10 bases/turn) and the helix has a uniformed diameter
Semiconservative Model: type of DNA replication in which the replicated double helix consists of one old strand, derived from the parental molecule, and one newly made strand
Base Pairing
Purines: Adenine and Guanine (double ring)
Pyrimidine: Thymine and Cytosine (single ring)
Consistent with Chargaff's rule
Held together by two hydrogen bonds
Antiparallel: 5' phosphate and 3' hydroxyl. If it were parallel it would have negative charges next to each other