DNA: the Code of Life

Basic Requirements For Genetic Material 1)Carry genetic information from cell to cell, generation to generation LOTS of info!!! 2)Copy itself with fidelity 3)Mutate occasionally and carry the mutation along with fidelity 4)Have a way to decode stored info and translate it into action in cells.

Discovery Of Structure 1868:Miescher first isolated deoxyribonucleic acid, or DNA, from cell nuclei

Discovery Of Structure 1951:Linus Pauling deduced the structure of proteins

Discovery Of Structure 1953:Watson and Crick put together a model of DNA

Discovery Of Function 1928:Griffith was working with S (virulent) and R (nonvirulent) strains of a pneumonia-causing bacterium Discovered "transforming factor"

Griffith’s Experiment LIVE lethal bacteria injected LIVE non-lethal bacteria injected DEAD lethal bacteria injected MIXTURE of LIVE nonlethal & DEAD lethal bacteria injected – –Blood showed LIVE lethal bacteria inside

Griffith’s Experiment CONCLUSIONS?

Discovery Of Function By 1920’s, chromosomes structure was known to be proteins & DNA PROBLEM: which carried the genetic material? Repeating nucleotides or Repeating polypeptides?????

Discovery Of Function 1944: Avery showed that the “transforming factor" was nucleic acid, not protein

Confirmation of DNA Function Used in experiments by Hershey and Chase to prove which of these (DNA) was the heredity material.

Confirmation of DNA Function Viruses called bacteriophages use bacterial cells for reproduction. Consist of only a protein coat and a nucleic acid core

Hershey & Chase Experiment “Labeled” viruses with either – – Radioactive PHOSPHORUS (found ONLY in DNA) or – – Radioactive SULFUR (found ONLY in proteins)

Hershey & Chase Experiment “Labeled” P virus into bacteria – –Bacteria = RADIOACTIVE – –Empty viral coats = NOT “Labeled” S virus into bacteria – –Empty viral coats = RADIOACTIVE – –Bacteria = NOT

Hershey & Chase Experiment CONCLUSION?

DNA is composed of 4 kinds of nucleotides Each nucleotide consists of: – – one 5-carbon sugar (deoxyribose) – – one phosphate group, and – – one of 4 bases adenine (A), guanine (G), thymine (T), cytosine (C). Molecular Structure of DNA

sugar (deoxyribose) phosphate group P

Molecular Structure of RNA sugar (ribose) phosphate group P

Purines - 2 Rings Adenine Guanine

Pyrimidines - 1 Ring Thyamine Cytocine

Purines vs Pyrimidines Chargaff showed that – – % of A always = % of T – – % of G always = % of C Purines always with pyrimidines Conclusions about their relationship within DNA???

Purines vs Pyrimidines

DNA Structure Rosalind Franklin and Maurice Wilkins used X-ray diffraction techniques to produce images of DNA molecules.

X-Ray Crytallography

DNA Structure 1) DNA = two strands of nucleotides twisted into a double helix 2) Helixes run anti- parallel to each other (in opposite directions) – – Watson’s discovery

DNA Structure

DNA Characteristics 3) Base pairs are formed by hydrogen bonding of A with T, and G with C

DNA Characteristics 4) Sequence of bases in a nucleotide strand is different from species to species

DNA Characteristics 5) The length of a strand of DNA (number of base pairs) is different from species to species. 6) More complex organisms generally have greater #’s of base pairs

DNA - the unwinding

DNA Replication Two strands of DNA unwind and expose their bases – – DNA polymerase opens strands – – Begins at the 5’ end

Unattached nucleotides pair with exposed bases – – DNA ligases assist in re-bonding – – DNA “swivelase” promotes re-winding into a double helix DNA Replication

DNA Polymerases attach at many sites along the original molecule Replication Bubbles

Results in DNA molecules that consist of one "old" strand and one "new" strand (semi- conservative). DNA Replication

Historical Discoveries Meselson & Stahl - invented density gradient centrifugation and used this to prove how DNA is replicated

Meselson-Stahl Experiment

DNA Repair DNA polymerases & DNA ligases engage in repair when they "read" the complementary sequence on the other strand and restore it

Organization Of DNA On Chromosomes Each eukaryotic chromosome contains one long DNA molecule (46 in humans)

DNA of eukaryotes is highly organized to prevent tangling Some histones (a protein) act as spools to wind the DNA into units called nucleo- somes

Organization Of DNA On Chromosomes Another histone stabilizes the arrangement and allows the beaded chain to form looped regions.

Designedby Anne F. Maben These images are for viewing only and may not be published in any form