Dna Structure And Function

DNA Structure and Function Chapter 13

Hunt for Fame, Fortune and • Researchers first believed that proteins held the key for DNA heritable information, because they were so diverse. • Frederick Griffith wanted to develop a vaccine against the bacterium Streptococcus pneumoniae • The colonies of one strain had a rough surface appearance, and the other colony seemed to be smooth. Griffin gave them the name of R and S. • The steps of Griffin experiment. – First he injected mice with live R cells. The mice did not develop pneumonia. The R strain was harmless. – Second he injected other mice with lice S cells. The mice died. The S strain was pathogenic, it caused the disease. – Third he killed S cells by exposing them to high temperature. The mice injected with dead S cells did not die. – Fourth he mixed live R cells with heat-killed S cells and injected it into the mice. The mice died. Their blood showed live S cells. – Heat had killed the S cells but did not destroy their hereditary material.

Cont. • Oswald Avery was intrigued by Griffith’s discovery. • He transformed harmless bacteria by mixing them with extracts of killed pathogenic cells. • He found that adding protein-digesting enzymes to the extracts had to effect, cell were still transformed. • He then added a DNA-digesting enzyme to extracts which prevented transformation. • After Avery experiments many other scientist started using bacteriophages in their experiments. • Bacteriophages- only infect bacteria. – Consist only of DNA and a coat of protein.

Cont. • Linus Pauling deducted the structure of protein (collagen). • In 1953 Watson and Crick built a model of DNA. They discovered the structure of DNA.

Discovery of DNA • Nucleotide- is found in DNA and it consist of a fivecarbon sugar, a phosphate group, and a nitrogencontaining base. • The nitrogen-containing bases are adenine (A), guanine (G), thymine (T), and cytosine (C). • T and C are pyrimidines with a backbone of carbon and nitrogen that forms a single ring. • A and C are purines which are larger molecules having two rings. • The amount of adenine to thymine is the same and the amount of guanine to cytosine is the same. • The nucleotides in DNA are joined to one another by way of condensation reactions that form long chains. • Scientist Rosalind Franklin made x-ray diffraction images of DNA.

Cont. • Pauling who discovered proteins made a big mistake by making all of the negatively charged phosphate groups pointing inward facing each other. They should have been facing outward so that they do not repel each other. • Watson and Crick came to the conclusion that DNA must consist of two strands held together by hydrogen bonds. • The bonds form when the two strands are running in opposite directions of each other and twist to form a double helix. • Only two kinds of base paring form along the molecules length A-T and G-C. • All DNA molecules show the same bonding pattern. • The constancy in DNA’s bonding pattern is the

DNA Replication and Repair • Semiconservative Replication – Enzymes easily break the hydrogen bonds between 2 nucleotide strands of DNA. – When the enzymes and proteins act on the DNA strand it unwinds and exposes the nucleotide bases. – Each of the parent strands stays intact and a companion strand is assembled according to paring rule of A-T and G-C. – As soon as the strand is done being coded the old strand and the new strand bind together and form a double helix. – Each parent cell is conserved during

Cont. • Replication Enzyme – Molecules that are over the cell cycle activate replication enzymes – Helicases unzip the hydrogen bonds which are individually weak and easy to break. – The 2 parent cells are prevented from winding back together because small proteins temporarily bind with them. – DNA polymerase catalyze the formation of two brandnew strands of DNA from free nucleotides. – They can only assemble in the 5’ to 3’ direction. – For the parent cells that runs in the opposite direction the nucleotides are assembled in short stretches on the parent strand and DNA ligases seal the stretches together in a continuous strand. – This complementary strand winds up with the old strand. – A free nucleotide has a tail of three phosphate groups dangling from the 5’ carbon end.

Cont. • AN individual may not survive if something changes in its DNA. • The wrong nucleotide may be base-paired with the parent template and unless this is fixed it can weaken the functions of the genes or protein products. • DNA proofreading mechanisms- fix most errors in replication and most of the strands breaks. – When they can not replication is arrested and control over the cell cycle come into play. – Mismatches that slip past the proofreaders are only one type of DNA damage. – Repair enzymes- can repair some changes; they recognize and snip out damaged site or