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2 History of DNA Early scientists thought protein was the cell’s hereditary material because it was more complex than DNA Proteins were composed of 20 different amino acids in long polypeptide chains copyright cmassengale

Frederick Griffith In 1928, Frederick Griffith carried out experiments on pneumonia bacteria in mice. Discovery: something in heat-killed virulent bacteria could be transferred to live, harmless bacteria and make them virulent.

Griffith’s Experiment

Oswald Avery Avery continued working with Griffith’s findings in hope of discovering what factor in bacteria carried the trait of virulence. Isolated proteins, carbohydrates, nucleic acids and applied them to non- virulent bacteria. Only nucleic acids (DNA) caused a change.

The History of DNA Alfred Hershey and Martha Chase –radioactive isotope tracer experiment –bacterial virus (bacteriophage T2) infects a host cell ( bacterium Escherichia coli) –found that T2 virus DNA, not its protein coat, enters the host cell –genetic information for replication of the virus

Using S 35 Bacteria grown in normal non- radioactive media T 2 grown in media containing S 35 incorporate S 35 into their proteins Blending causes phage protein coat to fall off T2 attach to bacteria and inject genetic material Is protein the genetic material? When centrifuged, phage protein coats remain in the supernatant while bacteria form a pellet. The supernatant is radioactive, but the pellet is not. Did protein enter the bacteria?

Using P 32 Bacteria grown in normal non- radioactive media T2 grown in P 32 containing media incorporate P 32 into their DNA Blending causes phage protein coat to fall off T2 attach to bacteria and inject genetic material Is DNA the genetic material? When centrifuged, phage protein coats remain in the supernatant while bacteria form a pellet The pellet is radioactive, but the supernatant is not. Did DNA enter the bacteria?

Erwin Chargaff Chargaff studied DNA itself, in hopes of providing some clues about its structure. Discovered that there are always equal amounts of the bases Adenine and Thymine, and equal amounts of Cytosine and Guanine. Chargaff proposed that these bases pair with one another in some way. Adenine and Thymine always join together A T Cytosine and Guanine always join together C G

Wilkins and Franklin Rosalind Franklin and Maurice Wilkins worked with X-ray crystalography to find more clues about the structure of DNA. Franklin’s X-ray images suggested a helical structure.

Franklin and Wilkins

Watson and Crick James Watson and Francis Crick were also working on discovering the structure of DNA. Applied Chargaff’s rule, assumed that A always pairs with T, C with G. Watson was not entirely convinced of the helical structure that Franklin had suggested, and his critique of her work led her to doubt herself.

Watson and Crick Wilkins consulted with Watson and Crick. Without Franklin’s knowledge, he handed them the data that he and Franklin had worked on. Watson immediately recognized the significance. He and Crick went to work on a model of DNA.

The First DNA Model

15 DNA DNA.DNA is often called the blueprint of life. In simple terms, DNA contains the instructions for making proteins within the cell.

16 The Shape of the Molecule DNA is a very long polymer. The basic shape is like a twisted ladder or zipper. This is called a double helix.

17 One Strand of DNA The backbone of the molecule is alternating phosphates and deoxyribose sugar The teeth are nitrogenous bases. phosphate deoxyribose bases

18 One Strand of DNA One strand of DNA is a polymer of nucleotides. One strand of DNA has many millions of nucleotides. nucleotide

19 Four nitrogenous bases Cytosine C Thymine T Adenine A Guanine G DNA has four different bases:

20 Two Kinds of Bases in DNA Pyrimidines are single ring bases.Pyrimidines are single ring bases. Purines are double ring bases.Purines are double ring bases. C C C C N N O N C C C C N N N N N C

21 Thymine and Cytosine are pyrimidines Thymine and cytosine each have one ring of carbon and nitrogen atoms.Thymine and cytosine each have one ring of carbon and nitrogen atoms. C C C C N N O N cytosine C C C C N N O O thymine C

22 Adenine and Guanine are purines Adenine and guanine each have two rings of carbon and nitrogen atoms.Adenine and guanine each have two rings of carbon and nitrogen atoms. C C C C N N N Adenine N N C C C C C N N O N Guanine N N C

23 Two Stranded DNA Remember, DNA has two strands that fit together something like a zipper. The teeth are the nitrogenous bases but why do they stick together?

24 C C C C N N O N C C C C N N O N N N C Hydrogen Bonds The bases attract each other because of hydrogen bonds. Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA. The bonds between cytosine and guanine are shown here with dotted lines

25 Hydrogen Bonds, Hydrogen Bonds, cont. When making hydrogen bonds, cytosine always pairs up with guanine Adenine always pairs up with thymine Adenine is bonded to thymine here C C C C N N N N N C C C C C N N O O C

26 DNA Replication

27 Replication Facts DNA has to be copied before a cell dividesDNA has to be copied before a cell divides DNA is copied during the S or synthesis phase of interphaseDNA is copied during the S or synthesis phase of interphase New cells will need identical DNA strandsNew cells will need identical DNA strands

What Is DNA Replication DNA Replication is the process in which the DNA within a cell makes an exact copy of itself. –Why does DNA replicate? –During which phase of the cell cycle does DNA replicate?

DNA Replication models

The Three Possible DNA Replication Models Conservative- would leave the original strand intact and copy it. Dispersive-would produce two DNA molecule with sections of both old and new along each strand. Semiconservative –would produce DNA molecule with both one old strand and one new strand.

DNA Replication Replication bubble Replication fork Replication fork Hydrogen bond Replication occurs during Interphase DNA replication is the process where an entire double-stranded DNA is copied to produce a second, identical DNA double helix.

DNA Replication Helicase unwinds the double helix starting at a replication bubble. The two strands separate as the hydrogen bonds between base pairs are broken. Two replication forks form and the DNA is unwound in opposite directions. DNA helicase

DNA Replication Helicase has completed unwinding the DNA strand. Single strand Binding Proteins (SSB) keep the two strands from re-annealing (coming back together).

DNA Replication PrimaseRNA Primer Primase is an RNA polymerase that makes the RNA primer. These primers “tell” the DNA polymerase where to start copying the DNA. Leading Strand Lagging Strand

DNA Replication DNA Polymerase The DNA polymerase starts at the 3’ end of the RNA primer of the leading stand CONTINUOUSLY. DNA is copied in 5’ to 3’ direction. DNA polymerase copies the lagging strand DIS- continuously. Leading Strand Lagging Strand 3’ 5’ 3’ Direction of Replication

DNA Replication Another DNA Polymerase removes the RNA primers and replaces them with DNA.

DNA Replication Finally the gaps in the sugar phosphate backbone are sealed by DNA ligase There are now 2 identical double helices of DNA. ligase