Rapid Pace of Disease Gene Discovery

Rapid Pace of Disease Gene Discovery Total disease genes identified- 1,112 (as of yr 2000) Susceptibility genes

1940’s- T.H. Morgan’s group showed that genes are located on chromosomes But…. Are _________ or DNA the genetic material?? Most believed it to be protein However, this was not consistent with experiments with microorganisms, like bacteria and viruses. DNA Structure and Function Chapter 10 1. The search for genetic material lead to DNA History

1928- Frederick _______- He studied Streptococcuspneumoniae, a bacterium that causes pneumonia in mammals. One strain, the R strain, was harmless. The other strain, the S strain, was ___________. Experiment- Griffith mixed _____________ S strain with live R strain bacteria and injected this into a mouse. Result- Mouse died, but the S strain was recovered from the mouse’s blood. S R Killed S R + Killed S • Griffith called this phenomenon, ______________-a change in genotype and phenotype due to the assimilation of a foreign substance (now known to be DNA) by a cell. Injection Dies Lives Lives Dies Live S found Fig. 16.1 Fig. 16.1

What was the transforming substance in Grifith’s experiments?? • Finally in 1944, Oswald_______, Maclyn McCarty and Colin MacLeod announced that the transforming substance was _____. • Still, many biologists were skeptical. • In part, this reflected a belief that the genes of bacteria could not be similar in composition and function to those of more complex organisms. • In 1952, Alfred Hershey and Martha Chase showed that DNA was the genetic material of the phage T2 • Phage T2 is a ______ that infects bacteria What is a virus?? • Viruses consist of a DNA (sometimes ____) enclosed by a protective coat of protein. • To replicate, a virus infects a host cell and takes over the cell’s metabolic _________________.

Fig. 16.2a • The T2 phage, consisting almost entirely of DNA and protein, attacks Escherichiacoli (E. coli), a common _______________bacteria of mammals. • This phage can quickly turn an E. coli cell into a ___________factory that releases phages when the cell ruptures.

The Hershey and Chase Experiment-Designed to determine the source of genetic material in the phage Procedure: Two parallel experiments #1 Grow T2 phage in the presence of _____________sulfur, marking the ___________ but not DNA • Recall that sulfur is a component of some_____________. ATP #2 Grow T2 phage in the presence of _____________ phosphorus , marking the _______ but not proteins • Recall that __________is a component of DNA Fig. 5.29 • Allow each batch to infect separate E. coli cultures. • Shortly after infection, use blender to “shake loose” any remaining phage components

Now centrifuge – bacteria pellet and phage do not. Tested the pellet and ____________ of the separate experiments for the presence of radioactivity. Hershey-Chase experiment (cont.) virus particle labeled with 35S virus particle labeled with 32P bacterial cell (cutaway view) label outside cell label inside cell

Results of Hershey-Chase experiment- If used radioactive sulfur (proteins labeled)- most of the radioactivity was in the______________, not in the pellet. If used radioactive phosphorous(DNA labeled)- most of the ______________ was in the pellet with the bacteria. Hershey-Chase experiment (cont.) • Conclusion- injected _______ of the phage provides the _________ information for new viruses.

Circumstantialevidence that DNA is the genetic material in eukaryotes Cells double the amount of DNA in a cell prior to ________________ and then distribute the DNA equally to each daughter Diploid sets of chromosomes have twice as much DNA as the __________ sets in gametes of the same organism.

1947- Erwin Chargaff had developed a series of rules based on a survey of DNA composition in organisms. Known that DNA was a polymer of nucleotides consisting of a nitrogenous base, deoxyribose, and a phosphate group. Known that there were four bases-adenine (A), thymine (T), guanine (G), or cytosine (C). Chargaff noted that the four bases are found in characteristic, but not necessarily equal, ratios • He also found a peculiar regularity in the ratios of nucleotide bases which are known as Chargaff’s rules.

Chargaff’s rules- 1. The number of adenines was approximately equal to the number of thymines (%T = %A). 2. The number of guanines was approximately equal to the number of cytosines (%G = %C). Human DNA is 30.9% adenine, 29.4% thymine, 19.9% guanine and 19.8% cytosine.

Nucleotide Bases ADENINE (A) GUANINE (G) phosphate group deoxyribose THYMINE (T) CYTOSINE (C)

By the beginnings of the 1950’s, the race was on to move from the structure of a single DNA strand to the three-dimensional structure of DNA. Among the scientists working on the problem were Linus ______, in California, and Maurice Wilkins and Rosalind ________, in London. Watson and Crick discovered the double helix by building models to conform to X-ray data

The basic structure of DNA was known by 1952 • The ________group of one nucleotide is attached to the _____ • of the next nucleotide in line. • The result is a “__________ ” of alternating phosphates and sugars, from which the bases project. T Bases A C G Fig. 16.3

The double helix- a _________ homology The _______________chains of each strand are like the side ropes of a rope ladder. Pairs of nitrogen bases, one from each strand, form rungs. The ladder forms a twist every ____ bases Fig. 16.5

In addition, Watson and Crick determined that chemical side groups off the nitrogen bases would form hydrogen bonds, connecting the two strands. Adenine could form two ________bonds only with thymine Guanine could form ______hydrogen bonds only with cytosine. This finding _________Chargaff’s rules. Fig. 16.6

The linear sequence of the four bases can be varied in countless ways. Each gene has a unique order of nitrogen bases. In April 1953, Watson and Crick published a succinct, one-page paper in Nature reporting their double helix model of DNA. The Nobel Prize in Chemistry was awarded in later years

In a second paper, Watson and Crick published their _____________for how DNA replicates. Base pairing is at the heart of DNA replication Base pairing enables existing DNA strands to serve as templates for new complimentary strands When a cell copies a DNA molecule, each strand serves as a _________ for ordering nucleotides into a new ________________ strand. 1.Separate strands Double-stranded DNA 2.Add complementary bases 3.Connect bases Fig. 16.7

DNA Replication • Each parent strand remains ________ • Every DNA molecule is half “____” and half “_____” new old old new

Bacterium E. coli (5 million base pairs- )- _______________ Human cells- (6 billion base pairs)- ___________ DNA replication requires a large number of enzymes and other proteins A. DNA replication is fast and efficient Time to copy the entire genome and divide into daughter cells: Accuracy • Less than one error per __________nucleotides. Machinery • More than a dozen enzymes and other proteins required B. Where does replication start? – Answer- The ______________________ Bacteria- one site in DNA recognized by specific proteins

Eukaryotes- ______________ of origin sites per chromosome. At each origin site, the DNA strands separate forming a replication “______ ” with ______________ at each end. The replication bubbles_______as the DNA is replicated and eventually ______ . Origin of replication (cont.) 15.3 15.2 15.1 14 13.3 13.1 12 11 11.1 11.2 12 13.1 13.2 13.3 14 15 21 22 23.1 23.3 31.1 31.2 31.3 32 33.1 33.3 34 35.1 Fig. 16.10

DNA polymerases add the ____________(nitrogen base, deoxyribose, and a triphosphate tail). Rate of elongation ____nucleotides/second in bacteria ___per second in human cells. The raw nucleotides are nucleoside triphosphates. Onephosphateconnects nucleotides Two phosphates are removed upon addition of nucleotide Fig. 16.11

At the replication fork the leading stand is copied ______________from a single primer the lagging strand is copied in ____________ using many primers. Summary of DNA replication Fig. 16.16

Nucleotide pairing errors occur at a rate of one error per __________ base pairs. Yet the final error rate is only one per billion nucleotides. Enzymes proofread DNA during its replication and repair damage in existing DNA How do we account for this discrepency?? Answer- ______________ a. ______________proofreads each new nucleotide against the template nucleotide as soon as it is added. If incorrect, it corrects it. b. Each cell continually monitors and repairs its genetic material, with over ______ repair enzymes identified in humans.

Rapid Pace of Disease Gene Discovery