DNA Structure and Function AP Biology

Big Idea 3 Living systems store, retrieve, transmit, and respond to information essential to life processes. Heritable information provides for the continuity of life DNA and in some cases RNA, is the primary source of heritable information

The Genetic Material Scientists confirmed that genes are on chromosomes Able to map Drosophilia chromosomes Did not know the composition of genes Chromosomes contain both proteins and nucleic acids.

3 requirements for genetic material 1. Store developmental, structural and metabolic information 2. Be stable so that it can be replicated 3. Undergo rare changes called mutations.

Transformation Frederick Griffith Trying to develop a vaccine against Streptococcus pneunoniae Discovered two strains S – strain ( smooth shiny colonies) mice injected die R- strain (rough appearing colonies) mice injected live

Griffith’s Transformation Experiment

The Transforming Substance Avery Isolated substances such as carbohydrates, proteins, and nucleic acids, use enzymes to destroy them. Repeated Griffith’s experiment If proteins were digested, transformation occurred If RNA was digested, transformation occurred. If DNA was digested, transformation did not occur.

Reproduction of Viruses Bacteriophages are viruses that infect bacteria. Viruses consist of a protein capsid surrounding a nucleic acid core Viruses use host cells to reproduce Must inject their genetic material into the host cell

Hershey-Chase Experiment Relied on the chemical difference between DNA and protein DNA contain P and Proteins contain S Use radioactive 32P and 35S to label DNA and proteins.

Bacteria and Bacteriophages

Hershey and Chase Experiments

Results of the experiments Phages with 32P infected bacteria Phages with 35S infected bacteria 32P was found in cells Results indicate that DNA is the genetic material

Chemistry of DNA Nucleic Acid Contains only 4 nucleotides Nitrogen containing base Adenine, Guanine, Thymine, and Cytosine Phosphate Pentose – 5 carbon sugar

The Structure of DNA DNA Contains Two pyrimidine bases Single rings Cytosine and Thymine Two purine bases Double rings Adenine and Guanine

Chargaff’s Rules Edwin Chargaff did an analysis of base content of DNA Found nucleotide content was not fixed, DNA has variability The % of each type of nucleotide differs from species to species. In each species the amount of A = the amount of T and the amount of G = the amount of C

Variation in Base Sequence Chargaff’s data suggest A is paired with T and G is with C Average human chromosome contains about 140 million base pairs Any 4 possible nucleotides can be present at each position, the total number of possible nucleotide sequence is 4140,000,000.

The Double Helix Using Rosalind Franklin’s X-ray diffraction photo of DNA Watson and Crick were able to determine the structure of DNA as a double helix Sugar phosphate backbone Paired bases on the inside Purines are paired with pyrimidines

R. Franklin’s Photo 51

Replication of DNA Process of copying DNA During replication each old strand of DNA serves as a template for a new strand Replication is termed semi-conservative replication One old strand is conserved or present in each daughter DNA.

Steps of Replication the old strands are unwound and “unzipped” The H bonds between base pairs are broken . Helicase enzyme unwinds the molecule. New nucleotides are positioned by complementary base pairing Nucleotides are joined together to form new strands Steps 2 and 3 are carried out by an enzyme complex called DNA polymerase.

Messelson & Stahl Experiment confirmed semi-conservative replication Use N with different densities Grew bacteria in medium containing heavy 15N. Then switched bacteria to medium with light 14N

After 1 division only hybrid DNA molecules in the cells –intermediate density After 2 divisions ½ DNA molecules were light and ½ were hybrid

Aspects of DNA Replication DNA strands are anti-parallel to allow for complementary base pairing One strand of DNA runs from 3’ to 5’ in one direction and the other runs 3’ to 5’ in the opposite direction

DNA polymerase synthesizes the daughter strand in the 5’ to 3’ direction. Join nucleotides to the free 3’ end of th previous nucleotide DNA polymerase cannot start synthesis of DNA chain RNA polymerase lays down a short RNA primer complementary to the strand Then DNA polymerase can join nucleotides to the 3’ end.

The leading strand- synthesized continuously toward the replication fork The lagging strand The parental strand runs in the 3’ to 5’ direction away from the fork The daughter strand begins at the fork Replication begins over and over by new DNA polymerase molecules as DNA winds Replication of the lagging strand is discontinuous Results in segments called Okazaki fragments.

Completing Replication RNA primers are removed While proofreading, DNA polymerase I removes the RNA primers and replaces with DNA nucleotides DNA ligase joins the fragments

The DNA molecule gets shorter with each replication DNA polymerase cannot replicate the end of the lagging strand after the RNA primer is removed The DNA molecule gets shorter with each replication Eukaryotic DNA molecules have a special nucleotide sequence called telomeres Telomeres do not code for proteins Series of repeats of short nucleotides sequence such as TTAAGGG

Mammalian cells grown in a culture divide about 50 times and then stop The loss of telomeres signals the cell to stop dividing Telomeres are added to chromosomes during gamete formation by the enzyme telomerase In cancer cell telomerase is turned on in error. Contributes to cancer cells ability to divide without limit.

Prokaryotic vs. Eukaryotic Replication Prokaryotic replication Circular loop of DNA Replication moves around DNA in one direction Always occurs in 5’ to 3’ direction 106 base pairs per minute Eukaryotic replication Begins at numerous origins Along the chromosome Bi- directional Starts at replication forks

Replication Errors Genetic Mutation Permanent change in the sequence of bases Mismatched After proofreading One mistake per 1 billion bases DNA damage Environmental factors Free radicals in cells UV radiation Organic chemicals Tobacco smoke Pesticide Pollutant

Genetic mutations need not harm an organism Some may be beneficial May cause cancer or genetic disease Without changes in genetic material, evolution would not be possible.