Chapter 14: DNA

I) Genetic Material – What is it? Stable molecule that is not easily damaged or altered DNA has many hydrogen bonds that hold it together Contains strong bonds in the “sugar-phosphate backbone” Phosphodiester Bonds – bond between deoxyribiose and phosphate Can be easily reproduced Accurately copied by enzymes that “unzip” the double helix Hi fidelity – very accurate to conserve genetic code of a species Complex for variability Four nitrogenous bases form 64 3-base codons Many sequences of codons can code for different proteins

II) Experiments that Proved DNA is the Hereditary Material of Organisms Frederick Griffith (1928) Bacterial Transformation Hereditary information from dead virulent bacteria was transferred into live non- virulent bacteria Live non-virulent bacteria became virulent Genetic Material was resistant to heat treatment – was not protein based because proteins denature in high temperatures Avery-McCarty-MacLeod (1944) Used heat to kill the virulent Streptococcus pneumonia bacteria and extracted RNA, DNA, carbohydrates, lipids and proteins Each molecule was added to a culture of live non-virulent bacteria to determine which was responsible for changing them into virulent bacteria. DNA was the only molecule that turned the non-virulent cells into virulent cells, which they concluded was the genetic material within cells.

Francis Crick and James Watson (1953) Hershey and Chase (1952) Determined the genetic material was the molecule DNA Traced the movement of DNA from viruses into bacterial cells Proved DNA was the molecule being transferred NOT protein Rosalind Franklin (1952) Studied DNA using X-Ray crystallography Determined the double helix structure (two strands, NOT 3) Determined the structure of the sugar phosphate backbone Could not interpret the base pairing Erwin Chargaff (1952) Determined the ratio of DNA bases to be equal A = T C = G Ratios are also species specific Francis Crick and James Watson (1953) Determined the base pairing of DNA Used Franklin’s data to help determine the overall structure Proposed how DNA could replicate by “unzipping” Could not have done their work without Franklin’s results

Hershey and Chase Experiment

Rosalind Franklin’s X-Ray Diffraction

Chargaff’s Rule Humans: 60% A-T 40% C-G Wheat: 55% A-T 45% C-G

James Watson and Friends

Watson and Bolen

Messelson and Stahl (1958) Determined how DNA replicates Modeled semiconservative replication The old or template strand is copied and becomes part of the new DNA molecule

III) DNA Structure DNA, and in some cases RNA, is the primary source of heritable information. 1. Double Helix Two alpha helices of deoxyribonucleic acid Contains repeating subunits called nucleotides Phosphate, sugar, nitrogenous base 2. Bonding Hydrogen bonds BETWEEN the nitrogenous bases Many weak bonds results in collectively strong bonding Phosphodiester Bonds Strong bonds in sugar-phosphate backbone 3. Complementary (opposite) Base-Pairing Adenine and Thymine pair Cytosine and Guanine pair Result of specific hydrogen bonding between bases Purines: Adenine and Guanine Pyrimidines: Cytosine, Uracil, Thymine (PYRamid stones were CUT) One side of the double helix complements the other side

4. Antiparallel Helices Each side of the DNA molecule is oriented in the opposite directions Permits base pairing Important in understating direction of replication and location of enzymes and other proteins Based upon the numbering of the carbons in the deoxyribose molecule

5. Semiconservative Replication Each DNA molecule is copied from the original strand Two copies of DNA produced in replication Each strand contains half new and half template DNA

IV) DNA Replication Genetic information is transmitted from one generation to the next through DNA or RNA Non-eukaryotic organisms have circular chromosomes One replication origin Eukaryotic organisms have multiple linear chromosomes Multiple replication origins Exceptions to this rule: Plasmids - small extra-chromosomal, double-stranded circular DNA molecules Prokaryotes, viruses and eukaryotes can contain plasmids

Eukaryotic Chromosome Structure

New DNA molecules are synthesized from a template or original DNA strand Topoisomerase – relaxes the supercoiling of DNA Gyrase – relieves strain on the helix as it is uncoiled DNA Helicase – breaks hydrogen bonds between nitrogenous bases DNA Primase – forms a primer of RNA in the 3’5’ Starting point for DNA polymerase to attach and begin copying DNA Polymerase – synthesizes new DNA from template “Reads” and moves along the DNA template in the 3’  5’ direction Adds nucleotides to the 3’ ends of the DNA template Exonucleases – remove incorrect DNA bases DNA polymerase I can move backwards one base to remove errors Ligase – links Okazaki fragments on the lagging strand Lagging strand – new DNA produced in short segments Leading strand – new DNA produced in one continuous strand

Bi-Directional Replication in DNA 1 Bi-Directional Replication in DNA 2 Replication 3 Replication 4

IV) Special Cases of DNA Replication Retroviruses Genetic information in retroviruses is a special case and has an alternate flow of information: from RNA  DNA Reverse transcriptase - enzyme that copies the viral RNA genome into DNA. This DNA integrates into the host genome Viral genes become transcribed and translated for the assembly of new viral progeny. Examples of retroviruses: HIV,