Chapter 12 DNA

The Genetic Code Genetic Code – the way in which cells store the program that they seem to pass from one generation of an organism to the next generation Evidence that DNA is the Genetic Material

1928 – Fred Griffith studied pneumonia caused by bacteria 1928 – Fred Griffith studied pneumonia caused by bacteria. He worked with 2 strains of bacteria, each containing different genetic information. S - strain – virulent produced capsule R- strain – nonvirulant – did not have capsule

12.1 Identifying the Substance of Genes “The Role of DNA” 1928 - Frederick Griffith Conclusion: Some of the genetic material from the dead, virulent bacteria (S, had entered the living, nonvirulent bacteria (R) changing them to the virulent form. This is called BACTERIAL TRANSFORMATION (one strain of bacteria had been transformed into another)

Discovery of the Role of DNA (cont’d) Oswald Avery, Maclyn McCarty, and Colin MacLeod wanted to know – What factor had transformed the bacteria? 1944 - Made “juice” from heat killed bacteria and treated “juice” with enzymes to destroy lipids, proteins, carbs, and RNA  transformation still occurred BUT when the treated the “juice” with enzymes to destroy DNA  transformation did not occur therefore, DNA was the TRANSFORMING FACTOR Scientists were still skeptical about the genetic material of higher organisms.

Discovery of the Role of DNA (cont’d) Hershey-Chase 1952 – hypothesized that bacteriophages (made of DNA and a protein coat) don’t enter bacteria intact, but the phages protein coat attaches to the bacterial cell wall and the phage then injects its DNA into the bacterial cell.

Hershey-Chase Experiment: Infected cells make more virus by injecting their DNA animation 1

Discovery of the Role of DNA (cont’d) Conclusion: DNA and not protein entered the bacteria – strong evidence that the genetic material of bacteriophages is DNA. DNA was the molecule that carried the genetic code

Discovery of the Role of DNA (cont’d) 1. Storing Information 2. Copying Information 3. Transmitting Information 10

12.2 The Structure of DNA 1. Rosiland Frankiln (& Maurice Wilkins) (early 1950’s)– produced photographs (using X-ray diffraction) showing DNA is twisted into a spiral or HELIX with the bases perpendicular to the length of the molecule. Picture also showed that DNA must be composed of more than one strand and that sugar-phosphate backbone is on the outside of the helix

Erwin Chargaff –Chargaff’s Rule – number of nucleotides containing A (adenine) equals the number of nucleotides containing T (thymine) and that the number of G (guanine) equals the number of C (cytosine)

James Watson and Francis Crick 1953 – used Franklin and Wilkins x-ray crystallography picture of DNA and information from Chargaff to make a model of DNA (still used today)

The Double Helix DNA is made of nucleotide subunits 5 C-sugar – DEOXYRIBOSE Sugar One or more phosphate groups Phosphate One of 4 possible nitrogen-containing bases Base

Model suggests that there are 2 strands of DNA and that the 2 strands are arranged like a ladder. Sides of the ladder = sugar and phosphate backbone (phosphodiester bonds) Rungs of the ladder = nitrogen bases – each rungs consists of 2 nitrogen bases COMPLEMENTARY Base Paired. (A=T) (C=G) (held together by H bonds) Purine – Adenine & Guanine- Double Ring (It’s 2x’s as good to be pure) Pyrmidine – Thymine & Cytosine- single ring

DNA (deoxyribonucleic acid) bases: Adenine, Thymine, Cytosine, and Guanine pyrimidines purines Pyrimidines: single ring bases Purines: double ring bases (2xs as good to be pure) Complimentary binding pattern: Adenine + Thymine (share 2 Hydrogen bonds) Cytosine + Guanine (share 3 hydrogen bonds)

The Structure of DNA Two polynucleotide strands wrapped around each other in a double helix A sugar-phosphate backbone Steps/”rungs” made of hydrogen-bound bases (A=T, C=G) Twist

12.3: DNA Replication Semi-conservative model was suggested by Watson and Crick but proven by Matthew Meselson and Franklin Stahl (1958) – each parent strand is a template for a new complimentary strand – end result is two identical DNA molecules each consisting of one old side “conserved” from parent and one new side

Takes place in the nucleus in eukaryotes Four Easy steps to remember: 1.Unwind 2. Unzip 3. Add new parts 4. 2 new molecules of DNA rewind

12.3 DNA REPLICATION: During “S phase” of interphase DNA Replication UNWIND/UNZIP 1.DNA helicase separates parental DNA and exposes bases (unwinds/unzips) 2.Single Stranded Binding Proteins (SSBP) hold strands apart, preventing them from recoiling Adding New Parts/Elongation 3. RNA primase lays down short segments of RNA (RNA primer) to which new strands of DNA can be made

4. DNA polymerase- attaches to the RNA primer and begins to elongate (attach free nucleotides to exposed bases) the strands. Done continuously on the leading strand, in short pieces (Okazaki fragments) on the lagging strand.

Why is there a Leading and a Lagging Strand????? The 2 strands of DNA are antiparallel DNA polymerase can only add nucleotides to the 3’ end (5’  3’) therefore both strands can not be made continuously

DNA replication begins at specific sites on double helix 5. DNA Polymerase replaces RNA primer with DNA nucleotides, it also proofreads new strand for base-pair errors (lagging strand requires many RNA primers) 6. DNA ligase joins sugar-phosphate backbone (“glues”) of Okazaki fragments (phosphodiester bonds link fragments) 2 New IDENTICAL Molecules of DNA REWIND replication forks Animation/tutorial Secret of Life Video

*Telomeres – tips of chromosomes are difficult to copy, the enzyme telomerase adds short repeated DNA sequences to telomeres, lengthening the chromosomes – this makes it less likely that important gene sequences will be lost during replication animation

The Big Picture Two strands unwind and unzip (HELICASE) splits H bonds between bases Add new parts – new nucleotides are added to the exposed strands by DNA POLYMERASE (RNA PRIMASE – first adds RNA nucleotides) DNA LIGASE “glues” 2 new identical molecules of DNA rewind

When DNA can repair mistakes and when it can’t DNA Repair enzymes work like a spell checker Cut out wrong sequences Undamaged strand is template Only 2 or 3 stable changes per year Mutations: some severe, others are not Inheritable changes occur in gametogenesis Now the “wrong” sequences are copied Ex: cystic fibrosis (CF): a deletion of 3 nucleotides in a certain gene Ex: sickle cell anemia: one nucleotide substitution in the hemoglobin gene Mutagen: a mutation causing substance (can break DNA) Ex: X-Rays, radioactivity, nicotine

Prokaryotic Cells vs. Eukaryotic Cells: DNA Replication