BIOL 1107 1nd EditionExam # 3 Study Guide Lectures: 19 - 24Each section below includes material from the lecture, homework and textbook.Lecture 19 & 20 Chapter 16: The Molecular Basis of Inheritance T.H Morgan’s group:- Showed that genes are located on chromosomes- Two components of chromosomes (DNA and protein) became candidates for genetic material- Role of DNA in heredity was first discovered by studying bacteria and viruses that infect themFrederick Griffith:- Discovery of genetic role of DNA began w/ research by this guy in 1928- He worked with two strains of a bacterium (pathogenic one + harmless one)- When he mixed heat-killed remains of pathogenic strain w/ living cells of harmless strain, some living cells became pathogenico Called this phenomenon transformation (change in genotype and phenotype due to assimilation of foreign DNA)Avery and his group:- 1944- announced transforming substance was DNA- Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria- Many biologists remained skeptical, because very little was known about DNAHershey and Chase:- Very important experiment- 1952- they performed experiments showing that DNA is the genetic material of a phase known as T2- To determine this, they designed experiment showing that only one of the two component of T2 (DNA or protein) enter an E. coli cell during infection- They concluded that injected DNA od phage provides genetic informationChargaf’s findings:- Base composition of DNA varies between species- In any species the # of A and T bases are equal and # of G and C bases are equal- The basis for these rules was not understood until discovery of double helix- 1950- he reported that DNA composition varies from one species to the nexto This evidence of diversity made DNA a more credible candidate for genetic materialo After DNA was accepted as the genetic material, challenge was to determine howits structure accounts for its role in heredity Maurice Wilkins & Rosalind Franklin:- They were using a technique called X-ray crystallography to study molecular structure- Franklin produced picture of DNA molecule using this technique- Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical- The X-ray images also enable Watson to deduce the width of the helix and the spacing of the nitrogenous bases- He pattern in the photo suggested that the DNA molecule was made up of two strands, forming a double helixWatson and Crick:- Built models of a double helix to conform to the X-rays and chemistry of DNA- Franklin had concluded that there were two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior- Watson built a model in which the backbones were antiparallel (their subunits run in opposite directions)- Pairing a purine with a pyrimidines resulted in a uniform width consistent with the X-ray data - Relationship b/n structure and function is manifest in double helix- W + C noted that specific base pairing suggested a possible copying mechanism for genetic material- They also reasoned that pairing was more specific, dictated by the base structures- Established that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C)- This Watson-Crick model explains Chargaff’s rules: in any organism the amount of A = T, and amount of G = C - Since two strands of DNA are complementary, each strand acts as a template for building a new strand in replication- In DNA replication, parent molecule unwinds and 2 new daughter strands are built based on base-pairing rulesMeselson and Stahl:- Watson and Crick’s semi-conservative model of replication predicts that when a double helix replicates, each daughter molecule will have on old strand (derive d or “conserved”from the parent molecule) and one newly made strand- Competing models were the conservative model (two parent strands rejoin) and the dispersive model (each strand is a mix of old and new)- Experiments by Meselson and Stahl supported semiconservative model- They labeled the nucleotides of old strands w/ heavy isotope of nitrogen, while any new nucleotides were labeled w/ lighter isotopeDNA Replication:- Copying of DNA is remarkable in its speed and accuracy- More than a dozen enzymes and other proteins participate in DNA replication- Replication begins at particular sites called origins of replication, where the two strands are separated, opening up a replication “bubble”- A eukaryotic chromosome may have hundreds or even thousands of origins of replication- Replication proceeds in both directions from each origin, until entire molecule is copied- At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating - Helicases- enzymes that untwist double helix @ replication forks- Single-stranded binding proteins- bind to and stabilize single-stranded DNA- Topoisomerase- corrects “overwinding” ahead of replication forks by breaking, swiveling and rejoining DNA strands - Primase- an enzyme which starts an RNA chain from scratch and adds RNA nucleotides one at a time using parental DNA as a template- Primer- short (5-10 nucleotides long), and the 3’ end serves as the starting point for new DNA strand- DNA polymerases- enzymes that catalyze the elongation of new DNA @ a replication forko Most DNA polymerases require a primer and a DNA template strando Rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cellso Cannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3’ endo Initial nucleotide strand is a short RNA primer- Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate- dATP supplies adenine to DNA and is similar to the ATP of energy metabolism- the difference is in their sugars: dATP has deoxyribose while ATP has ribose- As each monomer of dATP joins DNA strand, it loses two phosphate groups as a molecule of pyrophosphate - Antiparallel structure of double helix affects replication- DNA polymerases add nucleotides only to free 3’ end of a growing strand; therefore, a new DNA strand can elongate only in the 5’ to 3’ direction- Leading strand- DNA polymerase synthesizes this strand continuously, moving toward the replication