Lecture Notes: Chapter 16 - The Molecular Basis of Inheritance PDF

Q 1 -2 Chapter 16: The Molecular Basis of Inheritance  Lecture Objectives ✓Know the various experiments that led to the discovery of DNA & RNA ✓Know differences between DNA & RNA ✓Understand DNA replication ✓The editing process of DNA ✓Mistakes that can occur in DNA ✓Packaging of DNA...

Q 1 -2 Chapter 16: The Molecular Basis of Inheritance  Lecture Objectives ✓Know the various experiments that led to the discovery of DNA & RNA ✓Know differences between DNA & RNA ✓Understand DNA replication ✓The editing process of DNA ✓Mistakes that can occur in DNA ✓Packaging of DNA How does DNA replication transmit genetic info? We just heard about mitosis & meiosis and how DNA needs to REPLICATE to pass the genetic information down to offspring How does DNA replication transmit genetic info?  DNA is the genetic material Early in the 20th century, the identification of the molecules of inheritance posed a major challenge to biologists  The Search for the Genetic Material: T. H. Morgan’s group showed that genes are located on chromosomes, 1910-- Using fruit flies DNA and protein—became candidates for inheritable material Frederick Griffith in 1928- discovery of the genetic role of DNA discovered through research on bacteria & the viruses that infect them worked with two strains of a bacterium, one pathogenic and one harmless  Can a genetic trait be transferred between different bacterial strains? Transformation= change in genotype and phenotype due to assimilation of foreign DNA Evidence That DNA Can Transform Bacteria 1944- Oswald Avery, Maclyn McCarty, and Colin MacLeod identified the transforming substance as DNA Many biologists remained skeptical, mainly because little was known about DNA Further studies using: bacteriophages (phages)= viruses that infect bacteria widely used as tools by researchers in molecular genetics viruses= DNA (sometimes RNA) enclosed by a protective coat, often simply protein Phage T2 Reproductive Cycle Evidence That Viral DNA Can Program Cells In 1952,-- Alfred Hershey and Martha Chase showed that DNA is the genetic material of a T2 phage designed an experiment showing that only1 of 2 components of T2 (DNA or protein) enters an E. coli cell during infection They concluded that the injected DNA of the phage provides the genetic information More Evidence That DNA Is the Genetic Material In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next evidence of molecular diversity among organisms made DNA a more credible candidate for the genetic material Chargaff’s rules: base composition of DNA varies between species In any species the # of A & T bases is = and the # of G & C bases is equal why was not understood until the discovery of the double helix… The structure of a DNA strand  Building a Structural Model of DNA Maurice Wilkins & Rosalind Franklin used a technique called X-ray crystallography to study molecular structure Franklin produced a picture of the DNA molecule using this technique  Building a Structural Model of DNA Franklin’s images of DNA allowed James Watson to deduce that DNA was helical also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases The pattern in the photo suggested that the DNA molecule was made up of two strands, forming a double helix  Building a Structural Model of DNA Watson & Crick built models to conform to the X-rays & chemistry Franklin concluded there were 2 outer sugar-phosphate backbones, with the nitrogenous bases paired at the interior Watson built a model in which the backbones were antiparallel (their subunits run in opposite directions) Stick Model DNA Double Helix  Building a Structural Model of DNA At first, Watson &Crick thought the bases paired A with A, and so on, but such pairings did not result in a uniform width across Instead, pairing a purine (A or G) with a pyrimidine (C or T) resulted in a uniform width consistent with the X-ray data  Building a Structural Model of DNA Found pairing was more specific & dictated by the base structures Determined adenine (A) paired only with thymine (T), & guanine (G) paired only with cytosine (C) Their model explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = C Proteins work together in DNA replication & repair Inheritance relies on accurate replication of DNA prior to meiosis & transmission to the next generation Replication prior to mitosis ensures the faithful transmission of genetic info Watson & Crick’s base pairing suggested a possible copying mechanism → DNA replication Since the two strands are complementary, each acts as a template for building a new one yielding 2 exact replicas of the “parental” molecule A model for DNA replication: the basic concept  DNA replication: three alternative models Competing models were Semiconservative model predicts each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand Watson & Crick’s Conservative model (the two parent strands rejoin) & Dispersive model (each strand is a mix of old and new)  How does DNA Replication Inquiry: Does DNA replication follow the conservative, semiconservative, or dispersive model? Experiments by Matthew Meselson & Franklin Stahl supported the semiconservative model  DNA Replication: A Closer Look is remarkable in its speed & accuracy more than a dozen enzymes and other proteins participate! Replication in bacteria is best understood, but evidence suggests that the replication process in eukaryotes and prokaryotes is fundamentally similar  Getting Started DNA Replication begins at particular sites origins of replication- where two DNA strands are separated, opening up a replication “bubble” proceeds in both directions from each origin, until the entire molecule is copied eukaryotic chromosomes have hundreds or even thousands of origins of replication! Animation: Origins of Replication  Getting Started At the end of each bubble is a replication fork: Y-shaped region where parental DNA strands are being unwound Players involved: Helicases untwist the double helix at the replication forks Single-strand binding proteins bind to and stabilize single- stranded DNA Topoisomerase relieves the strain of twisting of the double helix by breaking, swiveling, and rejoining DNA strands It’s complicated!  Synthesizing a New DNA Strand DNA polymerases catalyze the elongation of new DNA at a replication fork can only add nucleotides to the 3’ end of a growing DNA strand Most DNA polymerases require a primer (5-10 bp) to which they can add nucleotides Synthesized by primase  Synthesizing a New DNA Strand nucleoside triphosphate- added to growing DNA strand dATP supplies adenine to DNA and is similar to the ATP of energy metabolism each joins the DNA strand, via a dehydration reaction, losing two phosphate groups (pyrophosphate)  Antiparallel Elongation: Leading Strand D N A polymerases add nucleotides only to the free 3′ end of a growing strand; therefore, it can elongate only in the 5′ → 3′ direction along one strand the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork  Antiparallel Elongation: Lagging Strand To elongate the lagging strand, DNA polymerase must work away from the replication fork The lagging strand is synthesized as a series of Okazaki fragments, which are joined together by DNA ligase  DNA Replication https://www.youtube.com/watch?v=s3nDTGg73AU Bacterial DNA replication Bacterial DNA replication proteins and their functions The “trombone” model: DNA replication complex --The proteins that participate in DNA replication form a large complex that is hypothesized to stay stationary as the DNA winds through  Proofreading & Repairing DNA DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides In mismatch repair enzymes replace incorrectly paired nucleotides that have evaded the proofreading process In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA Evolutionary Significance of Altered DNA Nucleotides Error rate after proofreading and repair is low but not zero sequence changes may become permanent and can be passed on to the next generation changes (mutations) are the source of the genetic variation upon which natural selection operates & are responsible for the appearance of new species!!  Replicating the Ends of DNA Molecules For linear DNA, the usual machinery cannot complete the 5′ ends of daughter DNA strands There is no 3′ end of a preexisting polynucleotide for DNA polymerase to add on to After each round it would produce shorter DNA molecules with uneven ends This is not a problem for prokaryotes, most of which have circular chromosomes!  Replicating the Ends of DNA Molecules A telomere= is repetitive DNA at the end of a eukaryotic chromosome’s DNA molecule postpones the erosion of genes near the ends of DNA molecules possibly connected to aging Telomerase catalyzes the lengthening of telomeres in germ cells Not present in somatic cells  Chromosomes A chromosome consists of a DNA molecule packed together with proteins bacterial chromosome: double- Eukaryotic chromosomes have stranded, circular molecule with a linear DNA molecules associated small amount of protein with a large amount of protein DNA is “supercoiled” in a region of the cell called the nucleoid  Eukaryotic Chromosome DNA is precisely combined with proteins in a complex called chromatin Chromosomes fit into the nucleus through an elaborate, multilevel system of packing Proteins called histones are responsible for the main level of DNA packing in interphase chromatin  Chromatin packing in a eukaryotic chromosome In a 10-nm chromatin fiber, the unfolded chromatin resembles beads on a string, with each “bead” being a nucleosome composed of DNA wound twice around a core of eight histones, two each of the four main histone types (H2A, H2B, H3, H4) amino end of each, the histone tail extends outward from the nucleosome and is involved in regulation of gene expression  DNA Packing https://www.youtube.com/watch?v=9kQpYdCnU14  Chromatin Packing Most chromatin is loosely packed during interphase & condenses prior to mitosis Euchromatin= loosely packed chromatin Heterochromatin= highly condensed chromatin a few regions (centromeres and telomeres) are packed as heterochromatin which makes it difficult for the cell to express genetic information coded in these regions  Lecture Objectives ✓Know the various experiments that led to the discovery of DNA & RNA ✓Know differences between DNA & RNA ✓Understand DNA replication ✓The editing process of DNA ✓Mistakes that can occur in DNA ✓Packaging of DNA