The Molecular Basis of Inheritance

Transcription

1 hapter 16 he Molecular Basis of Inheritance Dr. Wendy Sera Houston ommunity ollege Biology 1406 Key oncepts in hapter DN is the genetic material 2. Many proteins work together in DN replication and repair. 3. chromosome consists of a DN molecule packed together with proteins. Life s Operating Instructions In 1953, James Watson and Francis rick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DN Figure 16.1 What is the structure of DN? 1

2 Life s Operating Instructions, cont. Hereditary information is encoded in DN and reproduced in all cells of the body his DN program directs the development of biochemical, anatomical, physiological, and (to some extent) behavioral traits DN is copied during DN replication, and cells can also repair their DN oncept 16.1: DN is the genetic material Early in the 20th century, the identification of the molecules of inheritance loomed as a major challenge to biologists he Search for the enetic Material: Scientific Inquiry When. H. Morgan s group ( he Fly Room ) showed that genes are located on chromosomes, the two components of chromosomes DN and protein became candidates for the genetic material he role of DN in heredity was first discovered by studying bacteria and the viruses that infect them 2

3 Evidence hat DN an ransform Bacteria he discovery of the genetic role of DN began with research by Frederick riffith in 1928 riffith worked with two strains of a bacterium (Streptococcus pneumonia), one pathogenic and one harmless When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DN Experiment Living S cells (pathogenic control) Living R cells (nonpathogenic control) Heat-killed S cells (nonpathogenic control) Mixture of heatkilled S cells and living R cells Results Mouse dies Mouse healthy Mouse healthy Mouse dies Figure 16.2 an a genetic trait be transferred between different bacterial strains (riffith s experiment)? Living S cells very, Mcarty, & MacLeod In 1944, Oswald very, Maclyn Mcarty, and olin MacLeod announced that the transforming substance was DN! Many biologists remained skeptical, mainly because little was known about DN 3

4 100 nm Evidence hat Viral DN an Program ells More evidence for DN as the genetic material came from studies of viruses that infect bacteria Such viruses, called bacteriophages (or phages), are widely used in molecular genetics research virus is DN (sometimes RN) enclosed by a protective coat, often simply protein Figure 16.3 Virus infecting a bacterial cell Phage head DN ail sheath ail fiber enetic material Bacterial cell Bacteriophages infecting a bacterial cell Phage head ail sheath ail fiber DN Bacterial cell 4

5 nimation: Phage 2 Reproductive ycle Hershey & hase: Is protein or DN the genetic material of phage 2? In 1952, lfred Hershey and Martha hase showed that DN is the genetic material of a phage known as 2 hey designed an experiment showing that only one of the two components of 2 (DN or protein) enters an E. coli cell during infection hey concluded that the injected DN of the phage provides the genetic information Figure 16.4 Is protein or DN the genetic material of phage 2? Experiment Batch 1: Radioactive sulfur ( 35 S) in phage protein 1 Labeled phages 2 gitation frees outside infect cells. phage parts from cells. Radioactive protein 3 entrifuged cells form a pellet. 4 Radioactivity (phage protein) found in liquid entrifuge Batch 2: Radioactive phosphorus ( 32 P) in phage DN Radioactive DN Pellet entrifuge Pellet 4 Radioactivity (phage DN) found in pellet 5

6 nimation: Hershey-hase Experiment dditional Evidence hat DN Is the enetic Material It was known that DN is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group In 1950, Erwin hargaff reported that DN composition varies from one species to the next his evidence of diversity made DN a more credible candidate for the genetic material hargaff s Rules wo findings became known as hargaff s Rules he base composition of DN varies between species In any species the number of and bases are equal and the number of and bases are equal he basis for these rules was not understood until the discovery of the double helix 6

7 Sugar phosphate backbone end Nitrogenous bases hymine () Figure 16.5 he structure of a DN denine () ytosine () Phosphate end Sugar DN (deoxyribose) nucleotide Nitrogenous base uanine () nimation: DN and RN Structure Building a Structural Model of DN: Scientific Inquiry fter DN was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredity Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure Franklin produced a picture of the DN molecule using this technique 7

8 Figure 16.6 Rosalind Franklin and her X-ray diffraction photo of DN (a) Rosalind Franklin (b) Franklin s X-ray diffraction photograph of DN Building a Structural Model of DN, cont. Franklin s X-ray crystallographic images of DN enabled Watson to deduce that DN was helical he X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases he pattern in the photo suggested that the DN molecule was made up of two s, forming a double helix nimation: DN Double Helix 8

9 Figure 16.7 he structure of the double helix end Hydrogen bond end 3.4 nm 1 nm (a) Key features of DN structure 0.34 nm end (b) Partial chemical structure end (c) Space-filling model Video: Stick Model of DN (Deoxyribonucleic cid) Building a Structural Model of DN, cont. Watson and rick built models of a double helix to conform to the X-rays and chemistry of DN 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) 9

10 Building a Structural Model of DN, cont. t first, Watson and rick thought the bases paired like with like ( with, and so on), but such pairings did not result in a uniform width Instead, pairing a purine with a pyrimidine resulted in a uniform width consistent with the X-ray data Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width consistent with X-ray data Building a Structural Model of DN, cont. Watson and rick reasoned that the pairing was more specific, dictated by the base structures hey determined that adenine () paired only with thymine (), and guanine () paired only with cytosine () he Watson-rick model explains hargaff s rules: in any organism the amount of =, and the amount of = Figure 16.8 Base pairing in DN Sugar denine () Sugar hymine () Sugar Sugar uanine () ytosine () 10

11 oncept 16.2: Many proteins work together in DN replication and repair he relationship between structure and function is manifest in the double helix Watson and rick noted that the specific base pairing suggested a possible copying mechanism for genetic material We now call this copying mechanism DN replication he Basic Principle: Base Pairing to a emplate Strand Since the two s of DN are complementary, each acts as a template for building a new in replication In DN replication, the parent molecule unwinds, and two new daughter s are built based on basepairing rules Watson and rick s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old (derived or conserved from the parent molecule) and one newly made 2014 Pearson Education, Inc. nimation: DN Replication Overview 11

12 Figure 16.9 model for DN replication: the basic concept (a) Parental molecule (b) Separation of parental s into templates (c) Formation of new s complementary to template s DN Replication: loser Look he copying of DN is remarkable in its speed and accuracy More than a dozen enzymes and other proteins participate in DN replication etting Started Replication begins at particular sites called origins of replication, where the two DN s are separated, opening up a replication bubble eukaryotic chromosome may have hundreds or even thousands of origins of replication Replication proceeds in both directions from each origin, until the entire molecule is copied 12

13 0.5 µm 0.25 µm Figure (a) Origin of replication in an E. coli cell Origin of Parental (template) replication Daughter (new) Replication Bacterial fork chromosome Doubleed Replication bubble DN molecule (b) Origins of replication in a eukaryotic cell Origin of replication Double-ed DN molecule Bubble Eukaryotic chromosome Parental (template) Daughter (new) Replication fork wo daughter DN molecules wo daughter DN molecules nimation: Origins of Replication etting Started, cont. t the end of each replication bubble is a replication fork, a Y-shaped region where new DN s are elongating Helicases are enzymes that untwist the double helix at the replication forks Single- binding proteins bind to and stabilize single-ed DN opoisomerase corrects overwinding ahead of replication forks by breaking, swiveling, and rejoining DN s 13

14 Figure Some of the proteins involved in the initiation of DN replication Primase opoisomerase Replication fork RN primer Helicase Single- binding proteins etting Started, cont. DN polymerases cannot initiate synthesis of a polynucleotide; they can only add nucleotides to an existing end he initial nucleotide is a short RN primer n enzyme called primase can start an RN chain from scratch and adds RN nucleotides one at a time using the parental DN as a template he primer is short (5 10 nucleotides long), and the end serves as the starting point for the new DN Synthesizing a New DN Strand Enzymes called DN polymerases catalyze the elongation of new DN at a replication fork Most DN polymerases require a primer and a DN template he rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells 14

15 New emplate Sugar Phosphate Base DN polymerase OH P P i Pyrophosphate Nucleotide Figure Incorporation of a nucleotide into a DN 2 P i OH ntiparallel Elongation Leading Strand he antiparallel structure of the double helix affects replication DN polymerases add nucleotides only to the free end of a growing ; therefore, a new DN can elongate only in the to direction long one template of DN, the DN polymerase synthesizes a leading continuously, moving toward the replication fork nimation: Leading Strand 15

16 Figure Synthesis of the leading during DN replication Leading Overview Origin of replication Primer Lagging Lagging Overall directions of replication Leading Figure Synthesis of the leading during DN replication 1 DN pol III starts to synthesize leading. Parental DN Origin of replication RN primer Sliding clamp DN pol III 2 ontinuous elongation in the to direction ntiparallel Elongation Lagging Strand o elongate the other new, called the lagging, DN polymerase must work in the direction away from the replication fork he lagging is synthesized as a series of segments called Okazaki fragments, which are later joined together by DN ligase 16

17 Figure Synthesis of the lagging Leading Lagging Overview Origin of replication Lagging 2 1 Overall directions of replication Leading Leading Lagging Overview Origin of replication Lagging Figure Synthesis of the lagging emplate Primase makes RN primer. Origin of replication RN primer for fragment 1 3 DN pol III detaches. Overall directions of replication Leading DN pol III makes Okazaki fragment 1. Okazaki fragment 1 RN primer for fragment 2 Okazaki fragment DN pol III makes Okazaki fragment DN pol I replaces RN with DN. DN ligase forms bonds between DN fragments. Overall direction of replication nimation: Lagging Strand 17

18 Figure Summary of bacterial DN replication. Single- binding proteins Helicase Parental DN Leading template Leading DN pol III Primer Primase 5 DN pol III 4 Leading Lagging Lagging Overview Origin of replication Overall directions of replication 3 Leading Lagging DN pol I DN ligase 2 1 Lagging template nimation: DN Replication Review 18

19 he DN Replication omplex he proteins that participate in DN replication form a large complex, a DN replication machine he DN replication machine may be stationary during the replication process Recent studies support a model in which DN polymerase molecules reel in parental DN and extrude newly made daughter DN molecules Figure current model of the DN replication complex Leading template DN pol III Parental DN onnecting protein DN pol III Helicase Leading Lagging template Lagging BioFlix: DN Replication 19

20 Proofreading and Repairing DN DN polymerases proofread newly made DN, replacing any incorrect nucleotides In mismatch repair of DN, repair enzymes correct errors in base pairing DN can be damaged by exposure to harmful chemical or physical agents such as cigarette smoke and X-rays; it can also undergo spontaneous changes In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DN Figure Nucleotide excision repair of DN damage Nuclease DN polymerase DN ligase Evolutionary Significance of ltered DN Nucleotides Error rate after proofreading repair is low, but not zero Sequence changes may become permanent and can be passed on to the next generation hese changes (mutations) are the source of the genetic variation upon which natural selection operates 20

21 Replicating the Ends of DN Molecules he limitations of DN polymerase create problems for the linear DN of eukaryotic chromosomes he usual replication machinery provides no way to complete the ends, so repeated rounds of replication produce shorter DN molecules with uneven ends his is not a problem for prokaryotes, most of which have circular chromosomes. Ends of parental DN s Leading Lagging Figure Shortening of the ends of linear DN molecules Lagging Parental Last fragment RN primer New leading New lagging Next-to-last fragment Removal of primers and replacement with DN where a end is available Second round of replication Further rounds of replication Shorter and shorter daughter molecules elomeres Eukaryotic chromosomal DN molecules have special nucleotide sequences at their ends called telomeres elomeres do not prevent the shortening of DN molecules, but they do postpone the erosion of genes near the ends of DN molecules It has been proposed that the shortening of telomeres is connected to aging 21

22 Figure elomeres in Mouse hromosomes 1 µm elomerase If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce! n enzyme called telomerase catalyzes the lengthening of telomeres in germ cells he shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions here is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist oncept 16.3: chromosome consists of a DN molecule packed together with proteins he bacterial chromosome is a double-ed, circular DN molecule associated with a small amount of protein In a bacterium, the DN is supercoiled and found in a region of the cell called the nucleoid Eukaryotic chromosomes have linear DN molecules associated with a large amount of protein 22

23 hromatin In the eukaryotic cell, DN is precisely combined with proteins in a complex called chromatin hromosomes fit into the nucleus through an elaborate, multilevel system of packing Histones are proteins that are responsible for the first level of DN packing in chromatin. Figure hromatin packing in a eukaryotic chromosome DN double helix (2 nm in diameter) Nucleosome (10 nm in diameter) 30-nm fiber hromatid (700 nm) Histones DN, the Histones double helix Histone tail H1 Nucleosomes, or beads on a string (10-nm fiber) 30-nm fiber Loops Scaffold 300-nm fiber Replicated Looped chromosome domains (1,400 nm) (300-nm fiber) Metaphase chromosome nimation: DN Packing 23

24 hromatin, cont. hromatin undergoes changes in packing during the cell cycle t interphase, some chromatin is organized into a 10-nm fiber, but much is compacted into a 30-nm fiber, through folding and looping Interphase chromosomes occupy specific restricted regions in the nucleus and the fibers of different chromosomes do not become entangled Figure Painting hromosomes 5 µm hromatin, cont. Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis Loosely packed chromatin is called euchromatin During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions 24