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Chapter 16 The Molecular Basis of Inheritance PowerPoint® Lecture Presentations for Biology Eighth Edition Neil Campbell and Jane Reece Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings Overview: Life’s Operating Instructions • In 1953, James Watson and Francis Crick introduced an elegant double-helical model for the structure of deoxyribonucleic acid, or DNA • DNA, the substance of inheritance, is the most celebrated molecule of our time • Hereditary information is encoded in DNA and reproduced in all cells of the body • This DNA program directs the development of biochemical, anatomical, physiological, and (to some extent) behavioral traits Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-1 Concept 16.1: DNA is the genetic material • Early in the 20th century, the identification of the molecules of inheritance loomed as a major challenge to biologists Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Search for the Genetic Material: Scientific Inquiry • When T. H. Morgan’s group showed that genes are located on chromosomes, the two components of chromosomes—DNA and protein—became candidates for the genetic material • he was the one that worked on fruit flies and took 2 years to naturally get 1 mutant white eyed fly; • He bred them and ended up determining that some traits were sex linked • And that other traits were carried on other types of chromosomes • BTW, he was a Nobel Prize in 1933 for his contribution to genetics Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • The key factor in determining the genetic material was choosing appropriate experimental organisms • The role of DNA in heredity was first discovered by studying bacteria and the viruses that infect them Evidence That DNA Can Transform Bacteria • The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928 • Griffith worked with two strains of a bacterium, one pathogenic and one harmless Non pathogenic is the “rough” version pneumococcus Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings “smooth” version is the Pathogenic • Griffith mixed: – heat-killed remains of the pathogenic strain – living cells of the harmless strain, • He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Up to this time they thought the genetic factor could be protein or nucleic acids • Scientists didn’t believe it was nucleic acids because they were so simple (they didn’t know how nucleic acids worked) Griffith Continued 1. Inject live nonpathogenic= live mouse 2. Inject pathogenic= mouse dies 3. Inject heat killed= mouse alive 4. Inject heat killed pathogenic and live 5. Nonpathogenic= dead mouse What happened? The live nonpathogenic Took up the DNA from the dead pathogenic Incorporated it into its own genome, started Making copies of the pathogenic genome And killed the mice • In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming substance was DNA • Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Oswald Avery experiment • What they did: broke open the heat killed pathogenic bacteria • Ran 3 different experiments, each was designed to inactivate the 3 likely “agents of transformation” – RNA -they added Rnase to inactivate it – DNA –they added DNAse to inactivate it – Protein- they added protease to inactivate it • He then tested each for their ability to transform live nonpathogenic bacteria into pathogenic • They determined that DNA was the only one that caused the transformation (how did this experiment work?) • Many biologists remained skeptical, mainly because little was known about DNA Evidence That Viral DNA Can Program Cells • More evidence for DNA 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 Animation: Phage T2 Reproductive Cycle Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-3 Phage head Tail sheath Tail fiber Bacterial cell 100 nm DNA • In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2 – Remember: phages (or bacteriophages) • Are viruses that infect bacteria T2 happens to like to infect E. coli Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • To determine the source of genetic material in the phage, – they designed an experiment showing that only one of the two components of T2 enters an E. coli cell during infection • DNA or protein They concluded that the injected DNA of the phage provides the genetic information Fig. 16-4-1 EXPERIMENT Phage Radioactive protein Bacterial cell Batch 1: radioactive sulfur (35S) DNA Radioactive DNA Batch 2: radioactive phosphorus (32P) Batch 1 was labeled with radioactive sulfur which would incorporate Into the protein portion of the phage (the amino acid cysteine has sulfur in it) Batch 2 had radioactive phosphorus the binds to the DNA Inside The phage Fig. 16-4-2 EXPERIMENT Phage Empty protein Radioactive shell protein Bacterial cell Batch 1: radioactive sulfur (35S) DNA Phage DNA Radioactive DNA Batch 2: radioactive phosphorus (32P) Mechanically disrupted the Membranes by the use of a Homogenizer- fancy word for blender Fig. 16-4-3 The proteins are lighter than the DNA So the proteins float while DNA will Make a pellet; they found that the radiolabeled DNA Was found in the bacteria but not any of the radioLabeled protein because the bacteria “ingested” the viral DNA EXPERIMENT Phage Empty protein Radioactive shell protein Radioactivity (phage protein) in liquid Bacterial cell Batch 1: radioactive sulfur (35S) DNA Phage DNA Centrifuge Pellet (bacterial cells and contents) Radioactive DNA Batch 2: radioactive phosphorus (32P) Centrifuge Pellet Radioactivity (phage DNA) in pellet For the curious minded: • Proteins were recognized as a biological molecule in the 18th century (egg albumin, fibrin, wheat gluten, and blood serum albumin were distinguishable forms at this time) • They were recognized by their ability to coagulate or flocculate (come out of suspension) based on heating or the addition of acid • Frederic Sanger was the 1st to correctly sequence a protein: insulin • And won a Nobel for this in 1958 And how we found out about nucleic acids • Friedrich Miescher wanted to isolate the protein components of leukocytes (white blood cells). • Miescher made arrangements for a local surgical clinic to send him used, pus-coated patient bandages • once he received the bandages, he planned to wash them, filter out the leukocytes, and extract and identify the various proteins within the white blood cells. • he came across a substance from the cell nuclei that had chemical properties unlike any protein, – -a much higher phosphorous content – No sulfur – resistance to proteolysis (protein digestion) he found something new More FYI Mature red blood cells do not have a nucleus; probably because up to 1/3 of their total cell weight is taken up by hemoglobin so they can carry more oxygen White blood cells do have nuclei since they are only needed at specific sites and do not recirculate What DNA is made of: How to sequence DNA • Purines are double ringed structures= adenine and guanine • Pyrimidines are single ringed= cytosine and thymine and uracil Uracil is only found in RNA as a replacement for thymine • Chargaff’s rules state that in any species there is an equal number of A and T bases, and an equal number of G and C bases • Chargaff knew the percentages but didn’t know the reason for the %s: the double helix • In humans for example the four bases show up in these percentages: – A= 30.3% G=19.5% – T=30.3% C=19.9% Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Additional Evidence That DNA Is the Genetic Material • It was known that DNA is a polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group • In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next • This evidence of diversity made DNA a more credible candidate for the genetic material Animation: DNA and RNA Structure Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • The percentages are going to vary by species • Remember, the more you have in common biochemically with another species the more closely related you are to each other (this is part of the biochemistry link we talked about in evolution) Fig. 16-5 Sugar–phosphate backbone 5 end Nitrogenous bases 5’ means the 5th carbon of the 5 carbon sugar Thymine (T) Adenine (A) Cytosine (C) 3’ means the 3rd carbon of The 5 carbon sugar DNA nucleotide Phosphate Sugar (deoxyribose) 3 end Guanine (G) Building a Structural Model of DNA: Scientific Inquiry • After most biologists became convinced that DNA was the genetic material, the challenge was to determine how its structure accounts for its role • Up to this point scientists knew it had 4 nitrogenous bases, a phosphate acid and a sugar • They didn’t know the configuration But they thought in was a triple strand (Linus Pauling) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure • Franklin produced a picture of the DNA molecule using this technique • (Varying accounts, of course, about this story) She did a majority of the work involved in discovering the form of DNA • Frank and Wilkins didn’t really work together, they both ran separate research groups that were both working on the structure of DNA • They got off on a bad note: he initially thought she was a research assistant, and not a peer • Realizing his mistake didn’t really solve anything • Because of this there was tension between the two • Wilkins, withouth Rosalind knowing, showed Watson a photo that Rosalind Frank had taken • Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical Fig. 16-6 (a) Rosalind Franklin (b) Franklin’s X-ray diffraction photograph of DNA • The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases • The width suggested that the DNA molecule was made up of two strands, forming a double helix Animation: DNA Double Helix Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Watson and Crick thought the phosphate/sugar backbone was really on the inside and the nitrogenous bases were on the outside- so they had to flip their idea inside out Fig. 16-7a 5 end Hydrogen bond 3 end 1 nm 3.4 nm 3 end 0.34 nm (a) Key features of DNA structure (b) Partial chemical structure 5 end Fig. 16-7b (c) Space-filling model • Watson 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 antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • At first, Watson and Crick thought the bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width • Instead, pairing a purine (A or G) with a pyrimidine (C or T) resulted in a uniform width consistent with the X-ray Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-UN1 Purine + purine: too wide Pyrimidine + pyrimidine: too narrow Purine + pyrimidine: width consistent with X-ray data • Watson and Crick reasoned that the pairing was more specific, dictated by the base structures • They determined that adenine (A) paired only with thymine (T), and guanine (G) paired only with cytosine (C) • The Watson-Crick model explains Chargaff’s rules: in any organism the amount of A = T, and the amount of G = C Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-8 Adenine (A) Thymine (T) Guanine (G) Cytosine (C) Watson Crick and Wilkins Watson/Crick published first so got all the glory while Rosalind Franklin was published as a support article in the same edition of Nature. Watson/Crick/Wilkins were given the Nobel Prize in Physiology or medicine in 1962 Rosalind Franklin got Ovarian cancer at the age Of 37 died. Concept 16.2: Many proteins work together in DNA replication and repair • The relationship between structure and function is manifest in the double helix • Watson and Crick noted that the specific base pairing suggested a possible copying mechanism for genetic material Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings The Basic Principle: Base Pairing to a Template Strand • Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication • In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules Animation: DNA Replication Overview Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand • Competing models were the conservative model (the two parent strands rejoin- magically they thought) • the dispersive model (each strand is a mix of old and new like a patchwork) Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-10 Parent cell (a) Conservative model (b) Semiconservative model (c) Dispersive model First replication Second replication • Messelsohn and Stahl Experiment • Experiments by Matthew Meselson and Franklin Stahl supported the semiconservative model • Their experiment: – They labeled the nucleotides of the old strands with a heavy isotope of nitrogen (they cultured e. coli in the presence of the heavy isotope) • They also cultured e coli with the normal isotope of nitrogen Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • They then processed each batch of e. coli and mixed them in culture together • They centrifuge them (with cesium chloride, which is about the same weight as DNA) to make sure that the nitrogen-15 would actually separate out at as lower band than the nitrogen-14 The real experiment: • E. coli grown for 14 generations in the nitrogen-15 (NH4Cl was the nitrogen source) • They prepared the E. coli and separated some out to be grown in the nitrogen-14 (so old nitrogen-15 e. coli moved to a new medium with nitrogen-14) • Sampled the broth every 20 minutes (the replication time for e. coli) • Generation 0= all heavy • Next generations= neither heavy nor light • 2 generations= half light, half intermediate • After more generations= more light than intermediate • The first replication produced a band of hybrid DNA, eliminating the conservative model(why?) • A second replication produced both light and hybrid DNA, eliminating the dispersive model and supporting the semiconservative model • https://www.youtube.com/watch?v=ZTMyNmZr cwA Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-11a EXPERIMENT 1 Bacteria cultured in medium containing 15N 2 Bacteria transferred to medium containing 14N RESULTS 3 DNA sample centrifuged after 20 min (after first application) 4 DNA sample centrifuged after 20 min (after second replication) Less dense More dense Fig. 16-11b CONCLUSION First replication Conservative model If this were true, then they would Have had light and heavy, and no Intermediate, over time they would Have had more light and less heavy Semiconservative model Dispersive model Second replication DNA Replication: A Closer Look • The copying of DNA is remarkable in its speed and accuracy • More than a dozen enzymes and other proteins participate in DNA replication https://www.youtube.com/watch?v=8kK2zwjRV0 M (12:59) Crash course biology lesson 10 Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Getting Started • Replication begins at special sites called origins of replication, where the two DNA 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 the entire molecule is copied Animation: Origins of Replication Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-12a Remember: bacteria reproduce through binary fission (exact method not known) but the Circular DNA must Replicate before the cell can divide Origin of replication Parental (template) strand Daughter (new) strand Doublestranded DNA molecule Replication fork Replication bubble 0.5 µm Two daughter DNA molecules (a) Origins of replication in E. coli Fig. 16-12b THIS IS HAPPENING DURING THE S PHASE OF INTERPHASE Origin of replication Double-stranded DNA molecule Parental (template) strand Daughter (new) strand 0.25 µm Bubble Replication fork Two daughter DNA molecules (b) Origins of replication in eukaryotes • At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating • They elongate in BOTH directions at the same time BUT the new strand is only made in the 5’ to 3’ end • There are free nitrogenous bases floating around in the cytoplasm Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-13 Single-strand binding protein binds to and stabilizes single-stranded DNA until it can be used as a template know this Primase adds a few Single-strand binding Topoisomerase corrects “overwinding” proteins RNA nucleotides to start the replication ahead of replication forks by breaking, swiveling, and rejoining DNA strands have to Know this for the exam 3 Topoisomerase 5 RNA primer 5 3 5 Helicase Helicases are enzymes that untwist the double helix at the replication forks (breaks the h bonds) have to know this for the exam 3 • DNA polymerases (gotta know this enzyme) cannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3 end • The initial nucleotide strand is a short RNA primer called primase about 5-10 nucleotides long adds RNA nucleotides one at a time using the parental DNA as a template • Then new DNA can start being added to the 3’ end of the RNA primer Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Synthesizing a New DNA Strand • Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork • Most DNA polymerases require a primer and a DNA template strand • The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate The base would be an A, T, C, G • For instance, 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 the DNA strand, it loses two phosphate groups as a molecule of pyrophosphate • This is a COUPLED EXERGONIC REACTION that helps drive the polymerization reaction Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-14 The bases are held together by H bonds, the Phosphate is attached to the phosphate backbone by a phosphodiester Bond courtesy of the DNA polymerase New strand 5 end Sugar 5 end 3 end T A T C G C G G C G C T A A Base Phosphate Template strand 3 end 3 end DNA polymerase A Pyrophosphate 3 end C Nucleoside triphosphate 5 end C 5 end Antiparallel Elongation • The antiparallel structure of the double helix (two strands oriented in opposite directions) affects replication • DNA polymerases add nucleotides only to the free 3end of a growing strand; therefore, a new DNA strand can elongate only in the 5 to 3direction • The template strand is being read in the 3’ to 5’ direction Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork • (your book shows a leading strand and lagging strand on the same template side; remember: you have the origin of replication that are all over; so, depending on what side of the replication fork you are on you are either lagging or leading)) Animation: Leading Strand Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-15 Overview Origin of replication Leading strand Lagging strand Primer Lagging strand Leading strand Overall directions of replication Origin of replication 3 5 RNA primer 5 “Sliding clamp” 3 5 Parental DNA DNA poll III 3 5 DNA pol 3 replaces the RNA primer (takes over for and Starts to place nucleotides); It is moved along by a protein Called the “sliding clamp” Then DNA pol 2 digests Away the primer and Replaces it with the proper Nucleotides to fill the gap 5 3 5 Fig. 16-15a Overview Origin of replication Leading strand Lagging strand Primer Leading strand Lagging strand Overall directions of replication Fig. 16-15b Origin of replication 3 5 RNA primer 5 “Sliding clamp” 3 5 Parental DNA DNA pol III 3 5 5 3 5 • To elongate the other new strand, called the lagging strand, DNA polymerase must work in the direction away from the replication fork • The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase • It joins the phosphate backbones of all the Okazaki fragments into a continuous DNA strand Animation: Lagging Strand Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-16 Overview Origin of replication Lagging strand Leading strand Lagging strand 2 1 Leading strand Overall directions of replication 3 5 5 Template strand 3 RNA primer 3 5 3 1 5 3 5 Okazaki fragment 3 1 5 3 5 2 3 5 2 3 3 5 1 3 5 1 5 2 1 3 5 Overall direction of replication Fig. 16-16a Overview Origin of replication Leading strand Lagging strand Lagging strand 2 1 Leading strand Overall directions of replication Fig. 16-16b1 3 Template strand 5 5 3 Fig. 16-16b2 3 Template strand 3 5 5 3 RNA primer 1 5 3 5 Fig. 16-16b3 3 Template strand 3 5 5 3 RNA primer 3 1 5 5 Okazaki fragment 3 1 5 3 5 Fig. 16-16b4 3 5 5 Template strand 3 3 RNA primer 3 1 5 5 3 5 Okazaki fragment 3 1 5 3 5 2 1 3 5 Fig. 16-16b5 3 5 5 Template strand 3 3 RNA primer 3 1 5 5 3 1 5 3 5 2 3 3 5 Okazaki fragment 1 3 5 5 3 5 2 1 Fig. 16-16b6 3 5 5 Template strand 3 3 RNA primer 3 1 5 5 3 1 5 3 5 2 3 3 5 Okazaki fragment 3 5 1 5 3 5 2 1 5 3 1 2 Overall direction of replication 3 5 Table 16-1 You have to Know these! Fig. 16-17 Overview Origin of replication Lagging strand Leading strand Leading strand Lagging strand Overall directions of replication Single-strand binding protein Helicase 5 Leading strand 3 DNA pol III 3 Parental DNA Primer 5 Primase 3 DNA pol III Lagging strand 5 4 DNA pol I 3 5 3 2 DNA ligase 1 3 5 The DNA Replication Complex • The proteins that participate in DNA replication form a large complex, a “DNA replication machine” • The DNA replication machine is probably stationary during the replication process • Recent studies support a model in which DNA polymerase molecules “reel in” parental DNA and “extrude” newly made daughter DNA molecules Animation: DNA Replication Review Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Proofreading and Repairing DNA • DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides • In mismatch repair of DNA, repair enzymes correct errors in base pairing • DNA can be damaged by chemicals, radioactive emissions, X-rays, UV light, and certain molecules (in cigarette smoke for example) • In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-18 Nuclease Cuts the “problem” on two side DNA polymerase DNA pol replaces the mistake with The correct nucleotides DNA ligase DNA ligase seals the DNA together Replicating the Ends of DNA Molecules • Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes • The usual replication machinery provides no way to complete the 5 end (because no way to replace the primer), so repeated rounds of replication produce shorter DNA molecules Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-19 5 Leading strand Lagging strand Ends of parental DNA strands 3 Last fragment Previous fragment RNA primer Lagging strand 5 3 Parental strand Removal of primers and replacement with DNA where a 3 end is available 5 3 Second round of replication 5 New leading strand 3 New lagging strand 5 3 Further rounds of replication Shorter and shorter daughter molecules • Eukaryotic chromosomal DNA molecules have at their ends nucleotide sequences called telomeres (supposedly non coding areas) • Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules • It has been proposed that the shortening of telomeres is connected to aging Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-20 1 µm • If chromosomes of germ cells became shorter in every cell cycle, essential genes would eventually be missing from the gametes they produce • An enzyme called telomerase catalyzes the lengthening of telomeres in germ cellscompensating for the shortening Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • The shortening of telomeres might protect cells from cancerous growth by limiting the number of cell divisions • There is evidence of telomerase activity in cancer cells, which may allow cancer cells to persist Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Concept 16.3 A chromosome consists of a DNA molecule packed together with proteins • The bacterial chromosome is a doublestranded, circular DNA molecule associated with a small amount of protein • Eukaryotic chromosomes have linear DNA molecules associated with a large amount of protein • In a bacterium, the DNA is “supercoiled” and found in a region of the cell called the nucleoid Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Chromatin is a complex of DNA and protein, and is found in the nucleus of eukaryotic cells • Histones are proteins that are responsible for the first level of DNA packing in chromatin Animation: DNA Packing Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-21a Nucleosome (10 nm in diameter) DNA double helix (2 nm in diameter) H1 Histones DNA, the double helix Histones Histone tail Nucleosomes, or “beads on a string” (10-nm fiber) Fig. 16-21b Chromatid (700 nm) 30-nm fiber Loops Scaffold 300-nm fiber Replicated chromosome (1,400 nm) 30-nm fiber Looped domains (300-nm fiber) Metaphase chromosome • Chromatin is organized into fibers • 10-nm fiber – DNA winds around histones to form nucleosome “beads” – Nucleosomes are strung together like beads on a string by linker DNA • 30-nm fiber – Interactions between nucleosomes cause the thin fiber to coil or fold into this thicker fiber Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • 300-nm fiber – The 30-nm fiber forms looped domains that attach to proteins • Metaphase chromosome – The looped domains coil further – The width of a chromatid is 700 nm Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Most chromatin is loosely packed in the nucleus during interphase and condenses prior to mitosis • Loosely packed chromatin is called euchromatin –the normal kind we talked about • During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into heterochromatin –will appear clumpy • Dense packing of the heterochromatin makes it difficult for the cell to express genetic information coded in these regions Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings • Histones can undergo chemical modifications that result in changes in chromatin organization – For example, phosphorylation of a specific amino acid on a histone tail affects chromosomal behavior during meiosis Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings Fig. 16-22 RESULTS Condensin and DNA (yellow) Outline Condensin of nucleus (green) Normal cell nucleus DNA (red at periphery) Mutant cell nucleus