Search for Genetic Material Protein or Nucleic acid

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Search for Genetic Material Protein or Nucleic acid /DNA or RNA? • Griffith’s Transformation

Search for Genetic Material Protein or Nucleic acid /DNA or RNA? • Griffith’s Transformation Experiment • Avery’s Transformation Experiment • Hershey-Chase Bacteriophage Experiment

How do we know that all of our genetic information comes from DNA? •

How do we know that all of our genetic information comes from DNA? • What type of experiment would you design to determine that DNA is the source of all genetic information?

Search for the genetic material: 1. Stable source of information 2. Ability to replicate

Search for the genetic material: 1. Stable source of information 2. Ability to replicate accurately 3. Capable of change Timeline of events: • 1890 Weismann - substance in the cell nuclei controls development. • 1900 Chromosomes shown to contain hereditary information, later shown to be composed of protein & nucleic acids. • 1928 Griffith’s Transformation Experiment • 1944 Avery’s Transformation Experiment • 1953 Hershey-Chase Bacteriophage Experiment • 1953 Watson & Crick propose double-helix model of DNA • 1956 Gierer & Schramm/Fraenkel-Conrat & Singer Demonstrate RNA is viral genetic material.

Fig. 2. 2: Frederick Griffith’s Transformation Experiment - 1928 “transforming principle” demonstrated with Streptococcus

Fig. 2. 2: Frederick Griffith’s Transformation Experiment - 1928 “transforming principle” demonstrated with Streptococcus pneumoniae Griffith hypothesized that the transforming agent was a “IIIS” protein.

Fig. 2. 3: Oswald T. Avery’s Transformation Experiment - 1944 Determined that “IIIS” DNA

Fig. 2. 3: Oswald T. Avery’s Transformation Experiment - 1944 Determined that “IIIS” DNA was the genetic material responsible for Griffith’s results (not RNA). Peter J. Russell, i. Genetics: Copyright © Pearson Education, Inc. , publishing as Benjamin Cummings.

Avery, Mc. Carty, and Mac. Leod Repeated Griffith’s Experiment Oswald Avery Maclyn Mc. Carty

Avery, Mc. Carty, and Mac. Leod Repeated Griffith’s Experiment Oswald Avery Maclyn Mc. Carty Colin Mac. Leod

The Hershey-Chase Experiment Alfred Hershey & Martha Chase worked with a bacteriophage: A virus

The Hershey-Chase Experiment Alfred Hershey & Martha Chase worked with a bacteriophage: A virus that invades bacteria. It consists of a DNA core and a protein coat Protein coat DNA

Hershey-Chase Bacteriophage Experiment - 1953 Bacteriophage = Virus that attacks bacteria and replicates by

Hershey-Chase Bacteriophage Experiment - 1953 Bacteriophage = Virus that attacks bacteria and replicates by invading a living cell and using the cell’s molecular machinery. Fig. 2. 4 Structure of T 2 phage DNA & protein

Evidence That Viral DNA Can Program Cells • More evidence for DNA as the

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 T 2 Reproductive Cycle Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 2. 5: Life cycle of virulent T 2 phage:

Fig. 2. 5: Life cycle of virulent T 2 phage:

 • In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA

• In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T 2 • To determine the source of genetic material in the phage, they designed an experiment showing that only one of the two components of T 2 (DNA or protein) enters an E. coli cell during infection • They concluded that the injected DNA of the phage provides the genetic information Animation: Hershery-Chase Experiment Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 2 -6: Hershey-Chase Bacteriophage Experiment - 1953 1. T 2 bacteriophage is composed

Fig. 2 -6: Hershey-Chase Bacteriophage Experiment - 1953 1. T 2 bacteriophage is composed of DNA and proteins: 2. Set-up two replicates: • • Label DNA with 32 P Label Protein with 35 S 3. Infected E. coli bacteria with two types of labeled T 2 4. 32 P is discovered within the bacteria and progeny phages, whereas 35 S is not found within the bacteria but released with phage ghosts. 1969: Alfred Hershey

How did DNA: 1. Store information? 2. Duplicate itself easily? These questions would be

How did DNA: 1. Store information? 2. Duplicate itself easily? These questions would be answered by discovering DNA’s structure

Why was the discovery of DNA’s Structure so important? Its structure complimented Darwin’s Idea

Why was the discovery of DNA’s Structure so important? Its structure complimented Darwin’s Idea of Natural Selection Its structure complimented Mendel’s Principles of Genetics. Created tools for understanding how genes work and Gene technology

So, What is DNA Made Of?

So, What is DNA Made Of?

Nucleotide = monomers that make up DNA and RNA (Figs. 2. 9 -10) Three

Nucleotide = monomers that make up DNA and RNA (Figs. 2. 9 -10) Three components 1. Pentose (5 -carbon) sugar DNA = deoxyribose RNA = ribose (compare 2’ carbons) 2. Nitrogenous base Purines Adenine Guanine Pyrimidines Cytosine Thymine (DNA) Uracil (RNA) 3. Phosphate group attached to 5’ carbon

Chargaff's Rule of Base Pairing The rules of base pairing (or nucleotide pairing) are:

Chargaff's Rule of Base Pairing The rules of base pairing (or nucleotide pairing) are: • A with T: the purine adenine (A) always pairs with the pyrimidine thymine (T) • C with G: the pyrimidine cytosine (C) always pairs with the purine guanine (G)

James D. Watson & Francis H. Crick - 1953 Double Helix Model of DNA

James D. Watson & Francis H. Crick - 1953 Double Helix Model of DNA Two sources of information: 1. Base composition studies of Erwin Chargaff • indicated double-stranded DNA consists of ~50% purines (A, G) and ~50% pyrimidines (T, C) • amount of A = amount of T and amount of G = amount of C (Chargraff’s rules) • %GC content varies from organism to organism Examples: %A %T %G %C %GC Homo sapiens Zea mays Drosophila Aythya americana 31. 0 25. 6 27. 3 25. 8 31. 5 25. 3 27. 6 25. 8 19. 1 24. 5 22. 5 24. 2 18. 4 24. 6 22. 5 24. 2 37. 5 49. 1 45. 0 48. 4

James D. Watson & Francis H. Crick - 1953 Double Helix Model of DNA

James D. Watson & Francis H. Crick - 1953 Double Helix Model of DNA Two sources of information: 2. X-ray diffraction studies - Rosalind Franklin & Maurice Wilkins 3. Conclusion-DNA is a helical structure with 4. distinctive regularities, 0. 34 nm & 3. 4 nm. Fig. 2. 13

Double Helix Model of DNA: Six main features 1. Two polynucleotide chains wound in

Double Helix Model of DNA: Six main features 1. Two polynucleotide chains wound in a right-handed (clockwise) double-helix. 2. Nucleotide chains are anti-parallel: 3. Sugar-phosphate backbones are on the outside of the double helix, and the bases are oriented towards the central axis. 4. Complementary base pairs from opposite strands are bound together by weak hydrogen bonds. 5’ 3’ 3’ 5’ A pairs with T (2 H-bonds), and G pairs with C (3 H-bonds). e. g. , 5’-TATTCCGA-3’ 3’-ATAAGGCT-3’ 5. Base pairs are 0. 34 nm apart. One complete turn of the helix requires 3. 4 nm (10 bases/turn). 6. Sugar-phosphate backbones are not equally-spaced, resulting in major and minor grooves.

Fig. 2. 15

Fig. 2. 15

 • Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA

• Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical • 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

Fig. 2. 14 B-DNA

Fig. 2. 14 B-DNA

1962: Nobel Prize in Physiology and Medicine James D. Watson Francis H. Crick Maurice

1962: Nobel Prize in Physiology and Medicine James D. Watson Francis H. Crick Maurice H. F. Wilkins What about? Rosalind Franklin

Scientist 1952 1944 1928 Year Experiment Conclusion Griffith 1928 Mice/pneumoniacausing bacteria A transformation of

Scientist 1952 1944 1928 Year Experiment Conclusion Griffith 1928 Mice/pneumoniacausing bacteria A transformation of the bacteria was caused by some substance – genetic material? Avery 1944 Chemical experiments DNA (not protein) is the genetic material Hershey and Chase 1952 Viruses and bacteria/radioactive markers on proteins and DNA (not protein) is the genetic material Chargaff Early 1950’s Biochemical work Chargaff’s Rules: 1)Adenine=thymine Guanine=cytosine 2) amounts of A, T, G, C vary in different species Franklin Early 1950’s x-ray diffraction of DNA is a helix with 2 strands, nitrogen bases in center Watson and Crick 1953 Models only, used data Published the classic paper in from other scientists which the correct structure of DNA was first proposed

Fig. 2. 31

Fig. 2. 31

The Race to Discover DNA’s Structure was Over • DNA is made up of:

The Race to Discover DNA’s Structure was Over • DNA is made up of: – Four nucleotides: Adenine, Thymine, Guanine and Cytosine – These follow the rules of base-pairing: • Adenine bonds with Thymine • Guanine bonds with Cytosine – A sugar-phosphate backbone • DNA is arranged in an double-helix

DNA Replication • The double helix did explain how DNA copies itself • We

DNA Replication • The double helix did explain how DNA copies itself • We will study this process, DNA replication, in more detail

How does DNA replicate? Hypotheses: Conservative Semi-Conservative Dispersive

How does DNA replicate? Hypotheses: Conservative Semi-Conservative Dispersive

Meselson-Stahl Experiment 1. Bacteria cultured in medium containing a heavy isotope of Nitrogen (15

Meselson-Stahl Experiment 1. Bacteria cultured in medium containing a heavy isotope of Nitrogen (15 N)

Meselson-Stahl Experiment 2. Bacteria transferred to a medium containing elemental Nitrogen (14 N)

Meselson-Stahl Experiment 2. Bacteria transferred to a medium containing elemental Nitrogen (14 N)

Meselson-Stahl Experiment 3. DNA sample centrifuged after First replication

Meselson-Stahl Experiment 3. DNA sample centrifuged after First replication

Meselson-Stahl Experiment 4. DNA sample centrifuged after Second replication

Meselson-Stahl Experiment 4. DNA sample centrifuged after Second replication

DNA Replication The “parent” molecule has two complementary strands of DNA. Each is base

DNA Replication The “parent” molecule has two complementary strands of DNA. Each is base paired by hydrogen bonding with its specific partner: A with T G with C

DNA Replication The first step in replication is the separation of the two strands.

DNA Replication The first step in replication is the separation of the two strands.

DNA Replication Each parental strand now serves as a template that determines the order

DNA Replication Each parental strand now serves as a template that determines the order of the bases along a new complementary strand.

DNA Replication The nucleotides are connected to form the sugarphosphate backbones of the new

DNA Replication The nucleotides are connected to form the sugarphosphate backbones of the new strands. Each “daughter” DNA molecule consists of one parental strand one new strand.

 • Watson and Crick’s semiconservative model of replication predicts that when a double

• 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) and the dispersive model (each strand is a mix of old and new) Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

DNA Replication: A Closer Look • The copying of DNA is remarkable in its

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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Getting Started • Replication begins at special sites called origins of replication, where the

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 Animation: Origins of Replication copied Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 16 -12 Origin of replication Parental (template) strand Daughter (new) strand Doublestranded DNA

Fig. 16 -12 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 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

Fig. 16 -12 a Origin of replication Parental (template) strand Daughter (new) strand Doublestranded

Fig. 16 -12 a 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 -12 b Origin of replication Double-stranded DNA molecule Parental (template) strand Daughter

Fig. 16 -12 b 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

• At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating • Helicases are enzymes that untwist the double helix at the replication forks • Single-strand binding protein binds to and stabilizes single-stranded DNA until it can be used as a template • Topoisomerase corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 16 -13 Primase Single-strand binding proteins 3 Topoisomerase 5 3 5 Helicase 5

Fig. 16 -13 Primase Single-strand binding proteins 3 Topoisomerase 5 3 5 Helicase 5 RNA primer 3

 • DNA polymerases cannot initiate synthesis of a polynucleotide; they can only add

• DNA polymerases cannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3 end • The initial nucleotide strand is a short RNA primer Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

 • An enzyme called primase can start an RNA chain from scratch and

• An enzyme called primase can start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template • The primer is short (5– 10 nucleotides long), and the 3 end serves as the starting point for the new DNA strand Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Synthesizing a New DNA Strand • Enzymes called DNA polymerases catalyze the elongation of

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

• Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate • d. ATP supplies adenine to DNA and is similar to the ATP of energy metabolism • The difference is in their sugars: d. ATP has deoxyribose while ATP has ribose • As each monomer of d. ATP joins the DNA strand, it loses two phosphate groups as a molecule of pyrophosphate Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

Fig. 16 -14 New strand 5 end Sugar 5 end 3 end T A

Fig. 16 -14 New strand 5 end Sugar 5 end 3 end T A T C G G C T A A Base Phosphate Template strand 3 end DNA polymerase 3 end A T Pyrophosphate 3 end C Nucleoside triphosphate 5 end C 5 end

Antiparallel Elongation • The antiparallel structure of the double helix (two strands oriented in

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 3 end of a growing strand; therefore, a new DNA strand can elongate only in the 5 to 3 direction Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

 • Along one template strand of DNA, the DNA polymerase synthesizes a leading

• Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork 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

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 poll III 3 5 5 3 5

Fig. 16 -15 a Overview Origin of replication Leading strand Lagging strand Primer Leading

Fig. 16 -15 a Overview Origin of replication Leading strand Lagging strand Primer Leading strand Lagging strand Overall directions of replication

Fig. 16 -15 b Origin of replication 3 5 RNA primer 5 “Sliding clamp”

Fig. 16 -15 b Origin of replication 3 5 RNA primer 5 “Sliding clamp” 3 5 Parental DNA pol III 3 5 5 3 5

 • To elongate the other new strand, called the lagging strand, DNA polymerase

• 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 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

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 3 5 1 5 2 1 3 5 Overall direction of replication

Fig. 16 -16 a Overview Origin of replication Leading strand Lagging strand 2 1

Fig. 16 -16 a Overview Origin of replication Leading strand Lagging strand 2 1 Leading strand Overall directions of replication

Fig. 16 -16 b 1 3 Template strand 5 5 3

Fig. 16 -16 b 1 3 Template strand 5 5 3

Fig. 16 -16 b 2 3 Template strand 3 5 5 RNA primer 5

Fig. 16 -16 b 2 3 Template strand 3 5 5 RNA primer 5 3 1 3 5

Fig. 16 -16 b 3 3 Template strand 3 5 5 RNA primer 5

Fig. 16 -16 b 3 3 Template strand 3 5 5 RNA primer 5 3 3 1 Okazaki fragment 3 1 5 5 3 5

Fig. 16 -16 b 4 3 5 5 Template strand 3 RNA primer 5

Fig. 16 -16 b 4 3 5 5 Template strand 3 RNA primer 5 3 1 5 3 5 Okazaki fragment 3 3 3 1 5 5 2 1 3 5

Fig. 16 -16 b 5 3 5 5 Template strand 3 RNA primer 5

Fig. 16 -16 b 5 3 5 5 Template strand 3 RNA primer 5 3 1 3 5 1 5 5 2 3 5 Okazaki fragment 3 3 3 1 3 5 5 2 1 3 5

Fig. 16 -16 b 6 3 5 5 Template strand 3 RNA primer 5

Fig. 16 -16 b 6 3 5 5 Template strand 3 RNA primer 5 3 1 5 2 3 5 1 5 2 3 3 5 1 5 3 5 Okazaki fragment 3 3 3 3 5 1 5 2 1 Overall direction of replication 3 5

Table 16 -1

Table 16 -1

Fig. 16 -17 Overview Origin of replication Lagging strand Leading strand Lagging strand Overall

Fig. 16 -17 Overview Origin of replication Lagging strand Leading strand Lagging strand Overall directions of replication Single-strand binding protein Helicase 5 3 Parental DNA Leading strand 3 DNA pol III Primer 5 Primase 3 5 DNA pol III 4 3 5 Lagging strand DNA pol I 3 2 DNA ligase 1 3 5

The DNA Replication Complex • The proteins that participate in DNA replication form a

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

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 DNA polymerase DNA ligase

Fig. 16 -18 Nuclease DNA polymerase DNA ligase

Replicating the Ends of DNA Molecules • Limitations of DNA polymerase create problems for

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 ends, 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

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

• Eukaryotic chromosomal DNA molecules have at their ends nucleotide sequences called telomeres • 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

Fig. 16 -20 1 µm

 • If chromosomes of germ cells became shorter in every cell cycle, essential

• 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 cells Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

 • The shortening of telomeres might protect cells from cancerous growth by limiting

• 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

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

• 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 -21 a Nucleosome (10 nm in diameter) DNA double helix (2 nm

Fig. 16 -21 a Nucleosome (10 nm in diameter) DNA double helix (2 nm in diameter) H 1 Histones DNA, the double helix Histones Histone tail Nucleosomes, or “beads on a string” (10 -nm fiber)

Fig. 16 -21 b Chromatid (700 nm) 30 -nm fiber Loops Scaffold 300 -nm

Fig. 16 -21 b 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

• 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 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

• 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

• 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 Copyright © 2008 Pearson Education Inc. , publishing as Pearson Benjamin Cummings

 • Histones can undergo chemical modifications that result in changes in chromatin organization

• 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