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CHAPTER 16. THE MOLECULAR BASIS OF INHERITANCE. OVERVIEW. By the 1940’s, scientists knew that chromosomes carried hereditary material and consisted of DNA and protein. Most thought that protein was the genetic material because: -proteins were macromolecules
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CHAPTER 16 THE MOLECULAR BASIS OF INHERITANCE
OVERVIEW • By the 1940’s, scientists knew that chromosomes carried hereditary material and consisted of DNA and protein. • Most thought that protein was the genetic material because: -proteins were macromolecules -little was known about nucleic acids -properties of DNA seemed too uniform to account for the multitude of inherited traits
I. Concept 16.1: DNA is the genetic material A. Evidence That DNA Can Transform Bacteria 1. In 1928 Frederick Griffith provided evidence that the genetic material was a specific molecule 2. He conducted 4 sets of experiments using two strains of pneumococcus—smooth (S) (encapsulated cells with a polysaccharide coat and caused pneumonia) and rough (R) (no coat and did not cause pneumonia)
1st Experiment—injected live S into mouse and the mouse died (pathogenic bacteria) • 2nd Experiment—injected live R into mouse and mouse remained healthy (nonpathogenic bacteria) • 3rd Experiment—injected heat-killed S into mouse and mouse remained healthy (heat-killed bacteria which was pathogenic) • 4th Experiment—injected heat-killed S and live R into mouse and mouse died of pneumonia. Examined blood and contained live S cells.
3. He concluded from his experiments with Streptococcus pneumoniae that R cells had acquired from the dead S cells the ability to make the polysaccharide coats so this trait must be inheritable. 4. He could never explain the chemical nature of the “transforming agent.” 5. This phenomenon is now called transformation (the assimilation of external genetic material by a cell) 6. In 1944, Avery, McCarty, and MacLeod discovered that the transforming agent had to be DNA. 7. Others still believed thatproteinwas the genetic material.
B. Evidence That Viral DNA Can Program Cells 1. More evidence that DNA is the genetic material came from the studies of bacteriophages(bacterial viruses) 2. In 1952, Hershey and Chase performed experiments showing that DNA was the genetic material of a phage known as T2. • They designed an experiment to determine if protein or DNA was responsible for reprogramming a host bacterial cell. • This experiment provided evidence that nucleic acidsrather than proteins were hereditary materialin viruses.
C. Additional Evidence that DNA is the Genetic Material of Cells 1. Circumstantial Evidence • A eukaryotic cell doubles its DNA content prior to mitosis • During mitosis, the doubled DNA is equally divided between two daughter cells. • An organism’s diploid cells have twice the DNA as its haploid gametes.
2. Experimental Evidence • Provided by Chargaffin 1950 when he analyzed the DNA composition of different organisms. He found: DNA composition varies from species to species In every species studied, there was a regularity in base ratios. -# of adenine = # of thymine -# of guanine = # of cytosine • A=T and G=C became known later as Chargaff’s Rule. Explanation of Chargaff’s Rule came with Watson and Crick’s structural model for DNA.
D. Building a Structural Model of DNA 1. By the 1950’s DNA was accepted as the genetic material, and the covalent arrangement in a nucleic acid polymer was well established. The three dimensional structure was unknown, however. 2. Among the scientist working on the problem were Linus Pauling, in California, and Maurice Wilkins and Rosalind Franklin, in London. 3. The first to come up with the correct answer were two scientists who were relatively unknown at thetime— AmericanJames Watson, and Englishman Francis Crick.
Rosalind FranklinX ray crystallography identified that DNA was a double helix structure.
E. In April of 1953 Watson and Crick proposed the structure of DNA in a one page paper in the journal Nature. • Proposed structure: ladder-like molecule twisted into a spiral (double helix), with sugar-phosphate backbones as uprights and pairs of nitrogenous bases as the rungs. • Backbones of helix areantiparallel(run in opposite directions) • There is a specific pairing between nitrogenous bases (A with T; G with C) • Nitrogenous bases are held together by hydrogen bonds: A = T (2 hydrogen bonds) G ≡ C (3 hydrogen bonds)
DNA • Covalent bonds link the units of each nucleotide. • The two strands of DNA are held together by Hydrogen bonds between the base pairs. • In Watson’s model of DNA, the sugar-phosphate backbones were antiparallel- with their subunits running in opposite directions.
Base-pairing rule is significant because: 1. Explains Chargaff’s Rule 2. Suggest mechanism for DNA replication 3. Dictates combination of complementary base pairs, but places no restriction on the linear sequence (can be highly variable) 4. Hydrogen bonds stabilize the structure.
II. Concept 16.2: DNA Replication and Repair • In a second paper Watson and Crick published their hypothesis for how DNA replicates. • The model of DNA structure suggests a template mechanism for DNA replication. A. Steps to DNA Replication 1. Two DNA strands separate. 2. Each strand is a template for assembling a complementary strand. 3. Nucleotides line up singly along the template strand in accordance with the base-pairing rules (A—T; G—C) 4. Enzymes link the nucleotides together at their sugar phosphate groups.
B. Watson and Crick’s Model is a Semiconservative Model for DNA Replication. • When a double helix replicates, each of the two daughter molecules will haveone old or conserved strandfrom the parent molecule and one newly created strand. • In the late 1950’s Matthew Meselsonand Franklin Stahl provided the experimental evidence to support the semiconservative model of DNA replication.
C. A Closer Look at DNA Replication • The 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 special sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble” • These areas have a specific sequence of nucleotides. • Also creates a Replication Fork • 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
At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating • Helicasesare 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 • Topoisomerasecorrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands
DNA Polymerase- helps synthesize new DNA by adding nucleotides to a preexisting chain. • DNA Pol III- adds DNA nucleotide to RNA primer and continues adding nucleotides complementary to original DNA template strand. • DNA polymerases cannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3end • The initial nucleotide strand is a short RNA primer which is formed by an enzyme calledprimasewhich uses 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
D. Synthesizing a New DNA Strand • Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork • New nucleotides align themselves along the templates of the old DNA strands (A-T and C-G). • 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 • Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate
5. 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
E. 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 • Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork 4. To elongate the other new strand, called the lagging strand, DNA polymerase must work in the direction away from the replication fork 5.The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase
DNA Replication • DNA Pol I- replaces RNA nucleotides of the primer with DNA nucleotides- moving from 5’ to 3’. • DNA Ligase- seals Okazaki fragments and newly synthesized DNA into one continuous DNA strand. (joins sugar phosphate backbone)
F. 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)
F. Proofreading and Repairing DNA 4. In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA • Nucleotide Excision Repair- damaged segment of DNA is cut out, and gap is filled by DNA Pol I and DNA Ligase. • Nuclease- DNA cutting enzyme. Removes damaged DNA.
G. 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 • 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
