Chapter 16: Molecular Basis of Inheritance PDF

Chapter 16 - The Molecular Basis of Inheritance 16.1 DNA IS THE GENETIC MATERIAL genes - sequences of DNA that encode proteins genome - all of the chromosomes and other DNA in an organism’s cells HISTORY OF DNA STRUCTURE 1878 - Friedrich Mieschner – discovered nu...

Chapter 16 - The Molecular Basis of Inheritance 16.1 DNA IS THE GENETIC MATERIAL genes - sequences of DNA that encode proteins genome - all of the chromosomes and other DNA in an organism’s cells HISTORY OF DNA STRUCTURE 1878 - Friedrich Mieschner – discovered nucleic acid; showed protein-digesting enzymes would destroy nearly everything in a cell except the contents of the nucleus 1914 - Robert Feulgen – developed a method for staining DNA (brilliant crimson); showed that nuclear DNA was restricted to the chromsosomes; also showed somatic cells contained the same amount of DNA while gametes (sex cells like sperm and eggs) contain half that amount 1928 - Fred Griffith – showed that traits could be passed from a nonliving organism to a living organism (i.e. transformation of nonpathogenic rough pneumococci into pathogenic smooth pneumococci); this showed that hereditary information was not dependent on life but may be chemical in nature (some chemical was responsible for transformation) 1943 - Avery, MacLeod, McCarty – showed DNA was the agent of bacterial transformation (however, was it the actual hereditary information?) 1952 - Hershey & Chase – found that DNA, rather than protein was the hereditary information (i.e. radioactive phosphorus [in DNA] was passed from bacteriophage to offspring rather than radioactive sulfur [in protein]) 1953 - Watson & Crick – determined the structure of DNA (double helix made up if two complimentary strands of nucleotides); based on:  Emil Chargraff’s rule (A=T, G=C)  Rosalind Franklin’s X-ray diffraction studies (which provided W & C with a series of necessary measurements in the molecule)  structure was immediately accepted because it explained two of DNA’s functions: o long strands of various nucleotide combinations could be a code o complimentary strands explain how DNA might copy itself 16.2 MANY PROTEINS WORK TOGETHER IN DNA REPLICATION AND REPAIR HISTORY OF DNA REPLICATION 1953 - Watson-Crick template theory – once they described structure, they theorized that one strand could act as the template for the formation of a new, complimentary copy 1957 - Arthur Kornberg – used cellular extracts (enzymes, DNA, nucleotides, etc.) to carry out DNA replication in vitro 1958 - Meselson & Stahl – using radioactive 15N, showed that DNA replication was semiconservative (not conservative or random) CHAPTER 16 – THE MOLECULAR BASIS OF INHERITANCE 1  Enzymes and proteins of DNA replication in prokaryotes topoisomerase Topoisomerase – functions in supertwisting of DNA (corrects 'overwinding' ahead of DNA helicase replication forks by breaking, swiveling, and rejoining DNA strands) DNA helicase – unwinds and separates DNA strands; single-strand binding protein DNA polymerase I (SSB) stops the strands from coming back together DNA polymerase III primase – lays down a short complimentary strand of RNA as a primer for DNA DNA ligase synthesis (this is because DNA polymerase enzymes can only add nucleotides to the end of an already formed strand) primase DNA polymerase III – lays down complimentary strands of DNA running in 5' to 3' single-stranded direction; responsible for most of replication binding proteins DNA polymerase I – lays down complimentary strands of DNA running in 5' to 3' leading/lagging strand direction; also functions in removal of RNA primer and repairs nicks in lagging strand Okazaki fragments DNA ligase – seals nicks in backbone by catalyzing reaction between phosphates and sugars telomerase mismatch repair DNA Replication nucleotide excision 1. Topoisomerase relaxes supercoiling of prokaryotic chromosome repair 2. DNA helicase unzips complimentary strands nucleosome 3. SSB hold the two strands apart so they do not spontaneously rebind 4. Primase lays down short, complimentary strands of RNA heterochromatin 5. DNA polymerase III synthesizes the complimentary DNA strand, beginning at euchromatin the RNA primer and continuing in the 5' to 3' direction; because the enzyme is specific, it can only follow the DNA helicase along one strand (5' to 3') and cannot follow along on the 3' to 5' strand; on the 3' to 5' strand, new primer must be laid down periodically and the DNA polymerase III can only jump on and produce short segments of the strand (thus there is a leading strand [5' to 3'] and a lagging strand [3' to 5']); because of this the leading strand is continuously synthesized while the lagging strand is synthesized 1000 - 2000 base fragments flanked by RNA primers [Okazaki fragments] 6. DNA polymerase I patches the fragments in the lagging strand, removing primer and replacing it with DNA 7. DNA ligase seals nick in backbone  The DNA replication machine is probably present as a multienzyme complex; some evidence suggests that in eukaryotes it is anchored to the nuclear matrix  DNA polymerase also has a proofreading function and immediately removes mismatched bases and replaces them (reduces errors from 1/100,000 to 1/10,000,000,000)  Other errors in replication and errors due to changes in DNA (radiation, x-ray, uv) can also be fixed in mismatch repair  In nucleotide excision repair, a nuclease cuts out error and polymerase and ligase fix; nucleotide excision repair is important in the removal of thymine dimmers (caused by UV)  In eukaryotic DNA, it is impossible to replicate the 3' end of a DNA molecule cannot be removed because the primers cannot be removed and the ends cannot be replicated; thus chromosomes have telomeres at the end (repeated regions) that can become shorter without damaging genes; somatic cells have a limited life because of length of telomeres; germ cells have an enzyme called telomerase which allows them continue dividing because the ends are replicated CHAPTER 16 – THE MOLECULAR BASIS OF INHERITANCE 2 Differences between Prokaryotic / Eukaryotic DNA Synthesis 1. prokaryotic DNA is circular / eukaryotic DNA is linear 2. eukaryotic DNA is organized in nucleosomes (molecules wrapped around the basic protein, histone) thus replication is slow (50 bp/sec as opposed to 500 bp/sec) since the DNA must unwind and rewind 3. eukaryotic DNA replication begins at many points on the chromosome since there is so much DNA and the process is so slow 4. a major difference in the prokaryotic and eukaryotic mechanisms for segregation (sorting newly replicated DNA molecules into daughter cells) a. prokaryotes simply attach old and new strands of DNA to different points on membrane during replication; they then segregate when the cell divides b. eukaryotic segregation is more complicated since there is 10-12X more genes and several chromosomes instead of one; segregation involves organized packaging of molecules and changes in cytoskeletal structure (i.e. formation of microtubular spindle) - MITOSIS!!! 5. only eukaryotes have TRUE sexual reproduction; cells are diploid with two forms of each gene (alleles); these must be segregated in a special way to produce gametes for sexual reproduction which contain one of each type of allele - MEIOSIS!!! Replication in organelles (mitochondria, chloroplasts) is similar to prokaryotic replication (since DNA is circular and there are no histone proteins); different in that there are far fewer genes (40 as opposed to 3000 in bacteria); clearly, organelles are not self-sufficient and scientists believe that many organelles genes have moved to nucleus throughout evolution 16.3 A CHROMOSOME CONSISTS OF A DNA MOLECULE PACKED TOGETHER WITH PROTEINS Prokaryotic chromosome - 1000x smaller than eukaryotic, circular molecule associated with some basic proteins Eukaryotic chromosome - single linear DNA molecules precisely combined with a large amount of protein Eukaryotic Chromosome DNA associated with histone protein (highly basic with large amts of lysine, arginine; highly conserved throughout all organisms two molecules each of the four types of histones creates a protein core around which DNA is wrapped; resulting structures are nucleosomes histone tails of nucleosomes interact to cause 10-nm fiber to fold/coil into 30-nm fibers these fibers form looped domains attached to chromosome scaffold made of protein, creating 300-nm fiber looped domains coil themselves during mitosis into metaphase chromosomes; specific genes are always at specific points, meaning process is highly specific and precise euchromatin - areas of the chromatin that regularly decondense and are expressed heterochromatin - areas of the chromatin that remain permanently condensed CHAPTER 16 – THE MOLECULAR BASIS OF INHERITANCE 3