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Chapter 18

Chapter 18. Molecular Genetics. Goals for this Chapter:. Summarize the events and experiments that led to the discovery of the structure of DNA Explain how the interaction between DNA and proteins results in the accurate replication of genetic information

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Chapter 18

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  1. Chapter 18 Molecular Genetics

  2. Goals for this Chapter: • Summarize the events and experiments that led to the discovery of the structure of DNA • Explain how the interaction between DNA and proteins results in the accurate replication of genetic information • Design and construct models to simulate the structure and replication of DNA

  3. Goals for this Chapter: • Explain how genetic information is encoded in DNA molecules • Describe the processes through which genetic information is expressed in living cells • Design and perform a simulation to illustrate the steps of protein synthesis • Explain some of causes and effects of DNA mutations

  4. Goals for this Chapter: • Describe how random changes in nucleotide sequences provide a source of genetic variability • Explain how nucleotide sequences provide evidence that different species are related • Design and perform a simulation to illustrate the use of restriction enzymes and ligases to create recombinant DNA

  5. Goals for this Chapter: • Explain how the insertion of new DNA sequences into cells can transform organisms • Describe some of the social, environmental, and ethical issues associated with genetic technologies

  6. 18.1 – DNA Structure and Replication • In 1869, Friedrich Mieschner coined the term “nucleic acid” to describe the material found in the nuclei of cells • However, it took almost a century for scientists to understand the DNA was the material that carried hereditary information

  7. Isolating the Material of Heredity • In the early 1900s, Phoebus Levene identified two compounds in chromosomes – proteins and DNA • Scientists did not know what part (the DNA or protein) actually carried hereditary information • Two major experiments led to the identification of DNA as hereditary material:

  8. Griffith’s Transforming Principle

  9. Griffith’s Transforming Principle • Griffith’s experiment provided good evidence that DNA was the material responsible for passing on traits • However, scientists were not prepared to accept this explanation until more evidence was gathered

  10. Hershey & Chase • Hershey and Chase performed an experiment in 1952 that used radioactive labeling of compounds to trace hereditary material • They used two radioactive materials (sulfur-35, which would be found in proteins and phosphorus-32, which would be found in DNA) to label parts of a bacteriophage

  11. Hershey and Chase • In the case where the phosphorus marker on DNA was used, material was found inside the cell, while the sulfur markers on the proteins were not

  12. What is DNA? • DNA is deoxyribonucleic acid • It is a molecule used by cells to carry genetic information • The code in DNA is arranged into genes http://www.pbs.org

  13. What is Found in DNA? • DNA actually contains both proteins and nucleic acids • However, the proteins do not contain the genetic code • Our genetic code is contained in the nucleic acids found within the DNA structure http://www.accelrys.com

  14. The Structure of DNA • DNA consists of 6 chemicals: • Deoxyribose sugar • Phosphate • Adenine • Cytosine • Guanine • Thymine • The nitrogen bases are always found in complementary pairs http://student.ccbcmd.edu

  15. Chagraff’s Rule • In the 1940s, Edwin Chagraff determined that although nucleotides were not found in equal amounts, there are roughly the same amounts of complementary bases • For instance, if a sample of DNA has 15% thymine bases…

  16. Watson & Crick • To understand how DNA operates, its structure must be understood • James Watson & Francis Crick determined the helical structure of DNA at Cambridge University in 1953 • Their analysis of X-ray diffraction patterns of crystallized DNA molecules allowed them to determine the structure of DNA http://nitro.biosci.arizona.edu http://genome.jgi-psf.org

  17. Rosalind Franklin • Rosalind Franklin provided the X-ray diffraction analysis of crystallized DNA to Watson & Crick • Her work along with the work of Chagraff allowed Watson and Crick to develop the well-known double-helix model of DNA that we have today

  18. A Closer Look at DNA • As you can see, DNA is antiparallel, which means that the left hand strand runs the opposite direction of the right hand strand

  19. mRNA vs. DNA

  20. Genes and the Genome • Gene: • Genome:

  21. Placement of Genes • Genes are not equally spaced on chromosomes • For instance, chromosome 4 is relatively long (200 million bases), but has about 800 genes • Chromosome 19 has only 55 million bases in comparison, but has more than 1500 genes

  22. The Replication of DNA • The DNA molecule can make copies of itself • This is required to ensure that two new cells that arise from mitosis have the same genetic code • Replication occurs in a series of steps

  23. Initiation • Replication starts at a specific nucleotide sequence, called the replication origin • Our chromosomes have multiple replication origins, while the circular DNA of bacteria only have a single replication origin

  24. “Unzipping the Helix” • An enzyme known as DNA helicase unwinds the DNA at replication forks

  25. The action of helicase creates a “replication bubble” where the DNA has been unwound • At each end of the “bubble” are replication forks that branch out to unpaired single strands

  26. Elongation • Elongation: • Elongation is carried out by DNA polymerase enzymes • They act based on place placement of primers

  27. “Primers” • An enzyme known as primase places RNA primers at the sites where DNA replication is to begin

  28. “Polymerases” • There are 2 significant DNA polymerase enzymes • polymerase III attaches base pairs to the exposed DNA strand in the 5’ to 3’ direction (the 5’ and 3’ refer to the carbons in the deoxyribose sugar)

  29. One strand that is created is continuous (known as the leading strand), while other strands (lagging strand) is replicated in short segments • These short segments are known as Okazaki fragments, and they will be sealed later

  30. “Polymerases” • The enzyme polymerase I follows polymerase III and removes the RNA primers, replacing them with nucleotides • The result is two strands of DNA that are identical to their parent

  31. “Sealing the Deal” • At this point, the DNA still has small “nicks” in it • Another enzyme, known as ligase, repairs those nicks (assembles the Okazaki fragments into a single long DNA chain) • The completion of the two new DNA strands is known as termination Interactive Review

  32. The Final Product • As a result, we are left with two strands of DNA • DNA replication is semiconservative – each new strand has part of the older parent strand

  33. Gene Sequencing – Circa 1990s • We can now map genes by using restriction enzymes to chop the DNA into small segments • Each of these enzymes cuts at a specific DNA sequence • This produces segments of varying lengths, known as RFLPs (Restriction Fragment Length Polymorphisms)

  34. The RFLPs are then marked with radioactive dyes • Finally, the RFLPs are placed on a thin layer of gel through which a small electrical field is applied • Within the gel, the RFLPs are pulled along by the electrical field • The smaller, lighter fragments move the greatest distance • This creates a distinct banding pattern

  35. These bands can then be used to map genes • As well, this can be used for “DNA Fingerprinting” as each person’s pattern of bands is different

  36. Modern Analysis • Mapping genes using gel electrophoresis takes an incredibly long time • Now, DNA is still cut into fragments, but four different colours of dyes are used • A laser is run over the fragments and a computer records the reflected light • Each of the colours corresponds to a different nitrogen base

  37. Therefore, genes can be now mapped by computer at a rate of over a thousand base pairs in a minute (rather than months of work by hand) http://bioweb.wku.edu

  38. The Human Genome Project • The first map of the human genome was completed in 2000 • By 2003, a much more complete and comprehensive map was completed by an international team of scientists

  39. 18.2 – Protein Synthesis and Gene Expression • In the same year that Watson and Crick published their model of DNA, Frederick Danger established that proteins consist of long chains of amino acids • The sequence of the amino acids determines the shape and properties of the protein • Ultimately, the interactions between proteins drives how cells operate

  40. Scientists began to wonder if the sequence in DNA was related to the sequence of amino acids in a protein • It was soon shown that the genetic code in fact does determine the sequence of amino acids found in proteins

  41. Gene Expression • Genetic information flows from DNA to RNA to protein • This is known as the “central dogma” of gene expression

  42. DNA and Protein Synthesis • Although DNA contains very few different structural components, it is responsible for coding for huge amounts of information (about 25, 000 genes in a human) • The sequence of the base pairs is the key to coding for different proteins • Because there are only 4 nitrogen bases and 20 amino acids, 3 bases together can code for different proteins (two bases can only code for 16, while three can code for 64 possible combinations)

  43. Codons • A codon is a 3-base pair segment of DNA • Each codon corresponds to a particular amino acid, or it also may correspond to an initiator “go” or terminator “stop” command

  44. mRNA • to produce proteins, the DNA does not leave the nucleus • a carrier molecule known as messenger RNA (mRNA) is used to carry the code to the ribosomes which produce protein http://tigger.uic.edu

  45. Transcription • The DNA strand “unzips”, exposing the nucleotides • Nucleotides in the mRNA are arranged using the complementary nucleotides on the DNA as a blueprint • The mRNA chain fuses and is moved to the ribosome • The DNA strands rejoin http://fig.cox.miami.edu

  46. Translation and Protein Synthesis • the single-stranded mRNA attaches itself to the small ribosome like a ribbon • initiator codons in the mRNA turn on protein synthesis • transfer RNA (tRNA) molecules in the cytoplasm pick up amino acids and bring them to the mRNA

  47. Overview – Synthesis of Protein • DNA “unzips” • mRNA makes a complementary copy of the DNA • mRNA is taken to the ribosomes • The ribosomes match the mRNA with tRNA that carry amino acids • The amino acids form a chain, which becomes a protein • the mRNA “stop” codon is read, and synthesis stops Protein Synthesis Animation

  48. The Genome and Proteome • Genomics is the study of entire genomes and how the genes interact • However, study of the proteome (the proteins produced by the genome) is often more important because they are the functional parts of the genome

  49. Mutations and Genetic Recombination • Genomes are not constant • Mutations occur from time to time • Mutations occurring in body cells are called somatic cell mutations • However, only mutations occurring in reproductive cells (germ line mutations) will be passed on to offspring

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