Chapters 16 and 17

1. Chapters 16 and 17

2. Objectives • Describe the data that led Watson and Crick to suggest their model of DNA structure • Show how the double helix model of DNA conformed to prior observations of the chemistry of the molecule and, at the same time, suggested mechanisms for the roles of DNA in gene expression and chromosome replication • Define and explain the terms replication, transcription and translation

3. Define what is meant by semi-conservative DNA replication and describe the role of Okazaki fragments • Use the base-pairing rules to determine the nucleic acid sequences of new DNA strands, mRNA and tRNA from DNA sequences • Trace the steps of protein synthesis, including the locations of the processes, the involvement of complex molecular machinery and the requirement for inputs of energy • Explain why the DNA code must involve a reading frame of three bases

4. Introduction • Many basics of molecular biology determined using viruses that infect bacteria • viruses composed of protein coat and internal genetic material (DNA or RNA) • not living-no cell structure, no metabolism, cannot self-replicate • Bacterial viruses called bacteriophages • logical choice for experiments

5. Early experiments showed DNA genetic material • 1928-Griffith • Streptococcus pneumonia is a bacteria that causes pneumonia in mammals • 2 strains: 1 pathogenic and 1 nonpathogenic • Mix heat killed pathogenic strain with living nonpath strain and some of these cells became pathogenic

7. Transformation • Change in genotype and phenotype due to an assimilation of external DNA by the cell

8. 1952-Hershey and Chase • Showed that viruses can infect bacteria

10. DNA is the Genetic Material • Prior to the 1950’s, it was already known that DNA is a polymer of nucleotides, each consisting of three components • Each monomer (nucleotide) composed of phosphate group, 5C sugar, and nitrogenous base • DNA-deoxyribose • RNA-ribose • Each polymer contains four nucleotides named for the nitrogenous bases • DNA • thymine-T, cytosine-C, adenine-A, and guanine-G • RNA • uracil-U replaces T

11. DNA • DNA is hereditary information is enclosed in the chemical language of DNA • DNA is the blue print and program that directs the development of many traits

12. Watson and Crick • DNA is double-stranded helix • Watson-Crick model based on observations from many sources • chemical structure of bases • X-ray crystallographs from Rosalind Franklin • chemical analysis of DNA • A=T and G=C • ratios of A+T and G+C

13. final model that fit observations is double helix with sugar backbones on outside and hydrogen-bonded bases on inside • twisted rope ladder • A always bonds with T and G always bonds with C • led to proposed mechanisms of DNA function

16. Meselson and Stahl demonstrated that DNA replication is semi-conservative, confirming the Watson-Crick model

19. DNA Replication • Copying of the DNA is extremely fast and accurate • Prokaryotes= 5000 nuc per second • Errors occur in every 1 billion nuc • DNA replication depends on base pairing rules • requires accurate and complete copies of DNA • known as semi-conservative replication • each strand of DNA molecule acts as template for synthesis of new strand • bases added to growing strand using base-pairing rules

21. replication initiated at special sites called origins of replication

22. 3’ 5’

25. http://www.youtube.com/watch?v=DGGIhfe7YZg

26. Enzymes proofread DNA during replication and repair damage in existing DNA • mismatch repair - enzymes repair errors in base pairing during replication • excision repair - enzymes repair damage caused by physical and chemical agents

27. 3’ end of leading strand has potential to get shorter with repeated replication • solved by two mechanisms • telomeres - non-coding repeated DNA sequences at ends of linear molecules • telomerases - catalyzes lengthening of telomeres • shortening of telomeres may represent cell life - limiting factor

30. Chapter 16 Check list • What did Griffith do? • What did Hershey and Chase do? • Watson and Crick? • Meselson and Stahl? • What bases pair with each other? • DNA replication is _______? • Can stranded DNA have many replication sites? • What about circular DNA? • What are the enzymes that are used in replication, what do they do and what order do they occur in? • Define leading vs. lagging strands and the differences between them. • DNA synthesis starts with what kind of primer?

31. Gene Expression/Transcription and Translation(CH 17) • Genotype expressed as proteins-basis of phenotypic traits • one gene-one polypeptide hypothesis • flow of information is from DNA to RNA (transcription) to polypeptide (translation)

32. Genes specify proteins via transcription and translation • Evidence for this idea came from studies of metabolic defects • 1909, British physician Archibald Garrod was the first to dictate phenotypes through enzymes

33. Nutritional Mutants • Beadle and Tatum caused bread mold to mutate by using x-rays • Could not survive on minimal medium • Using genetic crosses they determined that their mutants fell into three classes, each mutated in a different gene

35. Genetic information written as codons and translated into amino acids occurs in two stages: • transcription of DNA in nucleus into mRNA • translation of mRNA into amino acid sequence in cytoplasm

36. two languages used by cell • nucleotides-five letters in alphabet (T or U, A, C, G) • amino acids-20 letters in alphabet • DNA= Code • MRNA=Codon • TRNA= Anticodon

37. smallest nucleotide word that can accommodate amino acid alphabet is a three-letter word • triplets of bases in nucleotides specify specific amino acids-known as codons • first codon deciphered was UUU-phenylalanine • forms the basis of the genetic code • universal • redundant but not ambiguous

39. Evolution of Genetic Code • The genetic code is nearly universal and it is shared by organisms from the simplest bacteria to the most complex animals

40. Transcription • Makes copies of information in DNA molecule in form of messenger RNA • occurs in nucleus in eukaryotes and cytoplasm in prokaryotes • involves RNA polymerase • initiated at promoter region and elongation continues until terminated at terminator region • Two other types of RNA-rRNA and tRNA-also transcribed

42. Promoters, specific nucleotide sequences at the start of a gene, are the site of RNA polymerase binding • Transcription factors help the RNA polymerase recognize the promoter sequence and bind to the DNA.

43. RNA polymerase moves along the DNA and creates the mRNA • Does the work of ALL the enzymes use in DNA replication

44. https://www.youtube.com/watch?v=WsofH466lqk

45. Eukaryotic mRNA’s are processed before leaving the nucleus • modified nucleotide G subunits added to 5’ end (cap) and a poly-A tail added to 3’ end • introns (non-coding sequences) excised from mRNA by spliceosomes and exons (coding sequences) linked together. • shuffling of exons contributes to protein diversity

47. Function and Evolution of Introns • The presence of introns allows for alternative RNA splicing • Proteins often have different structural and functional regions called domains • In many cases different exons code for the different domains

49. Translation • Occurs in cytoplasm • involves ribosomes, tRNA, mRNA enzymes and protein factors, and energy sources

50. http://www.youtube.com/watch?v=5bLEDd-PSTQ