Molecular Basis of Inheritance

1. Molecular Basis of Inheritance Chapter 16

2. Deciphering DNA

3. Introduction • DNA - genetic material • RNA – some viruses • Other role of RNA- function as an adapter • - structural and in some cases as catalytic molecules

4. Topics to be covered • Structure of DNA • Replication of DNA • Transcription process (making RNA to DNA) • The genetic code • The translation process – protein synthesis • Genomics • Essentials of human genome sequencing or project

5. The DNA and RNA • Genetic material • PROERTIES OF GENETIC MATERIALS: • able to replicate itself. • Stable both chemically and structurally. • Provide the scope for slow changes (mutation)- required for evolution. • Express itself in the form of ‘Mendelian characters’. • Have the ability to direct their duplications – rule of base pairing and complementarity.

6. Con……. • Chargaff's rules. • Chargaff's rules state that DNA from any cell of all organisms should have a 1:1 ratio (base Pair Rule) of pyrimidine and purine bases and, more specifically, that the amount of guanine should be equal to cytosine and the amount of adenine should be equal to thymine.

8. The Search for Genetic Material • Known • Genes on chromosomes • Chromosomes made of DNA and protein • Unknown • Which chromosomal component was the genetic material • Protein • Heterogeneous class of macromolecules with specific functions • Case stronger initially • Nucleic acids • Physical and chemical properties too uniform for amount of variation • Experimentation gradually changed perceptions • DNA’s role clarified by studying bacteria and their viruses

9. Con…….. • The discovery of nuclein by Miescher and the proportion of principal of inheritance by Mendel were same almost • Material from the nucleus of a cell, considered a single substance when first isolated in the late 1800s but later shown to consist of DNA and associated proteins.  • In 1926 – the mechanism of inheritance had reached the molecular level gradually.

10. Transforming Principle • Frederick Griffith - 1928 • Streptococcus pneumonia • On culture media - some are smooth or shiny colonies (S) -rough colonies (R) • Due to the presence and absence of mucous (polysaccharide) • crystal violet-nalidixic acid-gentamicinagar:CVNG)

12. Biochemical characteristics of transforming principle Oswald avery, Colin MacLeod and Maclyn McCarty

14. Frederick Griffith Streptococcus pneumoniae model S encapsulated and virulent; R non-encapsulated and non-virulent Heat killed S cells mixed with R cells created S cells Concluded that S cells have a chemical component that can transform other cells

15. Oswald Avery • Identified the transforming substance from Griffith’s work as DNA • Focused on DNA, RNA, and protein • Extract components from pathogenic bacteria • Each individually inactivated and tested for transformation ability • Degradation of DNA only substance to prevent • Not uniformly supported • Proteins better candidates • Doubted bacterial DNA similar to that of complex organisms • Little still known about DNA

16. Hershey and chase experiment • The proof – DNA is the genetic material – Alfred Hershey and Martha Chase.

19. Alfred Hershey and Martha Chase • Tracked protein and DNA of E. coli phage T2 • Bacteriophage is a virus that infects bacteria • Radioactive isotopes to label cells • Determined that DNA entered bacteria and directed virus reproduction not protein

20. THE DNA • Deoxyribonucleic acid (DNA)- genetic material in most organisms. • Long polymer of deoxyribonucleotides. • The no.of nucleotides or base pair in the DNA determines its length. • Length is characteristics of an organisms. • E.coil – 4.6 x 106 • Bacteriophage φ 174 – 5,386 bp • Lambda phage – 48,502bp • Human beings - 3.3 x 109 bp (haploid number)

22. Structure of a poly nucleotide chain (DNA/RNA) • a nucleotide is composed of a nitrogenous base, pentose sugar and a phosphate group. • A PENTOSE SUGAR:

23. A NITROGENOUS BASE • Nitrogen containing organic molecule – similar physical properties of a base. • i. purines - adenine and guanine • ii. Pyrimidines – cytosine, uracil and thymine • Adenine + ribose sugar adenosine • Adenine + deoxyribose sugar deoxyadenosine • Guanine + ribose sugar guanosine • Guanine + deoxyribose sugar deoxyguanosine • Cytosine + ribose sugar cystidine • Cytosine + deoxyribose sugar deoxycystidine

26. A PHOSPHATE GROUP • A nucleotide ( or deoxynucleiotide depending upon the type of sugar present) is formed. • Phosphate group is linked to 5’ OH of a nucleoside through a phosphoester linkage. • Two nucleotides when liked through a 3’ to 5’ phophodiester linkage form a dinucleotide. • a. one end with a free phosphate moiety at 5’ end of ribose sugar. • b. the other end with a free hydroxyl 3’ OH group.

28. Existing Knowledge of DNA • Polymer of nucleotides with 3 components • Pentose sugar (deoxyribose) and a phosphate group • Purines = two rings • Adenine (A) • Guanine (G) • Pyrimidines = one ring • Thymine (T) • Cytosine (C)

29. Discoveries related to structure and composition of DNA • Friedrich Meischer (1869) – nuclein • Levene (1910) – found DNA contain phosphoric acid and deoxyribose sugar. • Erwin Chargaff (1950) – chargaff rule • James Watson and Francis Crick – double helical structure

31. Where is DNA located?

32. Erwin Chargaff • The amount of A, T, G, and C in the DNA vary from species to species • Evidence of molecular diversity to increase DNA credibility • Chargaff’s rules • In each species, the amount of A = T while the amount of C = G • Importance unknown until discovery of double helix

33. Rosalind Franklin • X-ray diffraction image of DNA • DNA is helical in structure • Uniform in width and spacing between bases • Suggested that there were 2 strands = double helix • Concluded that sugar-phosphate backbones were on the outside • Evidence was groundwork for Watson and Crick

34. James Watson and Francis Crick • Double helix with anti-parallel strands • Sugar-phosphate backbone on outside • Paired nitrogenous bases on inside • Complimentary hydrogen binding of a purine and a pyrimidine • A with T form 2 bonds, G with C form 3 bonds • Consistent with Chargoff and Franklin • Awarded the Nobel Prize

35. Salient features of double – helix DNA • It is made of two polynucleotide chains, where the backbone is constituted by sugar- phoshate, and the bases project inside. • The two chains have anti – parallel polarity. It means, if one chain has the polarity 5’ to 3’ , the other has 3’ to 5’. • H- bond, purine comes opposite to a pyrumidine. This generates approximtely uniform distance between the strands of the helix. • The two chains are coiled in aright- handed fashion. The pitch of the helix is 3.4nm and there are roughly 10 bp in each turn consequently.

36. The distance between a bp in a helix is approximately equal to 0.34nm. • The plane of one base pair stacks over the other in double helix. This in addition to h- bonds, confers stability of the helical structure.

37. DNA Double-Helix Structure

38. Chromatin, chromosome and chromatid • Chromatin – DNA+ protein (histones ) histone octamer • During cell division chromatin condenses and form a structure called chromosome.

40. Chromatin structure

41. Packaging of DNA helix • The length of DNA in humans can be calculated as distance between two consecutive base pairs is • 0.34 nm = 0.34 x 10-9 • Total no. Of base pairs in a DNA helix in a typical mammalian cell = 6.6 x 10 9 • Length of this DNA helix = 6.6x10 x 0.34 x 10 -9 m • = 2.2 m (appro) • Total no. Of .base pair x distance between two consecutive base pair.

42. In Prokaryotes • DNA – held together with some proteins in a region known as nucleoid. • The DNA in nucleoid is organised in large loop held by proteins.

44. In Eukaryotes • Histone are the protein – rich in amino acids –lysine and arginine. • It both carries positive charges on their side chain. • Histone are a set of positively charged , basic protein. • Make a unit of eight molecules known as histone octamer. • - negatively charged DNA is wrapped round the positively charged histone octamer forming a structure called nucleosome.

45. There are five types of histone proteins- • H1, H2 A, H2B, H3, and H4 • DNA connecting the two adjacent nucleosome is called linker DNA. It bears H - one nucleosome contains approximately 200 bpof DNA helix. - the nucleosomes are the repeating units of chromatin. - the nucleosomes in chromatin look like’beads - on – string when we are observing under EM.

46. The chromatin is packed to form a solenoid structure of 30nm diameter. • Further supercoiling tends to form looped struture called chromatin fibre and then chromatid. • This further coils and condenses at metaphase stage of a cell division to form the chromosome.

47. Forms of chromatin • Euchromatin : • Looser DNA packaging • Lightly stained when observe under microscope. • Contains less DNA. • Trascriptionally active • Found in eukaryotes and in eukryotes

48. Heterochromatin • Tighter DNA packaging • Dark stained when observe under microscope. • Contains more DNA • Trascriptionally inactive • Found in prokaryotes

50. Histone octamer