1. Chemical nature of hereditary material

2. Discovery of Nuclein “Nuclein.”……………??????????? In 1868, Johann Friedrich Miescher, a young Swiss medical student discovered. He recovered an acidic substance from pus cells that he called “nuclein.” Miescher’s nuclein was unusual in that it contained large amounts of both nitrogen and phosphorus, two elements known at the time to coexist only in certain types of fat.

3. The key component of the acidic material in Miescher’s nuclein, was not documented until the 1940s. The role of nucleic acids in storing and transmitting genetic information was not established until 1944, and The double-helix structure of DNA was not discovered until 1953.

4. Functions of the Genetic Material The genetic material must replicate, control the growth and development of the organism, and allow the organism to adapt to changes in the environment.

5. Chromosomes are composed of two types of large organic molecules (macromolecules) called proteins and nucleic acids. The nucleic acids are of two types: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). During the 1940s and early 1950s, the results of elegant experiments clearly established that the genetic information is stored in nucleic acids, not in proteins. In most organisms, the genetic information is encoded in the structure of DNA. However, in many small viruses, the genetic information is encoded in RNA.

6. Proof That Genetic Information Is Stored in DNA

7. PROOF THAT DNA MEDIATES TRANSFORMATION

8. PROOF THAT DNA CARRIES THE GENETIC INFORMATION IN BACTERIOPHAGE T2

10. The Structures of DNA and RNA DNA is usually double-stranded, with adenine paired with thymine and guanine paired with cytosine. RNA is usually single-stranded and contains uracil in place of thymine.

11. NATURE OF THE CHEMICAL SUBUNITS IN DNA AND RNA Nucleic acids, the major components of Miescher’s nuclein, are macromolecules composed of repeating subunits called nucleotides. Each nucleotide is composed of (1) a phosphate group, (2) a five- carbon sugar, or pentose, and (3) a cyclic nitrogen-containing compound called a base ( Figure ). In DNA, the sugar is 2-deoxyribose (thus the name deoxyribonucleic acid); in RNA, the sugar is ribose (thus ribonucleic acid). Four different bases commonly are found in DNA: adenine (A), guanine (G), thymine (T), an cytosine (C). RNA also usually contains adenine, guanine, and cytosine but has a different base, uracil (U), in place of thymine. Adenine and guanine are double-ring bases called purines; cytosine, thymine, and uracil are single-ring bases called pyrimidines.

12. Both DNA and RNA, therefore, contain four different subunits, or nucleotides: two purine nucleotides and two pyrimidine nucleotides (Figure ). In polynucleotides such as DNA and RNA, these subunits are joined together in long chains ( Figure). RNA usually exists as a single-stranded polymer that is composed of a long sequence of nucleotides. DNA has one additional—and very important—level of organization: it is usually a double-stranded molecule.

15. Nucleotide = monomers that make up DNA and RNA (Figs.) Three components 1. Pentose (5-carbon) sugar DNA = deoxyribose RNA = ribose (compare 2’ carbons) 2. Nitrogenous base Purines Adenine Guanine Pyrimidines Cytosine Thymine (DNA) Uracil (RNA) 3. Phosphate group attached to 5’ carbon

16. nucleoside = sugar +base nucleotide = sugar + base + phosphate

17. Nucleotides are linked by phosphodiester bonds to form polynucleotides. Phosphodiester bond Covalent bond between the phosphate group (attached to 5’ carbon) of one nucleotide and the 3’ carbon of the sugar of another nucleotide. This bond is very strong, and for this reason DNA is remarkably stable. DNA can be boiled and even autoclaved without degrading! 5’ and 3’ The ends of the DNA or RNA chain are not the same. One end of the chain has a 5’ carbon and the other end has a 3’ carbon.

18. 5’ end 3’ end

19. Structure of DNA James D. Watson/Francis H. Crick 1953 proposed the Double Helix Model based on two sources of information: 1.Base composition studies of Erwin Chargaff indicated double-stranded DNA consists of ~50% purines (A,G) and ~50% pyrimidines (T, C) amount of A = amount of T and amount of G = amount of C (Chargraff’s rules) %GC content varies from organism to organism Examples:%A%T%G%C%GC Homo sapiens31.031.519.118.437.5 Zea mays25.625.324.524.649.1 Drosophila27.327.622.522.545.0 Aythya americana25.825.824.224.248.4

20. Structure of DNA James D. Watson/Francis H. Crick 1953 proposed the Double Helix Model based on two sources of information: 2.X-ray diffraction studies by Rosalind Franklin & Maurice Wilkins Conclusion-DNA is a helical structure with distinctive regularities, 0.34 nm & 3.4 nm. Fig. 2.11

21. The Double Helix

23. Double Helix Model of DNA: Six main features 1.Two polynucleotide chains wound in a right-handed (clockwise) double-helix. 2.Nucleotide chains are anti-parallel: 5’  3’ 3’  5’ 3.Sugar-phosphate backbones are on the outside of the double helix, and the bases are oriented towards the central axis. 4.Complementary base pairs from opposite strands are bound together by weak hydrogen bonds. A pairs with T (2 H-bonds), and G pairs with C (3 H-bonds). 5’-TATTCCGA-3’ 3’-ATAAGGCT-5’ 5.Base pairs are 0.34 nm apart. One complete turn of the helix requires 3.4 nm (10 bases/turn). 6.Sugar-phosphate backbones are not equally-spaced, resulting in major and minor grooves.

24. DNA STRUCTURE: ALTERNATE FORMS OF THE DOUBLE HELIX The Watson–Crick double-helix structure just described is called B-DNA. B- DNA is the conformation that DNA takes under physiological conditions (in aqueous solutions containing low concentrations of salts). The vast majority of the DNA molecules in living cells exist in the B conformation. In high concentrations of salts or in a partially dehydrated state, DNA exists as A-DNA, which is a right-handed helix like B-DNA, but with 11 nucleotide pairs per turn (Table ).DNA molecules almost certainly never exist as A-DNA in vivo. Certain DNA sequences have been shown to exist in a left-handed, double- helical form called Z-DNA. The function of Z-DNA in living cells is still not clear.

27. DNA STRUCTURE: NEGATIVE SUPERCOILS IN VIVO All the functional DNA molecules present in living cells display one other very important level of organization—they are supercoiled. This supercoiling causes a DNA molecule to collapse into a tightly coiled structure similar to a coiled telephone cord or twisted rubber band ( Figure). Supercoils are introduced into and removed from DNA molecules by enzymes that play essential roles in DNA replication and other processes.

28. If we rotate the free end in the same direction as the DNA double helix is wound (right-handed), a positive supercoil (overwound DNA) will be produced. If we rotate the free end in the opposite direction (left-handed), a negative supercoil (underwound DNA) will result.

29. Supercoiling (higher order structure)

30. Chromosomes in prokaryotes Prokaryotes are monoploid (mono one); they have only one set of genes (one copy of the genome). (“Monoploid” should not be confused with “haploid,” which refers specifi cally to the reduced chromosome number in gametes.) In most viruses and prokaryotes, the single set of genes is stored in a single chromosome, which in turn contains a single molecule of nucleic acid (either RNA or DNA). The length of the circular DNA molecule present in the chromosome of the bacterium Escherichia coli is about 1500 um. E. coli cell has a diameter of only 1 to 2 m, the large DNA molecule present in each bacterium must exist in a highly condensed (folded or coiled) configuration.

31. Chromosomes in Eukaryotes In contrast to prokaryotes, most eukaryotes are diploid, having two complete sets of genes, one from each parent. When chromatin is isolated from interphase nuclei, the individual chromosomes are not recognizable. Instead, one observes an irregular aggregate of nucleoprotein. Chemical analysis of isolated chromatin shows that it consists primarily of DNA and proteins with lesser amounts of RNA The proteins are of two major classes: (1) basic (positively charged at neutral pH) proteins called histones and (2) a heterogeneous, largely acidic (negatively charged at neutral pH) group of proteins collectively referred to as nonhistone chromosomal proteins histones are important in chromatin structure (DNA packaging) and are only nonspecifically involved in the regulation of gene expression. nonhistone chromosomal proteins do not play central roles in the packaging of DNA into chromosomes. Instead, they are candidates for roles in regulating the expression of specific genes or sets of genes.

32. James D. Watson Francis H. Crick Maurice H. F. Wilkins 1962: Nobel Prize in Physiology and Medicine