DNA Structure & Function - Student Notes PDF

DNA Dr. Afira Waqar Department of Biosciences History DNA was first isolated and characterized by Friedrich Miescher in 1869. He called the phosphorus-containing substance “nuclein.” In 1940s, with the work of Oswald T. Avery, Colin MacLeod, and Maclyn McCarty, was there any compelling...

DNA Dr. Afira Waqar Department of Biosciences History DNA was first isolated and characterized by Friedrich Miescher in 1869. He called the phosphorus-containing substance “nuclein.” In 1940s, with the work of Oswald T. Avery, Colin MacLeod, and Maclyn McCarty, was there any compelling evidence that DNA was the genetic material. Avery and his colleagues found that an extract of a virulent strain of the bacterium Streptococcus pneumoniae (causing disease in mice) could be used to transform a nonvirulent strain of the same bacterium into a virulent strain. They were able to demonstrate through various chemical tests that it was DNA from the virulent strain (not protein, polysaccharide, or RNA, for example) that carried the genetic information for virulence. Then in 1952, experiments by Alfred D. Hershey and Martha Chase —removed any remaining doubt that DNA, not protein, carried the genetic information. Rule of Erwin Chargaff Another important clue to the structure of DNA came from the work of Erwin Chargaff and his colleagues in the late 1940s. Examining dozens of species, they found that the four nucleotide bases of DNA occur in different ratios in the DNAs of different organisms. However, the base composition remains constant in different tissues of the same species, and does not vary with age, environment, nutritional state, or generation. Furthermore, regardless of the species, the number of adenosine residues is equal to the number of thymidine residues (that is, A = T), and the number of guanosine residues is equal to the number of cytidineresidues (G = C). From these relationships it follows that the sum of the purine residues equals the sum of the pyrimidine residues; that is, A + G = T + C. These quantitative relationships, sometimes called “Chargaff’s rules,” were a key to establishing the three dimensional structure of DNA. DNA Structure James Watson and Francis Crick relied on this accumulated information about DNA to set about deducing its structure. In 1953 they postulated a three-dimensional model of DNA structure It consists of two helical DNA chains wound around the same axis to form a right-handed double helix.  Complementary strands of DNA The two antiparallel polynucleotide chains of double-helical DNA are not identical in either base sequence or composition. Instead they are complementary to each other. Wherever adenine occurs in one chain, thymine is found in the other wherever guanine occurs in one chain, cytosine is found in the other. Hydrogen bonding between base pairs The DNA double helix, or duplex, is held together by hydrogen bonding between complementary base pairs and by base-stacking interactions. The complementarity between the DNA strands is attributable to the hydrogen bonding between base pairs the hydrogen bonds do not contribute significantly to the stability of the structure. Base-stacking interactions between successive G≡C or C≡G pairs are stronger than those between successive A═T and T═A pairs or adjacent pairs including all four bases. Because of this, DNA duplexes with higher G≡C content are more stable DNA Can Occur in Different Three- Dimensional Forms The Watson-Crick structure is also referred to as B-form DNA, or B- DNA. The B form is the most stable structure for a random sequence DNA molecule under physiological conditions and is therefore the standard point of reference in any study of the properties of DNA. Two structural variants that have been well characterized in crystal structures are the A and Z forms. Comparison of A, B, and Z forms of DNA  A common type of DNA sequence is a palindrome. A palindrome is a word, phrase, or sentence that is spelled identically when read either forward or backward; two examples are ROTATOR and NURSES RUN. In DNA, the term is applied to regions of DNA with inverted repeats, such that an inverted, self-complementary sequence in one strand is repeated in the opposite orientation in the paired strand, The self-complementarity within each strand confers the potential to form hairpin or cruciform (cross-shaped) structures When the inverted repeat occurs within each individual strand of the DNA, the sequence is called a mirror repeat. Mirror repeats do not have complementary sequences within the same strand and thus cannot form hairpin or cruciform structures. Sequences of these types are found in almost every large DNA molecule Hairpins and cruciforms. Palindromic DNA (or RNA) sequences can form alternative structures with intrastrand base pairing. (a)Hairpin structures involve a single DNA or RNA strand. (b) Cruciform structures involve both strands of a duplex DNA. Blue shading highlights asymmetric sequences that can pair with the complementary sequence either in the same strand or in the complementary strand. DNA Base Pairs and DNA Stability: Problem In samples of DNA isolated from two unidentified species of bacteria, X and Y, adenine makes up 32% and 17%, respectively, of the total bases. What relative proportions of adenine, guanine, thymine, and cytosine would you expect to find in the two DNA samples? What assumptions have you made? One of these species was isolated from a hot spring (64 °C). Which species is most likely the thermophilic bacterium, and why? Find a solution……………………………………..????????????????? Solution For any double-helical DNA, A = T and G = C. The DNA from species X has 32% A and therefore must contain 32% T. This accounts for 64% of the bases and leaves 36% as G≡C pairs: 18% G and 18% C. The sample from species Y, with 17% A, must contain 17% T, accounting for 34% of the base pairs. The remaining 66% of the bases are thus equally distributed as 33% G and 33% C. This calculation is based on the assumption that both DNA molecules are double-stranded. The higher the G+C content of a DNA molecule, the higher themelting temperature. Species Y, having the DNA with the higher G+C content (66%), most likely is the thermophilic bacterium; its DNA has a higher melting temperature and thus is more stable at the temperature of the hot spring. Thank you