The brief structures of dna

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The brief structures of dna

  1. 1. The brief Note on Structures of DNA Introduction: Deoxyribonucleic acid (DNA) is a molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses. Types of structures  Primary structure of DNA  Secondary structure of DNA  Unusual DNA Secondary structure  Tertiary structure of DNA Primary structure of DNA DNA and RNA are called as a nucleic acid (polymer of repeating subunits) Subunit are comprised of the three parts  Five carbon sugars: Subunit of RNA contains a pentose sugar called ribose and the DNA contains deoxyribose sugar. The sugars only differ in the presence or absence of the oxygen at 2’ position  A phosphate group : which are negatively charged and give DNA molecules a negative charge and play it’s a vital role in the phosphodiester bond formation  A Nitrogenous bases : The nucleobases are classified into two types: the purines, A and G and the pyrimidine, C , U and T . The uracil is specific for RNA According to the Chargaff’s rules [A] = [T] and [G] = [C] and [A] + [G] = [T] + [C] Nucleoside and Nucleotide When base is chemically linked to a 1’ carbon of the sugar it form nucleoside and when the phosphate group is attaches to the 5’ carbon of the same sugar it form nucleotide Nucleotide joins together by condensation reaction forming a DNA strand Significance of 5' to 3' direction A phosphate group attached to the 5′-end and hydroxyl group is attached to 3′-end carbon in the sugar-ring. This naming convention is important because nucleic acids can only be synthesized in vivo in the 5′-to-3′ direction, as the polymerase that assembles new strands only attaches new nucleotides to the 3′-hydroxyl (-OH) group, via a phosphodiester bond. The Length of DNA and RNA
  2. 2. The size of an individual gene or an organism's entire genome is often measured in base pairs because DNA is usually double-stranded. Hence, the number of total base pairs is equal to the number of nucleotides in one of the strands A kilo base (kb) is a unit of measurement in molecular biology equal to 1000 base pairs of DNA or RNA. The haploid human genome (23 chromosomes) is estimated to be about 3.2 billion base pairs long Secondary Structure of DNA The secondary structure of a nucleic acid molecule refers to the base pairing interactions within a single molecule or set of interacting molecules. The secondary structures of biological DNA's and RNA's tend to be different: biological DNA mostly exists as fully base paired double helices, while biological RNA is single stranded  Hydrogen bonds formed between the bases Two nucleotides on opposite complementary DNA or RNA strands that are connected via hydrogen bonds are called a base pair. Watson-Crick base pairing, adenine (A) forms a base pair with thymine (T) by 2 hydrogen bonds and guanine (G) pairs with cytosine (C) by 3 hydrogen bonds in DNA. Hydrogen bonding gives structural stability to the molecules  Base Stacking provides chemical stability to the DNA double helix This occurs due to hydrophobic effect of DNA. Stacking eliminates any gapes between the bases and excludes the maximum amount of h20 from the interior of the double helix Structure of the Watson –crick DNA double helix The term double helix refers to the structure formed by double-stranded molecules of nucleic acids .The double helical structure of a nucleic acid complex arises as a consequence of its secondary structure The two strands of DNA run in opposite directions to each other and are therefore anti-parallel, one backbone being 3′ (three prime) and the other 5′ (five prime). This refers to the direction the 3rd and 5th carbon on the sugar molecule is facing. Attached to each sugar is one of four types of nucleobases (informally, bases). It is the sequence of these four nucleobases along the backbone that encodes biological information. That sequence is called as a genetic code
  3. 3. Types of DNA B-DNA A-DNA Z-DNA  crystallized from water; water retained in the inner structure; predominant form under physiological conditions  10 base pairs/turn of helix  Right handed  Distance bet base pairs 0.34 nm  Diameter – 2.0 nm or 20 A  Dehydrated form of B- DNA  Right handed helix  11 base pairs/helix  Diameter = 26 A  Found in short stretches of native DNA and synthetic DNA  Left handed helix  12 base pairs/helix  Diameter = 18 A Unusual DNA secondary structures Slipped structures Present upstream of regulatory sequence , it is a tendem repeat in DNA is two or more adjacent copies of a pattern of nucleotide arranged in head to tail fashion Crucifrom structures These are inverted repeats are base sequence of identical compositions on the complementary strand .They are some time called as palindromes. The DNA become rearranged so each repeat pairs with the complementary sequence of its own strand of DNA instead of the complement of the other strand Triple-stranded DNA A triple-stranded DNA is a structure of DNA in which three oligonucleotides wind around each other and form a triple helix. In this structure, one strand binds to a B-form DNA double helix through Hoogsteen or reversed Hoogsteen hydrogen bonds.
  4. 4. Tertiary structure of the DNA  Circular (relaxed) – in E.coli; simian virus 40; bacteriophage; certain animal species  Supercoiled DNA – extra twisting in the linear duplex; allows DNA to be more compact in the cell; regulatory role in replication  Topoisomerase – change topology of the DNA  Quadruplex DNA – four stranded; in protozoan DNA; occur in G-rich regions; regulating and stabilizing telomeres and regulation of gene expression

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