Lecture 1: Molecular Basis Of Genetics

THE MOLECULAR BASIS OF GENETICS SBT 2130

MOLECULAR BASIS OF GENETICS TOPICS 1. DNA STRUCTURE 2. REPLICATION 3. RECOMBINATION 4. CHROMOSOME ORGANIZATION

DNA STRUCTURE Primary structure of nucleic acids DNA and RNA are macromolecular structures composed or regular repeating polymers formed from nucleotides. Nucleotides are basic building blocks of nucleic acids and are derived from nucleosides that are composed of two elements: a five-membered pentose carbon sugar (2-deoxyribose in DNA and ribose in RNA) and a nitrogenous base.

DNA STRUCTURE Primary structure of nucleic acids The carbon atoms of the sugar are designated ‘prime’ (1’. 2’, 3’, etc) to distinguish them from the carbon atoms of the nitrogenous bases, of which there are two types – purines and pyrimidines.

DNA STRUCTURE Primary structure of nucleic acids The purine bases (composed of fused five- and six membered rings), adenine (A) and guanine (G) are found in both RNA and DNA, as is the pyrimidine (a single six-membered ring) cytosine (C). The other pyrimidines are each restricted to one type of nucleic acid: uracil (U) occurs in RNA, whilst thymine (T) is limited to DNA.

DNA STRUCTURE Primary structure of nucleic acids A nucleotide, or nucleoside phosphate, is formed by the attachment of a phosphate to the 5’ position of a nucleoside by an ester linkage.

DNA STRUCTURE Primary structure of nucleic acids Such nucleotides can be joined together by the formation of a second ester bond by reaction between the terminal phosphate group of one nucleotide and the 3’ hydroxyl of another, thus generation a 5’ to 3’ phosphodiester bond between adjacent sugars; this process can be repeated idefinitely to give long polynucleotide molecules.

Chemistry of DNA Synthesis -Hydrogen bonds forms first. -Incoming nucleotide provides the energy (triphosphate like a “loaded spring”)

DNA SYNTHESIS IS CATALYZED BY DNA POLYMERASE (AND GROWS ONLY IN THE 5’->3’ DIRECTION)

POLYMERIZING & EDITING BY POLYMERASE INVOLVE TWO CATALYTIC SITES

DNA STRUCTURE Primary structure of nucleic acids DNA has two such polynucleotide strands; each strand has a polarity or directionality. The polarities of the two strands of the molecule are in opposite directions, and thus DNA is described as an ‘antiparallel’ structure.

ANTIPARALLEL NATURE OF DNA One strand in a double helix runs 5’ to 3’, whilst the other strand runs in the opposite direction 3’ to 5’. The strands are held together by hydrogen bonds between the bases.

DNA STRUCTURE It is conventional to write a nucleic acid sequence starting at the 5’ end of the molecule, using single capital letters to represent each of the bases, for example CGGATCT. Terminal phosphate groups can, when necessary, be indicated by the use of a ‘p’; thus 5’ pCGCTACT 3’ indicates the presence of a phosphate on the 5’ end of the molecule

DNA STRUCTURE Secondary structure of nucleic acids The two polynucleotide chains in DNA are usually found in the shape of a right-handed double helix, in which the bases of the two strands lie in the centre of the molecule, with the sugar-phosphate backbones on the outside. A crucial feature of this double-stranded structure is that it depends on the sequence of bases in one strand being complementary to that in the other.

DNA STRUCTURE Secondary structure of nucleic acids A purine base attached to a sugar residue on one strand is always hydrogen bonded to a pyrimidine base attached to a sugar residue on the other strand. Adenine (A) always pairs with thymine (T) or uracil in RNA, via two hydrogen bonds, and guanine (G) always pairs with cytosine (C) by three hydrogen bonds.

DNA STRUCTURE Secondary structure of nucleic acids When these conditions are met a stable doublehelical structure results in which the backbones of the two strands are, on average, a constant distance apart. The strands are designated as plus (+) and minus (-) and an RNA molecule complementary to the minus (-) strand is synthesised during transcription

THE DNA DOUBLE HELIX

DNA STRUCTURE Secondary structure of nucleic acids Although the three-dimensional structure of DNA may vary it generally adopts a double helical structure termed the B form or B-DNA in vivo. There are other forms of right-handed DNA, such as A and C, that are formed when DNA fibres are subjected to different relative humidities

THE VARIOUS FORMS OF DNA

The various forms of DNA serve to show that it is not a static molecule but dynamic and constantly in flux, may be coiled, bent or distorted at certain times.

DNA STRUCTURE Secondary structure of nucleic acids The major distinguishing feature of B-DNA is that it has approximately 10 bases for one turn of the double helix; furthermore distinctive major and minor grooves may be identified.

DNA STRUCTURE Secondary structure of nucleic acids RNA almost always exists as a single strand, it often contains sequences within the same strand which are self-complementary, and which can therefore base-pair if brought together by suitable folding of the molecule. A notable example is transfer RNA (tRNA), which folds up to give a clover leaf secondary structure.

SECONDARY STRUCTURE OF YEAST tRNA