MOLECULAR GENETICS

In 1869, F. Miescher isolated the compounds contained within the nuclei of a variety of cells and named it nuclein. Later on P. Levene discovered that there are two types of nucleic acid (the term nuclein). He concluded that there are three molecular parts, bonded together, forming repeating unit called a nucleotide. The genes were known to exist on chromosome and the chromosomes are composed of both proteins and DNA. It was not clear which of these, proteins or DNA, is responsible for inheritance. In 1928, Fredrick Griffith discovered the phenomenon of transformation using bacteria. During transformation, the dead bacteria could transfer the genetic material to transform other living bacteria. Some of the important experiments which gave the evidences of genetic material nature is given inTableB.2.3

DNA a Double Stranded Helix

Watson and Crick discovered that DNA is a double-stranded, helical molecular ’ladder’ with rungs of uniform length and with alternating sugar-phosphate units forming the ladder upright. Each rung pairs a purrine with a pyrumidine  i.e., A always pairs T and C with G. this is known as complementarity  of the bases (Fig. B.3.1).

  The main features of the molecular architecture of the DNA molecule are:

It is double –stranded helix of uniform diameter

It is right-handed.

It is antiparallel (the two strands run in opposite directions).

The outer edges of nitrogenous bases are exposed in the major and minor groove.

The genetic material performs four important functions:

The genetic material stores an organism’s genetic information.

Some important landmarks leading to the discovery of Genetic Material

Year	Scientist	Contributions
1869	F.Miescher	Isolated DNA and called it nuclein
1920	P.Levene	Discovered that there are two types of nucleic acid
1940s	O.Avery	The proportion of the bases varies in the nucleic acids of different organisms. The proportion of purine base sis always equal to the proportion of pyrumidine bases.
1950s	G. Beadle and E. Tatum	Developed the one-gene –one-enzyme theory, which states that the production of one enzyme is under the control of a particular unit of heredity or gene.
1952	A.Hershey and M. Chase	Confirmed that DNA (not protein) is the hereditary material.
1953	R.Franklin and M. Wilkins	Analyzed DNA by X-ray diffraction and concluded that bit is shaped like a helix (winding staircase).
1953	J. Watson and F.Crick	Developed double helical model of DNA

The genetic material is susceptible to mutation.

The genetic material is precisely replicated.

The genetic material is expressed as phenol-type.

DNA contains Repeated Sequences

The DNA of eukaryotic organism also contains repeated sequences and is called repetitive DNA. One type of repetitive DNA is located at the ends of eukaryotic chromosomes. These are called telomeres. Every time the cell divides the telomeres shorten. When the chromosome ends become too short for the cell to function, it dies. However in some cells the enzyme telomerase helps rebuild the repetitive sequence after the cell division.

DNA is a Self Replicating Unit

Meselson and Stahl showed that DNA replication is semi-conservative i.e each parent strand serves as template for a new strand. Thus the two replicated DNA molecules each contain one parental and one newly synthesized strand. The enzyme DNA polymerase catalyses the additions of nucleotides to the 3’ end of each strand. The added nucleotides are determined by complementary base pairing with the template strand. The replication complex is huge protein complex that attaches to the chromosome at the origin of replication on both the strand in 5’ to 3’ direction. Many proteins assist in DNA replication. DNA helicase separates the two strands and single Strand Binding proteins keep the strands from reassociating. 

   In prokaryotes, two interlocking circular DNAs are formed which are separated by an enzyme called DNA topoisomerase. The primase catalyes the synthesized continuously while the lagging strand in pieces called Okazaki fragments, later on joined by the enzyme ligase (fig. B.3.2). DNA polymerase makes about one error in 10⁵ bases replicated. DNA is also subjected to natural alteration and chemical damage. These errors can be repaired by three different mechanisms: proof reading, mismatch repair and excision repair. Knowledge of mechanism of DNA replication led to the development of techniques for making multiple copies of DNA sequences and for determining the nucleotide sequence of DNA molecules (see box)

The polymerase chain reaction (PCR) makes multiple copies of a DNA sequence. PCR is a cyclic process in which a sequence of steps is repeated over and over again. Kerry Mullis developed this technique in which following steps are involved:

• Double stranded fragments of DNA are separated into single strands by heating (denaturation )

• A short artificially synthesized primer along with the four deoxyribonucleotide  tiphosphate  (dATP, dTTP, dCTP and dGTP) and strands of DNA are placed in a test tube. To this mixture DNA polymerase is added.

• DNA polymerase catalyses the production of complementary new strands.