The genome of prokaryotes and eukaryotes contain thousands of genes. The genomes of various plants and animals are being investigated as the product of these genes can be of great use to mankind. The awareness of this information about the various genomes can help in identifying various diseases and the treatment of defective genes as well. The information can also throw light on the evolutionary process of the genes and the distribution of particular genes in the population.

Basic Facts About DNA

• It were the experiments of Fred Griffith which suggested that DNA was probably the genetic material. He worked with virulent S and nonvirulent R strain Pneumococcus bacteria and found that R strain could become virulent when it took in DNA from heat-killed S strain.


• Chromosomes are made of both DNA and protein and experiments on bacteriophage viruses by Hershey & Chase further proved that DNA was the cell’s genetic material.


• DNA  is a very large molecule made up of a long chain of sub-units. The sub-units are called nucleotides. Each nucleotide is made up of a sugar called deoxyribose, a phosphate group -PO4   and an organic base.


• It was Erwin Chargaff who showed the amounts of the four bases in DNA ( A,T,C,G) and gave the Chargaff rule which states that Adenine must pair with Thymine and Guanine must pair with Cytosine. These bases form weak hydrogen bonds. The amount of Adenine is equal to Thymine and amount of Cytosine is equal to Guanine.



A	=	30.3%
T	=	30.3%
G	=	19.5%
C	=	19.9%

• Rosalind Franklin took diffraction x-ray photographs of DNA crystals.


• In the 1950’s, Watson & Crick built the first model of DNA using Franklin’s x-rays.



Two strands coiled called a double helix and the sides made of a pentose sugar Deoxy -ribose bonded to phosphate (PO4) groups by phospho-diester bonds. The center made of nitrogen bases bonded together by weak hydrogen bonds. In fact, the DNA usually consists of a double strand of nucleotides .

• DNA is copied before a cell divides and it is duplicated during the S or synthesis phase of interphase. New cells get the identical DNA strands.


• DNA Replication begins at Origins of Replication where two strands open forming Replication Forks (Y-shaped region) and then these new strands continue to grow at the forks.


• As the 2 DNA strands open at the origin, Replication Bubbles form which keeps on moving as replication continues. Prokaryotes (bacteria) have a single bubble but eukaryotic chromosomes have MANY bubbles.


• Before replication begins, the enzyme Helicase unwinds and separates the 2 DNA strands by breaking the weak hydrogen bonds. Single-Strand Binding Proteins attach and keep the 2 DNA strands separated and untwisted.


• The enzyme Topoisomerase attaches to the 2 forks of the bubble to relieve stress on the DNA molecule as it separates.


• Before new DNA strands can form, there must be RNA primers present to start the addition of new nucleotides. The enzyme, Primase is synthesizes the RNA Primer. DNA polymerase then adds the new nucleotides.


• DNA polymerase can only add nucleotides to the 3’ end of the DNA so
  the NEW strand is built in a 5’ to 3’ direction. This is known as the Leading Strand  and is synthesized as a single strand from the point of origin toward the opening replication fork.


• The Lagging Strand is synthesized discontinuously against overall direction of replication.This strand is made in MANY short segments and is replicated from the replication fork toward the origin. These series of short segments on the lagging strand are called Okazaki Fragments which are joined together by an enzyme called, Ligase.


• DNA polymerase initially makes about 1 in 10,000 base pairing errors.
  Enzymes proofread and correct these mistakes. The new error rate for DNA that has been proofread is 1 in 1 billion base pairing errors.
  
  


• Chemicals & ultraviolet radiation damage the DNA in our body cells so the cells must continuously repair DAMAGED DNA. Excision repair occurs when any of over 50 repair enzymes remove damaged parts of DNA. DNA polymerase and DNA ligase replace and bond the new nucleotides together.