Molecular Evolution Dr. Singh Naresh Kumar, Ph.D Professor, College of Animal Science, KNU, ChunCheon, Kangwon
Molecular Basis of Inheritance Until 1940's scientists were know to chromosomes and that the chromosomes are composed of DNA and protein And that the chromosomes carry genes But what was gene, was not know Since physical and chemical properties of DNA was monotonous and repetitive, so it was thought DNA could not be genetic material. Since protein had heterogenic structure and greater specificity, so it was thought to be as genetic material.
Fred-Griffith in 1928 was working with streptococcus pneumoniae which is a bacteria used to cause respiratory tract infection including lungs. He grew colonies of Streptococcus and found two types of colonies i.e. Smooth and rough. He injected live bacteria to mice and found that bacteria from smooth colony killed the mice whereas bacteria from rough colony did not kill the mice. He also injected dead bacteria to mice and found that neither the smooth nor the rough form of bacteria killed the mice. He then combined the live rough and dead smooth bacteria and injected to mice. This mixture killed the mice. Griffith then concluded that there is something in the dead Smooth bacteria that has transformed Rough bacteria and made them smooth bacteria. This was nothing but the genetic material.
Later, In 1944, Avery, McCarty and Macleod isolated the DNA from smooth bacteria and concluded that it was the DNA that has transformed the rough bacteria to smooth bacteria. And claimed that the DNA was the genetic material. Later in 1952 Hershey and Chase concluded that DNA is the genetic material. He formed an experiment where he tagged T2 (bacteriophage) with p-32 and s-35 and infected the E.coli. He found lot of p-32 inside the E.Coli.
DNA (Deoxyribo nucleic acid) structure is double helix and been discovered by James Watson (USA) and Francis Crick (GB) solve the structure of DNA (by x-ray crystallograph of DNA). • DNA, the genetic material, is a macromolecule made of monomers called nucleotides. • DNA molecule resembles a twisted ladder. A sugar-phosphate backbone forms the sides of the ladder. • The phosphate group of one nucleotide is covalently bonded to a hydoxyl group on the sugar of the next. • Hydrogen bonding between bases creates the rungs of the ladder. • Each strand has a free phosphate group at the 5' end and a free hydroxyl group at the 3' end. • The numbers refer to the carbons in the sugar part of each nucleotide
X-ray crystallograph suggests: structure is double helix helix width uniform (2nm) bases stacked 0.34 nm apart others had shown that DNA is made from: lots of phosphate lots of deoxyribose sugar amount phosphate = amount deoxyribose sugar four different bases are: adenine (a big base); thymine (a small base) ; cytosine (a small base); guanine (a big base) amount of adenine = amount of thymine amount of guanine = amount of cytosine
Note: a. the sugar-phosphate-sugar- phosphate backbone b. note that bases connected to the sugars c. base pairing (A=T and G=C) d. antiparallel Meselson & Stahl predicted (assumed or hypothesized) that replication of DNA is dispersive which means all the DNA formed will have one old and one new strand