Editing Molecular biology (section)

== Griffith's experiment ==
{{Main|Griffith's experiment}}
[[File:01 dna.jpg|thumb| Griffith's experiment]]

In 1928, [[Frederick Griffith]], encountered a virulence property in [[pneumococcus]] bacteria, which was killing lab rats. According to Mendel, prevalent at that time, gene transfer could occur only from parent to daughter cells. Griffith advanced another theory, stating that gene transfer occurring in member of same generation is known as horizontal gene transfer (HGT). This phenomenon is now referred to as genetic transformation.<ref>{{Cite journal |last1=Ravenhall |first1=Matt |last2=Škunca |first2=Nives |last3=Lassalle |first3=Florent |last4=Dessimoz |first4=Christophe |date=May 2015 |title=Inferring Horizontal Gene Transfer |journal=PLOS Computational Biology |language=en |volume=11 |issue=5 |pages=e1004095 |doi=10.1371/journal.pcbi.1004095 |doi-access=free |pmid=26020646|pmc=4462595 |bibcode=2015PLSCB..11E4095R }}</ref>

Griffith's experiment addressed the pneumococcus bacteria, which had two different strains, one virulent and smooth and one avirulent and rough. The smooth strain had glistering appearance owing to the presence of a type of specific polysaccharide – a polymer of glucose and glucuronic acid capsule. Due to this polysaccharide layer of bacteria, a host's immune system cannot recognize the bacteria and it kills the host. The other, avirulent, rough strain lacks this polysaccharide capsule and has a dull, rough appearance.{{cn|date=May 2023}}

Presence or absence of capsule in the strain, is known to be genetically determined. Smooth and rough strains occur in several different type such as S-I, S-II, S-III, etc. and R-I, R-II, R-III, etc. respectively. All this subtypes of S and R bacteria differ with each other in antigen type they produce.<ref name="Verma-2004" />