Restriction endonucleases.

Restriction endonucleases are enzymes commonly found in bacteria which can fragment DNA. Cleavage of the DNA molecule takes place after recognition of a nucleotide sequence which is specific for each restriction endonuclease. Some of the enzymes split the DNA within the recognition sequence. This property allows these enzymes to be used for the systematic fragmentation of DNA molecules in structural and functional analyses of genetic material. The biological function of restriction endonucleases could be to provide a sort of primitive immune system which enables the cells to destroy foreign genetic material. Methylation of nucleotides lying within the recognition sequence serves to protect the cell's own DNA from being degraded by its restriction endonucleases. Restriction endonucleases were discovered during the investigation of the restriction of viral growth observed on changing hosts.     

The photocurrent response of human cones is fast and monophasic

Background  

The precise form of the light response of human cone photoreceptors in vivo has not been established with certainty. To investigate the response shape we compare the predictions of a recent model of transduction in primate cone photoreceptors with measurements extracted from human cones using the paired-flash electroretinogram method. As a check, we also compare the predictions with previous single-cell measurements of ground squirrel cone responses.