levels of damage. surviving t4 virus released from multicomplexes show no increase in mutation, indicating that mr of uv irradiated virus is an accurate process. the bottom figure shows the survival curves for inactivation of virus t4 by the dna damaging agent mitomycin c (mmc). in this case the survival curve for multicomplexes has no initial shoulder, suggesting that only the second recombinational repair process described above is active. the efficiency of repair by this process is indicated by the observation that a dose of mmc that allows survival of only 1 in 1,000 monocomplexes allows survival of about 70% of multicomplexes. similar multicomplex survival curves (without shoulders) were also obtained for the dna damaging agents p32 decay, psoralen plus near-uv irradiation (puva), n-methyl-n'-nitro-n-nitrosoguanidine (mnng), methyl methane sulfonate (mms) and nitrous acid. several of the genes found to be necessary for mr in virus t4 proved to be orthologs for genes essential for recombination in prokaryotes, eukaryotes and archaea. this includes, for instance, t4 gene uvsx which specifies a protein that has three-dimensional structural homology to reca from escherichia coli and the homologous protein rad51 in eukaryotes and rada in archaea. it has been suggested that the efficient and accurate recombinational repair of dna damages during mr may be analogous to the recombinational repair process that occurs during meiosis in eukaryotes. history bacteriophages were first discovered by the english scientist frederick twort in 1915 and félix d'hérelle in 1917. in the late 1930s, t. l. rakieten proposed either a mixture of raw sewerage or a lysate from e. coli infected with raw sewerage to the two researchers milislav demerec and ugo fano. these two researchers isolated t3, t4, t5, and t6 from e.coli. also, in 1932, the researcher j. bronfenbrenner had studied and worked on the t2 phage, at which the t2 phage was isolated from the virus. this isolation was made from a fecal material rather than from sewerage. at any rate, max delbrück was involved in the discovery of the t even phages. his part was naming the bacteriophages into type 1(t1), type 2 (t2), type 3 (t3), etc. the specific time and place of t4 virus isolation remains unclear, though they were likely found in sewage or fecal material. t4 and similar viruses were described in a paper by thomas f. anderson, max delbrück, and milislav demerec in november 1944. in 1943, salvador luria and delbrück showed that bacterial mutations for phage resistance arise in the absence of selection, rather than being a response to selection. the traditional wisdom among bacteriologists prior to 1943 was that bacteria had no chromosomes and no genes. the luria–delbrück experiment showed that bacteria, like other established model genetic organisms, have genes, and that these can spontaneously mutate to generate mutants that may then reproduce to form clonal lineages. that year, they also began working with alfred hershey, another phage experimenter. (the three would share the 1969 nobel prize in physiology or medicine, "for work on the replication mechanism and genetics of viruses".) the phage group was an informal network of biologists centered on max delbrück that carried out basic research mainly on bacteriophage t4 and made numerous seminal contributions to microbial genetics and the origins of molecular biology in the mid-20th century. in 1961, sydney brenner, an early member of the phage group, collaborated with francis crick, leslie barnett and richard watts-tobin at the cavendish laboratory in cambridge to perform genetic experiments that demonstrated the basic nature of the genetic code for proteins. these experiments, carried out with mutants of the riib gene of phage t4, showed, that for a gene that encodes a protein, three sequential bases of the gene's dna specify each successive amino acid of the protein. thus the genetic code is a triplet code, where each triplet (called a codon) specifies a particular amino acid. they also obtained evidence that the codons do not overlap with each other in the dna sequence encoding a protein, and that such a sequence is read from a fixed starting point. during 1962-1964 phage t4 researchers provided an opportunity to study the function of virtually all of the genes that are essential for growth of the phage under laboratory conditions. these studies were facilitated by the discovery of two classes of conditional lethal mutants. one class of such mutants is known as amber mutants. another class of conditional lethal mutants is referred to as temperature-sensitive mutants studies of these two classes of mutants led to considerable insight into numerous fundamental biologic problems. thus understanding was gained on the functions and interactions of the proteins employed in the machinery of dna replication, repair and recombination, and on how viruses are assembled from protein and nucleic acid components (molecular morphogenesis). furthermore, the role of chain terminating codons was elucidated. one noteworthy study used amber mutants defective in the gene encoding the major head protein of phage t4. this experiment provided strong evidence for the widely held, but prior to 1964 still unproven, "sequence hypothesis" that the amino acid sequence of a protein is specified by the nucleotide sequence of the gene determining the protein. thus, this study demonstrated the co-linearity of the gene with its encoded protein. a number of nobel prize winners worked with virus t4 or t4-like viruses including max delbrück, salvador luria, alfred hershey, james d. watson, and francis crick. other important scientists who worked with virus t4 include michael rossmann, seymour benzer, bruce alberts, gisela mosig, richard lenski, and james bull.