Question

3. (7) Suppose that you have selected E cofthat can grow only when the medium is supplemented with cysteine. You isolate one mutant that you believe is defective in a new gene require for cysteine biosynthesis- sX You have two strains of bacteria: Hitr cysX- galehislac+ purtrpefort cysX+gal-his-tac-pur-trp-tort A) What supplements would you need to add to minimal media to recover all cysX lach, fon recombinants? Would you add glucose? B) What recombinant genotypes would you recover on plates containing only minimal media, glucose, histidine, purines, and T1 phage? C) When you conduct an interrupted mating experiment with the Hlr and F. strains. Torf colonies with the following phenotypes first appear at the times shown in the table below: Minutes Phenotype cys Cys his Lac- pur. irp 38.5 9.5 14.5 Draw a bacterial chromosome and indicate the relative position of each of the markers and the cysXgene in the Hir strain. Be sure to label distances between markers. D) Based on the results in (C), you decide to also perform mapping of the cysX gene using P1 cotransduction P1 phage grown on an E. coli strain with the genotype cysX- tort trp are used to infect a recipient strain of Ecoli with the genotype cysX+for trp These bacteria are then plated on complete media in the presence of T1 phage. Two hundred for transductants are scored for their genotype with respect to the other markers. Here are the results 10 10 180 Genotype Number of colonies cysX+trp cysX trp- cysX-trpe cysX-trp What is the recombination frequency between trpand cysl? E) What is the recombination frequency between ton and cysX? F) What is the order of the three genes? Indicate the distance between them.

Answer 1

Answer A: The recombinant strain cysX+ lac+ tonR need cysteine as an additional supplement to the minimal medium. Glucose should not be added as there is no gene in the recombinant strain for glucose metabolism.

Answer B: his- lac- pur- trp+ gal+ cysX+ tonR. Even though there is mutations in the histidine, lactose, purine metabolism genes, those biomolecules are present in the medium additionally. The T1 phage resistant gene tonR is also present in the recombinant strain.

Answer C: With the interrupted mating experiment, the following bacterial chromosome and the relative positions of the genes cane be predicted.

cyst 9.5 lact his+ purt 13.5 galt 8 trp+ cys-

Answer D: Transduction can be used as a technique in mapping the bacterial genes. The recombination frequency between trp and cysX is (10/200) × 100 = 5%.

Answer E: The recombination frequency between ton and cysX is 100%. There is no co-transformation of ton and cysX genes. That means the genes are located far apart, hence, higher chance of crossing over and recombination.

Answer F: The order of the three genes and the distance between them is as follows,

cysX 5 trp 100 100 ton