Use the following figure to answer the questions below it a

Use the following figure to answer the questions below it.

a) List the map order of the genes.

b) The transfer of the lac gene plateaus at a lower number than the azi gene. What feature of conjugation accounts for this observation? Note: \"Because fewer bacteria receive the lac gene\" does not answer this question! You must explain how and why.

100 80 40 20 10 20 30 Time (minutes) (a) 50 azr lacr galt. 10 min Hfr str sr Origin 17 min 25 min Origin (b) F str

Solution

Order of genes from origin is azi^r, ton^r, lac⁺, and gal⁺. The gene closest to origin enters first and the gene located next to it will enter after the first gene. So, azi^r entered beginning from 9 minutes and ton^r from 11 minutes.

Few important points to note:

Please note that all bacteria in culture do not start participating in conjugation at exactly the same time. Conjugation process in cells is somewhat asynchronous. A significant proportion of cells may not participate in conjugation at all.

Reason why lac⁺ gene transfer hit plateau at low percentage than azi^r gene:

Conjugation involves transfer of DNA from Hfr strain to F- strain. For, This to happen, DNA should pass from one bacteria to another. While this is happening, DNA may break and process of transfer may halt spontaneously. This breaking/halting is a random process and its frequency is proportional to the duration of transfer. So, genes transferring late (these are the genes at more distance from origin) will transfer to less percentage of cells because there is more chance of occurring a spontaneous DNA break before they enter the cell. So, Lac gene hits plateau at low percentage.