in E. coli, the ability to utilize lactose as a carbon source requires the prese
ID: 271370 • Letter: I
Question
in E. coli, the ability to utilize lactose as a carbon source requires the presence of the enzymes B-galactosidase and B-galactoside permease. These enzymes are encoded by two closely linked genes, lacZ and lacY, respectively. Another gene, proC, controls, in part, the ability of E. coli cells to synthesize the amino acid proline. The alleles str^r and str^s, respectively, control resistance and sensitivity to streptomycin. Hfr H is known to transfer the two lac genes, proC, and str, in that order, during conjugation. A cross was made between Hfr H of genotype lacZ- lacY+ proC+ strs and an F- strain of genotype lacZ+ lacY- proC- strr. After about 2 hours, the mixture was diluted and plated out on medium containing streptomycin but no proline. When the resulting proC+ strr recombinant colonies were checked for their ability to grow on medium containing lactose as the sole carbon source, very few of them were capable of fermenting lactose. When the reciprocal cross (Hfr H lacZ+ lacY- proC+ strs X F- lacZ- lacY+ proC- strr) was done, many of the proC+ strr recombinants were able to grow on medium containing lactose as the sole carbon source. What is the order of the lacZ and lacY genes relative to proC? ( the answer is lacY—lacZ—proC, however what to the step by step explanation of getting there ) Please provide explanation. I do not need to know the answer, I need to know how to arrive to the answer which is lacY lac Z proC strr.
Need explanation to arrive to the correct answer. I know the answer i just need to know how one wouldsolve the problem to obtain the answer
Explanation / Answer
Answer:
Conjugation: It is a type of recombination bacteria which involves the transfer of DNA from one bacterial cell to another via direct cell to cell contact.
Plasmid divide donor or recipient. This plasmid called F-Plasmid/Fertility factor/F-Factor/ Sex pili. This pili helps in the making of contact between one bacterial cell to another.
Plasmids are in two conditions. 1) Independent 2) Integrated.
F- : Absence of F-Factor.
F+ andHfr: Presence of F-Factor, F+: is independent, Hfr: Integrated (Plasmid recombinant with bacterial chromosome).
If the conjugation between F- and Hfr: F-Factor from Hfr transfer to F- either completely or partially. The genes also carry along with F-Factor.
F-Factor of Hfr strain represented as 'F'. This F attached to the origin from which transfer starts and represented as 'O'.
'O' first enter and 'F' entered last. The conjugation between Hfr and F- is a long duration. If conjugation completes, F- converts to F+. If conjugation incomplete, then F- remains F-.
1) Cross made between:
Hfr: lacZ- lacY+ proC+ strs (Lac Z-: absence, lac Y+: permease present, ProC + : Proline present, strS: Sensitive to streptomycin).
F-: lacZ+ lacY- proC- strR (Lac Z+: present, lac Y-: permease Absent, ProC - : Proline Absent, strR: Resistance to streptomycin). Means F- Unable to synthesis proline and resistant to str. So F- can able to grow on medium supplemented with proline and str.
If conjugation completes:
Recombinant becomes: lacZ+ /lacZ- lacY- /lacY+ proC-/ proC+ strR /Strs (Lac Z and Lac Y are presnt, Proline can synthesis and srt resistant).
Results: "The conjugation mixture was diluted and plated out on medium containing streptomycin but no proline. When the resulting proC+ strR recombinant colonies were checked for their ability to grow on medium containing lactose as the sole carbon source, very few of them were capable of fermenting lactose".
Explanation:
a) In general F- (ProC- ) unable to grow on proline less medium. But Hfr can grow on proline less medium. ProC- converted to ProC+ . Here recombinant colonies are grown on medium containing streptomycin but no proline, Because, ProC+ transferred from Hfr to F- through conjugation.
b) If Conjugation is incomplete: Only a few of the genes which present immediately after 'O' will be transferred. Remaining genes did not transfer. Most of the colonies grow on proline less medium but most of the colonies unable to grow on Lactose containing a medium.
Note: Bacterial cells require glucose as carbon source. If Lactose is present, then activate the Lac operon, the enzymes obtained from LacZ ( galactosidase) split the lactose into glucose and galactose. Lac y (Permease) allow this lactose into cells. This glucose utilized by E.Coli cells.
c) From this results conclusion is After 'O' then immediately ProC+ is present, so most of the F- cells are converted from ProC- to ProC+ . This followed by Laz. Already F- is LacZ+. Even presence of this LacZ gene also unable to grow on Lactose containing a medium. Because Lac Y followed the LacZ. The conjugation is incomplete, Lac Y+ gene not transferred. So lactose unable to enter into a cell. Unable utilize the lactose.
Gene orde: LacY--LacZ--ProC--O
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2) Receprocal cross:
2) Cross made between:
Hfr: lacZ+ lacY- proC- strR (Lac Z+: present, lac Y-: permease Absent, ProC - : Proline Absent, strR: Resistance to streptomycin).
F-: lacZ- lacY+ proC+ strs (Lac Z-: absence, lac Y+: permease present, ProC + : Proline present, strS: Sensitive to streptomycin).
Result:"Many of the proC+ strR recombinants were able to grow on medium containing lactose as the sole carbon source".
Explanation:
a) In general F- (ProC+) able to grow on proline less medium. But Hfr unable to grow on proline less medium. Here recombinant colonies are grown on medium containing streptomycin but no proline, Because, ProC+ present in F- .
b) Most of the recombinant colonies grow on proline less medium and most of the colonies able to grow on Lactose containing a medium.
Note: Bacterial cells require glucose as carbon source. If Lactose is present, then activate the Lac operon, the enzymes obtained from LacZ ( galactosidase) split the lactose into glucose and galactose. Lac y (Permease) allow this lactose into cells. This glucose utilized by E.Coli cells.
c) From this, results conclusion is ProC+ is present in F- cells and str R transferred from Hfr to F-. This followed by Laz. F- is LacZ- So from Hfr Lac Z+ transfer to F-. So the presence of this LacZ gene able to grow on Lactose containing a medium. Because Lac Y followed the LacZ. In F- itself Lac Y+. Even in complete conjugation, No transfer of Lac Y fro Hfr to F-, the cells can able to grow on lactose.
d) But in first condition F- is Lac Y-. If conjugation incomplete the cells are unable to grow.
So gene order is:
LacY--LacZ--ProC--O
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Note: In the question, at the reciprocal cross, The given genotype is not representing the reciprocal cross. Please verify once.
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