This set of Cytogenetics Multiple Choice Questions & Answers (MCQs) focuses on “Gene Mapping in Bacteria by Conjugation”.
1. Conjugation can’t take place between_________________
a) F- and F+
b) F’ and F-
c) HFR and F-
d) HFR and F+
Explanation: For conjugation to take place one bacterium must have the F plasmid and other should lack it. HFR has the components of F plasmid within its genome so it can’t mate with another F+.
2. In an experiment you have met, bio, thr+ and leu + bacterial strain in test tube 1; and met+, bio+, thr and leu bacterial strain in other test tube 2. Which of the following will not comply with your experimental results?
a) Both the strains will grow in enriched media
b) 2nd strain will grow in met absent media
c) Mixture of two strains will grow in minimal media
d) 1st stain will not grow in leu absent media
Explanation: In this case, the 1st strain will have the ability to make leu as it is leu+. So it can grow in leu+ media. The ability of the mixture to grow in minimal media is due to the fact that gene transfer has taken place.
3. In case of mixing his+ leu- strain with his- leu+ strain we can see that the auxotroph behaves as prototroph for these two genes. This is because of________________
a) One is synthesizing one amino acid and giving it to the other type
b) They fuse to form similar diploid bacteria
c) Both will have the same genetic composition at the end
d) They share the RNA by extracellular processes
Explanation: Genetic transfer, mainly by conjugation will result in incorporation of the His+ Leu+ gene in both the strains at the end. Then they can produce both these amino acids by themselves and grow on minimal media. This is not by extracellular process but by contact.
4. Which gene in a plasmid is indispensible for it to participate in conjugation?
a) Pilli forming gene
b) Origin of replication
c) Sex determining gene
d) Origin of transfer
Explanation: There is no particular sex determining gene in bacteria, their pseudo sex is determined by presence or absence f fertility factor. However pilli forming gene is necessary for the conjugation channel, ori of transfer and ori of replication also maintain a plasmid.
5. Which of the following can transfer the genes in bacterial nucleoid as well?
a) F- plasmid
b) R factor
c) F’ plasmid
Explanation: While F’ can transfer one or two of the bacterial gene as well, but only HFR can transfer the gene from the nucleoid of the bacteria as it is the F plasmid incorporated in the central chromosome.
6. A HFR bacteria mates with an F- plasmid containing bacteria. What will be the type of bacteria after conjugation is complete?
Explanation: In HFR rarely the complete chromosome can be transferred from one bacterium to other due to the continuous motion of the medium. Here only partially the genes from Ori of transfer till the point where conjugation channel is broken gets transferred. As the full F plasmid can thus never be transferred so it is F-.
7. All the genes in the HFR that is transferred to the recipient cell before breakage of the conjugation channel is incorporated in the recipient genome. State whether it is true or false.
Explanation: It is true that all the genes in HFR till the point of breakage of conjugation channel will be transferred to recipient, but its incorporation in the recipient genome will depend on the recombination between the exogenote and endogenote.
8. In and interrupted mating experiment, it is seem that the after 10 mins AziR gene in recipient, after 20 minuites there is appearance of tonR and after 15 mins the recipient is lac+ , but the recipient is almost never his+. Where in the genome is the ori of transfer?
a) Between AziR and TonR genes
b) Between TonR and Lac+
c) Between Lac+ and His+
d) Between His+ and AziR
Explanation: As the origin of transfer is between His+ and AziR the sequence follows AziR, ->TonR->Lac+->His+ or the reverse direction. Thus, longer time is required for His to enter and in most cases before that the conjugation tube breaks.
9. If genes enter the different F- cells from different HFRs in the order as shown-
Azi 8 88 Ton 10 90 Lac 17 3 Gal 24 11
What is the order of the gene where columns are for different HFR and values denote time?
a) Azi, Lac, Gal, Ton
b) Lac, Gal, Azi, Ton
c) Gal, Lac, Ton, Azi
d) Tob, Azi, Lac, Gal
Explanation: The HFR can be produced by incorporation of the plasmid within the DNA at different positions. So here in two cases, the position is different. Considering the relative time of entry we can see a pattern which agrees with option 2. It can be in either orientation.
10. In an experiment of determining the order of gene in bacteria we use HFR which is gal+, Lac+, Ara+ and His+. If the recipient is negative for all these genes, and we selectively choose Gal + and get the pattern-
100 % recipients are Gal +
30% are His+
55% are Lac +
And only 5% are Ara+
What would be our inference?
a) Sequence of gene is Gal—Lac—His—Ara
b) As we are just selecting one gene we can’t tell the sequence
c) The HFR was incorporated between Gal and His only
d) The recipient selectively incorporated some genes
Explanation: While the HFR could incorporate in different positions by selecting one gene we are setting a start site for checking. Now the gene order agrees with the percent incorporation as the farther genes have a lower probability of incorporation.
11. For complete HFR to enter the recipient, how much time should the two bacterial cells be in contact?
a) 10 mins
b) 30 mins
c) 60 mins
d) 100 mins
Explanation: The two participating bacteria taking part in conjugation should actually stay connected by conjugation tube for 100 minutes for the entire chromosome i.e. HFR to get transferred. However, as they are constantly in motion such occurrence is rare.
12. You study the gene incorporation in recipient using HFR and get the following sequences-
P-W-A-K, W-L-I-K, A-K-I-L, W-P-R-T-W-L. What is the order of the gene?
Explanation: In this case, the F plasmid could have got incorporated in the genome at different positions which leads to different sequences. We have to determine the overlapping regions in order to find the complete sequence. It can be in either orientation so overlapping should be checked for both cases.
Sanfoundry Global Education & Learning Series – Cytogenetics.
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