Drugs added No. of colonies Drugs added No. of colonies A 1156 CD 786 B 1148 ABC 30 C 1161 ABD 42 D 1139 ACD 630 AB 46 BCD 36 AC 640 ABCD 30 AD 942 BC 51
Based on the above results, which one of the below options represents the most likely order of the drug-resistance genes?
The experiment aims to determine the order of drug resistance genes on the Pneumococcal chromosome using transformation. The key principle is that genes located physically close together on the chromosome are more likely to be transferred simultaneously into a recipient cell during transformation. This phenomenon is known as co-transformation. Therefore, the frequency of co-transformation between two genes is inversely proportional to the distance between them: higher co-transformation frequency implies closer linkage.
Transformation Experiment and Gene Linkage
Analyzing Co-transformation Data
We will examine the provided data, which lists the number of colonies obtained on media containing various combinations of drugs. The number of colonies indicates the success of transformation and co-transformation for the specified resistance markers.
Pairwise Linkage Interpretation
First, let's look at the results for single drug resistance and pairs of drugs:
| Drugs Added | No. of Colonies |
|---|---|
| A | 1156 |
| B | 1148 |
| C | 1161 |
| D | 1139 |
| AB | 46 |
| AC | 640 |
| AD | 942 |
| BC | 51 |
| BD | 36 |
| CD | 786 |
From the pairwise data, we observe the following co-transformation frequencies:
- A-D: 942 colonies. This is a very high number, indicating that genes A and D are very closely linked.
- C-D: 786 colonies. This is also a high number, suggesting C and D are closely linked.
- A-C: 640 colonies. This is a high number as well, indicating A and C are linked, but potentially less tightly than A-D or C-D.
- A-B: 46 colonies. Low frequency, suggesting A and B are distant.
- B-C: 51 colonies. Low frequency, suggesting B and C are distant.
- B-D: 36 colonies. Very low frequency, suggesting B and D are very distant.
Triplet Linkage Interpretation
Next, we consider combinations involving three or four drugs:
| Drugs Added | No. of Colonies |
|---|---|
| ABC | 30 |
| ABD | 42 |
| ACD | 630 |
| BCD | 36 |
| ABCD | 30 |
The results for triplets and quadruplets further confirm the linkage:
- ACD: 630 colonies. This high number confirms that genes A, C, and D are linked together in a cluster.
- ABC (30), ABD (42), BCD (36), and ABCD (30) all show very low colony counts. This strongly indicates that gene B is located far from the A-C-D cluster and is not frequently co-transformed with them.
Deducing the Gene Order
To establish the order of the linked genes A, D, and C, we compare their pairwise co-transformation frequencies. Higher frequencies correspond to shorter distances.
- A-D linkage (942 colonies) is the strongest, implying the shortest distance between A and D.
- C-D linkage (786 colonies) is the second strongest.
- A-C linkage (640 colonies) is the third strongest, implying the longest distance among these three pairs.
If we assume the order A-D-C:
- The distance A-D is short (high frequency 942).
- The distance D-C is medium (frequency 786).
- The distance A-C is long, being the sum of the A-D and D-C segments. Its frequency should be the lowest. This fits with the observed 640 colonies for A-C, as 942 > 786 > 640.
Therefore, the order of the linked genes is A-D-C.
Since gene B is found to be distant from A, D, and C, it is placed at the end, far from this cluster.
Final Gene Order Determination
Combining the linked order A-D-C with the distant gene B, the most likely overall order of the drug resistance genes is A-D-C-B.
This order is consistent with all observed co-transformation frequencies.
Important Questions from Ordering and Ranking (Notes)
- The sixth largest three-digit odd number among those with all digits distinct is
- There are five boys P, Q, R, S and T sitting in a row facing North such that P is right of Q and T is left of R and right of P. Q is right of S. Which boy is in the middle of the row?
Each of P, Q, R, S, T, U and V has an exam on a different day of a week starting from Monday and ending on Sunday of the same week. Q has an exam on Thursday and U has an exam on Saturday. T has an exam immediately before S. P has an exam on one of the days after V. R has an exam immediately after S. How many people have exams between R and P?
- Among five people of different ages, P is older than R. L is younger than M. R is older than Y. M is younger than Y. Who is the youngest?
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