91Ó°ÊÓ

A bacterial transformation is performed with a donor strain that is resistant to four drugs, \(A, B, C,\) and \(D,\) and a recipient strain that is sensitive to all four drugs. The resulting recipient cell population is divided and plated on media containing various combinations of the drugs. The following table shows the results. $$\begin{array}{lccc} \begin{array}{c} \text { Drugs } \\ \text { added } \end{array} & \begin{array}{c} \text { Number } \\ \text { of colonies } \end{array} & \begin{array}{c} \text { Drugs } \\ \text { added } \end{array} & \begin{array}{c} \text { Number } \\ \text { of colonies } \end{array} \\ \hline \text { None } & 10,000 & \text { BC } & 50 \\ \text { A } & 1155 & \text { BD } & 48 \\ \text { B } & 1147 & \text { CD } & 785 \\ \text { C } & 1162 & \text { ABC } & 31 \\ \text { D } & 1140 & \text { ABD } & 43 \\ \text { AB } & 47 & \text { ACD } & 631 \\ \text { AC } & 641 & \text { BCD } & 35 \\ \text { AD } & 941 & \text { ABCD } & 29 \\ \hline \end{array}$$ a. One of the genes is distant from the other three, which appear to be closely linked. Which is the distant gene? b. What is the likely order of the three closely linked genes?

Step by step solution

01

Identify Total Transformation Frequency

We observe that when no drugs are added, there are 10,000 colonies. This represents the total number of transformed colonies that is relevant to the study of transformation.

02

Analyze Single Drug Results

Check the colonies for individual drug resistances: A (1155 colonies), B (1147 colonies), C (1162 colonies), D (1140 colonies). These values represent the number of times each gene independently transformed the recipient cells.

03

Determine Gene Linkage from Paired Drugs

Observe the number of colonies when two drugs are present: AB (47), AC (641), AD (941), BC (50), BD (48), CD (785). Smaller numbers suggest closer linkage between genes, while larger numbers suggest greater separation.

04

Identify the Distant Gene

The smallest number of colonies in paired drugs is for AB (47) and CD (785). The same observation applies to BC, BD. These numbers indicate that genes A, B, and D are closely linked, while C is more distant, given the high number of colonies observed for passes with C.

05

Propose Order of Closely Linked Genes

Based on small colony numbers for certain combinations (AB = 47, BD = 48, and BC = 50), genes A, B, and D are likely closely linked. The fact that A and B have the smallest number of colonies when paired suggests they are adjacent in order. D also appears in combinations with relatively small colony numbers, suggesting it is nearby both of these genes. Thus, a likely order is C - A - B - D.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Gene Linkage

Gene linkage is a fascinating concept in genetics, where certain genes that are located close to each other on a chromosome tend to be inherited together during the sexual reproduction of organisms. When we say that genes are "linked," it means that they do not follow Mendel's law of independent assortment. This often happens because they are physically closer on the chromosome.

In the context of bacterial transformation, which refers to the uptake of foreign DNA by a recipient cell (specifically bacteria), gene linkage plays a vital role. Imagine chromosomes as tightly packed highways of DNA with several lanes; closely linked genes are like vehicles traveling close together in a convoy, rarely changing positions.

For example, when examining antibiotic resistance in bacteria, the number of colonies resistant to combinations of drugs can tell us about gene linkage. Smaller numbers of colonies appearing for particular drug combinations indicate that the respective genes are closely linked. This proximity reduces the chance of those genes being separated during the transformation process.

Antibiotic Resistance

Antibiotic resistance is the ability of bacteria to survive and multiply despite the presence of antibiotics that would typically kill them or inhibit their growth. This resistance is a significant concern in medicine as it can lead to treatment failures and increased healthcare costs.

In bacteria, antibiotic resistance can be spread through horizontal gene transfer, which includes transformation, conjugation, and transduction. In transformation, a bacterium takes up pieces of DNA from its environment, possibly acquiring genes that confer resistance to antibiotics.

In the exercise, observing the resistance patterns to four antibiotics (A, B, C, and D) helps us understand how these genes are organized. When only one drug is added, and several colonies resist it, it indicates independent transformation events. The combination of drugs that produce few colonies suggests tight gene linkage, while a higher colony count suggests that certain resistance genes, like gene C in this exercise, might be located further away and thus transfer separately.

Genetic Mapping

Genetic mapping is the process of determining the location and order of genes on a chromosome. This provides scientists with valuable insight into gene function and inheritance patterns.

In bacterial transformation studies, genetic maps can be constructed by analyzing the frequency of certain traits, like antibiotic resistance, appearing together. Close proximity on the chromosome means those genes are less likely to be separated by recombination.

To map genes, researchers often rely on recombination frequency, which is inferred from experiments like this one with antibiotic-resistant bacteria. If two resistance traits are often inherited together (indicated by fewer colonies surviving, as seen with pairs such as AB and BD), they are likely closely linked on the chromosome. Thus, mapping focuses on the order in which these genes appear: in our example, genes A, B, and D are tightly linked, while the gene for antibiotic C resistance appears to be more distant. This information enables scientists to determine the probable sequence on the chromosome (C being more distant from A, B, and D).