13. E. coli cells were grown in N¹⁵ medium for several generations and then shifted to normal medium for one generation. If the DNA isolated from the culture would be centrifuged on a CSCl equilibrium density gradient, the result will be

(1) a single band of double helix DNA consisting of one strand with N¹⁴ and another with N¹⁵ label.

(2) single band of double helix DNA consisting of N¹⁴and N¹⁵ in both the strands.

(3) two bands containing double helix DNA each containing both N¹⁴ and N¹⁵ label.

(4) two bands containing single stranded DNA one with N¹⁴ and other with N¹⁵ label.

Introduction

The use of nitrogen isotopes (^15N and ^14N) in bacterial DNA labeling combined with cesium chloride (CsCl) density gradient centrifugation has been pivotal in understanding DNA replication. When E. coli cells grown in heavy nitrogen (^15N) medium are shifted to normal nitrogen (^14N) medium, the resulting DNA after one generation shows characteristic banding patterns that provide insights into the mode of DNA replication.

Experimental Background

• E. coli cells are cultured in ^15N-containing medium for several generations, resulting in DNA fully labeled with heavy nitrogen.

• The cells are then shifted to ^14N medium and allowed to replicate once.

• DNA is extracted and subjected to CsCl equilibrium density gradient centrifugation, which separates DNA molecules based on density differences.

Expected DNA Banding Pattern After One Generation

• After one generation in ^14N medium, DNA molecules will be hybrid: each double helix contains one strand with ^15N (heavy) and one strand with ^14N (light).

• This results in a single band at an intermediate density position between the heavy (^15N) and light (^14N) DNA bands.

• The band represents double-stranded DNA molecules with one heavy and one light strand.

Analysis of Options

Correct Answer

(1) a single band of double helix DNA consisting of one strand with N14 and another with N15 label

Biological Significance

This result supports the semi-conservative model of DNA replication, where each daughter DNA molecule contains one parental (heavy) strand and one newly synthesized (light) strand. The experiment elegantly demonstrates how DNA replication conserves one original strand in each daughter molecule.

Keywords

E. coli DNA replication, N15 labeling, N14 medium shift, CsCl density gradient centrifugation, DNA banding pattern, semi-conservative replication, hybrid DNA, molecular biology, DNA strands, isotopic labeling.

Conclusion

When E. coli cells grown in ^15N medium are shifted to ^14N medium for one generation, DNA isolated and centrifuged in a CsCl density gradient forms a single hybrid band. This band represents double-stranded DNA molecules with one strand labeled with ^15N and the other with ^14N, confirming the semi-conservative nature of DNA replication.