Question

If E.coli was allowed to grow for 80 minutes then what would be the proportions of light and hybrid densities DNA molecules?

Answer 1

Hint: By following the semiconservative mode of DNA replication, the densities of light and hybrid DNA molecules can be calculated.

Complete answer:

As we know, DNA replication refers to the mechanism through which the parental DNA of a cell is copied to make the daughter DNA. Why is this happening? Well, simply because when a cell has to divide to form the daughter cells, its genetic material has to be evenly distributed. For that it is essential to make copies of the parental DNA first. 

That the replication of DNA follows a semiconservative mode was experimentally proved to us by Meselson and Stahl in 1958. It suggested that the double stranded parental DNA molecule unzips and each strand serves as a template for the synthesis of the daughter DNA strand. So, after the process of DNA replication is completed, there will be one parent and one new daughter strand in a molecule of DNA. The semiconservative model was based on the fact the nitrogen bases on one DNA strand form complementary base pairings with the bases on the other strand, such that Adenine forms double H-bonds with Thymine and vice versa; and Guanine forms triple H-bonds with Cytosine and vice versa. 

In their experiment, Meselson and Stahl used heavy isotopes of nitrogen (15N) to track the parental DNA and fed light nitrogen isotopes to daughter strands. The diagram below summarizes this experiment:

So, if we go by this experiment, we can calculate the densities as asked in the question. After 20 minutes, we have two heavy strands of DNA. After 40 minutes, when the heavy DNA would replicate on medium with light nitrogen, we will have two light strands of DNA. 

As, E. coli divides after every 20 minutes, after 80 minutes there will be four generations:

> After 20 minutes, 100% heavy DNA

> After 40 minutes, 50% heavy and 50% hybrid DNA

> After 60 minutes, 25% hybrid and 75% light DNA

> After 80 minutes, 12.5% hybrid and 87.5% light DNA

Note:

The other models suggested for DNA replication were:

Conservative Model, in which one molecule of DNA will have parental DNA while the other will have new DNA strands. Dispersive Model, in which the DNA molecules have parental and new DNA strands interspersed.