A Hybrid DNA was allowed to replicate of for 2 generation in N15 H4cl and for 1 generation in find the ratio Hybrid normal & heavy double strand DNA?

The initial DNA is a Hybrid DNA, meaning it consists of one N¹⁴ strand and one N¹⁵ strand (N¹⁴N¹⁵).
When this N¹⁴N¹⁵ DNA replicates in an N¹⁵ medium for the first generation:

• The N¹⁴ strand serves as a template, and a new N¹⁵ strand is synthesized, forming a N¹⁴N¹⁵ (Hybrid) molecule.
• The N¹⁵ strand serves as a template, and a new N¹⁵ strand is synthesized, forming a N¹⁵N¹⁵ (Heavy) molecule. After the 1st generation in N¹⁵ medium, we have: 1 Hybrid (N¹⁴N¹⁵) and 1 Heavy (N¹⁵N¹⁵) DNA molecule.

Now, the 1 Hybrid (N¹⁴N¹⁵) and 1 Heavy (N¹⁵N¹⁵) molecules replicate in the N¹⁵ medium.

• From the 1 Hybrid (N¹⁴N¹⁵) molecule:
  • N¹⁴ template + new N¹⁵ strand $\rightarrow$ N¹⁴N¹⁵ (Hybrid)
  • N¹⁵ template + new N¹⁵ strand $\rightarrow$ N¹⁵N¹⁵ (Heavy)
  • This yields: 1 Hybrid and 1 Heavy molecule.
• From the 1 Heavy (N¹⁵N¹⁵) molecule:
  • N¹⁵ template + new N¹⁵ strand $\rightarrow$ N¹⁵N¹⁵ (Heavy)
  • N¹⁵ template + new N¹⁵ strand $\rightarrow$ N¹⁵N¹⁵ (Heavy)
  • This yields: 2 Heavy molecules. After the 2nd generation in N¹⁵ medium, the total molecules are:
• Hybrid (N¹⁴N¹⁵): 1
• Heavy (N¹⁵N¹⁵): 1 (from original hybrid) + 2 (from original heavy) = 3
• Normal (N¹⁴N¹⁴): 0 So, at the end of 2 generations in N¹⁵ medium, we have 1 Hybrid and 3 Heavy DNA molecules.

Now, the 1 Hybrid (N¹⁴N¹⁵) and 3 Heavy (N¹⁵N¹⁵) molecules replicate in an N¹⁴ medium.

• From the 1 Hybrid (N¹⁴N¹⁵) molecule:
  • N¹⁴ template + new N¹⁴ strand $\rightarrow$ N¹⁴N¹⁴ (Normal)
  • N¹⁵ template + new N¹⁴ strand $\rightarrow$ N¹⁴N¹⁵ (Hybrid)
  • This yields: 1 Normal and 1 Hybrid molecule.
• From the 3 Heavy (N¹⁵N¹⁵) molecules:
  • Each N¹⁵N¹⁵ molecule replicates in N¹⁴ medium.
  • For each N¹⁵N¹⁵: N¹⁵ template + new N¹⁴ strand $\rightarrow$ N¹⁴N¹⁵ (Hybrid) (twice)
  • So, each N¹⁵N¹⁵ molecule yields 2 Hybrid (N¹⁴N¹⁵) molecules.
  • Since there are 3 Heavy molecules, they will yield $3 \times 2 = 6$ Hybrid (N¹⁴N¹⁵) molecules.

Combining the results from Step 3:

• Normal (N¹⁴N¹⁴) molecules: 1
• Hybrid (N¹⁴N¹⁵) molecules: 1 (from the initial hybrid's replication) + 6 (from the heavy molecules' replication) = 7
• Heavy (N¹⁵N¹⁵) molecules: 0

The ratio of Hybrid : Normal : Heavy double-strand DNA is:

$$\text{Hybrid} : \text{Normal} : \text{Heavy} = \boxed{\text{7:1:0}}$$