Refer to the following sequence of DNA: 5\' A T C G C G G T G 3\' 3\' T A G C G

Question

Refer to the following sequence of DNA: 5' A T C G C G G T G 3' 3' T A G C G C C A C 5' a. If this DNA was replicated in E. coli in the presence of radioactive thymine (^3 T) what replication products (2 products) will result (the symbol^3 T instead of T if the Thymine is radioactive in the sequence). b. If the products above went through one more round of replication in radioactive thymine, what replication products will result (4 products). c. If the products above went through one more round of replication in normal thymine, what replication products will result (8 products).

Explanation / Answer

Ans.     Original DNA strand is

5’- A T C G C G G T G -3’     - strand 1

            3’- T A G C G C C A C -5’     - strand 2

#A. All the resultant products (two daughter DNA) will be radiolabeled.

5’- A T C G C G G T G -3’     - strand 1 (original/ parental strand, not T3 labelled)

3’- T A G C G C C A C -5’     - strand 4 (T3 labelled strand)

            3’- T A G C G C C A C -5’     - strand 2 (original/ parental strand, not T3 labelled)

5’- A T C G C G G T G -3’    - strand 3 (T3 labelled strand)

T is T3 labelled.

Strand 3 and 4 belong to the new daughter strands synthesized during replication. Note that DNA replication is semi-conservative. So, each old parental (original) DNA stands has a new radiolabeled DNA strand after replication.

# B. As long as replication occurs in 3T medium, the resultant DNA molecules (both the strands) will be radiolabeled as #A.

5’- A T C G C G G T G -3’     - strand 1 (original/ parental strand, not T3 labelled)

3’- T A G C G C C A C -5’     - strand 5(T3 labelled strand)

            3’- T A G C G C C A C -5’     - strand 2 (original/ parental strand, not T3 labelled)

5’- A T C G C G G T G -3’    - strand 6 (T3 labelled strand)

           

3’- T A G C G C C A C -5’     - strand 4 (T3 labelled strand)

5’- A T C G C G G T G -3’    - strand 7 (T3 labelled strand)

5’- A T C G C G G T G -3’    - strand 3 (T3 labelled strand)

3’- T A G C G C C A C -5’     - strand 8 (T3 labelled strand)

#C. If DNA undergoes replication in normal T medium, the newly synthesized daughter strands will contain normal T residues.

5’- A T C G C G G T G -3’     - strand 1 (original/ parental strand, not T3 labelled)

3’- T A G C G C C A C -5’     - strand 9, normal T- new daughter strands

3’- T A G C G C C A C -5’     - strand 5(T3 labelled strand)

5’- A T C G C G G T G -3’     - strand 10, normal T- new daughter strands

            3’- T A G C G C C A C -5’     - strand 2 (original/ parental strand, not T3 labelled)

5’- A T C G C G G T G -3’     - strand 11, normal T- new daughter strands

5’- A T C G C G G T G -3’    - strand 6 (T3 labelled strand)

3’- T A G C G C C A C -5’     - strand 12, normal T- new daughter strands

3’- T A G C G C C A C -5’     - strand 4 (T3 labelled strand)

5’- A T C G C G G T G -3’     - strand 13, normal T- new daughter strands

5’- A T C G C G G T G -3’    - strand 7 (T3 labelled strand)

3’- T A G C G C C A C -5’     - strand 14, normal T- new daughter strands

           

5’- A T C G C G G T G -3’    - strand 3 (T3 labelled strand)

3’- T A G C G C C A C -5’     - strand 15, normal T- new daughter strands

3’- T A G C G C C A C -5’     - strand 8 (T3 labelled strand)

5’- A T C G C G G T G -3’     - strand 16, normal T- new daughter strands

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