Question

Radioactive nucleotides can be added to cells to study DNA replication. Basically, if radioactive nucleotides are added to a cell, any nucleotide strands that are already present remain non-radioactive but any new strands that are made while the radioactive nucleotides are present are radioactive.

In other words, the DNA template (or parent strand) would NOT be radioactive but the new DNA strands (daughter strands) would be. We can then extract the DNA for study or we can use this technique at a cellular level. If any DNA strands containing radioactive nucleotides exist in the cell, the cell itself is considered radioactive.

Think about a cell undergoing cell division...

Radioactive nucleotides are added to Cell 1, DNA replication occurs and once complete any "free"/single radioactive nucleotides are washed away (non-radioactive ones remain). If a DNA polymerase used radioactive nucleotides to build a DNA strand during replication that strand is still radioactive, there are just no "free" radio nucleotides left.

Think of homologous chromosomes from Cell 1 after the replication described above. If a homolog was radioactive, would radioactivity be in one sister chromatid or both? In a pair of homologues, would only one homolog be radioactive or both?

After the replication round described above Cell 1 divides to form two daughter cells (Cell 1.2A and Cell 1.2B). How many of the two daughter cells are radioactive?

If another round of cell division occurs, without further addition of radio-nucleotides, how many of the four daughter cells are radioactive? 1.2A is dividing to produce two daughter cells (1.2A.3A and 1.2A.3A), and 1.2A is dividing to produce two daughter cells (1.2B.3A and 1.2B.3B).

If a homolog was radioactive, would radioactivity be in one sister chromatid or both?

In a pair of homologues, would only one homolog be radioactive or both?

Two daughter cells (Cell 1.2A and Cell 1.2B). How many of the two daughter cells are radioactive?

If another round of cell division occurs, without further addition of radio-nucleotides, how many of the four daughter cells are radioactive?

Answer 1

Ans) a) if a pair of homolog was radioactive then radioactivity will be in both sister chromatids because DNA replication is semiconservative in nature means each newly synthesized sDNA duplex has one old parental strand and one newly synthesized daughter strand. So here newly synthesized DNA duplex in each sister chromatid have one old parental non radioactive strand and second newly synthesized radioactive daughter strand.

Ansb) in pair of homologous chromosomes both homologues are radioactive because DNA replication is semiconservative so each sister chromatid of a honologue is radioactive that means both homologous chromosomes are radioactive.

Ansc)at anaphase I homologous chromosome separate from each other and move towards opposite pole followed by telophaseI and cytokinesis two daughter cell each with one homologous chromosome is produced. Both daughter cell1.2Aand cell1.2 Bare  radioactive because each homologous chromosome is radioactive due to semiconservative nature of DNA replication.

Ansd) in anaphase II both sister chromatids are splitted from each other and move towards opposite pole followed by cytokinesis leads to formation of 4 daughter cells. These all 4 daughter cells have raadioactivity because each sister chromatid has radioactivity in their DNA strand.