Question

1. all cells have ribonucleotide resucatse (RNR) acitivty, even those cells that have terminally differentated and will never divide again. why do these cells need RNR?

2. The pool of dNTPs varies in size over the course of the cell cycle, and peaks at the beginning of S-phase (synthesis), while the NTP pool is relatively stable, and much larger. GIven what you know about the roles of these nucleotides, why does this make sense?

Answer 1

1] Ribonucleotide reductase [RNR] is an enzyme that catalyses the synthesis of dNDPs. The enzymes help to reduce ribonucleotides to deoxyribonucleotide. RNR are also needed to carry out various cellular functions like protein synthesis. Even the cells which have differentiated and will divide no more, require RNR to be able to perform their assigned cellular functions which may be synthesis and secretion of various types of cellular proteins in the form of enzymes, hormones etc. So, the presence of RNR IS ESSENTIAL

2] dNTP stands for deoxynucleoside triphosphate . These are the 4 nucleosides used by DNA polymerase to make copies of DNA or extend an annealed primer.

We know that in the course of a cell cycle, the DNA strands are synthesised and chromosomes are duplicated at the S-phase. During this phase the cell prepares itself for entering into the new cell cycle.

Since dNTPs contain the bases adenine, guanine, cytosine, thyamine and uracil as nucleosides, their amount just at the beginning of the S-phase is the highest , so that new strands of DNA can be easily synthesised in the cell nucleus.