Question

In a nucleus, the number of RNA nucleoside triphosphates is 10 times the number of DNA nucleoside triphosphates, but only DNA nucleotides are added during the DNA replication. Why?

Answer 1

A nucleotide is the basic building block of nucleic acids i.e RNA and DNA are polymers made of long chains of nucleotides. A nucleotide consists of a sugar molecule which is ribose in RNA or deoxyribose in DNA attached to a phosphate group and a nitrogen-containing base. For replication, the cell requires specific polymerases.

Complete answer:
A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates that contain deoxyribose sugar, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create two identical DNA duplexes from a single original DNA duplex which is separated during replication. During this process, DNA polymerase "reads" the existing DNA strands to create two new strands that match the existing one, called replication.

DNA polymerase adds nucleotides in a specific direction i.e three prime (3')-end of a DNA strand, one nucleotide at a time. Every time a cell divides, DNA polymerases are required to duplicate the cell's DNA, so that a copy of the original DNA molecule from the parent cell can be passed to each daughter cell. In this way, genetic information is passed down from generation to generation continuously. RNA polymerases are required for RNA formation as they specifically add nucleotides containing ribose sugar.

Thus, the DNA polymerase enzyme is highly specific to recognise only deoxyribonucleoside triphosphates and therefore, it cannot hold RNA nucleotides.

Note: DNA polymerase enzymes can be further subdivided into seven different families: A, B, C, D, X, Y, and RT. Retroviruses encode an unusual DNA polymerase called reverse transcriptase, which is an RNA-dependent DNA polymerase i.e polymerizes DNA from a template of RNA.