Q.Depending upon the chemical nature of the template (DNA or RNA) and the nature of nucleic acids synthesised from it (DNA or RNA), list the types of nucleic acid polymerases.

Nucleic acid polymerases are enzymes that catalyse the synthesis of nucleic acids by adding nucleotides to a growing chain. We classify them based on two criteria: the template they read and the product they synthesise.

When DNA serves as the template, three types emerge:

• DNA-dependent DNA polymerase – reads DNA and makes DNA; this is the enzyme responsible for DNA replication.

• DNA-dependent RNA polymerase – reads DNA and synthesises RNA; this carries out transcription in all organisms.

When RNA is the template:

• RNA-dependent RNA polymerase – reads RNA and produces RNA; found in certain viruses with RNA genomes. …

The classification of nucleic acid polymerases rests on a simple but powerful logic: what do they read, and what do they write? Every polymerase enzyme catalyses the synthesis of a nucleic acid chain by reading a template strand and adding complementary nucleotides one by one. The chemical nature of that template—whether it is DNA or RNA—and the nature of the product being assembled—again, DNA or RNA—together define four distinct categories of polymerases, each with its own biological role.

DNA-dependent DNA polymerases read a DNA template and synthesise a new DNA strand. This is the classic replication machinery of the cell. During DNA replication, the double helix unwinds, and each strand serves as a template for the synthesis of a complementary daughter strand. In prokaryotes like E. coli, DNA polymerase III is the main replicative enzyme, while DNA polymerase I fills in gaps and removes RNA primers. In eukaryotes, DNA polymerase α, δ, and ε handle replication, and other specialized polymerases repair damaged DNA. This category is fundamental to the faithful copying of genetic information from one generation to the next.

DNA-dependent RNA polymerases use a DNA template to synthesise RNA. This is transcription—the process by which genetic information encoded in DNA is copied into messenger RNA (mRNA), ribosomal RNA (rRNA), or transfer RNA (tRNA). In prokaryotes, a single RNA polymerase transcribes all genes. In eukaryotes, three main RNA polymerases divide the labour: RNA polymerase I transcribes most rRNA genes, RNA polymerase II transcribes mRNA and most small nuclear RNAs, and RNA polymerase III transcribes tRNA genes and the 5S rRNA gene. The product is always RNA, but the template is always DNA.

RNA-dependent DNA polymerases, also called reverse transcriptases, read an RNA template and synthesise DNA. This is the reverse of the usual flow of genetic information, hence the name. Retroviruses like HIV carry their genetic material as RNA and use reverse transcriptase to synthesise a DNA copy once inside a host cell. That DNA copy then integrates into the host genome. This enzyme is also crucial in the laboratory for making complementary DNA (cDNA) from mRNA, a technique widely used in molecular biology and genetic engineering. …

Build a 2x2 grid instead of a prose list: columns = product synthesised (DNA or RNA), rows = template read (DNA or RNA). Filling in the four cells gives DNA-dependent DNA polymerase, DNA-dependent RNA polymerase, RNA-dependent DNA polymerase (reverse tr …