Final answer:
Transcription is catalyzed by RNA polymerases, with RNA polymerase I synthesizing rRNAs, RNA polymerase II producing mRNAs, and RNA polymerase III involved in tRNA and small nuclear RNA genes production. The process involves the enzyme binding to a promoter, unwinding DNA, and synthesizing RNA using a DNA template.
Step-by-step explanation:
Transcription Catalyzed by RNA Polymerases
Transcription, a key process in gene expression, is catalyzed by enzymes known as RNA polymerases. In eukaryotic cells, there are three main types of RNA polymerase depending on the RNA being synthesized. RNA polymerase I is responsible for the synthesis of most ribosomal RNA (rRNA), RNA polymerase II catalyzes the transcription of protein-coding genes and produces messenger RNA (mRNA), and RNA polymerase III is mainly involved in transcribing transfer RNA (tRNA) and small nuclear RNA genes.
Transcription begins when an RNA polymerase binds to a DNA sequence known as a promoter. This process is aided by various transcription factors. The enzyme then unwinds the DNA to form an 'open complex' and begins synthesizing RNA by forming phosphodiester bonds between nucleotides, in a 5' to 3' direction, according to the complementary bases on the DNA template (3' to 5' direction).
The initiation of transcription involves the enzyme binding to the promoter region, unwinding the double helix to expose the template strand, and assembling an RNA strand that is antiparallel to the DNA template. Incorporation of nucleotides follows the rules of base pairing - uracil (U) pairs with adenine (A), adenine (A) pairs with thymine (T), guanine (G) pairs with cytosine (C), and cytosine (C) pairs with guanine (G) in the RNA strand.