Summary
Highlights
Transcription is the initial phase of protein synthesis, where cells create proteins. This process occurs in the nucleus of eukaryotic cells and results in an mRNA copy of a gene using DNA as a template. It is followed by translation, which happens at ribosomes outside the nucleus.
Transcription begins with RNA polymerase binding to a specific part of the DNA molecule. This initiates the process, followed by the breaking of hydrogen bonds between complementary DNA bases, separating the two DNA strands. Only one strand, the anti-sense strand, serves as a template for mRNA synthesis.
Free RNA nucleotides align with the exposed DNA bases on the anti-sense strand through complementary base pairing (adenine with thymine, guanine with cytosine, and uracil with adenine). RNA polymerase then catalyzes the formation of phosphodiester bonds, creating the sugar-phosphate backbone of the mRNA strand.
Transcription terminates when RNA polymerase reaches a stop codon on the DNA, signaling the end of the gene. RNA polymerase detaches from the DNA, and the newly formed mRNA strand is released. Afterwards, the DNA strands re-join and rewind into a helix.
After transcription, the mRNA strand exits the nucleus through a nucleopore and enters the cytoplasm. Here, it will be used in the next stage of protein synthesis: translation.
In eukaryotic cells, the initial mRNA strand, called pre-mRNA, contains both non-coding regions (introns) and coding regions (exons). Splicing is the process that removes introns and joins exons together to form mature mRNA. This mature mRNA then leaves the nucleus for translation. Prokaryotic cells, however, do not have introns, so their mRNA is ready for translation immediately after transcription.