Transcription produces RNA from DNA in living cells, with eukaryotic and prokaryotic processes differing due to the presence of membrane-bound organelles in eukaryotes. Eukaryotic transcription occurs in the nucleus and mitochondria, with three RNA polymerases involved in transcribing rRNA, mRNA, and tRNA. RNA nucleotides are added to the single strand of DNA to create RNA, which must travel through nuclear pores for translation in the cytoplasm.
Within all living cells, transcription is the process that produces strands of RNA, which are encoded based on the DNA found within the cells. The RNA is then used to make proteins within a cell’s cytoplasm. Within different organisms, the transcription process may differ slightly. This is especially true when looking at eukaryotes and prokaryotes; the term eukaryotic transcription describes the process within eukaryotes.
The main difference between eukaryotic organisms and prokaryotic organisms is the presence of membrane-bound organelles inside the cells. Eukaryotes have membrane-bound organelles, such as the nucleus and mitochondria, while prokaryotes do not. This is one of the first and most obvious distinctions between prokaryotic and eukaryotic transcription, as it determines where the process takes place. Eukaryotic transcription occurs within the nucleus and mitochondria, as that is where the DNA is found within these cell types. Consequently, translation must occur after transcription within eukaryotes as RNA must be transported to the cytoplasm from within the nucleus.
Almost all eukaryotic DNA is found within the nucleus, so this is the primary location for most of the transcript. When DNA is transcribed, a particular region of the DNA is unwound to be single-stranded. This region is called a cistron and ultimately codes for a protein after transcription and translation. In most cases, there are three enzymes involved in eukaryotic transcription as opposed to only one for prokaryotic transcription.
The enzymes involved in transcription are called RNA polymerases and the three different ones are RNA polymerase I (RNA Pol I), RNA polymerase II (RNA Pol II) and RNA polymerase III (RNA Pol III). The type of RNA determines which of the three polymerases is used during transcription. RNA Pol I transcribes ribosomal RNA (rRNA), which is used to make ribosomes within the cytoplasm and is where translation occurs. Messenger RNA, the strands that provide the code for proteins, is transcribed from RNA Pol II. The third enzyme, RNA Pol III, transcribes the DNA into transfer RNA (tRNA), which is used to transport the appropriate amino acids to the ribosomes to make protein strands.
During eukaryotic transcription, one of the RNA polymerases moves along the single strand of DNA. As it does so, it creates an RNA strand by adding an RNA nucleotide that is complementary to the one found within the DNA strand. The RNA nucleotides float freely within the nucleus and are attracted to the unbound single strand of DNA. Once the cistron has been transcribed, the new strand of RNA must travel through the pores of the nuclear membrane so that translation can take place in the cytoplasm.
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