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Unlike eukaryotic cells, a prokaryotic cell such as a bacterium generally does not have individual structures called organelles within it. There is typically no nucleus, mitochondria, or other areas where separate metabolic processes occur; everything is mostly free-floating inside the cell wall and plasma membrane. Like eukaryotic cells, there are usually strands of deoxyribonucleic acid (DNA) as well as ribonucleic acid (RNA) which can be copied through transcription. Prokaryotic transcription is typically controlled by an enzyme called prokaryotic RNA polymerase, which has to initiate the transcription of DNA, while termination of the process is usually triggered by other sequences of nucleotides.
When the RNA polymerase enzyme travels the length of a DNA strand, it unravels it at the transcription site and messenger, transfer, and ribosomal RNA can be made. There are typically two types of the enzyme in prokaryotic transcription; one is a core enzyme that can make copies but is unable to find the appropriate site on a gene. A holoenzyme form of the molecule is often able to initiate transcription at the specific region, and therefore is designed to locate the promoter sequences that tell the molecule when to start copying the DNA. The holoenzyme performs this function via a component called a sigma.
Prokaryotic transcription begins as the RNA polymerase attaches to the DNA promoter site. The molecule and the double-stranded structure, called a closed complex, can then interact and the DNA is opened into a single-stranded sequence near where transcription is started. This is called an open complex. The enzyme typically begins the transcription process by creating around 10 unusable transcripts, which are blocked from leaving the complex by a protein.
Once this protein is released, then the enzyme continues with transcription. There are sometimes differences in how strongly the RNA polymerase and proteins bind to DNA; the strength of this bond can be related to the statistical probability that a certain base will be at a given location. How closely the bases match this consensus sequence often determines how strong the bond will be.
Prokaryotic transcription of RNA usually occurs at about 40 nucleotides each second. Some proteins can change the rate at which this happens and the speed of copying certain sequences can also be different. Regulator genes often change how sequences are expressed depending on what the cell needs. Prokaryotic transcription can be terminated either when sequences in the RNA cause the molecular complex and DNA to separate, or when a specific protein binds to and travels up to the RNA polymerase enzyme.