RNA Transcription

pairs do not get paired, which shortens the genome over time. These ends are called telomeres and ae not coded. The base-pair sequence of TTAGGG is repetitively copied up to 1000 times to make the telomere. Telomerase is an enzyme that attaches to the end of the chromosome to add telomeres in cells like germ cells and adult stem cells, which remain “youthful” for extended periods of time. It isn’t active in adult somatic cells and may be responsible for the aging of cells because the telomeres shorten.

Plasmids also undergo DNA replication. While some replicate like bacterial DNA, others use the rolling circle replication technique. One piece of the double strand of DNA is nicked and then the DNA polymerase will help to make a copy of the un-nicked strand. The nicked strand may then circularize again and can get replicated. This leads to two copies of the plasmid.

RNA TRANSCRIPTION

Transcription happens when DNA is used as a copy to make an RNA transcript, which is single-stranded. It requires partial unwinding of the DNA segment and the formation of a transcription bubble. The antisense strand is the one that gets transcribed. This makes the RNA segment made a near-exact copy of the sense strand.

Transcription involves RNA polymerase, which works from the five-prime to threeprime end. RNA polymerase does not involve a primer but DNA polymerase does. Complementary RNA base pairs are added to match the DNA segment being transcribed. There are six subunits to the bacterial RNA polymerase. The sigma subunit is responsible for binding of the enzyme to a promotor site at the beginning of transcription.

Transcription always starts at a promoter site. The first DNA piece to be transcribed is called the initiation site. Nucleotides are added downstream from the initiation site. There are similarities in the promotor regions of all bacteria, which are the sites of RNA polymerase attachment. This usually involves what’s called a TATA box, which is a TATAAT sequence of DNA bases. Elongation happens at about 40 nucleotides per segment, getting made from the five-prime to three-prime end.

At some point, the RNA has been completely transcribed and the RNA polymerase dissociates from the template. This is called termination. There will be specific DNA sequences that act as signals for termination so that the RNA polymerase drops off and the RNA segment is freed.

Transcription also happens in eukaryotes with a few differences compared to prokaryotes. There are three different RNA polymerases in eukaryotes that transcribe different genes. One of the biggest differences is that one piece of eukaryotic RNA encodes for a single protein, while prokaryotic RNA encodes for multiple proteins at once. This is referred to as polycistronic RNA.

RNA is made in the nucleus but must exit the nucleus for protein translation. There are several modifications that need to happen before the RNA will be able to leave the eukaryotic nucleus in order to protect the RNA from being degraded. This allows eukaryotic messenger RNA to last several hours as opposed to prokaryotic messenger RNA, which lasts just a few seconds.

There will be a five-prime cap added to the RNA transcript in order to prevent degradation. In addition, a poly-A tail of a couple hundred adenine nucleotides to the three-prime end is added, also to prevent degradation and to signal the need for transport outside the nucleus.

It is only eukaryotic organisms that have exons that get expressed and introns that do not get expressed. The introns get spliced out of the pre-messenger RNA as part of the processing of the RNA. This RNA spicing process is done by a spliceosome, that contains small nuclear RNA. After each of these things happens, the RNA is transported outside the nucleus.