 In this video, I will describe the steps involved in transcription, including the enzymes involved and where the process occurs in the cell, and describe the post-transcriptional modifications of RNA. Transcription is the synthesis of a strand of RNA that is complementary to a gene. Then occurs in the nucleus, and then the resulting RNA that's produced can be a messenger RNA that will carry the information from DNA out of the nucleus in order to be translated by a ribosome to produce a protein. However, there are other forms of RNA produced by transcription. There's also R RNA, which is part of the ribosome structure. So R stands for ribosomal RNA, and there's T RNA, T stands for transfer. So transfer RNA functions to carry amino acids to the ribosome in order to provide those amino acids for the mechanism of translation. Here we can see the roles of ribosomal RNA and transfer RNA in the mechanism of translation. Ribosomal RNA is part of the structure of the ribosome, and ribosomal RNA actually forms the active site that enables the formation of the peptide bond. It catalyzes the formation of the peptide bond. And so the ribosome is an interesting example of an enzyme where RNA is a major catalyst instead of protein forming the enzyme. And then T RNA, transfer RNA we can see here, is functioning to carry amino acids into the ribosome during translation. The anticodon of the T RNA is complementary to the codon in the messenger RNA, and then the T RNA will transfer in an amino acid that corresponds to that codon following the genetic code to produce the primary sequence of the resulting polypeptide. Transcription has three basic steps. The first step known as initiation occurs when transcription factors a collection of proteins all bind to a sequence of the DNA known as the promoter. The promoter typically contains the sequence T A T A thymine adenine, thymine adenine. And because of that T A T A sequence, we often refer to the region of the promoter as the Tata box, that Tata box will be recognized by the transcription factors that initiate the process of transcription for that gene. These transcription factors will also enable gene transcription to be regulated so that RNA is not constantly being made from each of our genes, but can be made in larger or smaller quantities depending upon the needs of the cell. The enzyme RNA polymerase will also bind at the promoter region directed by the transcription factors, RNA polymerase will then start to produce the RNA nucleic acid that's complementary to the DNA sequence. RNA polymerase can function without the help of helicase to break the hydrogen bonds between complementary base pairs. RNA polymerase is able to break the hydrogen bonds itself and also catalyze the formation of the phosphodiester bond between the RNA nucleotides in order to initiate and then elongate the growing RNA polymer. Like DNA polymerase, RNA polymerase will only add nucleotides onto the 3 prime end, however RNA polymerase is able to initiate transcription without first having a primer. After elongation, the last step of transcription is termination and termination will be triggered by a termination sequence at the end of the gene. In transcription, post-transcriptional modifications will be required in order to produce the mature messenger RNA transfer RNA or ribosomal RNA. These post-transcriptional modifications include the addition of a 5 prime cap to a messenger RNA. This 5 prime cap includes a guanine trisphosphate, a modified nucleotide that's attached onto the 5 prime end of the RNA and will function as a signal to help guide the movement of the messenger RNA out of the nucleus and to the ribosome for translation. Then on the 3 prime end, a 3 prime tail is added. The 3 prime tail consists of a repeated adenine sequence where around 250 adenine nucleotides are added on at the 3 prime end and this helps to protect RNA from being degraded by RNA-ase nucleases enzymes that are constantly breaking down RNA in order to release the nucleotides and recycle them for producing new RNA. And then splicing is another mechanism that's required to produce a mature messenger RNA. Within the gene sequence, there are axons that are the sequences of DNA that code for the amino acids of a mature of a protein or a polypeptide that will be part of a protein. And then there are spaces of DNA sequence in between the axons. When a gene is transcribed, the RNA produced known as the pre-messenger RNA transcript includes introns that are separating the axons. Only the axons will be translated to produce a polypeptide and therefore the introns must be removed before translation. A structure known as a spliceosome will cut the introns out and join axons together in order to produce a mature messenger RNA that can then be translated by the ribosomes. The introns will then be degraded by nucleases and used in order to recycle those nucleotides to build new RNA in the process of transcription.