hello and welcome to our first video Lesson on chapter 21 transcription and RNA in this this lesson we want to look at the difference in structure of RNA as compared to DNA and also Define what constitutes a gene the processes of replication transcription and translation are all repetitive processes where building Polymers of either nucleotides or amino acids by adding monomers in a repetive to Fashion each has three major steps in the process of initiation the molecular machine binds to the template and Associates with itself the first monomer needed to form the polymer in the case of replication that machine is DNA polymerase transcription involves the machine RNA P polymerase and as we'll see in the next chapter the process of translation involving involves the machine the ribosome after we've initiated the synthesis of our polymer we continue in the process of elongation where we read the template and add the next monomer this is the truly repetitive part of each of these processes in replication it involves synthesizing DNA transcription we're making RNA and in Translation we're forming protein eventually we reach the end of the template and then begins the process of termination the molecular machine is released as well as the completed product let's define these terms replication transcription and translation in order to gain a better grasp of the overall purpose of each of these processes to replicate simply means to duplicate we're making an exact copy of the DNA to transcribe means to make a copy in a different form so in cellular transcription we're making a copy of the coding strand of DNA as we'll see in the form of RNA now it is a different form it's RNA rather than DNA and we'll look at those major differences in a moment but it is the same language the language of nucleic acid to translate means to render in another language the process of translation therefore involves converting the language of nucleic acid into the language of amino acids to form proteins and we'll look at that process in the next chapter let's next look at the two major differences between RNA and DNA the first and most important difference has to do with the nature of the sugar upon which we build those nucleotides in DNA we used the sugar deoxy ribos but in RNA we use aibos sugar so our nucleotides are ribonucleotides as highlighted by the red circle the major difference is that two two prime position carries a hydroxy group rather than simply a hydrogen atom so in other words the major difference between RNA and DNA has to do with a single atom an oxygen atom at that two prime position but it has much to do with the nature of the structures that can form in RNA as compared to those of DNA the position of that two prime o of the ribos prevents the formation of class classic Watson cric B helices in the RNA because of steric hindrance the two prime oxygen atom would come too close to three atoms of the adjoining phosphate and one atom in the next base as highlighted in the figure to the right in yellow so although RNA can form Watson CC based pairs of complimentary sequences due to hydrogen bonding interactions it cannot form a standard B type Helix it forms other structures as we'll see later the second major difference between RNA and DNA has to do with the nature of the nitrogenous bases the bases Adine and guanine the purine bases as well as the perimidine based cytosine are identical in DNA as well as RNA of course in DNA these bases are attached to deoxy ribos whereas in RNA they're attached to ribos there is one base that is different however in DNA we have the base thyine pictured on the far lower right and in RNA we have the base uracil as highlighted by the blue circles at the five number five carbon in the base thyine is carrying a methyl group whereas uracil is carrying simply a hydrogen atom let's next Define what is a gene we you're no doubt familiar with the fact that in the process of transcription we're going to generate messenger RNA or mRNA transcripts and in the process of translation that will be converted to a sequence of amino acids but let's remember there are ribosomal RNA molecules R RNA and transfer RNA molecules TRNA and others as we'll see in a moment that are not translated into protein in other words they function solely at the level of the RNA next we want to notice that in most cases one message one mRNA transcript will produce one polypeptide there are exceptions however in procaryotes many of their messages are polycistronic that is they carry multiple cistrons or genes and even in eukaryotic systems some mrnas contain the code for two proteins in overlapping sequences in other words the difference is where we start we can produce two different proteins we also want to acknowledge the fact that there may be control elements in the DNA that are required for the process of transcription but they do not manifest themselves they are not transcribed into the form of RNA this would include promoters and other regulatory regions as we'll see in a later lesson we also want to acknowledge that RNA transcripts undergo processing before they reach their final functional form in eukaryotic systems this involves the splicing of introns in a message a typical Gene consists of about eight exons or protein coding segments and we'll look at the process of splicing in a later lesson but keep in mind other RNA molecules are processed as well and this includes our RNA as well as TRNA so then we need to include in our definition for a gene that there are two types of genes first of all protein coding gen genes where the DNA is transcribed into RNA and then translated into protein and there are non-coding rnas in other words the gene is transcribed cribed into RNA and functions at the level of the RNA it's never translated into protein these are the so-called non-coding rnas as it turns out about 80% of the human genome that undergoes transcription produces non-coding rnas in other words if we were to express the entire genome 80% of the RNA molecules are never translated in this table for your book we have examples of non noncoding rnas it's not important you remember each of these we'll consider some of these in turn but simply to acknowledge the fact that many many in fact most of our genes are non-coding genes in our next video Lesson we'll look at an overview of transcription and we'll also look at the structure of Chromatin and how it's altered in order to prepare for transcription