 The protein coding regions of each mRNA is composed of our contiguous, known overlapping string of codons called an open reading frame or ORF. Each ORF specifies a single protein and starts and ends at internal sites within the mRNA. That is, the ends of an ORF are distinct from the ends of mRNA. So, if we have a mRNA which is so long and the ORF is an open reading frame, it must be somewhere within this mRNA. That is, the ORF starts here and ends here. So, the ends of ORF and the ends of mRNA cannot be the same. So, the ORF lies within the mRNA. Translation starts at the 5 prime end of the ORF and proceeds one codon at a time to the 3 prime end. So, it starts at 5 prime and ends at 3 prime. The first and the last codons of an ORF are known as start codon and end codon or stop codon. In bacteria, the start codon is usually AUG, that is, the 5 prime side is AUG and the 3 prime side is AUG. But apart from this, GUG and UUG act as a start codon. U-creatic cells always use AUG as the start codon. The start codon has two important functions. First, it specifies the first amino acid to be incorporated into the growing polypeptide chain. So, the first thing is that it will identify and specify the first amino acid. Second, it defines the reading frame for all subsequent codons. Because each codon is immediately adjacent to the next codon and because codons are 3 nucleotides long, any stretch of mRNA could be translated into three different reading frames. As you can see in this, these are three examples that are being read from three different ways. So, in the first case, we see that this is AUG and it starts from here. After starting from here, the entire reading frame stops at UGA. So, the amino acids that are being added in this are different. In the second case, if this mRNA starts from here, from UGA, obviously UGA is the stop codon, then it will start from here, A AG. Then in this, K, Q, F, L, Y. So, let's look at these two. First of all, both are the same. Here is A, here is Q, here is I, here is F, here is F, here is L. So, these proteins that are being formed, these proteins, the sequence of the amino acids is different. Even their sizes are different. In the third case, if it starts from here, then the entire protein is being formed and the stop codon has not yet been added. So, let's look at this too, E, S, N, F. The sequence of these two is completely different. So, in this way, these three different possible reading frames are read by mRNA. And as a result of this, different types of proteins are formed. Once translation starts, however, the reading frame is determined. So, when the translation starts, then the reading frame will be determined. After that, the change will not happen. Thus, by setting the location of the first codon, the start codon, determines the location of all the following codons. Stop codons, of which there are three. UAG, UGA, and UAA. These three work on the work of the stop codon. And wherever they come, they will define the end of ORF and signal termination of polypeptide synthesis. You can now understand the origin of the term open reading frame. Why is it called open reading frame? It is a contiguous stretch of codons read in a particular flame, as said by the first codon. That is open to translation because it lacks a stop codon. So, it is called open reading frame. mRNAs contain at least one ORF. The number of ORFs per mRNA is different between eukaryotes and prokaryotes. Eukaryotic mRNAs almost always contain a single ORF. In contrast, prokaryotic mRNAs frequently contain two or more ORFs. mRNAs containing multiple ORFs are known as polycystronic RNAs, while those encoding a single ORF are known as monocystronic RNAs. So, eukaryotes have monocystronic mRNAs and prokaryotes have polycystronic mRNAs. The polycystronic mRNAs found in bacteria, often encode proteins that perform related functions, such as different steps in the biosynthesis of an amino acid or nucleotide. So, there is a pathway and in this pathway, if different proteins are required or different enzymes are required, so one mRNA will encode different ORFs in different proteins which are related to one pathway and will encode these related proteins. So, here you can see the examples of monocystronic and polycystronic mRNAs. So, this is the case of polycystronic mRNA. You can see this is one reading frame, this is second reading frame and this is third reading frame. So, in a single mRNA, there are three reading frames, so three different proteins, protein alpha, protein beta and protein gamma are synthesized. But here, this is one mRNA and in this one mRNA, only one reading frame is present, so only one protein is synthesized.