RNA Processing - Cell Biology - PDF

RNA Cell Biology Processing Lokender Kumar Ph.D.  RNA Processing The newly made RNA, also known as the primary transcript (the product of transcription is known as a transcript) is further processed → before it is functional....

RNA Cell Biology Processing Lokender Kumar Ph.D.  RNA Processing The newly made RNA, also known as the primary transcript (the product of transcription is known as a transcript) is further processed → before it is functional. https://bio.libretexts.org/Bookshelves/Biochemistry/Book%3A_Biochemistry_Free_and_Easy_(Ahern_and_Rajagopal)/05%3A_Flow_of_Genetic_Information/5.05%3A_RNA_Processing Cell biology Lokender Kumar Ph.D.  RNA Processing The major difference in RNA processing, → between prokaryotes and eukaryotes, → processing of messenger RNAs. In bacterial cells, the mRNA is translated directly as it comes off the DNA template. In eukaryotic cells, RNA synthesis, which occurs → nucleus, → protein synthesis machinery, which is in the cytoplasm. Cell biology Lokender Kumar Ph.D.  RNA Processing Cell biology Lokender Kumar Ph.D.  RNA Processing Eukaryotic genes → introns, noncoding regions → interrupt the gene’s coding sequence. The mRNA copied from genes containing introns. These regions must be removed before the mRNA is sent out The process of removing the introns and rejoining the coding sections or exons, → splicing. Cell biology Lokender Kumar Ph.D.  RNA Processing The initial product of transcription of a protein coding gene is called the pre-mRNA (or primary transcript). After it has been processed and is ready to be exported from the nucleus, it is called the mature mRNA or processed mRNA. Cell biology Lokender Kumar Ph.D.  RNA Processing What are the processing steps for messenger RNAs? In eukaryotic cells, pre-mRNAs undergo three main processing steps: Capping at the 5' end Addition of a polyA tail at the 3' end Splicing to remove introns Cell biology Lokender Kumar Ph.D.  RNA Processing In the capping step of mRNA processing, a 7-methyl guanosine is added at the 5' end of the mRNA. The cap protects the 5' end of the mRNA from degradation by nucleases and also helps to position the mRNA correctly on the ribosomes during protein synthesis. Cell biology Lokender Kumar Ph.D.  RNA Processing An enzyme called PolyA Polymerase adds a "tail" of about 200 ‘A’ nucleotides to the 3' end. It plays a role in efficient translation of the mRNA, as well as in the stability of the mRNA. The cap and the polyA tail on an mRNA are also indications that the mRNA is complete (i.e., not defective). Cell biology Lokender Kumar Ph.D.  RNA Processing Cell biology Lokender Kumar Ph.D.  RNA Processing Introns are removed from the pre-mRNA → complex called the spliceosome. The spliceosome is made up of proteins and small RNAs → small nuclear ribonucleoproteins or snRNPs (pronounced SNURPS). The splicing machinery → recognize splice junctions (i.e., the end of each exon and the start of the next) in order to correctly cut out the introns and join the exons to make the mature, spliced mRNA. Cell biology Lokender Kumar Ph.D.  RNA Processing What signals indicate where an intron starts and ends? The base sequence at the start (5' or left end, also called the donor site) of an intron is GU while the sequence at the 3' or right end (acceptor site) is AG. There is also a third important sequence within the intron, called a branch point, that is important for splicing. Cell biology Lokender Kumar Ph.D.  RNA Processing In the first step, the pre-mRNA is cut at the 5' splice site (the junction of the 5' exon and the intron). The 5' end of the intron then is joined to the branch point within the intron. This generates the lariat-shaped molecule characteristic of the splicing process In the second step, the 3' splice site is cut, and the two exons are joined together, and the intron is released. Cell biology Lokender Kumar Ph.D. Cell biology Lokender Kumar Ph.D.  RNA Processing Many pre-mRNAs have a large number of exons that can be spliced together in different combinations to generate different mature mRNAs. This is called alternative splicing. Because of alternative splicing, each gene in our DNA gives rise, on average, to three different proteins. Once protein coding messages have been processed by capping, splicing and addition of a poly A tail, they are transported out of the nucleus to be translated in the cytoplasm. Cell biology Lokender Kumar Ph.D.  RNA Processing Cell biology Lokender Kumar Ph.D.