Label the promoter, transcription start site, exons, introns, splice donors SD and splice acceptors SA , transcription termination, 5' cap, and polyA tail. All three steps occur in the nucleus. All three steps occur in the cytoplasm. The cap and tail are added in the nucleus, but splicing occurs in the cytoplasm. Splicing occurs in the nucleus, but the cap and tail are added in the cytoplasm.
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 , and allows the production of many different proteins using relatively few genes, because a single RNA can, by combining different exons during splicing, create many different protein coding messages. Because of alternative splicing, each gene in our DNA gives rise, on average, to three different proteins. Alternative splicing may also be used to change or downregulate protein production from a gene, by retaining and intron that includes a premature stop codon, which will end protein synthesis before the final protein can be completed.
This role for alternative splicing may be useful for organisms that need to respond to a changing environment, such as temperature or light for plants. Profiling of mRNAs after cold treatment in tea plants revealed that These precursor RNAs pre-RNAs, or primary transcripts contain in their sequences the information necessary for their function in the cell. Both prokaryotes and eukaryotes process their ribosomal and transfer RNAs. The four rRNAs in eukaryotes are first transcribed as two long precursor molecules.
Enzymes then cleave the precursors into subunits corresponding to each rRNA. In bacteria, there are only three rRNAs and all are transcribed in one long precursor molecule that is cleaved into the individual rRNAs. Some of the bases of pre-rRNAs are methylated for added stability. The eukaryotic ribosome is composed of two subunits: a large subunit 60S and a small subunit 40S.
The bacterial ribosome is composed of two similar subunits, with slightly different components. Each different tRNA binds to a specific amino acid and transfers it to the ribosome. Mature tRNAs take on a three-dimensional structure through intramolecular basepairing to position the amino acid binding site at one end and the anticodon in an unbasepaired loop of nucleotides at the other end.
There are different tRNAs for the 21 different amino acids. Most amino acids can be carried by more than one tRNA. Structure of tRNA : This is a space-filling model of a tRNA molecule that adds the amino acid phenylalanine to a growing polypeptide chain. The amino acid phenylalanine is attached to the other end of the tRNA. In archaea and eukaryotes, each pre-tRNA is transcribed as a separate transcript. Multiple nucleotides in the pre-tRNA are chemically modified, altering their nitorgen bases.
On average about 12 nucleotides are modified per tRNA. This process is called RNA splicing. First the gene is copied in full to produce a primary transcript. This includes both introns and exons.
The splicing process is catalyzed by protein complexes called spliceosomes that are composed of proteins and RNA molecules called snRNAs. Errors in splicing are implicated in cancers and other human diseases. What kinds of mutations might lead to splicing errors?
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