BIO302 Final Term Solved Papers

operon model?

 Monod and Jacob proposed the operon model in 1961to explain how the lac system is regulatedThe term operon refers to two or more contiguous genes and the genetic elements that regulate their transcription in a coordinate fashionPromoters had not yet been discovered when Monod & Jacob proposed the operon model but were readily incorporated into the operon model after their discovery.

 The five major featurs of the model are:-

 1. The products of the lacZ, lacYand lacAgenes are encoded in a single polycistroniclac mRNA molecule.

 2. The promoter for this mRNA molecule is immediately adjacent to the lac region.

 Promoter mutations (p-) that are completely incapable of making β-galactosidase, permease, and transacetylase have been isolated. The promoter is located between lacI and lacO.

 3. The operator is a sequence of bases (in the DNA) to which the repressor protein binds.

 4. When the repressor protein is bound to the operator, lac mRNA transcription can’t take place.

 5. Inducers stimulate lac mRNA synthesis by binding to the repressor. This binding alters the repressor’s conformation so it can’t bind to the operator. In the presence of an inducer, therefore,

 The operator is unoccupied and the promoter is available for initiation of mRNA synthesis. This state is called derepression.

 This simple model explains many of the features of the lac system and of other negatively regulated genetic systems.

 However, lac operon is also subjected to positive regulation.

7: RNA editingexplain?

RNA editing is a molecular process through which some cells can make discrete changes to specific nucleotide sequences within a RNA molecule after it has been generated by RNA polymerase. RNA editing is relatively rare, and common forms of RNA processing (e.g. splicing, 5'-capping and 3'-polyadenylation) are not usually included as editing. Editing events may include the insertion, deletion, and base substitution of nucleotides within the edited RNA molecule.

RNA editing has been observed in some tRNA, rRNA, mRNA or miRNA molecules of eukaryotes and their viruses, archaea and prokaryotes.[1] RNA editing occurs in the cell nucleus and cytosol, as well as within mitochondria and plastids. In vertebrates, editing is rare and usually consists of a small number of changes to the sequence of affected molecules. In other organisms, extensive editing (pan-editing) can occur; in some cases the majority of nucleotides in a mRNA sequence may result from editing.

RNA-editing processes show great molecular diversity, and some appear to be evolutionarily recent achievements that arose independently. The diversity of RNA editing phenomena includes nucleobase modifications such as cytidine (C) to uridine (U) and adenosine (A) to inosine (I) deaminations, as well as non-templated nucleotide additions and insertions. RNA editing in mRNAs effectively alters the amino acid sequence of the encoded protein so that it differs from that predicted by the genomic DNA sequenc

Editing by insertion or deletion

RNA editing through the addition and deletion of uracil has been found in kinetoplasts from the mitochondria of Trypanosomabrucei[3] Because this may involve a large fraction of the sites in a gene, it is sometimes called "pan-editing" to distinguish it from topical editing of one or a few sites.

Pan-editing starts with the base-pairing of the unedited primary transcript with a guide RNA (gRNA), which contains complementary sequences to the regions around the insertion/deletion points. The newly formed double-stranded region is then enveloped by an editosome, a large multi-protein complex that catalyzes the editing.[4][5] The editosome opens the transcript at the first mismatched nucleotide and starts inserting uridines. The inserted uridines will base-pair with the guide RNA, and insertion will continue as long as A or G is present in the guide RNA and will stop when a C or U is encountered.[6][7] The inserted nucleotides cause a frameshift and result in a translated protein that differs from its gene.

The Effect of Uracil Insertion in pre-mRNA transcripts

The mechanism of the editosome involves an endonucleolytic cut at the mismatch point between the guide RNA and the unedited transcript. The next step is catalyzed by one of the enzymes in the complex, a terminal U-transferase, which adds Us from UTP at the 3’ end of the mRNA.[8] The opened ends are held in place by other proteins in the complex. Another enzyme, a Uspecific exoribonuclease, removes the unpaired Us. After editing has made mRNA complementary to gRNA, an RNA ligase rejoins the ends of the edited mRNA transcript.[9][10] As a consequence, the editosome can edit only in a 3’ to 5’ direction along the primary RNA transcript. The complex can act on only a single guide RNA at a time. Therefore, a RNA transcript requiring extensive editing will need more than one guide RNA and editosome complex