Question

-Stages of transcription (in detail for each step) - what components are required -Modifications of RNA (on the ends of mRNA, on the interior of mRNA) -why are these modifications important? -Ways to cut out introns (i.e. Splicesomes) -Alternative splicing Translation -TRNA structure and function -What controls accurate translation -wobble effect of tRNAS -general concept of how translation works using mRNA, ribosome, anticodon, tRNA -3 stages of translation (in detail) -initiation -elongation -termination

Answer 1

Answer: Transcription is a process that involves copying of a particular segment of DNA called gene in to a RNA molecule. One of the two strands of DNA called template strand is used as template to synthesis RNA molecule. RNA polymerase is the enzyme that catalyze the synthesis of new RNA molecule.

Stages of transcription: Transcription involves three stages, Initiation, elongation and termination.

Stage 1-Transcription initiation: The process of transcription initiates when RNA polymerase binds to a region of DNA of the gene called as promoter. Every gene has its own promoter and it is the site where RNA polymerase and other protein attaches and forms a transcription bubble. In bacteria, promoter contain two DNA sequences called -10 and -35 elements. RNA polymerase recognizes these sequence and binds to it. In eukaryotes, promoter contains TATA box that is recognized by transcription factors(TF). TF helps RNA polymerase and other molecules to bind to DNA of the gene and forms transcription bubble.

Stage 2- Transcription elongation: After initiation, RNA polymerase reads the template strand in 3' to 5 direction and synthesis RNA strand by adding complementary RNA nucleotides (A, G, C, U) in 5' to 3' direction.

Stage 3-Transcription termination: RNA polymerase continue to elongate the RNA molecule until it reaches and recognizes a terminator sequence on the DNA of the gene. In bacteria, termination occurs in two ways, Rho-dependent and Rho independent mechanism. In Rho-independent mechanism, a protein called Rho factors binds to RNA at Rho factor binding site and terminates transcription by pulling the RNA and template strand apart. In Rho-independent mechanism, RNA polymerase stalls and falls after reaching a region rich in C and G in the RNA. This regions form hairpin and stalls the RNA polymerase leading to instability and release of RNA polymerase. This transcript is readily used for translation in bacteria.

Components required for transcription are RNA polymerase, transcription factors and nucleotides.

In eukaryotes, RNA produced from transcription is called as Pre-RNA and is modified and processed to form a mature RNA.

Modification of RNA and their importance:

Once, the pre-mRNA is formed it is modified in the following ways.

1. Addition of 5' cap: 5' cap is a modified guanine molecule that is attached to the first nucleotide of the RNA at 5'end. It helps the RNA from degradation and assists in attaching to ribosome for translation.

2. Polyadenylation at 3' end: Poly-A-tail is added to the last nucleotide at the 3'-end that protect the RNA from degradation, helps in RNA export to cytoplasm from nucleus.

3. Pre-RNA contains both exons (coding sequence) and introns (non-coding sequence). Introns needs to be removed from the pre-RNA as these are junk sequence and the exons need to attached to form a mature RNA.

Ways to cut out introns

After addition of 5'-cap and poly-a tail, introns are chopped off from the Pre-RNA by a complex called as splicesomes. Splicesomes are protein RNA-complex that cuts introns from the pre-RNA and paste the exons to form a mature RNA.

Alternative splicing: It is process by which pre-RNA is spliced in more than one or two different ways to form more than one mature mRNA from the same gene. For example, if a pre-RNA has five exons (Exon 1, Exon2, Exon3, Exon 4, and Exon 5), alternative splicing will form more than one mRNA by attaching different combinations of the exon (mRNA 1 with Exon1-Exon3-Exon 4-Exon5, mRNA 2 with Exon 1-Exon2-Exon4-Exon5 and mRNA 3 with Exon2-Exon4-Exon5).