BS Final-Chapter 15-The Synthesis and Processing of RNA

Explain how RNA polymerase recognizes where transcription should begin.

-Often with the help of a sigma protein, the RNA polymerase recognizes the promoter region (TATA box) in order to initiate transcription.

Explain how RNA is modified after transcription in eukaryotic cells.

A poly-a tail is added to the 3' end (adenine) and a 5' cap of modified guanine is added to the 5'end The function of the 5'G-cap and poly(A) tail is to stabilize mRNA and protect it against degradation by exonucleases ( which degrade unmodified RNA - e.g. of viral origin). Additionally the 5'G-cap is important for transport from the nucleus to the cytoplasm and for the initiation of translation by the ribosomes.

Inrons vs exons

Exons= coding regions Introns=non-coding regions In most eukaryotic genes, coding regions (exons) are interrupted by noncoding regions (introns). During transcription, the entire gene is copied into a pre-mRNA, which includes exons and introns. During the process of RNA splicing, introns are removed and exons joined to form a contiguous coding sequence.

Describe the functional and evolutionary significance of introns.

FUNCTIONAL Introns may control gene activity Alternate splicing can give >1 protein from the same gene EVOLUTIONARYE Exons code for functional domains of proteins Widely spaced functional components allows recombination → new proteins

What is RNA splicing?

In molecular biology and genetics, splicing is a modification of the nascent pre-messenger RNA (pre-mRNA) transcript in which introns are removed and exons are joined. For nuclear encoded genes, splicing takes place within the nucleus after or concurrently with transcription.

Explain why, due to alternative RNA splicing, the number of different protein products an organism can produce is much greater than its number of genes.

Many genes are known to give rise to two or more different polypeptides , depending on which are exons. One gene can encode for more than one kind of polypeptide.

Compare DNA replication with transcription.

REPLICATION - you make a copy of DNA. leading strand of the unwound DNADNA replication involves unzipping the double helix with helicase and attaching a RNA primer with primase, and DNA polymerase III attaches nucleotides to the primers on the leading strand, and the lagging strand is built with okazaki fragments that are joined together with DNA ligase. Topoisomerase straightens out the DNA strand as it is being unzipped and single-strand binding proteins hold the DNA template in place while it is being copied. TRANSCRIPTION - you make complementary mRNA. from the Transcription involves the the building of an mRNA and does not require helicase to unzip the DNA double helix. In the initiation step, RNA polymerase attaches to the promoter region and the DNA strands unwind and RNA polymerase transcribes the mRNA strand, called the elongation phase. The mRNA strand comes out while it is still being transcribed, and the DNA double helix is rewound after RNA polymerase passes it. In the termination stage, the RNA transcript (mRNA) is released and the RNA polymerase detaches from the DNA strand.

Describe the role of the promoter, the terminator, and the transcription unit.

RNA polymerase recognizes where transcription should begin by attaching and initiating at the sequence known as a PROMOTER. RNA polymerase does not need a primer. The RNA polymerase stops transcription when it reaches the TERMINATOR, or the sequence coding for "stop" . The TRANSCRIPTION UNIT is the stretch of DNA that is transcribed into RNA

Explain the general process of transcription, including the three major steps of initiation, elongation, and termination.

Transcription has three basic steps; initiation, elongation, and termination. During initiation; in eukaryotes, the transcription factors mediate the RNA polymerase to bind to the promoter sequence of the DNA, the polymerase unwinds the DNA for the initiation at this point to start transcribing the template strand Elongation: RNA polymerase moves along the DNA template strand and untwists the double helix while doing so, this allows for 10 to 20 nucleotides to be paired with DNA at a time.The new RNA nucleotides peel away from the DNA template and the double helix reforms. Termination: In eukaryotes; the termination sequence causes the polymerase to detach from the DNA and release the transcribed RNA.