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Transcription and RNA Processing


MCAT Biochemistry

Eukaryotic transcription and RNA processing are key elements of the central dogma which encompasses the flow of genetic information in organisms. DNA is transcribed into mRNA, before undergoing translation to make proteins. Eukaryotic transcription takes place in the nucleus, involves different RNA polymerases, and afterwards, the RNA undergoes post-transcriptional processing.

Transcription is broken down into three stages: initiation, elongation, and termination. In initiation, general transcription factors and RNA polymerase II bind to the promoter, forming the transcription pre-initiation complex. Elongation involves RNA polymerase II reading the template DNA strand and adding complementary bases at the 3’ end. Termination occurs when RNA polymerase encounters a specific termination sequence on the DNA template strand, causing the release of the RNA transcript and the dissociation of the transcription machinery. Transcription is regulated by activators and repressors that bind to enhancer and silencer sites, respectively. In eukaryotic cells, the initial RNA transcript will undergo post-transcriptional processing, which includes addition of the 5’ cap, poly A tail, and splicing to generate the mature mRNA.

Lesson Outline

<ul> <li>Eukaryotic transcription and RNA processing.</li> <ul> <li>The central dogma: DNA is transcribed into mRNA, which then undergoes translation to make proteins.</li> <li>Eukaryotic transcription takes place in the nucleus.</li> <li>Eukaryotes use different RNA polymerases, unlike prokaryotes.</li> <li>Eukaryotic cells undergo post-transcriptional processing, but prokaryotes don't.</li> </ul> <li>Components of a eukaryotic gene:</li> <ul> <li>Promoter: Conserved sequence of adenine-thymine repeats (TATA or CAAT boxes).</li> <li>Two additional regulatory sites: Enhancers and silencers.</li> <li>Transcription factors and RNA polymerase bind to the promoter to initiate transcription.</li> </ul> <li>Transcription stages:</li> <ul> <li>Initiation: Formation of transcription pre-initiation complex.</li> <li>Elongation: RNA polymerase II reads the template DNA strand and adds complementary bases.</li> <li>Termination: RNA polymerase may continue chain elongation even after the initial RNA transcript is produced.</li> </ul> <li>Key regulators of transcription:</li> <ul> <li>Enhancer and silencer sites.</li> <li>Activator proteins bind to enhancer sites to increase gene expression.</li> <li>Repressor proteins bind to silencer sites to slow down gene expression.</li> </ul> <li>Transcription inhibitors:</li> <ul> <li>Amatoxins: Present in death cap mushrooms, inhibit RNA polymerase II.</li> <li>Actinomycin D: Chemotherapy drug that inhibits RNA polymerase.</li> <li>Rifampin: Antibiotic that blocks bacterial RNA polymerase.</li> </ul> <li>Post-transcriptional processing:</li> <ul> <li>Initial RNA transcript (heterogeneous nuclear RNA) contains introns and exons.</li> <li>5' cap added and poly A tail added to 3' end.</li> <li>Introns removed and exons joined together to produce mature mRNA.</li> </ul> </ul>

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What are the key differences between eukaryotic transcription and prokaryotic transcription?

Eukaryotic transcription takes place within the nucleus, while prokaryotic transcription occurs in the cytoplasm. Eukaryotes require a more complex set of transcription factors and utilize three different RNA polymerases, compared to just one RNA polymerase in prokaryotes. Additionally, eukaryotic transcription involves post-transcriptional processing, such as the addition of a 5' cap, poly-A tail, and splicing, which are not present in prokaryotes.

What is the role of RNA polymerase in the transcription process?

RNA polymerase is a crucial enzyme involved in the transcription of DNA to RNA. It binds to the promoter region of the DNA, unwinds the double helix, and synthesizes an RNA molecule in the 5'-3' direction complementary to the DNA template strand. RNA polymerase is responsible for initiation, elongation, and termination of transcription.

How do transcription factors contribute to the regulation of gene expression?

Transcription factors are proteins that assist RNA polymerase in binding to the promoter region of a gene. They control the rate of transcription and regulate gene expression by either activating or repressing the initiation of transcription. Transcription factors can be specific to particular types of genes or operate ubiquitously across various genes in the cell. Their interaction with upstream regulatory elements, such as enhancers or silencers, further modulates gene expression.

What are the three main stages of the transcription process and their respective significance?

The transcription process is composed of three main stages: initiation, elongation, and termination. In initiation, RNA polymerase and transcription factors bind to the promoter, forming the transcription initiation complex. This marks the beginning of gene transcription. During elongation, RNA polymerase synthesizes the RNA molecule by adding nucleotides to the growing strand, following the DNA template. During termination, RNA polymerase recognizes a termination signal within the DNA sequence, leading to the release of the newly synthesized RNA molecule and disassembly of the transcription machinery.

What is the spliceosome and its role in post-transcriptional processing?

The spliceosome is a large multi-component protein-RNA complex that is responsible for removing introns, the non-coding regions of precursor mRNA, during post-transcriptional processing in eukaryotes. The spliceosome identifies exon-intron boundaries and catalyzes the splicing reaction, which involves cleaving the introns and joining the exons to create a contiguous mature mRNA molecule. This splicing process is essential for proper gene expression, as it ensures that only the correct protein-coding information is ultimately translated into proteins.