Module 11 - Transcription and Translation Flashcards
Describe the central dogma.
The central dogma states that the flow of genetic information occurs from DNA to RNA, to make a functional product, a protein.
Mention the basic function of an RNA polymerase.
RNA polymerase is an enzyme that catalyzes the synthesis of an RNA molecule on a DNA template from ribonuleoside triphosphate precursors.
Prokaryotes have ___ RNA polymerase, while eukaryotes have ____.
one(1), three(3)
Mention the types of RNA and describe their function.
- mRNA (messenger RNA): codes for protein
- rRNA (ribosomal RNA): form the basic structure of the ribosome and catalyze protein synthesis
- tRNA (transport RNA): adaptors between mRNA and amino acids, central to protein synthesis
- snRNA (small nuclear RNA): function in variety of nuclear processes, including pre-mRNA splicing
Describe the initation stage of transcription in eukaryotes
Transcription begins when TFIID binds to TATA box via one of its component, TBP (TATA binding protein). It then recuit other factors along with RNA Pol. II to form the pre-initiation complex. Then, TFIIH activates the transcription process by unwinding DNA at TATA site using its helicase activity and phosphorylates the C-Terminal Domain tail at Ser-5 (of the RNA Pol. II) using its kinase activity, activating the RNA Pol. II
Note. The initiation complex involve the following:
- General Transcription Factors
- RNA Pol. II
- Mediator complex
- Transcriptional Activators
- Chromatin remodeling complexes
- Histone-modifying enzymes
Describe the three stages of translation.
Initiation:
Eukaryotic inititaition factors recognises the mRNA 7mG 5’ cap and recruits the Met tRNA and the 40S ribosomal subunit. The Met-tRNA/40S complex scans along the mRNA until it finds AUG after which the large ribosomal subunit also binds.
Elongation:
The previous tRNA exits fom the E site, while the next amino acid-tRNA occupies the A site based on the codon sequence. As a new peptide bond is formed in the P site, the 60S subunit moves 1 base up towards the 3’, followed by the 40S and the process repeats.
Temination:
Release factor binds to the A-site when STOP codon occurs which breaks down the whole complex.
Explain the roles of the pre-initiation complex and TATA box.
Pre-inititation complex is composed of critical proteins that are required for transcription. One of its main function is to bind to TATA box sequence to melt the DNA sequence and starrt transcription.
Explain the roles of key General Trasncription Factors (2).
- TFIID: recognizes TATA box (through TBP and TAFs) sequence near transcription starting point
- TFFIH: Unwinds DNA at the transcription start point (helicase activity); phosphorylate CTD which change conformation and activate RNA Pol. II
Describe the mRNA splicing steps.
mRNA splicing occurs during transcription using spliceosome which are attached to the CTD after the phosphorylation of Ser-2 as well as Ser-5. Splicesome recognises specific sequence characteristic of the intron where it must start with GU and ends with AG, as well as A in the middle of the sequence. Spliceosome is made up of snRNPs (a ribonucleoprotein). Splicing occurs when, with the help of the snRNPs, the 2’ hydroxyl group of the Adenosine attacks the 5’ exon sequence, after which the exposed hydroxyl group of released sequence attack the 5’ of the 3’ exon sequence, releasing a loop of intron called lariat.
Explain the relationship between codons, the Genetic Code, and tRNA.
The nucleotide sequence is translated into the language of amino acids. However, there 64 possible combinations of codon with onlly 20 amino acids along with the stop codon. Hence, multiple codons can specify the same code. tRNA provide the link between a particular codon and its corresponding amino acid which is covalently attached. Regions of complementary base pairings give tRNA a very stable secondary structure.
Describe the basic structure and function of the ribosome.
Ribosome is a ribonucleoprotein complex composed of rRNAs and ribosomal proteins that catalyses the synthesis of protein using the information provided by the mRNA. It is composed of a large (60S) and small (40S) subunit, forming the 80S ribosome.
Understand the role of the Release Factor.
When the stop codon is encountered, it binds to the A site of the ribosome causing the complex to break down and stops the translation process.
RNA Polymerase I, II, III encodes for _______, __________, _________ respectively.
rRNA, mRNA, tRNA
Describe what happens during the elongation stage of transcription.
When Ser-5 of the C-terminal Domain in RNA Pol. II is phosphorylated by TFIIH, capping proteins binds to the tail. As the transcription process began and about 20-30 nucelotides have been produced, the capping enzymes adds 7-methylguanosine to the end of the pre-mRNA, forming a 5’ to 5’ triphosphate bridge. This action stabilizes the sequence as well as act as a recognition site for translation.
Then, Ser-2 of the CTD gets phosphorylated by other kinases, which allow splicing factors to bind and splicing occurs. While this occurs, the Ser-5 starts to become dephsphorylated and proteins involved in the 3’ end processing bind to CTD.
Describe the termination stage of transcription.
The termination of transcription occurs when a specific sequence motif (AATAAA) is encountered. This is recognised by the Cleavage and Polyadenylation Specificity Factor (CPSF) and Clevage Stimulation Factor (CstF), which had bind to CTD due to the loss of phosphorylation of Ser-5. They then transfer to mRNA and cause cleavage of the message at the motif site. Poly-A Polymerase (PAP) then adds mutliple adenosines to create the poly-A tail. Finally the Poly-A Binding Protein (PABP) bind and protect it, preserving the stability of the mRNA.
How does each amino acid get paired with its tRNA?
