Lecture 12

Question 1

Describe the steps of information flow in transcription and and apply correct terminology to these processes.

Answer 1

Transcription
i) Initiation
RNA polymerase binds to the promoter on the 3’ end of the DNA template strand. This initiates RNA synthesis.
ii) Elongation
RNA polymerase moves downstream unwinding the DNA and elongates the RNA transcribed to the 3’ end.
iii) Termination
RNA transcription is released and the polymerase detaches from the DNA

Question 2

Describe the steps of information flow in translation and and apply correct terminology to these processes.

Answer 2

Translation
i) Codon recognition
Charged tRNA brings amino acids to the A site and base pair with the codon
Delivery requires a set of protein factor and free energy
ii) Peptide bond formation
Covalent bond is formed between two amino acids attached to the tRNA at the P and A site
Energy from aa-tRNA catalyst is rRNA
iii) Translocation
Ribosome move along one codon, they then transfer tRNA with a growing polypeptide chain from A site to P site.

Question 3

Describe the process of transcription of a gene and explain the roles of RNA polymerase, promoters and protein transcription factors in this process

Answer 3

Transcription begins when RNA polymerase binds to a promoter sequence near the beginning of a gene (directly or through helper proteins). RNA polymerase uses one of the DNA strands (the template strand) as a template to make a new, complementary RNA molecule. Transcription ends in a process called termination.

Question 4

Predict the mRNA sequence that would be transcribed from a given DNA template

Answer 4

5’ TGTACTA 3’

‘3 ACAUGAU 5’

Question 5

Compare and contrast the process of RNA transcription with that of DNA replication.

Answer 5

Replication is the duplication of two strands of DNA, whereas transcription is the formation of a single identical DNA from the two stranded DNA.
Second, there are different proteins involved in replication and transcription (uracil and thymine).
Third, the results of replication are two daughter cells, whereas transcription results with a protein molecule. Lastly, in transcription DNA serves as a template for RNA synthesis.

Question 6

Describe the major steps in processing of eukaryotic pre-mRNA transcripts

Answer 6

The conversion of the initial primary transcript synthesized by RNA polymerase II into a functional mRNA.

Question 7

Why is the processing of Eukaryotic pre-mRNA necessary?

Answer 7

• Necessary for the protection of enzymes that quickly digest uncapped RNAs generated by RNA processing, such as spliced-out introns and RNA transcribed downstream from a polyadenylation site.
• Introns must be removed to generate the correct coding region of the mRNA.
• In higher eukaryotes, including humans, alternative splicing is intricately regulated in order to substitute different functional domains into proteins, producing a considerable expansion of the proteome of these organisms.

Question 8

Overview of mRNA processing in eukaryotes.

Answer 8

1. ) Shortly after RNA polymerase II initiates transcription at the first nucleotide of the first exon of a gene, the 5’ endof the nascent RNA is capped with 7-methyguanylate
2. ) Transcription by RNA polymerase II terminates at any one of multiple termination sites downstream from the poly(A) site, which is located at the 3’ end of the final exon. The primary transcript is then cleaved at the poly(A) site.
3. ) A string of adenosine (A) residues is added.
4. ) For short primary transcripts with few introns, splicing follows cleavage and polyadenylation (as shown). For large genes with multipleintrons, introns are often spliced out of the nascent RNA during its transcription, before transcription of the gene is complete.

The 5’ cap and the sequence adjacent to the poly(A) tail are retained in mature mRNAs.

Question 9

Steps in RNA processing:

Answer 9

1. ) Use of alternative exons during pre-mRNA splicing
2. ) Use of alternative poly(A) sites
3. ) Properly processed mRNAs are exported to the cytoplasm. Improperly processed mRNAs are blocked from export to cytoplasm and are degraded the exosome complex containing multiple ribonucleases.
4. ) Translation initiation factors bind to the 5’ cap cooperatively with poly(A)-binding protein I bound to the poly(A) tail and initiate translation.
5. ) mRNA degraded in cytoplasmic P bodies that function in translational repression.
   - Deadenylated and decapped by enzymes
   - Degraded by cytoplasmic enzymes
   - This degradiation is controlled, thereby regulating the mRNA concentration and, consequently, the amount of protein translated.