DNA Transcription

Question 1

What are the required factors for transcription?

Answer 1

1. DNA template
2. Free ribonucleotides (NTPs)
3. RNA polymerase
4. Transcription factors

Question 2

What happens during transcription?

Answer 2

1. Initiation of transcription
2. Elongation of transcription
3. Termination of transcription

**Post-transciptional modification

Question 3

What happens during initiation of transcription?

Answer 3

1. RNA polymerase II cannot recognise and bind to a promoter on its own, requiring the aid of transcription factors to search for and attach to promoter DNA regions (TATA box, found ~30 base pairs from start site)
2. Transcription factors recognise the TATA box in the promoter and binds to DNA, facilitating the binding of RNA polymerase II.
3. Transcription initiation complex consists of transcription factors and RNA polymerase II.
4. Formation of transcription initiation complex causes DNA double helix to unwind at initiation site.
5. Template DNA strand (one of the exposed (-) DNA strands in 3’ -> 5’ direction) is used as a template for transcription to synthesis (+) RNA. Only (+) RNA can be translated into functional proteins.

Question 4

What happens during the elongation of transcription?

Answer 4

1. As RNA polymerase moves along the template strand for DNA, it adds the corresponding free ribonucleotides to the 3’ end of the growing RNA chain.
   - New RNA chain synthesised in the 5’ → 3’ direction
   - Adjacent ribonucleotides joined tgt by phosphodiester bonds, forming a continuous mRNA chain
2. RNA polymerase moves along the template strand from the 3’ to 5’ end.
   - As transcription complex continues down template strand, the newly formed RNA chain falls away from DNA template, DNA strands rewind into a double helix

Question 5

What are the significance of using RNA polymerase for transcription? (idk if this is how I phrase it)

Answer 5

1. One gene can be transcribed simultaneously by several molecules of RNA polymerase II —> allows cells to produce a particular protein in large amounts in a short time
2. RNA polymerase has no proofreading capability —> transcription hence produces many more copying errors than DNA replication

Question 6

What happens during termination of transcription?

Answer 6

1. Transcription proceeds till RNA polymerase transcribes the polyadenylation signal sequence (TTATTT) on DNA which codes for a polyadenylation signal (AAUAAA) in the pre-mRNA.
2. Proteins associated with the growing RNA chain cleave the growing RNA transcription 10-3 nucleotides downstream the AAUAAA signal, terminating transcription, releasing the primary RNA transcript.
3. RNA polymerase II dissociates from the DNA, DNA rewinds to form a double helix.

**Pre-mRNA must be processed and undergo post-transcriptional modification before leaving nucleus via nuclear pores.

Question 7

What does post-transcriptional modification include (pre-mRNA to mature mRNA)?

Answer 7

1. Addition of a 5’ 7-methylguanosine cap
2. Addition of a 3’ poly-A tail
3. Removal of introns and splicing together of exons

Question 8

What happens during capping and polyadenylation?

Answer 8

1. Capping: addition of a 7-methylguanosine to the first base of the transcript, forming a 5’ cap (occurs during elongation)
2. Polyadenylation: Addition of about 200 adenine (A) residues to form poly-A tail at the 3’ end (occurs immediately after cleavage behind the AAUAAA signal)

**Without the addition of the poly-A tail, the transcriptional product will be rapidly degraded

Question 9

What happens during RNA splicing?

Answer 9

1. Pre-mRNA transcript undergoes processing where introns (non-coding sequences) are spliced, while exons (coding sequences) are simultaneously spiced together to form the mature mRNA.
2. Catalysed by spliceosome (large protein complex assembled from proteins and small nuclear RNA molecules) that recognise splice sites in the pre-mRNA sequence.
3. Mature mRNA leaves the nucleus via nuclear pores in the nuclear envelope.