U4L2 - Transcription and Translation

Question 1

Explain what RNA contains + types

Answer 1

• Contains the nitrogenous base “U” (uracil) instead of “T” (thymine
• Contains ribose sugar instead of deoxyribose; Single stranded
• The three types are: Ribosomal RNA (rRNA) - makes up the structure of a ribosome, Messenger RNA (mRNA) - contains the “message” from the DNA,
  Transfer RNA (tRNA) - brings amino acids to the mRNA (translates the code)

Question 2

What are the stages of Transcription and what happens?

Answer 2

DNA —> mRNA (happens in nucleus)
1) Initiation
2) Elongation
3) Termination

Question 3

Where does transcription start?

Answer 3

• Promoter sequence (nucleotide sequence just before gene) is where the RNA polymerase will bond
• This sequence contains TATA box: Located 25 nucleotides away from the site where transcription is initiated and is the region of the promoter that allows RNA polymerase to bin
• Transcription Factors attach first then RNA Polymerase bonds to complete assembly

Question 4

What happens during Initiation?

Answer 4

1) RNA Polymerase binds to the DNA promoter
2) DNA is unwound, and a template is exposed

Question 5

What happens during Elongation?

Answer 5

1) mRNA is synthesized from 5’ —> 3’ from the template. Therefore, DNA template strand 3’ —> 5’
Note: The template strand is called the coding strand.
2) mRNA synthesis continues. DNA that has already been transcribed recoils into a helix
3) RNA polymerase terminates at the end of the gene

Question 6

What happens during Termination?

Answer 6

1) mRNA transcription finishes, RNA polymerase is released
2) termination sequence signals RNA polymerase to stop

Question 7

How does Transcription differ from Replication?

Answer 7

Transcription = creation of mRNA, uses RNA Polymerase

Replication = creation of new DNA strand, uses DNA Polymerase

Question 8

What are the Post-transcriptional Modifications

Answer 8

1) 5’ cap (methylated G nucleotide) added
2) Poly-A tail (series of A nucleotides) added to the 3’ end
(Note: cap and tail are protection from degradation and recognition by ribosome)
3) Spliced out introns (non-coding segments of gene interspersed among the coding segments are called exons)
- Introns removed and exons joined together to form mature mRNA
- snRNPs (“snurps”) recognise regions where exons and introns meet, and they bind to those areas