lesson 5 transcription

Question 1

what is the purpose of transcription

Answer 1

the purpose of transcription is the first step in gene expression and involves the production of an mRNA molecule from a DNA template

Question 2

what are the three stages and events of transcription

Answer 2

1. initiation
2. elongation
3. termination
   - transcription in prokaryotes and eukaryotes is very similar
   – in eukaryotes mRNA undergo a few modifications before it can leave the nucleus

Question 3

what goes on in the initiation phase

Answer 3

the correct transcription start site is selected and the transcription machinerhy composed of a large protein dna complex is assembled.
- for each gene, only one strand of DNA is transcribed. this strand is called the antisense strand or the template strand.
RNA polymerase binds to the dna at a specific point called a promoter region (upstream of the gene)
- a sequence of nucleotides (A-T BASE PAIRS) in DNA that inidicates where the rna polymerase complex shoulld bind to initiate transcription
- once the rna polmerase complex is bound to the DNA, it unwinds and opens a section of the double helix.

Question 4

what goes on in elongation phase

Answer 4

• the rna polymersae complex moves from 5 prime to 3 prime direction to synthesize an mRNA molecule that is complementary to the antisense strand.
  t is replaced with U
  hundreds of mrna moelcules can be synthesized simultaneously as long as the rna polymerase complex has moved from the promotor region, allowing another rna polymerase complex to bind. rna polymerase does NOT PROOFREAD.

Question 5

WHAT GOES. on in termination phase

Answer 5

• specific nucleotide sewuences in the dna template serve as a signal to stop transcription
• when the rna polymerase complex reaches this signal, it detaches from the dna strand.
• the new mRNA strand is released from the transcription asssemby and the dna double helix reforms.

Question 6

what is involved in mRNA processing?

Answer 6

• 5’ cap
• 3’ poly-A-tail
• removal of introns

Question 7

reason for processing 1

Answer 7

cells have nucleases in cytoplasm that chomp up any rna or dna out there
- nucleases cant get thru the nuclear membrane so dna is safe
- mRNA sent out int othe cytoplasm msut be protected
– methyl cap added to the 5’ end: protects it from digestion and plays a role in the initation of translation
– poly-a tail is added to the 3’ end by a poly-apolymerase and protects it from degradtion by nucleases
mRNA is still chomped up into nts and recycled but the poly-a gives it time

Question 8

reason 2

Answer 8

eukaryotic dna is composed of noncoding dna
the introns are the sections not expressed and the exons are sections express
spliceosomes cut out the introns and join the remaining exons together

Question 9

summary of modifications

Answer 9

1. dna strand is stranscriped
2. the 5’ end capped with a modified nucleotide
3. poly-a tail added to the 3’ end; itnrons removed
   exons joined
4. mature mRNA transported into cytosol
5. translation can begin