Transcription

Transciption: mRNA Synthesis

Transcription is the process of making RNA from a DNA template

mRNA = messenger RNA

• Contains the message from the gene

• Delivers the message from nucleus to cytoplasm

  • Instructions to build proteins

Stage 1 - Intiation

Goal: Start RNA synthesis

Steps:

• RNA polymerase binds to the promoter region on DNA

• DNA unwinds locally (by RNA polymerase), forming a transciprtion bubble

• RNA polymerase positions itself at the start site of the gene

Promoter region - Allows RNA polymerase to bind to the DNA before the gene

• Made of A - T

Antisense (template strand)

• DNA strand of the gene that is transcribed

Sense (coding) strand

• DNA strand of the gene that is NOT transcribed

• The strand could be a coding strand for one strand but a template strand for another

Transcription Factor: A protein that helps regulate transciption

• Tells RNA polymerase where, when, and how strongly to make RNA from a gene

Stage 2 - Elongation

Goal: Build the RNA strand

Steps:

• RNA polymerase binds to transcription factors in promoter region

• RNA polymerase reads the antisense strand 3 prime to 5 prime

• RNA is synthesized 5 prime → 3 prime, adding complementary nucleotides

• RNA strand (single-stranded) grows as the polymerase moves along the DNA

• DNA rewinds behind the polymerase

Stage 3 - Termination

Tells RNA polymerase to stop transcription

• Specific order or bases

• DNA helix reforms

• mRNA comes off the RNA polymerase

Post Transcription Modification

Precursour mRNA is released and double helix reforms

• RNA formed within the DNA double helix

• Will undergo 3 modifications to convert it to mature mRNA (that will leave the nucleus)

1. Poly-A tail

   • 50 - 250 A are added to the 3 prime end by poly-A polymerase.

   • Tail enables effecient translation and protects from RNA digesting enzymes in the cytosol

2. 5 prime cap

   • 7 G added to act as an intial attachment for ribosome

3. Introns (non-coding sequence) must be spliced out

   • Remove introns and join exons together

   • Splicing occurs in a spliceosome

     • Complex formed between pre-mRNA and SNRNPs (small ribonucleoproteins)

   • SnRNPs recognize and bind intron sequences, looping the pre-mRNA to bring 2 econs close together

     • Releases the introns and join exons

   • Exons may be joined in different combonations to produce different mRNAs from a single gene

     • alternative splicing

     • Explains why 20k human genes produce 100k proteins