Transcription
Step 1: Initiation
- The enzyme RNA polymerase binds to DNA and unwinds the helix near the beginning of the gene
- This binding occurs at a Promoter (a specialized sequence on one strand of DNA, just up from the start of a gene)
- A key element of the promoter in eukaryotes is the TATA Box: a section of DNA with a high % of thymine and adenine bases which is recognized by RNA polymerase enzyme
Step 2: Elongation
- Once RNA polymerase binds open the DNA double helix, it starts building a single-stranded RNA molecule
- RNA is made in the 5’ à 3’ direction using the 3’ à 5’ DNA strand as a template
- The opposite strand of DNA (the strand not being copied) is known as the coding strand (contains the same base pair sequence as the RNA being made à except with thymine in place of uracil)
- The new RNA molecule elongates as the nucleotides are added.
- The RNA winds temporarily with the template strand of the DNA into a hybrid RNA-DNA double helix
- Once RNA polymerase has progressed past the beginning of a gene, another molecule of RNA polymerase may start producing another RNA molecule if there is room
- Most genes are undergoing transcription of many RNA polymerase molecule spaced closely between them.
- When cells require a particular protein they usually produce thousand or even millions of copies.
- With many copies of mRNA being made, many ribosomes can mass-produce that protein required
step 3: termination
- The transcription of a protein-coding gene is terminated when RNA polymerase recognizes a termination sequence
- In eukaryotes, one termination sequence is a string of adenines, which are transcribed as a string of uracil on the RNA.
- Nuclear proteins bind to the polyuracil site and stop transcription
- The newly formed RNA, then breaks off from the DNA template strand.
- Transcription stops, and RNA polymerase can now bind to another promoter region