Animated Notes!
You need the PowerPoint Animation Player.
Click here if you do not have it.
- Transcription is the synthesis of an RNA strand from a DNA template.
- A gene's protein building instructions are transcribed to messenger RNA (mRNA).
- mRNA carries the code from DNA to the ribosomes where translation into a protein occurs.
- Transcription occurs in three stages:
- 1. Initiation:
- RNA polymerase binds to DNA at a specific sequence of nucleotides called the promoter.
- The promoter contains an initiation site where transcription of the gene begins.
- RNA polymerase than unwinds DNA at the beginning of the gene.
- 2.Elongation:
- Only one of the unmound DNA strands acts as a template for the RNA synthesis.
- RNA polymerase can only add nucleotids to the 3' end of the strand so like DNA, RNA must be synthesized in the 5' to 3' direction.
- Free ribonucleotides triphosphates from the cytoplasm are paired up with their commplementary base on the exposed DNA template.
- RNA polymerase joins the ribonucleoside triphosphates to form an mRNA strand.
- As RNA polymerase advances, the process continues.
- The DNA that has been transcribed, re-winds to form a double helix.
- Termination:
- RNA polymerase continues to elongate until it reaches the terminator, a specific sequence of nucleotides that signals the end of transcription.
- Transcription stops and mRNA polymerase and the new mRNA transcript are released from DNA.
- The DNA double helix reforms.
- The termination sequence usually consists of a series of adjancent adenines preceded by a nucleotide palindrome.
- This gives an RNA molecule that assumes a stem-and loop configuration.
- This configuration stops RNA polymerase from transcribing any further.
