Translation: The Three-act Play

Initiation

Act 1: Initiation

• The mRNA comes to the cytoplasm, with a starting condon, which is  AUG, on it.
• A small ribosomal subunit binds with mRNA.
• A tRNA (transfer RNA) carrying a specific amino acid, Met, at one end and having a specific nucleotide triplet, anticodon UAC, at the other end comes to bind with the starting codon.
• Initiation factors brings the large ribosomal subunit to mRNA, placing the tRNA in the P site.

Elongation

Act 2: Elongation

• Another tRNA carrying amino acid recognizes its corresponding condon at the A site.
• An RNA molecule catalyzes the formation of a peptide bond between the polypeptide in the P site withe the new amino acid in the A site.
• The polypeptide chain is transferred to the tRNA at the A site. 
• The ribosome moves the tRNA with the attached poplypeptide form the A site to the P site. This process need energy provided by GTP.
• The first tRNA enters the E site, and as the third tRNA attaches to the A site, it exits the E site to the cytoplasm to pick another amino acid.
• The process keeps going on.

Termination

Act 3: Termination

• When one of the three stop condons(UAG, UAA and UGA) reaches the A site, a release factor cut the bond between the polypeptide chain and its tRNA at the P site.
• Polypeptide, which is known as protein, is released.
• Translation complex disassembles.

Transcription: The Three-act Play

Overview
(from http://www.mun.ca/biology/desmid/brian/BIOL2060/BIOL2060-21/CB21.html)

ACT 1: Initiation

• Transcription factors recognize the promotor, TATA box, in the template strand of DNA, and bind to it.
• The template strand is also called antisense; the other strand, coding strand, is also called the sense.
• RNA polymerase II binds to transcription factors to form a transcription initiation complex, transcription begins.

ACT 2: Elongation

• RNA polymerase II unwinds the double helix.
• RNA polymerase II adds the complimentary nucleotides to the 3' end of the growing strand.
• When adding the complimentary nucleotides to the RNA transcript, every thymine is replaced by uracil.
• As RNA polymerase II moves forward, the double helix behind re-forms, and the newly transcribed RNA molecule, RNA transcript, peels away.
• Many RNA polymerase II can work on a single DNA simultaneously.

ACT 3: Termination

• When RNA polymerase II transcribes a terminator, AAUAAA, transcription stops.
• The pre-mRNA is cut, and RNA polymerase II is released from the DNA.
• In eukaryotic cells, before the mRNA enter the cytoplasm, enzymes work on modifying them.
• G-cap (modified guanine) is added to the 5' end of pre-mRNA.
• PolyA-tail is added to the 3' end of pre-mRNA.
• Pre-mRNA combines with snRNPs and other proteins to form a spliceosome.
• The snRNA in the snRNPs base pair with nucleotides at the ends of the noncoding segments on the pre-mRNA, introns.
• Introns are excised from the pre-mRNA, then, the coding regions, exons, are spliced together in the spliceosome.
• The snRNA acts as a ribozyme, and the RNA acts as an enzyme.
• The spliceosome comes apart, releasing mRNA.