The process of transcription

Ribonucleic acid (RNA) resembles DNA in being composed of nucleotides, however the nucleotides are slightly different

• DNA nucleotides are composed of a base, deoxyribose and a phosphate
• RNA nucleotides are composed of a base, ribose and a phosphate
• Ribose differs from deoxyribose by the substitution of a hydroxyl (OH) for a hydrogen (H) at the 2' position of the sugar ring
  • This difference makes RNA more unstable since this hydroxyl makes the backbone of the molecule more susceptible to degradation
• In addition, RNA has no thymine which is replaced by a similar molecule called uracil (thymine is simply uracil with an added methyl group)
  • Uracil base pairs with adenine (U·A)

The DNA molecule acts as the template for synthesis of an RNA copy

• As in replication, RNA nucleotides can be assembled into a chain using the existing chain of DNA as a guide to indicate the order necessary
• This process is catalyzed by a special enzyme called RNA polymerase
• Only a single strand of each transcribed region is copied into RNA. This strand encodes a protein (the "sense" strand) the opposite strand does not (the "antisense" strand)

RNA polymerase begins synthesis of the RNA chain at a region called a "promoter". The role of the promoter is to position the polymerase correctly so that the right region will be transcribed.

• RNA is synthesized by base pairing between DNA nucleotides and new RNA nucleotides
• Since the normal double-helical structure of the DNA would not allow this to occur, the helix must be opened up, breaking the DNA base pairs
• RNA polymerase catalyzes this opening of the helix, and directs the synthesis of a faithful copy of the DNA into RNA
• The enzyme proceeds down the DNA molecule unwinding base pairs ahead of it while those behind reform, and copying the DNA into RNA
• At a special signal in the DNA called a "terminator" the polymerase stops synthesis of the RNA, releases it, and dissociates from the DNA

I noted the fact that nucleic acids have a polarity (the two strands of DNA run in opposite directions)

• This polarity is expressed in terms of the structure of the sugar moiety
• The phosphate group bridges between the 5' and 3' positions of adjacent nucleotide sugars
• The DNA polymerase synthesizes DNA from the 5' to the 3' direction (the free end at the beginning is a 5', and the free end at the end is a 3')
• RNA polymerase has the same polarity-from 5' to 3'
  • In fact, "5' end" is a synonym for "the beginning" and "3' end" is a synonym for "the end" of the RNA
    

In prokaryotes, the RNA product of transcription is directly used in the synthesis of proteins. In eukaryotes the RNA is modified after transcription

• Special modifications are made at the 5' and 3' ends
  
• Internal regions of the RNA can be excised, with rejoining of the adjacent segments of RNA
  • By analogy to editing of film or tape, this process is called "splicing