Chapter 28: Nucleosides, Nucleotides and Nucleic Acids

DNA : Protein Biosynthesis and RNA

• There are several types of RNA : messenger or mRNA, transfer or tRNA and ribosomal or rRNA
• Remember that RNA differs to DNA in that it is based on ribose as the carbohydrate and has uracil instead of thymine.

• Occurs in the cell nucleus and is the process by which DNA is copied as mRNA.
• The process is similar to that of DNA replication on the previous page (review ?), but there are some differences.
• The mRNA is not a copy of the whole DNA strand, just a portion of maybe 500 to 6000 nucleotides.
• The portion is "controlled" by a promoter sequence and a terminator sequence (on and off switches respectively).
• Only about 10 DNA base pairs are "unwound" at any one time... this helps "expell" the growing mRNA.
• mRNA synthesis also starts at the 5' end, forming the phosphate esters by reaction with the 3' -OH with ATP, GTP, CTP or UTP.
• This reaction is catalysed by RNA polymerase.
• mRNA is composed of bases triplets, represented by a three letter code for a specific amino acid,  e.g. AGC = serine.
• This three letter code is called a codon that is read from the 5' to the 3' end.
• Note that a triplet of the 4 bases allows for 4³ = 64 combinations.... plenty for the 20 amino acids.
• The codon AUG also functions as a start codon and UAA, UAG and UGA as stop codons (on and off switches respectively) for protein biosynthesis.

• Occurs in the cell cytoplasm and is the process by which RNA is used for protein synthesis.
• The ribosome (2/3 nucleic acid, 1/3 protein : 3 rRNA subunits totalling 4545 nucleotides)
• tRNA are relatively small nucleic acids that transfer amino acids to the ribosome.
• A tRNA base triplet that is complimentary to a mRNA codon is called an anticodon
• The rRNA translates the original DNA code into the amino acid sequence for the protein building up from the N-terminus.

© Dr. Ian Hunt, Department of Chemistry