2.1.17 Biology- RNA

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RNA is different:

RNA is structurally different from DNA in a number of important ways:

·         the sugar molecule that makes up the nucleotides is ribose

·         the nitrogenous base uracil (U) is found instead of the organic base thymine (T)

·         the polynucleotide chain is usually single-stranded

·         three forms of RNA molecule exist


The base-pairing rule applies:

RNA nucleotides never contain the nitrogenous base thymine. This means the RNA that is made up of nucleotides containing the purines adenine (A) and guanine (G), and the pyrimidines cytosine (C) and uracil (U).

As in DNA, cytosine can form hydrogen bonds with guanine. Uracil is very similar to thymine and can form hydrogen bonds with adenine. So base-pairing of A-U, C-G can still occur.

The base-pairing rule means that molecules of RNA can be made that are complimentary to molecules of DNA. This is because exposed DNA nucleotides can have free RNA nucleotides hydrogen-bonded to them and then the sugar phosphate backbone is sealed up to form a chain of RNA nucleotides. This is the base of copying the genetic code of the DNA base sequence (transcription).


Three forms of RNA:

RNA molecules come in three forms:

·         Messenger RNA (mRNA) is made as a strand complimentary to one strand of a DNA molecule (the template strand) – it is therefore a


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