Reading the Instructions

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RNA is different

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

the sugar molecule that makes up the nucleotides is ribose

the nitrogenous base uracil is found instead of thymine

the polynucleotide is usually single stranded

three forms of RNA molecule exist

The base pairing rules apply

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

As in DNA, cytosine can form hydrogen bonds with guanine. Uracil is very similar to thymine and can form hydrogen bonds with adenine.

The base pairing rules mean that molecules of RNA can be made that are complementary to molecules of DNA. This is because exposed DNA nucleotides can have free RNA nucleotides hydrogen bonded to them and then the sugar-phosphate nucleotides. This is the basis of copying the genetic code of the DNA base sequence.

Three forms of RNA

RNA molecules exist in three forms:

messenger RNA is made as a strand complementary to one strand of a DNA molecule - it is therefore a copy of the other DNA strand of the double helix

ribosomal RNA is found in ribosomes

transfer RNA carries amino acids to the ribosomes, where they are bonded together to form polypeptides.

What are the instructions for?

the sequences of bases on DNA make up codes for particular protein molecules. They code for the sequence of amino acids in a protein.

The sequence coding for a particular protein can be exposed by splitting the hydrogen bonds that hold the double helix together in that region

RNA nucleotides form a complememtary strand (mRNA). This mRNA molecule is a copy of the DNA coding strand or gene.

The mRNA peels away from the DNA and leaves the nucleus through a nuclear pore.

The mRNA attaches a ribosome.

Then tRNA molecules bring amino acids to the ribosome in the correct order, according to the base sequence on the mRNA.

The amino acids are joined together by peptide bonds to give a protein with a specific primary structure.

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