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direct the synthesis of RNA molecules, which fall into three classes. Figure 5.10

indicates the locations of nucleic acids in prokaryotic and eukaryotic cells.

• Messenger RNA(mRNA) contains sequences of ribonucleotides which code for the
  amino acid sequences of proteins. A single mRNA codes for a single polypeptide chain
  in eukaryotes, but may code for several polypeptides in prokaryotes.


• Ribosomal RNA(rRNA) forms part of the structure of ribosomes, which are the sites
  of protein synthesis. Each ribosome contains only three or four different rRNA
  molecules, complexed with a total of between 55 and 75 proteins.


• Transfer RNA(tRNA) molecules carry amino acids to the ribosomes, and interact with
  the mRNA in such a way that their amino acids are joined together in the order
  specified by the mRNA. There is at least one type of tRNA for each amino acid.

In eukaryotic cells alone a further group of RNA molecules termedsmall nuclear RNA

(snRNA) is present which function within the nucleus and promote the maturation of

mRNA molecules. All RNA molecules are associated with their respective binding

proteins and are essential for their cellular functions. Nucleic acids from prokaryotic

cells are less well compartmentalised although they serve similar functions.

5.4.2 The packaging of DNA

The DNA in prokaryotic cells resides in the cytoplasm although it is associated with

nucleoid proteins, where it is tightly coiled and supercoiled by topoisomerase

enzymes to enable it to physically fit into the cell. By contrast eukaryotic cells have

Nucleus

Plasma

membrane Transcription

DNA nuclear RNASmall

Nucleolus

rRNA

tRNA

rRNA

mRNA

mRNA

tRNA rRNA

Mitochondria/Chloroplasts

tRNA

DNA

Ribosomes

Translation

Proteins

Fig. 5.10Location of DNA and RNA molecules in eukaryotic cells and the flow of genetic information.

150 Molecular biology, bioinformatics and basic techniques