Control Centre

This is a cell from a developing seed cotyledon of a broad bean Vicia faba plant. In the centre you can see the nucleus, floating like an icy-blue planet in a universe of cytoplasm - mission control, home the the DNA molecules that encode all the instructions for making the cell and, indeed, the whole plant. I stained the cell with a fluorescent dye that binds to the DNA in the nucleus and fluoresces blue when it's illuminated with ultraviolet light - the DNA (chromatin) is visible as the bright blue flecks on the nuclear membrane. The large, brightly fluorescing spot inside the nucleus is the nucleolus, where DNA is transcribed into ribosomal RNA subunits that are transported out of the nucleus and assembled into ribosomes in the cytoplasm. There the ribosomes form part of the machinery that translates the genetic code carried by messenger RNA, which is transcribed from the DNA in the nucleus, into proteins that are assembled from amino acid subunits. The nucleolus is highly visible in this cell because cotyledon cells in the seed manufacture and store proteins that are used when the seed germinates, to support the early growth of the seedling; this is a very busy nucleus and nucleolus because at this stage the cell is making a lot of protein. During early cotyledon development the cells are loosely packed together and you can see large triangular intercellular spaces between them. Later these will disappear, as the cells become packed full of proteins, lipids and starch, the cell walls thicken and the seed dries out during the seed ripening process. N.B. The cell walls are fluorescing blue in this image because of their own inherent biophysical properties, not because they contain DNA like that which is fluorescing blue in the nucleus.

The Inner Workings of an Onion

Onions have long been a favourite source of material for microscopists who want to explore the inner workings of a cell. Peel apart the onion bulb scales and it's easy to strip away the skin of cells that coats the scales; mount these in water on a microscope slide and large, brick-shaped translucent cells are easily visible and reveal the nucleus, that contains the DNA and controls the life of the cell. The centre of the cell is occupied by a large fluid-filled vacuole, with cytoplasm squeezed between it and the cell walls. Watch for a while and it soon becomes apparent the the cytoplasm is constantly streaming around the cell walls, carrying with it minute organelles like the mitochondria, they provide the energy that keeps the cells alive. Sometimes the cytoplasm is drawn out in strings across the vacuole, like stretched-out chewing gum. The whole of the cell is in a constant state of motion. So, next time you're about to chop an onion and chuck it in the frying pan, pause for a moment and contemplate the marvellous process shown in these video clips, which is going on in hundreds of thousands of cells in the living onion in your hand.