Protozoan DNA. Coloured transmission electron micrograph (TEM) of aggregates of DNA (deoxyribonucleic acid, purple) from an Oxytricha sp. protozoan. These aggregates have formed because the ends of the strands of DNA, known as telomeres, have bound to telomere-binding protein molecules (pink). Telomeres consist of short repeated sequences of DNA and proteins, and are looped. The loop structure prevents the end of the chromosome being eroded by enzymes present in the cell nucleus and prevents enzymes from joining two chromosomes together. Telomeres play an important role in cellular aging. As cells replicate and age the telomere steadily shrinks. When the telomere reaches a certain length the cell stops dividing and dies. Magnification: x26,