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Chloroplasts, the tiny green structures where plants and algae capture sunlight, were once free-living bacteria. More than a billion years ago a larger cell swallowed a cyanobacterium, a bacterium that could photosynthesise, and instead of digesting it, kept it. Its descendants live on in every leaf.

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The idea was first proposed in 1905 by the Russian biologist Konstantin Mereschkowski, building on the German botanist Andreas Schimper's observation in 1883 that chloroplasts closely resemble cyanobacteria. It was largely ignored until the American biologist Lynn Margulis revived it in 1967, and DNA evidence later confirmed it. The clues are still visible. Chloroplasts are wrapped in two membranes, multiply by splitting in two like bacteria, and carry their own small, usually circular, DNA. Over time most of their genes moved to the host cell's nucleus: a chloroplast keeps only about 100 of the thousands of genes its ancestor had, and about 95 per cent of the proteins it needs are now made from instructions in the nucleus. A similar merger had happened earlier and produced mitochondria, which make most of the energy in complex cells.

Türkçe oku
Kloroplastlar, yani bitkilerin ve alglerin güneş ışığını yakaladığı minik yeşil yapılar, bir zamanlar serbest yaşayan bakterilerdi. Bir milyar yılı aşkın süre önce daha büyük bir hücre, fotosentez yapabilen bir siyanobakteriyi yuttu, ama onu sindirmek yerine içinde tuttu. O bakterinin torunları bugün her yaprakta yaşıyor.

Bu fikri ilk kez 1905'te Rus biyolog Konstantin Mereschkowski ortaya attı. Alman botanikçi Andreas Schimper 1883'te kloroplastların siyanobakterilere çok benzediğini fark etmişti; Mereschkowski bu gözlemden yola çıktı. Fikir, Amerikalı biyolog Lynn Margulis 1967'de yeniden gündeme getirene kadar pek ilgi görmedi; sonradan DNA kanıtları onu doğruladı. İpuçları bugün de ortada. Kloroplastlar iki zarla çevrilidir, bakteriler gibi ikiye bölünerek çoğalır ve genellikle halka biçiminde, kendilerine ait küçük bir DNA taşır. Zamanla genlerinin çoğu konak hücrenin çekirdeğine taşındı: Bir kloroplast, atasının binlerce geninden yalnızca 100 kadarını korur; ihtiyaç duyduğu proteinlerin yaklaşık yüzde 95'i bugün çekirdekteki genlere göre üretilir. Benzer bir birleşme daha önce de yaşanmış ve karmaşık hücrelerin enerjisinin çoğunu üreten mitokondrileri ortaya çıkarmıştı.

Sources and further reading

Background reading: Chloroplast
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