DNA system provided the first clear suggestion that DNA carries genetic information. In 1933, while studying virgin sea urchin eggs, Jean Brachet suggested that DNA is found in the cell nucleus and that RNA is present exclusively in the cytoplasm. At the time, "yeast nucleic acid" (RNA) was thought to occur only in plants, while "thymus nucleic acid" (DNA) only in animals. The latter was thought to be a tetramer, with the function of buffering cellular pH. In 1937, William Astbury produced the first X-ray diffraction patterns that showed that DNA had a regular structure. In 1943, Oswald Avery, along with
Extrachromosomal DNA that cell matures and expands, containing fully mature plastids. Extrachromosomal circular DNA (eccDNA) is present in all eukaryotic cells, is usually derived from genomic DNA, and consists of repetitive sequences of DNA found in both coding and non-coding regions of chromosomes. EccDNA can vary in size from less than 2000 base pairs to more than 20,000 base pairs. In plants, eccDNA contains repeated sequences similar to those that are found in the centromeric regions of the chromosomes and in repetitive satellite DNA. In animals, eccDNA molecules have been shown to contain repetitive sequences that are seen in satellite DNA, 5S