Why are the plastids semiautonomous body ?

Cell organelles which contain their own DNA and replicate independently of the nucleus are said to be semi-autonomous. The plastids are partly independent or semi-autonomous as they can manufacture some of the proteins required for their functioning with the help of their DNA, RNAs, enzymes and ribosomes and obtain the others from the cytoplasm formed under the control of the nuclear DNA. In addition, the plastids can divide to increase their number.