Give one significant contribution of each of the following scientists:(i) Birbal Sahni (ii) H. Boyer (iii) Hargobind Khorana (iv) Robert Koch

(i) Birbal Sahni: Birbal Sahni was a renowned Indian paleobotanist who made significant contributions to the study of the fossils of the Indian subcontinent. He is best known for his research on the fossils of the Gondwana system. Sahni also founded the Birbal Sahni Institute of Palaeobotany in Lucknow, India.

(ii) H. Boyer: Herbert Boyer is a famous American biochemist known for his contributions to the creation of the first genetically engineered organism. He, along with Stanley Cohen, developed the technique of DNA cloning, which allowed genes to be transplanted between different biological species. Their discovery laid the foundation for the development of genetic engineering.

(iii) Hargobind Khorana: Hargobind Khorana was an Indian-American biochemist who shared the Nobel Prize in Physiology or Medicine in 1968 for his work on the interpretation of the genetic code and its function in protein synthesis. He was the first scientist to chemically synthesize oligonucleotides, the building blocks of DNA and RNA.

(iv) Robert Koch: Robert Koch was a German physician and microbiologist who is considered one of the founders of modern bacteriology. He identified the specific causative agents of tuberculosis, cholera, and anthrax. He also formulated Koch's postulates, which are a series of guidelines designed to establish a causal relationship between a microbe and a disease.