Sydney Brenner (13 January 1927 – 5 April 2019) was a South African biologist. In 2002, he shared the Nobel Prize in Physiology or Medicine with H. Robert Horvitz and Sir John E. Sulston. Brenner made significant contributions to work on the genetic code, and other areas of molecular biology while working in the Medical Research Council (MRC) Laboratory of Molecular Biology in Cambridge, England. He established the roundworm Caenorhabditis elegans as a model organism for the investigation of developmental biology, and founded the Molecular Sciences Institute in Berkeley, California, United States.

Brenner was born in the town of Germiston in the then Transvaal (today in Gauteng), South Africa, on 13 January 1927. His parents, Leah (née Blecher) and Morris Brenner, were Jewish immigrants. His father, a cobbler, migrated to South Africa from Lithuania in 1910, and his mother from Riga, Latvia, in 1922. He had one sister, Phyllis and also one brother, Isaac.

He was educated at Germiston High School and the University of the Witwatersrand. Having joined the university at the age of 15, it was noted during his second year that he would be too young to qualify for the practice of medicine at the conclusion of his six-year medical course, and he was therefore allowed to complete a Bachelor of Science degree in Anatomy and Physiology. During this time he was taught physical chemistry by Joel Mandelstam, microscopy by Alfred Oettle and neurology by Harold Daitz. He also received an introduction to anthropology and paleontology from Raymond Dart and Robert Broom. The histologist Joseph Gillman and director of research in the Anatomy Department persuaded Brenner to continue towards an honours degree and beyond towards an MSc. Brenner accepted though this would mean he would not graduate from medical school and his bursary would be discontinued. He supported himself during this time by working as a laboratory technician. It was during this time, in 1945, that Brenner would publish his first scientific works. His masters thesis was in the field of cytogenetics and publications during this time in the field Brenner would later call Cell Physiology.

In 1946 Wilfred Le Gros Clark invited Brenner to his Department of Anatomy in Oxford, during a visit to South Africa. Brenner was persuaded to finish his medical education instead. Brenner returned to medical school where he failed Medicine, nearly failed Surgery and achieved a First Class in Obstetrics and Gynecology. Six months later Brenner had finished repeating Medicine and Surgery and in 1951 received the degrees of Bachelor of Medicine, Bachelor of Surgery (MBBCh).

Brenner received an 1851 Exhibition Scholarship from the Royal Commission for the Exhibition of 1851 which enabled him to complete a Doctor of Philosophy (DPhil) degree at the University of Oxford as a postgraduate student of Exeter College, Oxford, supervised by Cyril Hinshelwood.

Following his DPhil, Brenner did postdoctoral research at the University of California, Berkeley. He spent the next 20 years at the Laboratory of Molecular Biology in Cambridge. There, during the 1960s, he contributed to molecular biology, then an emerging field. In 1976 he joined the Salk Institute in California.

Together with Jack Dunitz, Dorothy Hodgkin, Leslie Orgel, and Beryl M. Oughton, he was one of the first people in April 1953 to see the model of the structure of DNA, constructed by Francis Crick and James Watson; at the time he and the other scientists were working at the University of Oxford's Chemistry Department. All were impressed by the new DNA model, especially Brenner, who subsequently worked with Crick in the Cavendish Laboratory at the University of Cambridge and the newly opened Medical Research Council (MRC) Laboratory of Molecular Biology (LMB). According to Beryl Oughton, later Rimmer, they all travelled together in two cars once Dorothy Hodgkin announced to them that they were off to Cambridge to see the model of the structure of DNA.

Brenner made several seminal contributions to the emerging field of molecular biology in the 1960s (see Phage group). The first was to prove that all overlapping genetic coding sequences were impossible. This insight separated the coding function from structural constraints as proposed in a clever code by George Gamow. This led Francis Crick to propose the concept of a hypothetical molecule (later identified as transfer RNA or tRNA) that transfers the genetic information from RNA to proteins. Brenner gave the name "adaptor hypothesis" in 1955. The physical separation between the anticodon and the amino acid on a tRNA is the basis for the unidirectional flow of information in coded biological systems. This is commonly known as the central dogma of molecular biology, i.e. information flows from nucleic acid to protein and never from protein to nucleic acid. Following this adaptor insight, Brenner conceived of the concept of messenger RNA during an April 1960 conversation with Crick and François Jacob, and together with Jacob and Matthew Meselson went on to prove its existence later that summer. Then, with Crick, Leslie Barnett, and Richard Watts-Tobin, Brenner genetically demonstrated the triplet nature of the code of protein translation through the Crick, Brenner, Barnett, Watts-Tobin et al. experiment of 1961, which discovered frameshift mutations. Brenner collaborating with Sarabhai, Stretton and Bolle in 1964, using amber mutants defective in the bacteriophage T4D major head protein, showed that the nucleotide sequence of the gene is co-linear with the amino acid sequence of the encoded polypeptide chain.