Seconday Structure - Structures produced in polypeptide chains involving interactions between amino acids within the chain. Especially  helical and  pleated sheet structures. Also applies to the complex folding of nucleic acids as, for example: the clover leaf structure of tRNA.

Segregation - Equal separation of alleles for each trait during gamete formation, in which one allelle of each gene goes to each gamete.

Sequencing - Determining the order of nucleotides in a DNA or RNA molecule or the amino acid order in a protein

Sex chromosomes - The X and Y  chromosomes in humans that determine the sex of an individual . Females have two X chromosomes in diploid cells; males have an X and Y  chromosome.

Sex-linked disease - Any disease or abnormality that is determined by the sex chromosomes; "hemophilia is determined by a gene defect on an X chromosome". Sex-linked diseases will usually manifest themselves at different frequencies in males and females.

Sigma-factor - This is the 5th subunit of RNA polymerase. It is an initiation factor of 86 KDa. It promotes attachment to specific initiation sites on DNA. Following attachment, the sigma factor is released.

Small Ribosomal Subunit - This is the 30S subunit of the ribosome. It is composed of 16S rRNA and 21 proteins.

Somatic cells - Any cell in an organism except gametes and their precursors.

Stop codon - The three codons, UAA , UAG  and UGA , that do not code for an amino acid but act as signals for the termination of protein synthesis.

Svedberg constant (S) - A unit for measuring the rate at which a particle sediments in a centrifuge.