Each multicellular organism begins as a single cell that has the potential to develop into any cell type. As development proceeds, cells become committed to particular fates. The results of cloning experiments demonstrated that this process arises from differential gene expression.
In the early Drosophila embryo, determination is brought about through a cascade of gene control.
The dorsal–ventral and anterior–posterior axes of the Drosophila embryo are established by egg-polarity genes, which are expressed in the female parent and produce RNA and proteins that are deposited in the egg cytoplasm. Initial differences in the distribution of these molecules regulate gene expression in various parts of the embryo. The dorsal–ventral axis is defined by a concentration gradient of the Dorsal protein, and the anterior–posterior axis is defined by concentration gradients of the Bicoid and Nanos proteins.
Three types of segmentation genes act sequentially to determine the number and organization of the embryonic segments in Drosophila. The gap genes establish large sections of the embryo, the pair-rule genes affect alternate segments, and the segment-polarity genes affect the organization of individual segments. Homeotic genes then define the identity of individual Drosophila segments.
Homeotic genes control the development of flower structure. Three sets of genes interact to determine the identity of the four whorls found in a complete flower.
Apoptosis, or programmed cell death, is a highly regulated process that depends on caspases—proteins that cleave proteins. Apoptosis plays an important role in the development of many animals.
The immune system is the primary defense network in vertebrates. In humoral immunity, B cells produce antibodies that bind foreign antigens; in cellular immunity, T cells attack cells carrying foreign antigens.
Each B and T cell is capable of binding only one type of foreign antigen. When a lymphocyte binds to an antigen, the lymphocyte divides and gives rise to a clone of cells, each specific for that same antigen—the primary immune response. A few memory cells remain in circulation for long periods of time and, on exposure to that same antigen, can proliferate rapidly and generate a secondary immune response.
Immunoglobulins (antibodies) are encoded by genes that consist of several types of gene segments; germ-line DNA contains multiple copies of these gene segments, which differ slightly in sequence. Somatic recombination randomly brings together one version of each segment to produce a single complete gene, allowing many combinations. Diversity is further increased by the random addition and deletion of nucleotides at the junctions of the segments and by a high mutation rate.
The germ-line genes for T-cell receptors consist of segments with multiple varying copies. Somatic recombination generates many different types of T-cell receptors in different cells. Junctional diversity also adds to T-cell-receptor variability.
The major histocompatibility complex encodes a number of histocompatibility antigens. The MHC antigen allows the immune system to distinguish self from nonself. Each locus for the MHC contains many alleles.