⇒ Cell Communication
⇒ G Proteins
⇒ Kinases and Phosphatases
⇒ The Cell Cycle
Nonproliferating cells are in the "no growth" (G0
) stage of the cell cycle.
cells are expressing only those genes that are needed to maintain life
and carry out any specialized function they normally have. To begin the
first growth stage (G1
) of the cell cycle, cells must be signaled, usually
by more than one growth factor. The binding of growth factors to their
receptors may stimulate the receptor's tyrosine kinase activities, resulting
in autophosphorylation and in the phosphorylation of other kinases.
Most of these secondary kinases are serine-threonine kinases. In some
cases, activated receptors stimulate G proteins that subsequently stimulate
enzymes that produce second messengers. The second messengers
often stimulate serine-threonine kinases. The serine-threonine kinases
phosphorylate transcriptional activators or repressors, stimulating and inhibiting
them, respectively. This results in the expression of a number of
genes, in particular, the genes for G1
phase cyclins: CDPKs, RNA polymerase,
DNA helicase, and DNA polymerase. The accumulation of these
proteins during G1
is essential for DNA replication in the S phase of mitosis.