Phototaxis and Circadian Rhythm

$15.00 – $45.00

Chlamydomonas cells perceive light using a primitive “eye”. The light receptor is a rhodopsin, similar to the visual pigment in human and animal eyes. A specialized cellular region beneath this light receptor, the orange “eyespot” or stigma, contains a layer of globules containing a carotenoid pigment. The carotenoid layer helps focus the light on the true receptor, the rhodopsin. Movement can be toward or away from a light source, depending on its intensity. Phototactic accumluation of cells can be demonstrated using just a petri dish partially covered with black electrical tape. A more elaborate apparatus for observation under a microscope, based on a design by Tony Moss, can also be constructed. These experiments can be extended to include analysis of several mutant strains that are unable to carry out efficient phototaxis because they lack the eyespot structure, are defective in the signal transduction pathway that enables the cells to respond to light, or can’t swim normally. Some strains of Chlamydomonas show a strong circadian rhythm of phototaxis. This can be demonstrated in cells entrained for a week on a light-dark cycle.

Phototaxis mutant kit: Cultures of wild-type and mutant strains in liquid for observation over a few days; no preparation of media or subculturing is required. For classroom use, we ship the cultures on agar and provide stock solutions for making the liquid medium. Some background material and references are also included.

Circadian rhythm kit: Wild-type strain CC-124 for the circadian rhythm experiment, plus stock solutions for culture medium, and instructions.

Combined phototaxis and circadian rhythm kit: To examine phototaxis and do the circadian rhythm experiment.