Question

What do you understand by life cycle? Describe the different kinds of life cycles found in Protista.

Answer 1

The sequences of events between any given phase in one generation and that similar phase in the next succeeding generation constitute the life cycle. Two types of life cycles are seen in protists.

1. Life cycle showing zygotic meiosis: It occurs in some dinoflagellates and cellular slime moulds. The zygote is 2n that divides by meiosis (also called zygotic meiosis) and produces vegetative cells with 1n chromosome number. These cells divide repeatedly by mitosis and all the resulting daughter cells maintain the 1n number of chromosomes. Some of the vegetative cells produce gametes. When these gametes combine in fertilization, a zygote is formed and the life cycle is completed.
   
2. Life cycle showing gametic meiosis: This is found in the majority of protozoan protists, diatoms, and acellular slime moulds. The organism spends most of its life cycle in the 2n condition. The gametes are only 1n (haploid) that are produced by meiosis (also called gametic meiosis). The gametes fuse to form a zygote that grows to form the diploid individual.