The Life Cycle Biology Worksheet printable pdf download

EEB3240/5240

Life cycles:

Hand-out 1: The life cycle (due next Tuesday)

Purpose:

This exercise is designed for you to develop a better understanding of the different stages

of life cycles. This exercise consists in a purely theoretical approach to this objective. Although the

exercises may at times seem repetitive, you are encouraged to proceed through all of them, as this

will allow you to become familiar with the terminology

A life cycle is the “complete sequence of phases in the growth and development of any organism

from the zygote formation to the gamete formation.

The key words are: zygote and gametes, two stages of any life cycle. How would you define these

terms?

Zygote: ___________________________________________________________________

__________________________________________________________________________

Gametes: __________________________________________________________________

__________________________________________________________________________

If you were given a cell, and asked whether this cell was a zygote or a gamete, what feature would

you use to differentiate them? __________________________________________________

This difference is the key to understanding the concept of life cycles. Why? Well, imagine now a life

cycle reduced to these two stages. How would these stages be connected in an organism undergoing

sexual reproduction? In other words, what mechanisms account for the transition from one stage to

the other?

Let’s start with the transition from “gamete” to “zygote”. The zygote has twice the number of

chromosomes than the gamete. How is this achieved? Sexual reproduction involves the fusion of

two cells. These cells are the gametes. “Gamete” is a generic term. An egg is a gamete and so is a

sperm. Eggs and sperms are strikingly different in morphology (this is called anisogamy), but

sometimes, the gametes cannot be told apart (isogamy). The fusion of gametes, also called

syngamy, thus leads to a cell that bears two sets of chromosomes, one from each gamete.

Remember that a life cycle is a “complete sequence of phases”. It is thus necessary for the zygote-

phase to be connected to the gamete-phase. A transition from a cell with two complements to one

with a single set of chromosomes is thus needed. Simple! The zygote only has to be divided in two

cells. In theory this is correct, but is practice, the mechanism is somewhat more sophisticated than a

simple division.

Meiosis is a set of two successive divisions in which the chromosome number is reduced from

diploid (2N) to haploid (N). Meiosis thus differs from the simple division we imagined above, by

the occurrence of one additional division. It is during this division that chiasmata are established

between homologous chromosomes.

The Life Cycle Biology Worksheet