A level Biology exam revision resources written by A level Examiners





 

Organisms have a variety of different life cycles. The life cycle of humans is a good example to introduce the components of the cycles. The figure 1, below, shows the life cycle of a human.

Figure 1 : The life cycle of a human
The life cycle of a human

Gametes = specialised sex cells e.g. in humans - sperm and eggs

Diploid = stages where the organism, or cells, have two copies of each chromosome (one of maternal origin and one of paternal origin). The diploid state is often referred to as 2n because there are 2 copies of each chromosome present

e.g. human adults have 46 chromosome, two sets of 23, therefore 2n=46.

Haploid = stages where the organism, or cells, have only one copy of each chromosome. The haploid state is often referred to as n because there is only one copy of each chromosome present

e.g. human gametes have 23 chromosomes n=23.

Meiosis = the type of cell division where the chromosome number is halved from the diploid number (2n) to the haploid number (n). This is necessary so that the chromosome number stays the same, from one generation to the next.

Mitosis = the type of cell division where the chromosome number is unchanged, either 2n → 2n or n → n.

In humans the diploid phase is much longer lasting than the haploid gamete phase. The gamete cells are not capable of remaining alive for long on their own.

Many fungi have a different life cycle, where the adult form is a haploid organism that grows from a spore. A spore is a specialised cell from which a new organism can grow without fertilisation taking place. An example of this kind of life cycle is shown in figure 2, below.

Figure 2 : The life cycle of a fungi
The life cycle of a human

In this example the fungi produces zoospores, these are motile spores (spores which can move). The dotted line indicates an alternative cycle of reproduction that can occur, showing that this fungus can reproduce sexually or asexually.

Plants often show a type of life cycle called alternation of generations. In this type of cycle the organism has a haploid adult form and a diploid adult form. A typical plant life cycle can be seen in figure 3, below.  Exam Advice

Figure 3 : The life cycle of a plant
The life cycle of a human


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