Meiosis: The number of cells increases.

The idea

In the context of reproduction, this statement refers to the events immediately following fertilisation. When a male and female gamete fuse, they form a single diploid cell called a zygote. This new cell then divides by mitosis, producing identical diploid cells. As this process repeats, the overall number of cells increases rapidly, forming an embryo. For example, a single fertilised human egg divides to form a ball of cells that implants in the uterus. As the embryo develops, these cells begin to differentiate into specialised types, such as muscle or nerve cells, to form the tissues and organs of the new organism.

What earns the marks

• Fertilisation involves the fusion of two haploid gametes to form a single diploid cell known as a zygote.
• The newly formed zygote divides by mitosis to produce genetically identical diploid cells.
• Repeated mitotic divisions cause the overall number of cells to increase, forming an embryo.
• As the embryo develops, the cells differentiate into specialised cells to form different tissues and organs.

Common mistakes and how to put them right

• Stating that the fertilised egg divides by meiosis.

  Put it right: The zygote divides by mitosis for growth, whereas meiosis is only for making gametes.

• Confusing the increase in cell number with an increase in chromosome number.

  Put it right: The chromosome number remains constant (diploid) during these mitotic divisions.

• Forgetting to mention differentiation.

  Put it right: State that as the cell number increases, the cells also differentiate to form specialised tissues.

Exam tips

• Clearly state that the division after fertilisation is mitosis, not meiosis.
• Link the increase in cell number directly to the formation of an embryo and subsequent cell differentiation.