CIE A Level Biology (9700) 2019-2021

Revision Notes

16.1.2 Haploidy & Diploidy

Haploid & Diploid Cells

  • A diploid cell is a cell that contains two complete sets of chromosomes (2n)
    • These chromosomes contain the DNA necessary for protein synthesis and cell function
    • Nearly all cells in the human body are diploid with 23 pairs (46) of chromosomes in their nucleus
  • Haploid cells contain one complete set of chromosomes (n)
    • In other words they have half the number of chromosomes compared to diploid cells
    • Humans have haploid cells that contain 23 chromosomes in their nucleus
    • These haploid cells are called gametes and they are involved in sexual reproduction
    • For humans they are the female egg and the male sperm
  • Haploidy and diploidy are terms that can be applied to cells across different species
    • They describe the number of sets of chromosomes, not the total number of chromosomes

Exam Tip

Red blood cells are an exception when it comes to chromosome number as they don’t have a nucleus!

You may be asked to estimate the number of chromosomes that would be present in the haploid cell of a species.  For example, dogs have 78 chromosomes in their diploid cells. When trying to find the number of chromosomes in their haploid cells simply remember that diploid is 2n and haploid is n, meaning you just need to divide the number of chromosomes by 2. So dogs have 39 chromosomes in their haploid cells!

The Need for Reduction Division during Meiosis

  • During fertilization the nuclei of gametes fuse together to form the nucleus of the zygote
  • Both gametes must contain the correct number of chromosomes in order for the zygote to be viable. If a zygote has too many or too few chromosomes it may not survive
  • For a diploid zygote this means that the gametes must be haploid
    • n + n = 2n
  • Meiosis produces haploid gametes during sexual reproduction
  • The first cell division of meiosis is a reduction division
    • This is a nuclear division that reduces the chromosome number of a cell
    • In humans the chromosome number is reduced from 46 (diploid) to 23 (haploid)
  • The reduction in chromosome number during meiosis ensures the gametes formed are haploid

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.

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