Haploidy & Diploidy (CIE A Level Biology)

• 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 individual) 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 (no pairs) 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

Haploid and Diploid Cells Diagram

Haploid (n) and Diploid (2n) cells

• During fertilisation 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
  • Where n is the haploid number of chromosomes and 2n is the diploid number of chromosomes
• Meiosis produces haploid gametes during sexual reproduction
• The first cell division (this is not referring to the first stage) 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

Haploidy and Diploidy Diagram

The maintenance of chromosome number through reduction division in a mammalian life cycle