Haploidy & Diploidy (Cambridge (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

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