Reproduction is a fundamental part of existence for most organisms, and for the majority of animals and plants that you see around you, reproduction means combining the chromosomes of two individuals in order to generate a diploid organism that has genetic material from both parents.
A diploid cell, or organism, is represented by having two sets of chromosomes, unlike haploid cells, which only possess one set of chromosomes. In humans, haploid cells are only present as gametes—sperm or eggs—which will combine during fertilization to create a diploid zygote.
What is a Diploid Cell?
As mentioned above, a diploid cell is simply one that contains two sets of chromosomes. This means that the cell or organism has genetic material from both parents, which has provided it with genetic diversity. Every cell in the human body is diploid in nature, with the exception of eggs and sperm. We are considered, therefore, diploid organisms, like nearly every other eukaryotic organism—plant, animal or fungi.
Diploid cells replicate through a process of mitosis, in which the DNA is replicated, followed by a cellular division event, resulting in two identical daughter cells that are also diploid. In humans, there are 46 chromosomes in a diploid cell, representing 22 autosomes and 1 sex chromosome. This number varies between other animals, ranging from 2 chromosomes in a particular species of roundworm up to 254 chromosomes in the unassuming hermit crab! Diploid plants display an even greater range, with some species having more than 1,000 chromosomes.
The vast majority of animals, including humans, remain diploid for the entirety of their life cycles, but in some other organisms, fluctuation between diploid and haploid is possible. Plant species, for example, vacillate between a diploid multicellular state (sporophyte) and a haploid unicellular state (gametophyte). You will find this variation in alternating generations.
In fungus, the diploid section of the life cycle is quite short. When gametes unite, they form a diploid zygote, but this zygote quickly undergoes meiosis, resulting in four haploid spores. These haploid cells will then multiply via mitosis, remaining haploid throughout the remainder of the life cycle until the next reproductive event.
A Final Word
The diploid nature of eukaryotic organisms is one of the defining characteristics of higher life forms; holding two sets of chromosomes in each of our cells guarantees genetic diversity and recombination events, leading to the endless variety in our forms of life. This also allows different species to evolve more effectively through natural selection, as diploid organisms experience more genetic variation and mutations that can affect their phenotype.