Haploid: Definition, Chromosome Number, Examples

Haploid describes a cell or organism that contains one complete set of chromosomes, represented as n. In humans, sperm and egg cells are haploid and each normally contains 23 chromosomes. Haploidy is essential to sexual reproduction because it allows two chromosome sets to be restored when haploid gametes unite during fertilization, rather than causing the chromosome-set number to double in every generation.

What is Haploid?

Haploid is a condition in which only one complete set of chromosomes is present in a cell. The number of complete chromosome sets of a cell is called ploidy. In haploid cells, one chromosome from each type is present. It is represented by n.

When compared with a diploid cell (2n), the haploid cell contains half the chromosome number. In humans, sperm and egg cells are haploid cells.

Ploidy and Haploid Number (n)

Haploid chromosome set before and after DNA replication, showing that sister chromatids double DNA content without changing the cell from n to 2n.

Ploidy refers to the number of complete chromosome sets present in a cell. Depending upon the chromosome sets, cells may be haploid, diploid or polyploid. Haploid is a condition where only one complete set of chromosomes is present, represented by n. The number of chromosomes present in this single set is called the haploid number (n). In a diploid organism (2n), this number is half of the chromosome number present in the diploid cell.

Ploidy is based on the number of complete sets of homologous chromosomes, not on the number of DNA molecules present in the cell. A chromosome before replication contains one DNA molecule. During DNA replication, this DNA is copied and the chromosome now consists of two identical sister chromatids. The DNA content therefore becomes double, but no new chromosome set is added. The replicated chromosome is still considered as one chromosome. A haploid cell can therefore contain replicated chromosomes with two chromatids and still remain haploid (n).

Haploid Cells in Humans

The following are the haploid cells found in humans–

Sperm cell- Sperm is the male gamete, which contains 23 chromosomes (n). Of these, 22 chromosomes are autosomes while the remaining one may be X or Y chromosome. These are formed during spermatogenesis by meiotic division.
Egg cell (ovum)- It is the female gamete. The egg contains 23 chromosomes, 22 autosomes and one X chromosome. During oogenesis, reduction of diploid chromosome number to haploid takes place by meiosis.
Secondary spermatocytes and spermatids- After meiosis I, secondary spermatocytes are haploid. These undergo the second meiotic division, forming haploid spermatids. Spermatids later develop into sperm cells.

Haploidy in Cell Division

Haploidy means the presence of one complete set of chromosomes (n) in a cell. It is based on the number of chromosome sets, not on the number of chromatids present in each chromosome. Therefore, a haploid chromosome can still contain two sister chromatids.

Meiosis I and Ploidy Reduction

Meiosis I is the actual reduction division of meiosis. Here, the homologous chromosomes of every pair are separated and distributed into two different cells.
The original cell is diploid (2n), containing two homologous sets of chromosomes. After meiosis I, only one chromosome from every homologous pair is present in each daughter cell. Thus, chromosome set changes from 2n to n. This is referred to as reductional division.
Sister chromatids do not separate during this division. They remain attached with each other as one replicated chromosome.
So, the cells formed after meiosis I are already haploid (n), although each chromosome still consists of two sister chromatids. The presence of two chromatids does not make the cell diploid. Ploidy depends on homologous chromosome sets.

Meiosis diagram showing a diploid cell becoming haploid after homologous chromosomes separate in meiosis I, followed by sister chromatid separation in meiosis II.

Meiosis II and Sister Chromatids

Meiosis II mainly involves separation of sister chromatids. The two chromatids of each replicated chromosome are separated and move into different daughter cells.
No another DNA replication takes place between meiosis I and meiosis II. The chromosomes entering meiosis II are already duplicated from the earlier DNA replication.
The cells entering meiosis II are haploid. This point is important. Meiosis II does not change a diploid cell into haploid because that reduction has already occurred during meiosis I.
A haploid cell can therefore contain chromosomes having two sister chromatids. Haploid does not mean single-chromatid chromosome. After meiosis II, the sister chromatids are separated, but the ploidy remains n.

Mitosis in Haploid Cells

Mitosis generally maintains the existing ploidy of a cell. During this process, chromosomes are first replicated and the sister chromatids are later separated into the daughter nuclei.
Therefore, mitosis can also occur in a haploid cell (n). The haploid parent cell can produce haploid daughter cells.
DNA replication temporarily gives each chromosome two sister chromatids. Still, one chromosome set remains one set. Thus, the cell remains haploid before and after mitotic division.

Examples of Haploidy Across Organisms

Haploidy is not restricted to gametes only. In several organisms, one chromosome set (n) is present for a larger part of life cycle. A vegetative body, complete generation or even the whole individual can be haploid.

Plant alternation-of-generations cycle showing meiosis producing haploid spores, mitotic growth of the gametophyte, gamete production, fertilization, and the diploid sporophyte.

Plants

The diploid sporophyte (2n) produces haploid spores by meiosis. Thus, chromosome number is reduced during spore formation. These spores are not gametes.
A spore germinates and undergoes mitotic divisions, forming the gametophyte (n). It already contains a single set of chromosomes.
Gamete formation- The haploid gametophyte produces gametes through mitosis. Meiosis is not needed again here because the gametophyte is already haploid.
In mosses, the main visible plant body is the haploid gametophyte. A well-developed multicellular stage is therefore haploid. Ferns also have a free-living haploid gametophyte, but it is much smaller compared to the diploid sporophyte.
Fusion of two haploid gametes forms the diploid zygote (2n). The diploid condition is brought back by fertilization.

Fungi and Algae

Many fungi have a mainly haploid life cycle. The vegetative body may remain haploid, with nuclei multiplying through mitosis for an extended period.
During sexual reproduction, two haploid nuclei unite. A diploid nucleus is produced. This condition can be short, and meiosis again gives rise to haploid spores.
Ulothrix– Its vegetative filament is haploid. This is a multicellular body having only one chromosome set, rather than a single reproductive cell. The filament produces gametes, which fuse to form the diploid zygote.
Meiosis takes place later in the zygote of Ulothrix. Haploid meiospores are formed and new filaments develop from these spores by mitotic divisions.
The normal vegetative cells of Chlamydomonas reinhardtii are also haploid. They divide through mitosis. During sexual reproduction, these cells form gametes and gametes of opposite mating types fuse to produce a diploid zygote.
Haploid return- Meiosis later takes place in the zygote and the haploid vegetative condition is formed again. Thus, in fungi and some algae, haploidy may continue through several mitotic divisions and can represent an extended vegetative or multicellular stage.

Haplodiploidy in Some Insects

Haplodiploidy is found in some bees, ants and wasps. In this system, haploid and diploid individuals occur in the same life cycle.
An unfertilized egg can develop into a haploid male (n). Only one chromosome set is present, received from the mother.
Fertilized eggs generally give rise to diploid females (2n). They contain two chromosome sets, maternal and paternal.
Here, it is the complete male individual that is haploid. Haploidy is not confined only to the gametes.
This pattern is referred to as arrhenotokous haplodiploidy. Haploid males and diploid females are the common form found in these insects.

Haplodiploidy pathway showing an unfertilized egg developing into a haploid male and a fertilized egg developing into a diploid female.

Biological Significance of Haploidy

Haploidy has an important role in reproduction, genetic selection and life cycle of organisms. Since only one chromosome set is present, the effect of an allele may be expressed directly.

Allele expression- Only one allele of a gene is present at a locus. So, its effect is not masked by another dominant allele as in a heterozygous diploid condition.
Mutation exposure- Recessive harmful mutations may be directly expressed in the haploid stage. These mutations can therefore become exposed to natural selection.
Chromosome balance- Meiosis reduces the chromosome sets from diploid (2n) to haploid (n). Fertilization again restores the diploid condition and maintains the chromosome number through generations.
Haploid growth- Haploidy is not always limited to gametes. In fungi, some algae and plant gametophytes, haploid cells divide by mitosis and may form an extended or multicellular stage.
Genetic selection- Alleles present in haploid cells are directly exposed to selection. This can influence removal of harmful alleles, adaptation and maintenance of genetic variation.
Life cycle- Haploidy forms an important part of the haploid-diploid cycle. Animals generally have haploid gametes, whereas fungi, algae and plants may have a larger haploid phase.
Genetic studies- Haploid cells are useful for studying gene functions because only one genome copy is present. A mutation can show its effect without first making it homozygous.

Key Facts about Haploid for Exams

Key term	Exam fact
Haploid (n)	A haploid cell contains one complete set of chromosomes. It is represented by “n”.
Chromosome set	Only one member of each homologous chromosome pair is present. Thus, one chromosome set is present instead of two.
Diploid (2n)	Diploid cells contain two chromosome sets, whereas haploid cells contain one.
Meiosis	Meiosis reduces a diploid cell into haploid cells. One DNA replication is followed by two divisions.
Meiosis I	It is the actual reduction division. Homologous chromosomes separate and the chromosome sets become 2n to n.
Meiosis II	Sister chromatids separate in this division. The cells are already haploid before meiosis II starts.
Sister chromatids	A haploid cell can still have chromosomes made of two sister chromatids. Two chromatids do not make the cell diploid.
Gametes	Sperm and egg are haploid in animals. Fusion of two haploid gametes forms the diploid zygote.
Fertilization	It restores the diploid chromosome number (2n) by combining two haploid chromosome sets.
Plant spores	The diploid sporophyte produces haploid spores by meiosis. Spores are not gametes.
Gametophyte	It is the haploid generation of plants. The gametophyte produces gametes by mitosis, as it is already haploid.
Haploid mitosis	Haploid cells can divide through mitosis. The daughter cells remain at the same haploid ploidy level.
Alleles	Only one copy of each chromosome is present in the haploid condition. Therefore, there is no homologous chromosome carrying a second allele at the corresponding locus.
Main point	Haploid refers to chromosome sets, not the number of chromatids or amount of DNA.

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