Haploid

Introduction

A cell’s ploidy is the number of complete sets of chromosomes contained in it. The sets of chromosomes are derived from their paternal and maternal chromosome copies. On the basis of the number of sets of chromosomes seen, the cells can be categorized as –

  • Haploid (one set of chromosomes)
  • Diploid (two sets of chromosomes)
  • Polyploid (more than two complete sets of chromosomes)

Further, ploidy can be divided into two classes on the basis of type of change in the number of chromosomes –

  • Euploidy
  • Aneuploidy

In this article, we learn about the haploid condition and haploid cells in detail.

Haploid condition – Definition

The haploid condition of a cell is when it comprises a single set of chromosomes. Haploid can refer to the number of chromosomes in the sperm cells or eggs (gametes).

Gametes in humans are haploid cells that have 23 chromosomes, each set exists in diploid cells. Haploid cells are represented as “n” in terms of ploidy, diploid cells are represented as “2n”. In humans, 2n=46.

In humans, all the somatic cells of the body are haploid cells; it contains half the chromosomes contained in the normal diploid cells. It is through the process of meiosis that haploid gametes are produced. Hence, there is half reduction in the number of chromosomes in a parent diploid cell. In organisms such as algae, there is a haploid portion of their life cycle. Male ants are haploid entities all through their life cycle.

Haploid cells in Humans

Gametes or sex cells are haploid cells. The somatic cells in humans contain two sets of genes. It produces haploid gametes by the process of gametogenesis, wherein meiosis causes the chromosomal set to be reduced by half. This is vital as at the time of fusion of gametes, the resulting zygote is diploid in nature. This way, the integrity of the number of chromosomes is preserved all through the subsequent generations.

Human sex cells precisely comprise 23 chromosomes, hence gametes have half the diploid number of the human somatic cells. Two cells after fertilization fuse forming a zygote now with two sets of chromosomes. Then the zygote develops to grow through mitosis. This chromosome number of 46 in humans for each somatic cell gets maintained.

Haploid cells Examples

Cells in the bodies of humans are diploid all throughout their lifespan, with a few exceptions. The male and female gametes obtained from your parents united, forming the first cell of your body – the zygote. It replicated the obtained sets of DNA before it started dividing into two identical daughter cells.

Then, the cells continued to replicate and divide till it formed a cluster ball, the blastula, that started folding and differentiating into different parts of the body. Cells remain diploid as they continue to replicate via the cell division process of mitosis. Some parts of the tissues experience meiosis instead of mitosis. Meiotic division causes the splitting of the homologous chromosomes, thus reducing the ploidy of the daughter cells which are generated. The special gametes (sex cells) are the only haploid cells in the body.

Haploid number

The chromosome number is the number of chromosomes generic for a given species. The chromosome number in any particular species reproducing asexually is constant while in sexually reproducing entities, the number of chromosomes in the cells of the body (somatic cells) usually is diploid, represented as 2n (pair of each chromosome), which is twice the number seen in haploid (n) number seen in the gametes.

It is during the process of meiosis that the haploid number is produced. In some of the sexually reproducing entities, the offspring can be produced from the unfertilized eggs, and hence are haploid. A male bee is a good example.

Haploid chromosome – Haploid number of chromosomes

The number of chromosomes contained in the cell’s nucleus constituting one complete chromosome set is the haploid number. Haploid number is commonly represented by “n” which stands for the number of chromosomes. For each species, the haploid number is unique. The total number of chromosomes is referred to as the chromosome number.

The haploid number in humans is given as 2n=46, n = 23, this is because the haploid human cells possess one set of 23 chromosomes – 22 sets of autosomal chromosomes (non-sex chromosomes) and 1 set of sex chromosomes.

Haploid vs Diploid

Haploid and diploid cells can be seen occurring in different species. The main difference between diploid and haploid cells lies in the sets of chromosomes found in the types of cells they occur in. Haploid cells compromise half the chromosome number of diploid cells; these are germ cells mostly, while diploid cells are somatic cells.

Diploid cells

Diploid cells are those cells which contain two sets of chromosomes, paired chromosomes, one set obtained from each of the biological parents. Except for the sex cells in humans, all other cells are diploid cells (2n) having 23 pairs of chromosomes whereas sex cells or gametes are haploid in nature (ploidy = n) having only one set of chromosomes.

Key differences between Haploid and Diploid cells

Tabulated below are the main differences between haploid and diploid cells.

Haploid cells

Diploid cells

Sets of chromosomes

One set of chromosome

Two sets of chromosome

Types of cells

Mostly these are germ cells

These are somatic cells

Resultant of which type of cell division

Meiosis

Mitosis

Genetically identical

These cells are not identical genetically, as a result of the cross-over process occurring during meiosis

These cells are identical genetically due to mitotic cell division

Cell types in humans

Sex cells in humans are haploid

All cells in humans except for the sex cells are diploid cells, referred to as the somatic cells

Examples

Sperm cells, eggs

Muscle cells, bone cells, nerve cells

How does the resulting diploid cell ultimately have 2 complete sets of chromosomes?

In simple words, it is due to the process of meiosis.

Parent cells replicate their DNA before meiotic cell cycle starts, thereby doubling its organelle numbers and mass in a phase called interphase. Cells can proceed through meiosis I, meiosis II and the final division.

Cells pass through different phases two times as it proceeds through both meiotic divisions. Towards the end of the meiosis I, parent cells divide into two daughter cells. Identical sets of the originally replicated chromosomes are subsequently produced. After a stage, each daughter cell has a full copy of DNA.

Then the cells advance towards meiosis II, which as a result causes the sister chromatids to separate and hence the cells divide. This causes the female and male sex cells to have half the number of chromosomes as seen in the case of parent cells.

Sexual reproduction can then occur. Gametes unite, forming the zygote or fertilized eggs at the time of sexual reproduction. Zygote derives half their genetic content from the mother and half from the father. This is how the resultant diploid cells have two complete sets of chromosomes.

Reduction and Chromosome doubling

If chromosome doubling does not occur in meiosis before the cell divides, each of the four daughter cells would contain a quarter of the chromosomes of their parent cells only. In this case, the diploid parent cell would end up generating four cells with a half chromosome set only.

When such gametes fuse forming the offspring, the individual would be haploid and would turn out to be a different species altogether as compared to the parent and hence its inability to reproduce.

If the process of chromosome doubling in mitosis does not take place before the process of cell division occurs, each of the two daughter cells would contain one chromosome set only from their parent cell. This can lead to individuals with cells having varying numbers of chromosomes, in turn resulting in genetic mutations.

In the event when chromosomes do not reduce in meiosis, the combined two cells for the formation of offspring would lead to offspring with double the number of chromosomes of either of their parents or a chromosome set of 4n. Then, this offspring would turn into another species. In mitosis, if the chromosome reduction does not take place, the daughter cells would each comprise a 4n chromosomal set, and hence would be discordant with the other entities of the population.

Maintenance of the chromosomal number

Meiosis and mitosis are organized to make sure the chromosomal number of the offspring mirrors that of its parent. There is nondisjunction when the process of Karyokinesis does not appropriately occur – the chromosomes do not separate into their sister chromatids, which leads to the daughter cells having an unequal chromosome number.

After nondisjunction, monosomy takes place in the daughter cells, which contain few chromosomes. Such a cell then contains one few chromosomes or one chromosome, while it must have contained two. Then, daughter cells show trisomy – containing one extra chromosome or three chromosomes where it had to contain 2. Polyploidy takes place when cells comprise more than two haploid chromosome sets. Allopolyploidy occurs when the parent cells are not of the same species, which leads to the daughter cells containing a full diploid chromosome set from each of the parent cells. A good example of this case is the mule, which is sterile – it is a cross between donkey and horse.

You just learnt about haploid cells, their examples, haploid number, chromosome number, understood diploid cells, along with its difference with haploid cells.

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