All About Haploid Cells in Microbiology

Cross section biomedical illustration of meiosis where each duplicated chromosome has a mixture of genetic material and threads forming in cell to pull apart the pairs of chromosomes
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In microbiology, a haploid cell is the result of a diploid cell replicating and dividing twice through meiosis. Haploid means "half." Each daughter cell produced from this division is haploid, meaning that it contains half the number of chromosomes as its parent cell.

Cell with nucleus in mitosis and multiplication of cells
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Haploid Vs. Diploid

The difference between diploid and haploid cells is that diploids contain two complete sets of chromosomes and haploids contain only one set of chromosomes. Haploid cells are produced when a parent cell divides twice, resulting in two diploid cells with the full set of genetic material upon the first division and four haploid daughter cells with only half of the original genetic material upon the second.

Meiosis

Prior to the start of the meiotic cell cycle, a parent cell replicates its DNA, doubling its mass and organelle numbers in a stage known as interphase. A cell can then go through meiosis I, the first division, and meiosis II, the second and final division.

A cell goes through multiple stages twice as it progresses through both divisions of meiosis: prophase, metaphase, anaphase, and telophase. At the end of meiosis I, the parent cell splits into two daughter cells. Homologous chromosome pairs containing the parent chromosomes that were replicated during interphase then separate from each other and sister chromatids—identical copies of the originally replicated chromosome—remain together. Each daughter cell has a complete copy of DNA at this point.

The two cells then enter meiosis II, at the end of which the sister chromatids separate and the cells divide, leaving four male and female sex cells or gametes with half the number of chromosomes as the parent.

Following meiosis, sexual reproduction can occur. Gametes randomly join to form unique fertilized eggs or zygotes during sexual reproduction. A zygote gets half its genetic material from its mother, a female sex gamete or egg, and half from its father, a male sex gamete or sperm. The resulting diploid cell has two complete sets of chromosomes. 

Mitosis

Mitosis occurs when a cell makes an exact copy of itself then splits, producing two diploid daughter cells with identical sets of chromosomes. Mitosis is a form of asexual reproduction, growth, or tissue repair.

Haploid Number

The haploid number is the number of chromosomes within the nucleus of a cell that constitutes one complete chromosomal set. This number is commonly denoted as "n" where n stands for the number of chromosomes. The haploid number is unique to the type of organism.

In humans, the haploid number is expressed as n = 23 because haploid human cells have one set of 23 chromosomes. There are 22 sets of autosomal chromosomes (or non-sex chromosomes) and one set of sex chromosomes.

Humans are diploid organisms, meaning they have one set of 23 chromosomes from their father and one set of 23 chromosomes from their mother. The two sets combine to form a full complement of 46 chromosomes. The total number of chromosomes is called the chromosome number.

Haploid Spores

In organisms such as plants, algae, and fungi, asexual reproduction is accomplished through the production of haploid spores. These organisms have life cycles known as alternation of generations that alternate between haploid and diploid phases.

In plants and algae, haploid spores develop into gametophyte structures without fertilization. A gametophyte produces gametes in what is considered the haploid phase of the life cycle. The diploid phase of the cycle consists of the formation of sporophytes. Sporophytes are diploid structures that develop from the fertilization of gametes.