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 (meiosis). Each daughter cell is haploid. They have half the number of chromosomes as their parent cells. Haploid means "half."

For example, gametes are haploid cells that are produced by meiosis. Meiosis happens when it's time to reproduce an organism. Like in the sexual reproduction of a human, a zygote or fertilized egg, gets half its genetic material from the mother, contained in the sex gamete or cell of the egg, and half its genetic material from the father, which is contained in the male sex gamete or sperm. In the process of sexual reproduction, haploid sex cells unite at fertilization and become a diploid cell.

Cell with nucleus in mitosis and multiplication of cells
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Haploid Versus Diploid

A haploid cell differs from a diploid cell because instead of a diploid cell creating two new cells with equal numbers of chromosomes (as diploids do with mitosis), the "parent" diploid cell does the second division soon after the first. A diploid cell divides twice to produce four haploid daughter cells, with half the genetic material.

So, in this case, a diploid is the opposite of a haploid. It forms two strands or doubles. It duplicates all the genetic material.

Mitosis occurs when a cell is going to make an exact copy of itself as in the case of asexual reproduction, growth, or tissue repair. DNA replication occurs once, followed by a single division. The parent and daughter cells are both diploid, which means they have a double set of chromosomes.

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 abbreviated as "n," where n stands for the number of chromosomes. The haploid number will be different for different organisms.

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 (non-sex chromosomes) and one set of sex chromosomes.

As a human, you are a diploid organism, meaning you have one set of 23 chromosomes from your father and one set of 23 chromosomes from your mother. The two sets combined provide a full complement of 46 chromosomes. This total number of chromosomes is called the chromosome number.

More About Meiosis

Haploid cells are produced by meiosis. Prior to the start of the meiotic cell cycle, the cell replicates its DNA and increases its mass and organelle numbers in a stage known as interphase.

As a cell progresses through meiosis, it goes through the various stages of the cell cycle: prophase, metaphase, anaphase, and telophase, twice. At the end of meiosis I, the cell divides into two cells. Homologous chromosomes separate, and sister chromatids (chromosomes) remain together.

The cells then enter meiosis II, which means they divide again. At the end of meiosis II, sister chromatids separate, leaving each of the four cells with half the number of chromosomes as the original cell. 

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 that can alternate between a haploid phase and a diploid phase. This type of life cycle is known as alternation of generations.

In plants and algae, haploid spores develop into gametophyte structures without fertilization. The gametophyte produces gametes and is considered the haploid phase in 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.