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Haploid is the quality of a cell or organism having a single set of chromosomes. Organisms that reproduce asexually are haploid. Sexually reproducing organisms are diploid (having two sets of chromosomes, one from each parent). In humans, only their egg and sperm cells are haploid.

A human brain cell has 23 pairs of chromosomes, for a total of 46 chromosomes. Brain cells are called diploid cells because they have chromosomes…

Polyploid cells arise in the extraembryonic placenta, as well as in somatic tissues such as the brain, bone marrow, muscle, liver, mammary glands, and other epithelial tissues. In contrast, the only haploid cells are the egg and sperm.

Haploid describes a cell that contains a single set of chromosomes. The term haploid can also refer to the number of chromosomes in egg or sperm cells, which are also called gametes. In humans, gametes are haploid cells that contain 23 chromosomes, each of which a one of a chromosome pair that exists in diplod cells.

Meiosis and Mitosis

gametes

In order to generate haploid gametes, diploid germ cells need to undergo several rounds of cell division and build several new cells. Therefore, meiosis allows the germ cells to make a new collection of genes (haploid) from the usual two (diploid) copies.

In many animals, the germ cells originate in the primitive streak and migrate via the gut of an embryo to the developing gonads. There, they undergo meiosis, followed by cellular differentiation into mature gametes, either eggs or sperm. Unlike animals, plants do not have germ cells designated in early development.

There are 22 chromosomes pairs (autosomes) and a pair of sex chromosomes (allosomes). Thus, a single set of genes is formed from the two copies. Meiosis, therefore, helps the germ cells make a single set of genes (haploid) from the normal two copies (diploid).

Your genes are so tiny you have around 20,000 of them inside every cell in your body! Human genes vary in size from a few hundred bases to over a million bases.

Most genes are the same in all people, but a small number of genes (less than 1 percent of the total) are slightly different between people. Alleles are forms of the same gene with small differences in their sequence of DNA bases. These small differences contribute to each person’s unique physical features.

Venter, who is founder of both the J. Craig Venter Institute and Synthetic Genomics Inc., has been working to create a version of the bacterium Mycoplasma mycoides with only the bare minimum of genetic instructions required for life. As it turns out, he says, at least 473 genes are essential to sustain life in a lab.

Mycoplasma genitalium

A bacterium living in special cells inside an insect has the smallest genome of any known cellular lifeform, a new study finds.

The human genome is about 5,000 times larger than that of Mycoplasma genitalium, which causes gonorrhea-like symptoms in humans. Scientists study it in part because it contains only 517 cellular genes, the fewest known in single-celled organisms.

In a 1995 Science paper, Venter’s team sequenced the genome of Mycoplasma genitalium, a sexually transmitted microbe with the smallest genome of any known free-living organism, and mapped its 470 genes.

Smallest number: The female of a subspecies of the ant, Myrmecia pilosula, has one pair of chromosomes per cell. Its male has only one chromosome in each cell.

nucleotide

The DNA molecule consists of two strands that wind around one another to form a shape known as a double helix. Each strand has a backbone made of alternating sugar (deoxyribose) and phosphate groups.