People have 23 or 46 chromosomes

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Chromosomes are the carriers of genetic information and are located in the cell nuclei. Humans have 46 chromosomes in their body cells (exception: germ cells), including 44 so-called autosomes and two sex chromosomes (gonosomes). Read everything you need to know about chromosomes: definition, structure, function and important health disorders!

What is a chromosome?

Chromosomes are the carriers of all genetic information of an organism and are located in the cell nuclei.

Most of the time, the chromosomes exist as a rod-like or hook-like single strand, called a chromatid. One then speaks of a single chromatid chromosome.

Shortly before cells and nuclei divide, the chromosomes have to be doubled. This creates two identical chromatids that are connected to one another at one point (the centromere). The two chromatids of these two-chromatid chromosomes are pulled apart during nuclear division (mitosis) at the centromere and towards opposite cell poles. In between, a new cell membrane forms - two new daughter cells, each with a complete set of chromosomes, have emerged.

Chromosomes: structure

On the one hand, chromosomes consist of DNA (English abbreviation for deoxyribonucleic acid), which stores the genetic information in the form of individual genes. On the other hand, chromosomes contain special proteins (such as histones).

In the time between successive cell divisions (interphase), chromosomes are loosened up as chromatin in the cell nucleus. But as soon as a cell and nucleus division occurs, the chromatin is transferred into a more densely packed form of transport - the chromosomes.

Read more about chromatin in the article "Chromatin"

How many chromosomes does a person have?

The normal number of chromosomes (humans) in all nucleated body cells (except egg and sperm cells) is 46. Strictly speaking, there are 22 pairs of autosomes and 2 sex chromosomes (gonosomes). One speaks here of a double (diploid) set of chromosomes.

In each autosome pair, one specimen comes from the mother and one from the father. The same applies to the two sex chromosomes: each parent “contributed” a gonosome to the conception (fusion of egg and sperm cells).

The egg cells and sperm cells (sperm) have a simple (haploid) set of chromosomes: When the germ cells are formed, the normal double set of chromosomes must first be halved (in the course of the so-called reduction division or meiosis). Only in this way does the fusion of egg and sperm cells create a cell with a double set of chromosomes, from which a new person then grows.

What is the function of chromosomes?

The chromosomes contain the entire genetic material of an organism - for example the genetic information about hair color, body size and possibly also the risk of certain diseases. In every body cell, however, only that genetic information is active that the cell needs for its special function. The other genes are shut down.

The chromosomes also determine the sex of a person: the sex chromosomes in women are two X chromosomes. Men, on the other hand, have an X and a Y chromosome as a pair of gonosomes.

You can read more about the gonosomes in the articles: "X chromosome" and "Y chromosome"

Where are chromosomes located?

They are located in the nucleus of every cell in the organism - in humans, animals and higher plants.

What problems can chromosomes cause?

A chromosome aberration (anomaly) is when the structure or number of chromosomes deviates from the "normal state". In the first case there is a structural and in the second a numerical aberration.

Down syndrome (trisomy 21) is a numerical aberration: there is an additional (third) chromosome 21. Typical signs are, for example, short stature, flat back of the head, round and flat face and slanted eyes. In addition, people with Down syndrome are prone to heart defects, malformations in the digestive tract, hearing and visual disorders and leukemia, among other things. Often, however, skills such as a pronounced musical talent also emerge.

Klinefelter syndrome is another numerical chromosome aberration. It affects the male sex: those affected have an extra X chromosome. Possible consequences: The testicles are very small, the mammary glands are often enlarged (gynecomastia). In addition, those affected are mostly sterile.

Another chromosome aberration is Ullrich-Turner syndrome, which occurs in women: One of the two X chromosomes is missing or has structural defects. This usually leads to short stature, malformed ovaries and infertility.

In the rare cat-cry syndrome (cri-du-chat syndrome), part of chromosome 5 is missing. The affected babies utter high-pitched monotonous screams like those of a cat. In addition, a small head, a wide bridge of the nose, a noticeably small lower jaw, and psychomotor and intellectual disabilities are symptoms of this chromosomal aberration.

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