Why Do Most Humans Have 23 Pairs of Chromosomes?

你可能听说过137 is the magic number, but if you ask a geneticist, they'll tell youthe real magic is in 46。为什么46？因为那是几乎每一个中发现的染色体总数人类细胞- 确切地说是23对 - 那些类似线程的小结构包装了有关您是谁以及使您与众不同的所有信息。

脱氧核糖核酸and Chromosomes

To understand what chromosomes are, you first have to understand what脱氧核糖核酸是。DNA正式称为脱氧核糖核酸，是在所有动植物中发现的复杂分子。它几乎在生物体身体的每个细胞中都发现，并且包含使该生物启动和运行（以及开发和繁殖）所需的所有信息。DNA也是生物传递遗传信息的主要方式。在繁殖过程中，一部分DNA从父母传递到后代。简而言之，DNA是说明您完全独特生物学的故事。

您可以想象，DNA必须很长时间才能拥有所有重要的信息。而且是 - 如果您将DNA仅在一个单元格中伸展到全长，它将长约6.5英尺（2米），如果将DNA放在所有细胞中，链about twice the diameter of the solar system。

幸运的是，单元格非常聪明，并且具有将所有信息包装到太空效率的包裹中的巧妙方式。输入：染色体。

它的名字扎根于希腊语（色度）和身体（soma）的希腊语单词，the chromosome is a cell structure(or body) that scientists can spot under a microscope by staining it with colored dyes (get it?). Each chromosome is made up of protein and — you guessed it — DNA.

Every chromosome contains exactly one molecule of DNA, to be exact, and that long string of genetic info is tightly wrapped around the protein (called a histone), which acts like a spool, efficiently bundling the lengthy, info-rich molecule into the perfect size and shape to fit inside the nucleus of a cell. Every human cell has 23 pairs of chromosomes for a total of 46 (aside from精子和鸡蛋细胞， 哪个eachcontain只要23染色体）。

为什么要23对？

The magic number of 46 (23 pairs) per cell isn't universal among living things. First, though, humans also happen to be a "diploid" species, which means that most of our chromosomes come in matched sets called homologous pairs (the two members of each pair are called homologues). A lot of animals and plants are diploid, butnot all of them have a total number of 46。例如，蚊子的二倍体染色体六个，青蛙有26个，虾的染色体高达508个染色体！

但是，为什么人类有23对？它发生在期间进化。贝伦·赫尔（Belen Hurle）博士说：“人类有23对染色体，而其他所有大猿（黑猩猩，bo骨，大猩猩和猩猩）都有24对染色体。”Hurle是国家人类基因组研究所at the National Institutes of Health. "This is because in the human evolutionary lineage, two ancestral ape chromosomes fused at their telomeres [tips], producing human chromosome 2. Thus, humans have one fewer pair of chromosomes. This is one of the main differences between the human genome and the genome of our closest relatives."

Now let's go back to that sperm and egg issue — these cells only haveone homologous chromosomefrom each pair and are considered "haploid." Here's why: When a sperm and egg fuse, they combine their genetic material to form one complete, diploid set of chromosomes. And if you think about it, that makes perfect sense. It means each parent contributes one homologue to a homologous pair of chromosomes in their child's cells.

Consider血型as a clear example: People with AB blood type inherited two different gene variations on their two homologous chromosomes — one for A and one for B — that, when combined, produced AB.

Too Many or Too Few Chromosomes

现在，您知道了健康人的教科书示例，几乎每个细胞都有23对染色体，但生活并不总是xf187手机版一本教科书。如果某些东西或多或少导致染色体开发，会发生什么？来自标准46的染色体的增益或损失（called aneuploidy) occurs either during the formation of reproductive cells (sperm and egg), in early fetal development or in any other cell of the body after birth.

One of the more common forms of aneuploidy is "trisomy," which is the presence of an extra chromosome in the cells. One well-known result of trisomy is Down syndrome, which is a condition caused by three copies of chromosome 21 in each cell. This extra chromosome leads to a total of 47 chromosomes per cell, rather than 46.

The loss of one chromosome in a cell is called "monosomy," and describes a condition in which people have just one copy of a specific chromosome per cell as opposed to two. Turner syndrome, in which women have only one copy of the X chromosome per cell versus the regular two, is considered a form of monosomy.

There are other variations of aneuploidy as well, and in extreme cases, they may compromise a person's life. Additionally, cancer cells also have alterations in their number of chromosomes. Unlike the variations that happen in reproductive cells, these changes occur in other cells of the body, so they're not inherited.

Originally Published: Jul 3, 2019