2.26: Chromosomes

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How is it assured that every cell in your body has the same DNA?

Chromosomes, like those shown here, must form prior to cell division, to ensure that each daughter cell receives a complete set of genetic material. Essentially, each new cell receives half of each "X-shaped" chromosome.

Chromosomes

In eukaryotic cells, the nucleus divides before the cell itself divides. The process in which the nucleus divides is called mitosis. Before mitosis occurs, a cell’s DNA is replicated. This is necessary so that each daughter cell will have a complete copy of the genetic material from the parent cell. How is the replicated DNA sorted and separated so that each daughter cell gets a complete set of the genetic material? To understand how this happens, you need to know more about chromosomes.

Chromosomes are coiled structures made of DNA and proteins. Chromosomes are the form of the genetic material of a cell during cell division. It is this coiled structure that ensures proper segregation of the chromosomes during cell division. During other phases of the cell cycle, DNA is not coiled into chromosomes. Instead, it exists as a grainy material called chromatin.

Chromatids and the Centromere

DNA condenses and coils into the familiar X-shaped form of a chromosome, shown in Figure below, only after it has replicated. (You can watch DNA coiling into a chromosome at the link below.) Because DNA has already replicated, each chromosome actually consists of two identical copies. The two copies are called sister chromatids. They are attached to one another at a region called the centromere.

Chromosomes and Genes

The DNA of a chromosome is encoded with genetic instructions for making proteins. These instructions are organized into units called genes. Most genes contain the instructions for a single protein. There may be hundreds or even thousands of genes on a single chromosome.

Human Chromosomes

Human cells normally have two sets of chromosomes, one set inherited from each parent. There are 23 chromosomes in each set, for a total of 46 chromosomes per cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are actually 23 pairs of chromosomes per cell. Each pair consists of chromosomes of similar size and shape that also contain the same genes, but different alleles (alternate forms of a gene). The chromosomes in a pair are known as homologous chromosomes.

Summary

Chromosomes are coiled structures made of DNA and proteins.
Chromosomes form after DNA replicates; prior to replication, DNA exists as chromatin.
Chromosomes contain genes, which code for proteins.
Human cells normally have 46 chromosomes, made up of two sets of chromosomes, one set inherited from each parent.

Review

What are chromosomes? When do they form?
Identify the chromatids and the centromere of a chromosome.
Explain how chromosomes are related to chromatin. Why are chromosomes important for mitosis?
How many chromosomes are in a normal human cell?

Image	Reference	Attributions
	[Figure 1]	Credit: Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation;User:KES47/Wikimedia Commons;Zappy's Source: CK-12 Foundation ; commons.wikimedia.org/wiki/File:Chromosome_en.svg ; commons.wikimedia.org/wiki/File:PLoSBiol3.5.Fig7ChromosomesAluFish.jpg License: CC BY-NC 3.0
	[Figure 2]	Credit: Zappy's Source: CK-12 Foundation License: CC BY-NC 3.0