Chromosomes

Chromosomes

Just before a eukaryotic cell divides, a number of thread-like structures called chromosomes gradually become visible in the nucleus. They are easily seen, because they stain intensely with particular stains. They were originally termed chromosomes because ‘chromo’ means coloured and ‘somes’ means bodies. The number of chromosomes is characteristic of the species. For example, in human cells there are 46 chromosomes, and in fruit fly cells there are only eight.

The structure of chromosomes

Before studying nuclear division, you need to understand a little about the structure of chromosomes. The above figure is a simplified diagram of the structure of a chromosome just before cell division. You can see that the chromosome at this stage is a double structure. It is made of two identical structures called chromatids, joined together. This is because during the period between nuclear divisions, which is known as interphase, each DNA molecule in a nucleus makes an identical copy of itself. Each chromatid contains one of these DNA copies, and the two chromatids are held together by a narrow region called the centromere, forming a chromosome. The centromere can be found anywhere along the length of the chromosome, but the position is characteristic for a particular chromosome. Each chromatid contains one DNA molecule. DNA is the molecule of inheritance and is made up of a series of genes. Each gene is one unit of inheritance, coding for one polypeptide that is involved in a specific aspect of the functioning of the organism. 

The fact that the two DNA molecules in sister chromatids, and hence their genes, are identical is the key to precise nuclear division. When cells divide, one chromatid goes into one daughter cell and one goes into the other daughter cell, making the daughter cells genetically identical. 

So much information is stored in DNA that it needs to be a very long molecule. Although only 2nm wide, the total length of DNA in the 46 chromosomes of an adult human cell is about 1.8 metres. This has to be packed into a nucleus which is only 6 μm in diameter. This is the equivalent of trying to get an 18 km length of string into a ball which is only 6 cm in diameter! In order to prevent the DNA getting tangled up into knots, a precise scaffolding made of protein molecules is used. The DNA is wound around the outside of these protein molecules. The combination of DNA and proteins is called chromatin. Chromosomes are made of chromatin. Chemically speaking, most of the proteins are basic (the opposite of acidic) and are of a type known as histones. Because they are basic, they can interact easily with DNA, which is acidic. 

The precise details of chromatin structure are complex and you do not need to remember them, but they provide you with useful background knowledge. The solution to the packing problem is controlled coiling of the DNA. Coils can themselves be coiled to form ‘supercoils’; these may then be looped, coiled or folded in precise ways which are still not fully understood. We do, however, understand the basic unit of structure. This is called a nucleosome. (Although you do not need to know about nucleosomes, this will help you to understand how DNA forms chromosomes.) The nucleosome is cylindrical in shape, about 11 nm wide by 6 nm long. It is made up of eight histone molecules. The DNA is wrapped around the outside of the cylinder, making 1⅔ turns (equivalent to 147 base pairs) before linking to the next nucleosome. The DNA between the nucleosomes (linker DNA, 53 base pairs in length) is also held in place by a histone molecule. Nucleosomes line up like a string of beads to form a fibre 10nm wide. This string can be further coiled and supercoiled, involving some non-histone proteins. The extent of coiling varies during the cell cycle, the period between one cell division and the next. 

The chromosomes seen just before nuclear division represent the most tightly coiled (condensed) form of DNA. Between nuclear divisions, some uncoiling occurs. In fact, chromatin exists in two forms - euchromatin and heterochromatin. Euchromatin is loosely coiled, whereas heterochromatin is tightly coiled, as in the chromosomes seen at nuclear division. During the period between divisions (interphase) the majority is in the form of euchromatin. This is where the active genes  are located. The genes in the heterochromatin are mostly inactive. Chromatin is easily stained – the more tightly coiled it is, the more densely it stains. It is therefore easy to see chromatin in the light microscope (nuclei stain easily) and in the electron microscope the two forms of chromatin are clearly visible. 

Chromatin is at its most condensed in chromosomes at the metaphase stage of mitosis. The fact that all the information of the cell is tightly packed makes it easier to separate the information into two new cells. This is one of the main functions of chromosomes. Chromosomes also possess two features essential for successful nuclear division, namely centromeres and telomeres.