Chromosomes

What are chromosomes?

Chromosomes are molecules of DNA and proteins. They are located in the nucleus of the cell and their function is to carry genes. A gene is a part of DNA that contains code for the synthesis of a protein or an RNA molecule.

You can think of genes as of recipes that you use in the kitchen to make various dishes. In the recipe, you'd find ingredients and steps that you need to follow to make your favourite meal. Genes are actually very similar and contain all the instructions on how to make a protein.

However, there are also parts of DNA that do not code for any protein or an RNA molecule. These noncoding segments are sometimes referred to as junk DNA. Even though these parts of DNA don't contain instructions on the synthesis of a protein, they are still very important, although their precise role remains unknown.

If you want to refer to both the coding and noncoding segments of the DNA, you can use the term genome, which is used to refer to all the genetic material in the organism.

How many chromosomes do we have?

In humans, there are 24 types of chromosomes. There are 22 chromosomes that are numbered from 1 to 22 and these we call autosomes. Then, there are two sex chromosomes X and Y. As you probably know, men have one X chromosome and one Y chromosome. Women, on the other hand, have two X chromosomes. Human cells contain 2 copies of each chromosome - that means, 2 copies of chromosome 1, chromosome 2, etc., together making 46 chromosomes in total. There are, however, some cells that don't have follow this rule, e.g. germ cells, which only have 1 copy of each chromosome. A full set of these 46 chromosomes is called a karyotype.

What's the structure of a chromosome?

A chromosome has two arms - a p arm and a q arm. Between the two arms, there is a structure called centromere, which is very important during mitosis. At each end of a chromosome is a telemore, which protects the ends of a chromosome and is responsible for their proper replication. Every chromosome consists of 1 molecule of DNA and proteins. Together, they form a centromere. Here, you can see two centromeres because we're looking at a picture of a chromosome after replication, in other words, a chromosome with 2 molecules of DNA.

What are the different types of chromosomes?

Chromosomes vary in size and structure. For that reason, we distinguish different types of chromosomes. Metacentric chromosomes have p a q arm of the same length. Submetacentric chromosomes have their p arm shorter than their q arm. Acrocentric chromosomes have very short p arm. Telocentric chromosomes have only q arms.

What are the basic structural units of a chromosome?

As we said, chromosomes consist of a single molecule of DNA and proteins that modify its structure. This complex of DNA and proteins is called chromatin. Chromatin fibre is also the basic subunit of a chromosome. Chromatin proteins can be divided into two classes: the histones and the nonhistone chromosomal proteins. Histones are very important and for that reason they are present in chromosomes in almost the same amount as the DNA. They form a specific structure of the chromatin, which is called a nucleosome. Nucleosome is a basic structural unit of the chromatin.

We often describe the chromatin fibre as a series of beads on a string. The string is in fact the DNA and the bead is a nucleosome. Because the DNA connects the individual nucleosomes, it is also sometimes referred to as the linker DNA.

Every nucleosome consists of DNA that is wrapped around it and a complex of histones. It is, in fact, a complex of eight histone proteins - H2A, H2B, H3, and H4, all of them present in two molecules. This structure of eight histone proteins is sometimes referred to as the histone octamer.

A series of nucleosomes forms the chromatin fibre. The histone that allows this structure to be formed is the H1 histone and its function is to put the individual nucleosomes close together.

The structure of the chromatin and its condensation can be adjusted and cells have several mechanisms to do so. One of these are chromatin-remodelling complexes, complexes of proteins that change the position of DNA wrapped around the nucleosome making the DNA more or less accessible to other proteins in the cell.

Cells can also chemically modify histones to change the structure of the chromatin again making the DNA more or less accessible.

The consequence of these mechanisms is that the structure of the chromatin is not the same in every region of the chromosome. Parts of chromosomes that contain genes that are being expressed are more extended, while regions of chromosomes that contain genes that are not being expressed are more condensed (i.e., less extended).

Regions of the chromosome that are highly condensed are called heterochromatin. Heterochromatin is usually concentrated around the centromere and in the telomeres.

Regions of the chromosomes that are less condensed are called euchromatin. As we already said, if we want to express a gene, we want its DNA extended, which means that DNA in the heterochromatin is usually not expressed.

References:
Alberts, B. (2014). Essential Cell Biology. Garland Science.
Cooper, G. M., & Hausman, R. E. (2007). The cell: A molecular approach. ASM.
Pollard, T. D., Earnshaw, W. C., Lippincott-Schwartz, J., & Johnson, G. T. (2017). Cell biology. Elsevier.
Snustad, D. P., & Simmons, M. J. (2012). Principles of Genetics. Wiley.