• Queen Mary University of London
  • Barts Health NHS

46 – a magical number!

In terms of the human body, 46 (the number of people in our first group of volunteers, whose pictures and comments are here) is a truly magical number – it’s the total number of chromosomes that are found in each and every human cell*.

These 46 chromosomes carry the genetic information that’s passed from parent to child through heredity. It is the very detail of this genetic material – in the DNA – that makes most people (other than identical siblings) totally unique.

The total number of chromosomes in an organism, such as an animal or plant, is important and differs for different species. Some insects, for instance, only have one or two chromosomes. Meanwhile, giraffes have 62, chickens have 78, mice have 40, cabbages 18, and strawberries only 14. Humans, like many other species, are called ‘diploid’. This is because our chromosomes exist in matching pairs – with one chromosome of each pair being inherited from each biological parent.

Every cell in the human body contains 23 pairs of such chromosomes; our diploid number is therefore 46, our ‘haploid’ number 23. Of the 23 pairs, 22 are known as autosomes. The 23rd pair is made up of the sex chromosomes, called the ‘X’ and ‘Y’ chromosome. This is the pair of chromosomes that is responsible for ‘sex-linked’ medical conditions that pass through some families, such as the blood disorder haemophilia, which affects mainly males. Females have a pair of X chromosomes, males have an X and Y chromosome. 

The term ‘chromosome’ itself comes from the Greek for colour (chroma) and body (soma) and came about because scientists noticed that special dyes stained the chromosomes in a certain colourful way.

An individual chromosome, too small to be seen by the naked eye, is made up of a single molecule of double-stranded DNA (deoxyribonucleic acid) and protein. These long molecules of DNA are ‘coiled’ up around proteins called histones. Uncoiled and placed end to end, the DNA molecules from just one cell would be as long as six feet – yet once packaged up into chromosomes they can fit into the cell’s nucleus.

Together, the genetic material contained in an individual’s chromosomes forms the ‘genome’, with specific sections of DNA being called ‘genes’.

As mentioned above, the DNA molecule is formed in the shape of a double helix, similar in shape to a spiralling twisted ladder. Each side of the helix consists of a sugar-phosphate backbone with a ‘nucleotide base’ forming the half-rung. In DNA there are four nucleotide bases – adenine, thymine, cytosine and guanine, abbreviated to A, T, C and G respectively.

These bases help form the double helix when two base pairs link together, with A only linking to T and C only linking to G. This ‘complementary’ linking of bases is particularly important during cell division (a process called mitosis), when the DNA double helix ‘unzips’ and a new strand of DNA is produced to form two double strands.

These bases are also important because they code for proteins of the body. Sets of three bases form ‘codons’ which code for particular amino acids (the building blocks of proteins). The sequence GGG, for instance, codes for the amino acid ‘glycine’ and the sequence GTC for ‘valine’.

Each gene has the coding information for a protein or polypeptide, or a sequence of another type of nucleic acid called RNA (ribonucleic acid). Thus, the human genome codes for all the proteins in the body. These proteins can have different functions in the body. For instance, some proteins such as keratin, which is found in hair and nails, are structural. Other proteins may serve as enzymes – molecules that help certain chemical reactions in the body. Yet others can play a role in passing messages from one cell to another (cell signalling molecules) or in protecting humans against disease (e.g. antibodies).

In 2003 scientists around the world announced the results of an immense collaborative scientific effort, the final sequencing of the entire human genome – a sequence of around 3 billion base pairs. This was a scientific breakthrough of immense importance.

The names of the volunteers whose genome was sequenced for the human genome project aren’t known, but the contribution of these volunteers was vital for the success of the project; without their input researchers would not have made the advancements in genetics that they have done. Thanks to these volunteers, scientists now understand more about human genetic information, genes – a field of study called genomics. Eventually, further genomic studies should help scientists to develop better diagnostic tools and treatments.

Studying the genetic sequence of different individuals allows scientists to work out which genes are responsible for how our cells work or which genes are associated with particular human features – such as some medical conditions and how people might respond to medicines. For instance, some individuals may have an abnormality in their genetic sequence resulting in a particular medical condition. Such conditions are called genetic disorders, examples of which are the blood disorders sickle cell disease and thalassemia. Other medical conditions, such as type II diabetes, might be associated with particular genes or genetic sequences. Knowing who has these may help get better and faster treatment. This is what is behind East London Genes & Health, and it’s why we’re looking for 100,000 volunteers. Just like the volunteers that helped the human genome project, these volunteers will be helping researchers to know more about genes and helping improve health.

 

*except sperm cells and egg cells in the ovaries which are "haploid" and have 23 chromosomes (with the 23rd being an X in the eggs, and either an X or a Y in the sperm), and not 46 "diploid" like other cells.

 

Further reading:

Wellcome Trust Sanger Institute (our scientific partner) has some fantastic videos, facts and debates:  http://www.yourgenome.org/

Deoxyribonucleic acid (DNA) – http://www.genome.gov/25520880

Chromosomes – http://www.genome.gov/26524120

Genes – http://ghr.nlm.nih.gov/handbook/basics/gene

Genetic disorders – http://www.nlm.nih.gov/medlineplus/geneticdisorders.html

All about the human genome project (HGP) – http://www.genome.gov/10001772#al-1

Genomics – http://www.genome.gov/18016863

Genome-wide association studies – http://www.genome.gov/20019523

Talking glossary of genetic terms – http://www.genome.gov/Glossary/

 

Written by Julie N Reza and David van Heel