Genes and Chromosomes; Part 1 - an Introduction
This article, the first in a four-part series, introduces genes and chromosomes.
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Yamni Nigam, PhD, MSc, BSc, is lecturer in biomedical science; Alyson Bexfield, BSc, PhD, is senior research assistant; both at the School of Health Science, Swansea University.
Like the vast majority of living creatures, humans are formed when their father’s sperm meets their mother’s egg. These two ‘half’ cells (gametes) join (fertilisation) and form one complete cell, the zygote. This grows and divides over the following nine months, differentiating into different tissues and organs, to form a whole human being.
How does this zygote grow and develop into a unique and whole human being? How does it know what to do, which proteins to make inside the cells, what shape to make the nose and what colour to make the hair? It is all down to genes.
Genes and chromosomes
Information carried by the egg cell and sperm cell, in the form of genetic material, unite when the two gametes fuse at fertilisation. Genetic material resides in the nucleus of every human cell (with the exception of red blood cells, which have no nuclei). It dictates the function of each cell and the production of all the proteins the body requires, for example, to grow, to fight infection and even to behave.
The nucleus of each cell contains long threads known as chromosomes. These are made of protein and the chemical deoxyribonucleic acid (DNA). Periodically, along the chromosomes, a small patch of DNA containing specific instructions about how to make a particular protein occurs. Such a coded area of information is called a gene. The region of the chromosome on which a particular gene is located is known as its locus (Montague et al, 2008).
Each cell in the body contains an exact copy of all the many genes and, with the exception of identical twins, their combination is unique to the individual.
The genes carry information on how to make different proteins that determine features such as physical appearance, health and metabolism, although not all genes are needed by every cell. Just a few will be ‘switched on’ in any one cell at any one time. For example, the gene that determines hair colour is found in every cell but will only have an effect in cells where hair grows, such as the scalp. In all other cells it will be ‘switched off’ and inactive (Jones and Jones, 1997). Each living organism has its own set of genes, the total number of which is known as its genome.
Normal human body cells (somatic cells) have a total of 46 chromosomes, all containing many genes. The chromosome complement is called the karyotype and represents the total number, size and shape of all chromosomes within a cell. The 46 chromosomes present in the nucleus exist as 23 pairs (Fig 1). When chromosomes are present in pairs like this, the cell is termed diploid.
Before fertilisation, both gametes undergo a process called meiosis (to be discussed in article 2 next week), in which the numbers of chromosomes are halved. Gametes therefore have 23 chromosomes and are termed haploid. Upon fertilisation, the fusion of these two gametes produces a diploid zygote, with the normal 46 chromosomes. One chromosome in each pair therefore comes from each parent.
Genotype and phenotype
The genotype is the term given to the inheritable, internal genetic code of an individual cell. The information contained within it determines the phenotype, the outward, physical manifestation of the genotype, such as the generation of a particular protein, eye colour or nose shape.
The two chromosomes in each pair are generally homologous - matched in size and shape. However, the 23rd pair looks different in the cell of a normal female compared with that of a normal male (Fig 1). This is because these are the sex chromosomes, which determine the sex of the individual.
Females’ sex chromosomes are both alike and are called X chromosomes - one comes from each parent. Males have one X (from the mother), and another smaller one, known as a Y chromosome, which comes from the father (Fig 2).
While the 22 pairs of non-sex chromosomes - known as autosomal chromosomes (or autosomes) - contain genetic information guiding a range of characteristics, they have no effect on the sex of the individual. This is determined purely by the sex chromosomes.
Although normal karyotypes contain 22 pairs of autosomes and two sex chromosomes, some individuals have missing or additional chromosomes. Such changes to the karyotype are accompanied by developmental anomalies. For example, Down’s syndrome is caused by having three copies, or extra parts, of chromosome 21, and results in an individual with certain characteristics including mild to moderate learning disabilities. Extra sex chromosomes may also give rise to anomalies, for example Klinefelter syndrome results from an extra X chromosomes in males (XXY), and gives rise to shy, often sterile men with a higher incidence of dyslexia and speech problems. In Turner syndrome, there is only one sex chromosome (X instead of XX or XY). Individuals are female but with underdeveloped sexual characteristics, often of short stature, with a ‘caved-in’ chest appearance.
The structure of DNA
As mentioned earlier, chromosomes are made up of proteins and DNA. This nucleic acid is a huge molecule, made up of building blocks called nucleotides, which join together to form polynucleotide chains (Turnpenny and Ellard, 2007). Each nucleotide is composed of three parts - a phosphate molecule, a pentose (five-ringed) sugar (deoxyribose), and a nitrogenous base (Fig 3).
Four nitrogenous bases occur in DNA - adenine (A), guanine (G), cytosine (C) and thymine (T), after which the four nucleotides are named. DNA exists as two polynucleotide strands, each coiled into a helix. The two strands are joined together by hydrogen bonds between the nitrogenous bases of opposite nucleotides.
The bonding of the nitrogenous bases within DNA is very specific. Adenine always bonds with thymine, while guanine always bonds with cytosine. Therefore, two strands forming a DNA molecule are not identical - they are complementary. The specific bonding pattern explains how exact copies of a DNA molecule can be made every time a cell replicates (this will be discussed in the second article in this series).
A chromosome consists of a single, long DNA helix that contains many genes. The majority of the DNA has no currently recognised function, although our understanding of this may change as scientific knowledge progresses. However, only certain regions of DNA - the genes - contain special information coding for the expression and production of vital cellular proteins.
The human genome project
In 1990 the human genome project was initiated to discover the number, size and exact location of genes on each chromosome in human cells. The project was completed in 2003 and gave us access to a great deal of information about our genes. For example, there are around 22,000 genes in every human cell (Gregory et al, 2006), and chromosome 1 is the largest of our 23 pairs of chromosomes. It contains 3,141 genes and is one of the most medically important chromosomes. Over 350 diseases, including cancers and neurological disorders, are associated with disruptions in its sequence.
Gregory, S.G. et al (2006) The DNA sequence and biological annotation of human chromosome 1. Nature; 441: 315-321.
Jones, M., Jones, G. (1997) Advanced Biology. Cambridge: Cambridge University Press.
Montague, S.M. et al (2008) Physiology for Nursing Practice (3rd ed). New York, NY: Elsevier.
Turnpenny, P., Ellard, S. (2007) Emery’s Elements of Medical Genetics (13th ed). London: Elsevier/Churchill Livingstone.