Genetics
A Synopsis
The outstanding character of all living organisms – from viruses and bacteria to elephants and huge trees – is that they contain genetic material that enables them to multiply. This genetic material is the blueprint for all characters that are passed on to the offspring. The following is a brief summary of the processes involved.
Genetic material
The genetic material of all organisms on Earth is nuclein acids. They form long chains of which the active constituents are linked molecules known as nitrogen bases. There are only four different nitrogen bases in every type of nuclein acid – but by occurring in different sequences, they form an infinite variation of codes.
The two types of genetic material are ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). In organisms with cells – therefore all known organisms with the exception of viruses -DNA forms the blueprint for new body cells and sex cells, while RNA acts as “messengers” in the cell and is involved in the formation of proteins. Viruses, however, have only either RNA or DNA.
Procaryotes (bacteria and proto bacteria) have no separate cell nuclei, the DNA forming a single circle. In eucaryotes (organisms with cell nuclei) the genetic material is more or less arranged in X-shaped structures called chromosomes.
Eucaryote cells also contain organelles (mini-organs) called mitochondria and – in plants – plastids with their own genetic material. They are circular, indicating that the organelles descend from free-living prokaryotes. The heredity units on the chromosomes are called genes.
Chromosome numbers and ploidy
Each eucaryote organism has a fixed number of chromosomes, although this may differ in the sexes of some species. In man this number is 46 – but it can vary from as little as one in certain male ants to more than a thousand in some ferns.
Human chromosomes occur in two sets, which means that for each of the 23 chromosomes in a set, there is one in the other set with more or less the same composition. The exception is the sex chromosomes, of which humans have one pair.
Females have two identical sex chromosomes, called X chromosomes, but men have only one X chromosome and a much smaller so-called Y chromosome. This form of sex determination occurs in nearly all mammals and many other organisms. The number of chromosomes per set is abbreviated to n. In the case of man, it is written as n=23 and 2n=46 in its abbreviated form.
Organisms with two sets of chromosomes are called diploid (noun as well as adjective), those with only one haploid and those with more than two poliploid. Poliploids can be triploid (3n), tetraploid (4n), and so forth.
Most organisms are diploids. Haploids are extremely vulnerable because they do not have an extra copy should anything go wrong with a chromosome. In honeybees and many related insects of the order Hymenoptera, males – which are relatively unimportant in this species – are haploid.
Poliploidy may cause problems with, among others, organ formation and propagation and is rare in animals. It is reasonably common in plants that have fewer specialised organs, a less specific build and usually no special sex chromosomes. Unequal ploidy tends to cause more sterility than equal ploidy. For example, bananas, which are triploid, very seldom contain seeds – but hexaploid (6n) wheat is fertile.
When sets are complete, they are referred to as euploids. Aneuploidy arises when there are one or more chromosomes too few or too many. This usually has a far-reaching effect on the organism and is often fatal.
In man only a few types of aneuploids are viable. The best-known form of aneuploidy is Down Syndrome, which is caused by an additional chromosome 21.
Cell division
Cell division is essential for growth and renewal, as well as for the formation of sex cells. Two different types of cell division occur.
The division of body cells is known as mitosis. During this division the number of chromosomes remain constant and the original cell divides into two, each a replica of the original cell. This is possible because each chromosome “zips open” and divides into two.
While the process is taking place, the removed part is substituted piece by piece out of building substances in the cell and linked together. These replicated, identical chromosomes of each pair move to opposite poles of the cell and constriction takes place between them.
Another process is necessary for the formation of sex cells as they are to be fertilised: the chromosome number must be halved so that the new cell that is formed after fertilisation will have the original number of chromosomes. This takes place through meiosis, or reduction division.
Here the chromosomes separate from each pair and temporarily attach to each other. The chromosomes exchange genetic material before they move to opposite poles. The difference between mitosis and this stage of meiosis is that the chromosomes do not divide.
The two cells that are formed each have only half of the normal chromosome number. A second division similar to mitosis now takes place: the four cells are forerunners of sperma, ovicells or other sex cells. The number of chromosomes in a sex cell is known as x. (Compare this with n above – although x and n are often the same for a certain organism, the concepts are not the same.)
Traditional applications of genetics
Man has realised for many centuries that characters are transferred from one generation to the next – and this knowledge has been used long before the nature of genes was known. Thousands of years ago farmers all over the world realised that they could improve the quality of their field crops by selecting and sowing only the best seed, while in the choice of marriage partners family history of serious diseases was taken into account.
More recently it became possible, among others:
To identify the carriers of certain genetic diseases and to determine through prenatal tests whether women in high-risk groups were expecting affected babies.
To cure certain genetic diseases, for example through organ transplants, and to control others with treatment.
To exclude paternity through blood tests in many cases. More recently, tests were developed to prove paternity.
To determine the sex of babies before birth.
To breed many new garden and agriculture plants and animals.
New and future applications of genetics
During the past decade or two tremendous development took place in the area of genetic research. It not only became possible to study genes close-up, but also to manipulate them and to change the genetic composition of organisms. These techniques are often controversial.
Some people have ethical and religious problems with tampering with the blueprint for life. Others foresee serious practical consequences should manipulated organisms get out of control. It is also feared that only the privileged will have access to the new technology products, and that the gap between rich and poor will further increase.
On the other hand, proponents of biotechnology are of the opinion that genetic manipulation has many possibilities for the treatment of diseases, the development of agricultural supercrops that will help alleviate famine, and the limitation of the use of insecticides.
Of the latest developments are:
The Human Genome Project (see also article by this name), whereby almost the entire genetic code of man has been determined. Deciphering of the code is at present done internationally and researchers hope to be able to eventually determine precisely where every gene is located on every chromosome.
It might soon be possible to analyse the genetic composition of an embryo and to discover what the baby developing from it will look like, as well as to which diseases he or she will be susceptible. In future parents will probably be able to choose which of various embryos they wish to have implanted and doctors will be able to correct genetic abnormalities early in pregnancy.
It will even be possible to compile an individual health programme for every person that would be ideal for his or her specific genetic profile. The emphasis can be shifted to the prevention rather than the treatment of serious diseases such as cancer and heart diseases.
Genetic manipulation of agricultural crops. Instead of only relying on cross-breeding to improve crops, genes from other species are transferred to improve yields or to render plants more resistant to insect pests.
Cloning. After a sheep, Dolly, was the first mammal to be cloned in 1997 from the cell of a dead animal, a variety of other clones have been created. Cloning can be used to create whole herds of exceptionally superior animals, to increase stocks of endangered species and even to cultivate organs for transplantation.
Research on the relationship between organisms. By studying certain genes that occur in a variety of organisms, the relationship between different forms of life can be determined. This does not only make the classification of living organisms much more accurate than in the past, but should also provide important information on how certain structures, tissues and organs have developed through evolution.