Genetic Concepts  

DNA
Information about our DNA, hereditary material that makes us to who we are

Chromosomes
Information about our chromosomes, long wires DNA, on which all the genes are located

Genes
Information about our genes, separate units of DNA, which each have their own function

Mutations
nformation about mutations, mistakes in DNA, which are responsible for hereditary diseases


DNA

Our hereditary material is made of a chemical substance called Deoxy Nucleic Acid (abbreviated: DNA).
This DNA actually is a long chain of four different nucleotides: adenine (A), cytosine (C), guanine (G) and thymine (T). The DNA chain is very long and contains approximately 3 billion of these nucleotides. The DNA chain is made out of 46 different DNA chains, which are called chromosomes. These chromosomes harbour all the different genes, which is a separate unit of DNA with a specific function. Man has approximately 25,000 genes. Most of these genes are responsible for the production of a specific protein. Changes in DNA are called mutations. These mutations can be the cause of hereditary diseases.



Chromosomes

In each of our cells there exists a nucleus which contains the different chromosomes.
These chromosomes are long chains consisting of hereditary material (DNA).



Chromosomes can be classified according to size and form, and numbered of large to small. These classified chromosomes form a karyotype. In each cell we have 2 copies of each chromosome, of which one originates from our father and one from our mother. So there are 23 chromosome pairs, and the total sum of chromosomes is 46. In the germline (egg from the woman and sperm from the man), however,  there are 23 chromosomes, and only one copy of each chromosome pairs is present. When the 23 chromosomes from the egg meet with the 23 chromosomes from sperm cell, a fertilised egg or zygote with 46 chromosomes arises. From this zygote the child will develop. 44 of the 46 chromosomes (22 pairs) are present in both women and men : these chromosomes called autosomes. The remaining 2 chromosomes are called sex chromosomes. There are 2 sort of sex chromosomes : the X and the Y chromosome.
A woman has two X-chromosomes and no Y chromosome, because she gets an X chromosome from both her father and her mother. A man has two different sex chromosomes: an X chromosome that he inherits from his, and a Y chromosome  which comes from its father. A man can make therefore both sperm cells warrants with an X chromosome, and an Y chromosome. A woman has only eggs with an X chromosome. If a sperm cell fertilising the egg (which always contain an X chromosome) contains an Y chromosome the resulting child will be a boy (XY). If a sperm cell contains an X chromosome, a girl (XX) will develop.
Because we have 2 copies of each autosome, we have 2 copies of each autosomal genes. Genes which lie on the X chromosome have 1 copy in a man and 2 in a woman. Genes on the Y chromosome are present in 1 copy in men, whereas women do not have these genes.



Genes

The chromosomes exist from separate units of DNA called genes, which all have their own function. We have approximately 25,000 genes in each cell. Most of these genes have been discovered during the Human Genome Project, a very important scientific project in which research workers from the all over the world took part in order to decipher the DNA code. A large part of this project has already been completed, and  most of our genes have been identified. Geneticists are now trying to elucidate the function of all these genes. Most genes encode one or more specific proteins, and each of these proteins has its own function in cell. These proteins determine all our characteristics, such as how we look (the colour of our eyes, etc), but also all genetic disease.



Mutations

A change in DNA is called a mutation. Because genes are responsible for the production of a protein, a  mutation can lead to a change of the form or function of that protein, or to a reduced or increased production of this protein. In some cases this causes a hereditary disease. Mutations can arise by several mechanisms, but they mainly arise by mistakes in the copying of  DNA during the billion cell divisions. In view of the number of cell divisions it is not surprising that there arise mutations. Some environmental factors such as radiation or medication such as chemotherapy can also cause mutations. Mutations can be transmitted from parent to child. This explains why some diseases are hereditary. In the genetic laboratory one can study DNA to look for mutations. Of course one can not study all genes at once, but only that gene (s) that is taught to carry a mutation. Even the study of a single gene can be labour-intensive because the mutation can be located everywhere in that gene. In a lot of cases one must therefore study the whole gene which exists out of thousands of nucleotides in order to identify the mutation. This explains why many genetic tests are expensive and have a long turnaround time.






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