The Secrets of Life (1) - Chromosomes, DNA and Genes

Our bodies are made up of millions of cells. Inside the nucleus of each cell, there are chromosomes (染色體). An ordinary human has 46 chromosomes in each cell made up of 23 pairs. The first 44 chromosomes (called autosomes - 常染色體) are numbered 1 to 22 according to size from the largest to smallest. The last 2 chromosomes (called sex chromosomes - 性染色體) could only be either X or Y chromosomes. A female has 2 X chromosomes whereas a male has one X and one Y chromosomes. Hence, a ordinary female's chromosomes are described as 46,XX and male as 46,XY. Usually, a mother passes 23 chromosomes (one from each pair) to her child through her egg and a father likewise passes 23 chromosomes through his sperm. When the egg and the sperm join together at the time of conception, they form the first cell of the child which now has 46 chromosomes as the parents. Hence, one half of the chromosomes are inherited from the mother and the other half from the father. The cell continues to multiply (duplicated from the first cell) as the child continues to grow, resulting in exactly 46 chromosomes in every cell of the body.

Chromosomes, when stretched out, can be as long as 12 centimetres. All of the 46 chromosomes together is about 2 metres long, yet coiled and fit into a single cell nucleus which is about 2 micrometres (0.00002 metres). Each chromosome has a short (p) arm and a long (q) arm separated by a narrow constriction called centromere. Under the microscope and with special stains, the chromosome can be seen as a series of alternate light and dark bands. The bands are labeled p1, p2, p3, q1, q2, q3, etc., counting from the centromere out toward the ends. At higher resolutions, sub-bands (also numbered from the centromere out towards the end) can be seen within the bands. At even higher resolutions, sub-sub-bands can also be seen. The first digit in the band label refers to the band, the second digit the sub-band and the digit after the period the sub-sub-band. For example, a band labelled 16 p13.1 refers to chromosome 16, p-arm, band 1, sub-band 3 and sub-sub-band 1 accordingly.

Chromosomes are made up of material called deoxyribonucleic acid (DNA-脫氧核糖核酸). Thus, each chromosome is a DNA molecule. The DNA molecule is made up of 2 very long chains of 4 chemical units: Adenine (A), Guanine (G), Thymine (T) and Cytosine (C). These 4 chemical units form the bases (as 'letters') of the genetic codes which contain all the information for human growth and development. The bases are arranged in pairs such that A is only paired with T, and G only with C. The bases are further grouped into triplets (e.g. TGA, CTG, ACT etc.) to form '3-lettered words'. Each triplet gives instruction for the cell to produce a particular type of amino acid. A sequence of triplets (e.g. TGACTGACTGTCATG ..... ) enables the cell to assemble the amino acids in the correct order for the cell to function. A complete sequence of triplets that contains particular instructions to perform a particular function is called a gene (基因). Hence, a gene is a basic unit that carries the genetic information for a particular function for human growth and development. Some genes may be responsible for producing certain type of cells like blood, hair or lung etc. Others may be responsible for producing materials with personal characteristics like colour of eye and skin, height or shape of the nose etc. Genes are labelled by their functions. (e.g. gene HBA1, haemoglobin alpha 1 located in chromosome 16, is responsible for producing a protein necessary for red blood cells.)

There are hundreds and thousands of genes in a chromosome, and it is estimated that there are about 25,000 - 35,000 genes in a single human cell. The number of chemical bases (letters A, T, G or C) in a gene varies as dependent on the function of the gene, but may range from 1, 000 to several hundred thousand. In total, there are about 3.1 billion bases (letters) in a single human cell. The arrangement of these 3.1 billion bases is very specific and unique for each individual, and hence the DNA sequence in the cell can be used like a fingerprint for individual identification. When there are changes in the chromosomes or genes (e.g. due errors during the conception of the egg and sperm or external factors like radiation etc.), the normal sequence of genetic codes in the genes (and thus the chromosomes) will be disturbed. Hence, the normal function to produce the necessary materials for the cell to develop will be affected. As result, problems in health and development may occur.

As of today, our knowledge in genes is still quite incomplete (including the exact number of genes and functions of all the genes). But with our increasing knowledge in the secrets of the genetic codes in the genes (and hence the DNA sequence), we can understand more and more about life including how we inherit our personal characteristics from the our parents, the process of aging and deceases and what problems we could expect on health and development. These can well be determined even before birth of the child by appropriate prenatal testings .....................