DNA, genes and chromosones

An introduction to DNA, genes and chromosones - learn how it all starts.

Genes contain the blueprints for life

All living organisms contain genes. The traits of a human being, for example, are based on the expression of approximately 80,000 different genes. The unique composition of these genes is based on our inheritance from our parents.
There are some basic rules in biology. One is that ducks produce ducklings, not calves or lion cubs, just as cats produce kittens and dogs produce puppies. Both animals and humans also tend to produce offspring that look much the same as themselves. Short people tend to get short kids and tall people tend to get tall kids, tend to, however, being the operative words: there are many examples to the contrary. Tall people sometimes get short kids. For centuries scientists have tried to discover why offspring look much like their parents but do not resemble them 100 per cent. We now know that genes, passed from parents to offspring, determine all hereditary characteristics such as the colour of hair, eyes or flower petals.

Genes reside in the chromosomes of our cells

The genes that are inherited from parents contain all the biological instructions for building a human being. Nearly every cell in our bodies has its own set of these instructions.However, not all of the genes in each cell function all of the time. In specialized cells, such as nerves or skin, only genes that hold instructions for these types of cell are expressed.
The genes are placed one after another in long strings which are organized and bundled in rod-like structures called chromosomes. The chromosomes are housed in the nucleus of the cell. Every human being has 23 pairs of chromosomes, one set from each parent.
Microorganisms like bacteria or fungi also have genes. They have fewer than humans, though, and most often the genes are all placed in one circular chromosome that flows freely in the cell.

Genes are made of DNA

The 46 chromosomes in our cells are made of the chemical substance called deoxyribonucleic acid or DNA. There is a lot of DNA in each cell. If unwound, the DNA in one cell would be almost two metres long.
DNA is the starting point for every part of our body. But the DNA only stores the information that is needed to create an organism. In the body, the genes are transcribed into proteins. Proteins are the building blocks of both the cells and the enzymes that construct and maintain the body.
Not all the DNA in our bodies contains genes. Actually, most of the DNA in our bodies is so-called junk DNA. It doesn't code for any proteins and is considered by many scientists to be nothing more than waste from our evolutionary past.

DNA is a helical structure held together by four bases

The structure of DNA (deoxyribonucleic acid) is astonishingly simple. The DNA molecule is made of two long strings of sugar "backbones" held together by pairs of so-called bases, just like a ladder. The entire molecule is twisted around its own axis, creating a double helix.
Only four different kinds of base are included in the base pairs that hold together the helix. The four bases are called A, T, G and C according to the first letter of their scientific names: adenine, thymine, guanine and cytosine. Each base complements another base's chemical properties so that A always pairs up with T and G always pairs up with C.
The four chemical letters are placed in long rows along the DNA's "backbones". A specific sequence of letters, or bases, makes up a word, or gene. Each gene can be from a few hundred to many thousands of base pairs long.

Genes are transcribed into proteins via RNA

Every gene in the DNA codes for one protein. The unique order of the base pairs in each gene specifies the sequence of amino acids in the long string of amino acids from which the corresponding protein is made.
The DNA, however, does not code for the protein directly. It uses an intermediate called RNA. When the cell needs a new protein, it starts by making an RNA copy of the gene from the DNA. The copy is made by "unzipping" the two sugar "backbones" of the DNA. After that, the sequence of bases is re-created using one of the DNA strands as a template. The RNA molecule is thus a "mirror image" of the sequence of bases on one of the DNA strands.
When the RNA copy is ready, it binds itself to small molecules called ribosomes. The ribosomes read the code in the bases and translate it into the chain of amino acids that constitutes a protein.

Three bases in a gene correspond to one amino acid in the protein

Small molecules called ribosomes translate the bases in the RNA copy of the gene into proteins. The ribosome attaches itself to the end of the RNA and slowly moves along the string, reading the sequence made of the four different bases: C, T, G and C.
Three bases in a row specify one amino acid. For example, the base combination ACT codes for a threonine amino acid, while GAT codes for an aspartic acid. Every time the ribosome reads three bases it sticks the corresponding amino acid to the one it found when reading the previous three bases. Slowly the ribosome makes a long string of amino acids which it carries behind it.
There are 64 possible ways you can put four different bases in a row of three. As there are only 20 different kinds of amino acids, some combinations of base code for the same amino acid. Other combinations function as a stop sign. When the ribosome reads that specific combination, it releases the string of amino acids into the cell.

DNA is copied when cells divide

The genetic information in an organism's cells originates from the parents. An egg or sperm cell contains half the genes of the person who produces it. When the egg and sperm cells fuse in a mother's uterus, the two combine into a complete set of chromosomes and hence genes.
After fertilization, the egg immediately starts dividing. Just before each division, the cell copies the entire set of genes. This is done by "unzipping" the original DNA string, separating the base pairs that hold the two sugar "backbones" in the DNA together. As soon as the two strands are separated, new base pairs which complement the separated bases are put in place. In the end, there are two identical molecules, each with one old and one new strand. Once the DNA is copied, one set goes into each new cell.
Many simple organisms like bacteria and primitive fungi consist of only one cell. Dividing the cell and the DNA is their way of producing new offspring.

New genes are produced by natural mutations

The copying of DNA is an ongoing process throughout life. Every time a cell needs to be replaced, a new one is formed from the existing tissue. Even though the cells have special mechanisms to prevent mistakes, they do sometimes happen. Such mistakes are called mutations, and they occur all the time. The most common mutations to occur when cells copy their DNA are when one set of base pairs replaces another. Other mutations are more complex. Sometimes long sequences of base pairs are missing or are wrongly placed. Even entire chromosomes can be turned around or placed in the wrong position.
Some mutations are fatal or lead to diseases such as cancer, whilst others go undetected. The latter are the raw material of evolution. When the same genes in two different organisms are not completely alike, one may work better in a particular situation than the other. When the environment changes, the particular composition of gene variants in one organism may lead to survival while other die. This is how new genes evolve from old ones and new species evolve from others doomed to destruction.