DNA, genes and chromosones
An introduction to DNA, genes and chromosones - learn how it all starts.
Genes contain the blueprints for life
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
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
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
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
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
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
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.