The Gene

The human genome contains around 100.000 genes, distributed between the 23 pairs of chromosomes in a cell.
The gene consists of three major structures:

1) The gene regulatory segment - contains structures involved in the initiation and regulation of transcription.

2) Exons -the protein coding part of the gene.

3) Introns -the non-coding part of the gene.

The basic idea is that a gene serves as a recipe for a particular protein or in some cases RNA. This is true in most cases, although to make some proteins you need more than one recipe. Like when you bake a cheesecake, you need one recipe for the cake and one for the glazing.
One example of such a protein is hemoglobin, the oxygen-transporter in our bodies, which is the product of two different genes.
Different genes are active in different types of cells - a liver cell does not express the same genes as a brain cell for example. This in turn means that different cell types, depending on what function they have, produce different sets of proteins. Some proteins though are vital for the survival of a cell, and their corresponding genes are therefore active in all cell types, so called "housekeeping genes".

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