Trong thời hạn gen cũng có phần của DNA hoạt động (exonic) và không hoạt động transcribable (intronic) cung cấp phức tạp hơn nữa trong quy định và cho phép một gen mã cho nhiều hơn một protein. DNA được đóng gói trong nhiễm sắc thể. Trong các sinh vật đa bào có hai bản sao của mỗi nhiễm sắc thể, | 12 Genes environment and their interactions . heterozygous both gene products maybe formed or the abnormal may inhibit the production of the normal. Between genes are large portions of DNA that do not actively code for proteins. Some parts of this are the site where molecules called transcription factors bind with DNA to signal to a gene whether it should be activated or not. Within genes there are also portions of active exonic and inactive intronic transcribable DNA that offer further complexity in regulation and allow one gene to code for more than one protein. DNA is packaged in chromosomes. In multi-cellular organisms there are two copies of each chromosome except the sex chromosomes in each mature cell and one copy in the gametes. In species that use chromosomes for sexual determination birds and mammals in one gender females in mammals males in birds the two sex chromosomes are identical and in the other gender there are two different sex chromosomes X and Y chromosomes in mammals in each non-gamete cell. There are only about 35 000 genes in the human genome - but 35 000 protein switches are not in themselves enough to regulate the complex range of body functions that need to be regulated particularly if they are just on off switches. The complexity is created in a number of ways. Firstly while there may be only 35 000 genes there are various mechanisms to switch on or off part of the alphabet within a gene generally called splicing variants so as to produce more than one peptide from a gene. Secondly transcription factors not only turn a gene on or off but can regulate the degree to which it is turned on . expressed or turned Thirdly after the protein is made the intracellular machinery may lead to enzymatic processing breaking it up into smaller bits or leading to addition or subtraction of phosphate groups which is a way in which molecules transfer energy or by addition of sugar moieties to the protein. These latter processes are called .
