Genomic organization

The hereditary material i.e. DNA (deoxyribonucleic acid) of an organism is composed of a sequence of four nucleotides in a specific pattern, which encodes information as a function of their order.

There is a more than the 300-fold difference between the genome sizes of yeast and mammals, but only a modest 4- to 5-fold increase in overall gene number (see the figure on the right).

[3] The expansion of repetitive elements (such as Alu sequences) has even infiltrated the transcriptional units of the mammalian genome.

This results in transcription units that are frequently much larger (30–200 kb), commonly containing multiple promoters and DNA repeats within untranslated introns.

[citation needed] The vast expansion of the genome with noncoding and repetitive DNA in higher eukaryotes implies more extensive epigenetic silencing mechanisms.

Genome sizes and corresponding composition of six major model organisms as pie charts. The increase in genome size correlates with the vast expansion of noncoding (i.e., intronic, intergenic, and interspersed repeat sequences) and repeat DNA (e.g., satellite, LINEs, short interspersed nuclear element (SINEs), DNA ( Alu sequence ), in red) sequences in more complex multicellular organisms. This expansion is accompanied by an increase in the number of epigenetic mechanisms (particularly repressive) that regulate the genome. Expansion of the genome also correlates with an increase in size and complexity of transcription units, except for plants. P = Promoter DNA element.