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The 23 human chromosome territories during prometaphase in fibroblast cells. The last level of organization concerns the distinct positioning of individual chromosomes within the nucleus. The region occupied by a chromosome is called a chromosome territory (CT). [43] Among eukaryotes, CTs have several common properties.
The human genome is a complete set of nucleic acid sequences for humans, encoded as the DNA within each of the 24 distinct chromosomes in the cell nucleus. A small DNA molecule is found within individual mitochondria. These are usually treated separately as the nuclear genome and the mitochondrial genome. [1]
In eukaryotes, DNA is located in the cell nucleus, with small amounts in mitochondria and chloroplasts. In prokaryotes, the DNA is held within an irregularly shaped body in the cytoplasm called the nucleoid. [97] The genetic information in a genome is held within genes, and the complete set of this information in an organism is called its genotype.
Each human cell contains roughly two meters of DNA. [1]: 405 During most of the cell cycle these are organized in a DNA-protein complex known as chromatin, and during cell division the chromatin can be seen to form the well-defined chromosomes familiar from a karyotype. A small fraction of the cell's genes are located instead in the mitochondria.
In the human genome, there are three important classes of TEs that make up more than 45% of the human DNA; these classes are The long interspersed nuclear elements (LINEs), The interspersed nuclear elements (SINEs), and endogenous retroviruses. These elements have a big potential to modify the genetic control in a host organism. [30]
Deoxyribonucleic acid (DNA) is a nucleic acid containing the genetic instructions used in the development and functioning of all known living organisms. The chemical DNA was discovered in 1869, but its role in genetic inheritance was not demonstrated until 1943. The DNA segments that carry this genetic information are called genes.
Nuclear DNA is known as the molecule of life and contains the genetic instructions for the development of all eukaryotic organisms. It is found in almost every cell in the human body, with exceptions such as red blood cells. Everyone has a unique genetic blueprint, even identical twins. [7]
Each level in the hierarchy represents an increase in organisational complexity, with each "object" being primarily composed of the previous level's basic unit. [2] The basic principle behind the organisation is the concept of emergence —the properties and functions found at a hierarchical level are not present and irrelevant at the lower levels.