Genome



The word Genome is used in reference to the total genetic composition of an organism or a cell. Every organism has a genome that contains all of the biological information needed to build itself and maintain life processes. The biological information contained in a genome is encoded in its deoxyribonucleic acid (DNA) and is divided into discrete units called genes. When activated the genes send out instructions specifying cellular machinery to assemble proteins. This process called gene expression is needed to build and maintain all biological systems.

The term genome can include both DNA and RNA, and the chromosomes within the nucleus as well as the DNA in mitochondria or chloroplasts. In eukaryotes the nuclear genome refers to the information content contained in a haploid set of chromosomes, and its size is generally given as its total number of base pairs. For example, The normal human genome consists of 3 billion base pairs in 24 chromosomes (22 autosomes and an X or Y sex chromosome). In contrast, the genome of a bacteria is contained within a single chromosome. The viral genome can be either DNA or RNA.

Deoxyribonucleic Acid

 * Main Article Deoxyribonucleic Acid

In humans, as in other higher organisms, a Deoxyribonucleic Acid (DNA) molecule consists of two strands that wrap around each other to resemble a twisted ladder whose sides, made of sugar and phosphate molecules, are connected by rungs of nitrogen- containing chemicals called bases. Each strand is a linear arrangement of repeating similar units called nucleotides, which are each composed of one sugar, one phosphate, and a nitrogenous base. Four different bases are present in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). The particular order of the bases arranged along the sugar- phosphate backbone is called the DNA sequence; the sequence specifies the exact genetic instructions required to create a particular organism with its own unique traits.

The two DNA strands are held together by weak hydrogen bonds between the bases on each strand, forming base pairs (bp). Genome size is usually stated as the total number of base pairs; the human genome contains roughly 3 billion bp.

Each time a cell divides into two daughter cells, its full genome is duplicated; for humans and other complex organisms, this duplication occurs in the nucleus. During cell division the DNA molecule unwinds and the weak bonds between the base pairs break, allowing the strands to separate. Each strand directs the synthesis of a complementary new strand, with free nucleotides matching up with their complementary bases on each of the separated strands. Strict base- pairing rules are adhered to adenine will pair only with thymine (an A- T pair) and cytosine with guanine (a C- G pair). Each daughter cell receives one old and one new DNA strand. The cells adherence to these base- pairing rules ensures that the new strand is an exact copy of the old one. This minimizes the incidence of errors (mutations) that may greatly affect the resulting organism or its offspring.

Organellar Genome
Not all genetic information is found in nuclear DNA. Both plants and animals have an organelle (a "little organ" within the cell) called the mitochondria. Each mitochondrion has its own set of genes. Plants also have a second organelle, the chloroplast, which also has its own DNA. Cells often have multiple mitochondria, particularly cells requiring lots of energy, such as active muscle cells. This is because mitochondria are responsible for converting the energy stored in macromolecules into a form usable by the cell, namely, the adenosine triphosphate (ATP) molecule. Thus, they are often referred to as the power generators of the cell.

Unlike nuclear DNA (the DNA found within the nucleus of a cell), half of which comes from our mother and half from our father, mitochondrial DNA is only inherited from our mother. This is because mitochondria are only found in the female gametes or "eggs" of sexually reproducing animals, not in the male gamete, or sperm. Mitochondrial DNA also does not recombine; there is no shuffling of genes from one generation to the other, as there is with nuclear genes.