Introduction to DNA скачать в хорошем качестве

Introduction to DNA

Within a cell, the genetic material of an organism is packaged within the nucleus in long structures called chromosomes. A chromosome contains double-stranded molecules, known as DNA, which have a double-helix shape. Each of the strands of a DNA molecule are made up of small repeating units called nucleotides. Each nucleotide has three parts: a phosphate group, a sugar called deoxyribose, and one of four different nitrogenous bases. These bases are called adenine, guanine, cytosine, and thymine. The bases of each strand align along the center of the double helix, and bind to each other by hydrogen bonds. Only certain pairs of bases will bond with each other: adenine will only bond with thymine, and guanine will only bond with cytosine. The sequence of these four bases on the strands of DNA represent the genetic information within the cell. These sequences serve as templates, directing the cell to make proteins; the order of bases determining the type of protein produced. Protein synthesis occurs in two stages: transcription and translation. During transcription, specific enzymes within the nucleus 'read' the sequence of bases in the DNA template to produce an intermediate molecule called messenger RNA (mRNA), which has a complementary structure to the template. During translation, this mRNA molecule binds to a ribosome, and is read by other enzymes to produce a protein. DNA is also important in cell division. When a cell divides, it must duplicate all of its DNA. It does this by separating the two strands of the existing DNA molecules. Enzymes then use each strand as a template to produce two complete sets of each DNA molecule. This is called DNA replication.