The chains of histones are coiled in turn to form a solenoid, which is stabilised by the histone H1. The histone octamer at the centre of the nucleosome is formed from two units each of histones H2A, H2B, H3, and H4. A strand 150 to 200 nucleotides long is wrapped twice around a core of eight histone proteins to form a structure called a nucleosome. In eukaryotes, DNA molecules are tightly wound around proteins - called histone proteins - which provide structural support and play a role in controlling the activities of the genes. This DNA is tightly packed into structures called chromosomes, which consist of long chains of DNA and associated proteins. The nucleus of each cell in our bodies contains approximately 1.8 metres of DNA in total, although each strand is less than one millionth of a centimetre thick. The label eukaryote is taken from the Greek for 'true nucleus', and eukaryotes (all organisms except viruses, Eubacteria and Archaea) are defined by the possession of a nucleus and other membrane-bound cell organelles. These genes are known, collectively, as the human genome. Read more about gene expression and regulationĪ human being has 20,000 to 25,000 genes located on 46 chromosomes (23 pairs). regulatory sequences, which play a role in determining when and where the protein is made (and how much is made).non-coding regions, called introns, which do not specify amino acids.coding regions, called exons, which specify a sequence of amino acids.Genes consist of three types of nucleotide sequence: It consists of a specific sequence of nucleotides at a given position on a given chromosome that codes for a specific protein (or, in some cases, an RNA molecule). The gene is the basic physical and functional unit of heredity. The process is catalysed by the enzyme DNA polymerase, and includes a proofreading mechanism. The enzyme helicase breaks the hydrogen bonds holding the two strands together, and both strands can then act as templates for the production of the opposite strand.
#Sugar phosphate backbone chromosome code
A codon table sets out how the triplet codons code for specific amino acids.For example, AGA and AGG both code for the amino acid arginine. Each amino acid can be coded for by more than one codon. All life on Earth uses the same code (with a few minor exceptions). There are a two points to note about the genetic code: So, for example, the codon AGC codes for the amino acid serine, and the codon ACC codes for the amino acid threonine.
A gene is a sequence of nucleotides along a DNA strand - with 'start' and 'stop' codons and other regulatory elements - that specifies a sequence of amino acids that are linked together to form a protein. Individual codons code for specific amino acids. The four-letter 'alphabet' of A, T, G and C forms 'words' of three letters called codons. It is the order of the bases along a single strand that constitutes the genetic code. Second, it can store information in the linear sequence of the nucleotides along each strand. First, it can be copied or 'replicated', as each strand can act as a template for the generation of the complementary strand. This structure explains two of the most important properties of the molecule.
The double helix of the complete DNA molecule resembles a spiral staircase, with two sugar phosphate backbones and the paired bases in the centre of the helix. The bases link across the two strands in a specific manner using hydrogen bonds: cytosine (C) pairs with guanine (G), and adenine (A) pairs with thymine (T). Each of the sugar groups in this sugar-phosphate backbone is linked to one of the four nitrogenous bases.ĭNA's ability to store - and transmit - information lies in the fact that it consists of two polynucleotide strands that twist around each other to form a double-stranded helix. The backbone of the polynucleotide is a chain of sugar and phosphate molecules. a five-carbon sugar molecule (deoxyribose in the case of DNA).a nitrogenous base: cytosine (C), guanine (G), adenine (A) or thymine (T).It belongs to a class of molecules called the nucleic acids, which are polynucleotides - that is, long chains of nucleotides.Įach nucleotide consists of three components: DNA (or deoxyribonucleic acid) is the molecule that carries the genetic information in all cellular forms of life and some viruses.
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