STRUCTURE OF DNA

DNA stands for deoxyribonucleic acid. It is a type of nucleic acid, one of four major types of macromolecules which are essential for life, along with proteins, lipids, and polysaccharides. DNA is a double helix, made up of two single DNA strands. Each DNA strand is in turn composed of many monomers called nucleotides – specifically deoxyribonucleotides. As a result, each strand is a polynucleotide.
A nucleotide has three components: a nitrogenous base, a phosphate group, and a 5-carbon sugar, which for DNA nucleotides is deoxyribose.
The sugar and phosphate groups alternate to form DNA’s “backbone”. The phosphates act like connectors between the sugars by forming phosphodiester bonds between the 3rd C of one sugar ring and the 5th C of the next sugar ring. These special Cs are termed the 3’ and 5’ Cs. As a result, there is a directionality to this backbone. Each strand has a 5’ end with a terminal phosphate group, and a 3’ end with a terminal hydroxyl group.
There are four nitrogenous bases: adenine (A), guanine (G), thymine (T), and cytosine (C). Each of these attaches to deoxyribose via a glycosidic bond. The sequence of these four bases is the means by which DNA encodes genetic information, which can then be used to carry out all the cell’s functions. The central dogma of biology describes how this genetic information is accessed and used by the cell: DNA is transcribed into RNA, which is then translated into proteins.
The four nitrogenous bases can be divided into two groups – cytosine and thymine are pyrimidines, while adenine and guanine are purines. You can use this mnemonic to remember that: Climbing the pyramid, George avoided two pursuers.
The two single strands of DNA composing the DNA double helix are held together via hydrogen bonds, and they run antiparallel; that is, in opposite directions. One strand is 3’-5’, while the other is 5’-3’. Each nitrogenous base on one strand binds to its complementary base pair on the other strand. Purines hydrogen bond to pyrimidines. Specifically, adenine binds only to thymine via two hydrogen bonds, and cytosine binds only to guanine via 3 hydrogen bonds.
Due to this complementary binding, each of the two DNA strands contains the exact SAME information. When the two strands are separated during DNA replication, each of them acts as a template which contains all the information needed for recreating the other. The two DNA strands can be separated, or “melted” in several ways – through mechanical force by enzymes, through high temperatures, or through extreme pH levels.
Going back to the central dogma of biology – DNA gets transcribed into RNA, which then gets translated into protein. However, not all sequences of DNA are actually meant to be transcribed into useful RNA sequences. Those sequences of DNA which are actually meant act as a template for transcription of RNA which then gets translated into functional proteins are called noncoding, or “antisense” sequences. The DNA which is complementary to an antisense sequence is termed a coding, or sense sequence.
Note that transcribed RNA is complementary to the noncoding sequence, and so it exactly matches the coding sequence. Note that both strands of DNA can have sense and antisense sequences. Furthermore, sense and antisense sequences can overlap; for example, there can be overlapping genes, where one protein is encoded on one strand and another is encoded on the opposite strand by the sequence read in the opposite direction.
Since transcription can only proceed in the 3’-5’ direction along the template DNA strand, the RNA being synthesized is always 5’-3’. Thus, a sense sequence - for example, CAAT - will never be transcribed, although if the reverse sequence TAAC is an antisense sequence, it can be transcribed.
The twisting of the double helix creates more interesting features in DNA. The grooves are not equally spaced. One of these grooves is called the major groove, while the other is called the minor groove.
In its relaxed state, each DNA strand makes one loop around the axis of the double helix once every 10.4 base pairs. If the DNA is being twisted tighter in the same direction, this is called “positive supercoiling”, while being twisted in the other direction is called negative supercoiling.
DNA is found in the nucleus in eukaryotes, and in the nucleoid in prokaryotes. In both eukaryotes and prokaryotes, the DNA is packaged into chromosomes, which are a combination of DNA and proteins. Chromosomes make the DNA more condensed and organized, and allow the cell to have controlled access to specific portions of DNA. Eukaryotes have linear chromosomes, while prokaryotes have circular ones. The genome is the complete set of chromosomes in a cell.
3D DNA models: https://3dprint.nih.gov/discover/3DPX-010790?fbclid=IwAR0zguuJv1KESiMDLynat5Z5zoISkz5awZQtlVCazcDVOuNokxvwyK4U2e0
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Видео STRUCTURE OF DNA канала Neural Academy