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12. Life
| 12.18 |
A greater explanation of DNA |
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| Deoxyribonucleic acid (DNA) is the basic genetic material of most living organisms. Although a large and apparently complex molecule, the structure of DNA is in fact astonishingly simple. | ||||||
| A single DNA molecule consists of two separate strands wound around each other to form a double-helical (spiral) structure. Each strand is made up of a combination of just four chemical components known as nucleotides- all of which have the same basic composition. | ||||||
| Each nucleotide consists of a sugar molecule (deoxyribose) linked to a phosphate group to form the helical backbone; different nucleotides are distinguished only by the identity of the nitrogen-based unit called the nucleotide base bonded to the sugar molecule. | ||||||
| The four bases are: | ||||||
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| The bases lie in the central region of the double helix, with each base linked by hydrogen bonds to specific complementary base on the partner strand. | ||||||
| The base pair rule states that wherever you have an A on one strand, there will be a T at the same relative position on the other strand; wherever you have a G on one strand, you will have a C on the other strand. | ||||||
| In addition, the number of molecules of A in a sample always equals the number of molecules of T. Similarly, the number of C molecules always equals the number of molecules of G. | ||||||
| DNA is therefore basically a linear information macro molecules much like the long strips of computer tape used in the first computers. | ||||||
| A typical DNA base sequence might be: | ||||||
| 5'-AGCTTATTGCATAAGCGCGAT-3' | ||||||
| 5' and 3' These refer to the left-hand and the right-hand ends respectively of a DNA or RNA base sequence. | ||||||
| 12.10.1 | The genetic code | |||||
| The word genetic code and "genes" refer to lengths of DNA bases, sometimes 100 to 1000 bases long. The words also stand for the current understanding of DNA as being a chemical code for the storage of information on the production of Amino Acids, by the grouping of three bases per one Amino Acid to build specific proteins and molecules( to be discussed further in this chapter). The order of bases in the DNA is determined by patterns that arose in the remote past, sometimes millions of years ago. That order has usually been accurately preserved is one of the astonishing facts of biology. Changes that have occurred in the base sequence for any reason (mutations) are also copied with the same degree of accuracy. | ||||||
| For example, the following sequence of bases, read in triples could be classed as part of a gene. | ||||||
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| In all work conducted to date, only a portion of all DNA for all animals appears to code specific proteins and specific chemical structures, as much of the code sequence does not appear to make functioning physical structures produced by ribosomes. Over 50% of all genetic code for all animals appears not to code physical structures for proteins and functioning systems. | ||||||
| This is currently called "Junk DNA" by geneticists working in the field of de-coding the triplet-DNA-Amino Code system. | ||||||
| 12.10.2 | Chromosomes | |||||
| Chromosomes as discussed previously are the lengths of DNA grouped together, between 5000 and 50,000 genes per Chromosome. | ||||||
| Not only do the genes between species vary, but the number and length of chromosomes. | ||||||
| For instance a garden pea for instance has 14 chromosomes, a potato 48 and a crayfish 200. | ||||||
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Summary of human chromosomes (1 to 22, plus X and Y) |
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| 12.10.3 | Human Genetic Code and Human Chromosomes | |||||
| There are 46 chromosomes in the living cell of a human being and these chromosomes carry the genetic information that decides how a person will grow- whether he or she will be dark or fair, short or tall, blue-eyed or brown. But the sex cells, the female egg and the male sperm each have only 23 chromosomes. They fuse at conception to make a cell containing 46 chromosomes, half from each partner and it is this mixing of two sets of characteristics that creates the diversity of human life. Thus, Chromosomes are often spoken of as 23 pairs. | ||||||
| The major difference between humans are the 23rd chromosomes - X and y. The Chromosome X is much larger and has more genetic information than the smaller y chromosome. | ||||||
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| However, sperm produced by men can be of two types, X or y. | ||||||
| In terms of bases, the entire DNA code of the human being is around 3 Billion, or 3 gigabytes of memory potential for chemical triple-base code. However up to 85% of the entire DNA code appears to code chemical material, the rest is currently classed as "junk DNA' by the experts in this field. This non-coding material is found throughout the code for functioning and redundant genes as well as on its own. It is also estimated that only around 5% of all DNA of a human being is functioning, the rest (10%) being redundant genes. | ||||||
| There is estimated to be 100,000 genes in the genetic pool for humans. | ||||||
| If the DNA contained in the 46 human chromosomes were laid out end to end it would stretch several metres in length. Thus there is a very large amount of genetic detail embedded in these very long DNA strands, yet they are folded and compacted into the tiny space of the cell nucleus, which is only a few microns in diameter. | ||||||
| 12.10.4 | RNA | |||||
| Ribonucleic acid. A very long molecular polymer very similar to DNA made up of the informational bases A (adenine), G (guanine), C (cytosine) and U (uracil). The 'messages' which encode amino acid (protein) sequences copied from the genes are made of RNA. RNA is usually single stranded. For example, a typical messenger RNA base sequence might be: | ||||||
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5'-AGCUUAUUGCAUAAGCGCGAU-3' |
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| However, unlike DNA, cellular RNA is made up of only single strands; yet it is made up of the same type of basic building blocks as DNA, and its chemical composition is very similar to DNA. In RNA, T (thymine) becomes U (uracil). | ||||||
| The process of copying a DNA sequence into RNA sequence is called transcription. This takes place in the cells nucleus. | ||||||
| While a RNA sequence can be copied back into a DNA sequence (reverse transcription), a sequence of amino acids in a protein can never act as a copying template for the reverse flow of protein sequence into RNA. | ||||||
| Let us now look at the main Genetic copying process of cells. | ||||||
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