why do we have d n a
Your genes are part of what makes you the person you are. You are different from everyone alive now and everyone who has ever lived. But your genes also mean that you probably look a bit like other members of your family. For example, have you been told that you have 'your mother's eyes' or 'your grandmother's nose'? Genes influence what we look like on the outside and how we work on the inside. They contain the information our bodies need to make chemicals called proteins. Proteins form the structure of our bodies, as well playing an important role in the processes that keep us alive. Genes are made of a chemical called DNA, which is short for 'deoxyribonucleic acid'. The DNA molecule is a double helix: that is, two long, thin strands twisted around each other like a spiral staircase. The DNA double helix showing base pairs The sides are sugar and phosphate molecules.
The rungs are pairs of chemicals called 'nitrogenous bases', or 'bases' for short. There are four types of base: adenine (A), thymine (T), guanine (G) and cytosine (C). These bases link in a very specific way: A always pairs with T, and C always pairs with G. The DNA molecule has two important properties. It can make copies of itself. If you pull the two strands apart, each can be used to make the other one (and a new DNA molecule). It can carry information. The order of the bases along a strand is a code - a code for making proteins. A gene is a length of DNA that codes for a specific protein. So, for example, one gene will code for the protein insulin, which is important role in helping your body to control the amount of sugar in your blood. Genes are the basic unit of genetics. Human beings have 20,000 to 25,000 genes. These genes account for only about 3 per cent of our DNA. The function of the remaining 97 per cent is still not clear, although scientists think it may have something to do with controlling the genes.
If you took the DNA from all the cells in your body and lined it up, end to end, it would form a strand 6000 million miles long (but very, very thin)! To store this important material, DNA molecules are tightly packed around proteins called histones to make structures called
chromosomes. The packaging of DNA into chromosomes Human beings have 23 pairs of chromosomes in every cell, which makes 46 chromosomes in total. A photograph of a person's chromosomes, arranged according to size, is called a karyotype. The sex chromosomes determine whether you are a boy (XY) or a girl (XX). The other chromosomes are called autosomes. The karyotype of a male human being The largest chromosome, chromosome 1, contains about 8000 genes.
The smallest chromosome, chromosome 21, contains about 300 genes. (Chromosome 22 should be the smallest, but the scientists made a mistake when they first numbered them! ). The DNA that contains your genes is stored in your cells in a structure called the nucleus. This work is licensed under a. If you really wanted to, and if you had enough money, you could clone your beloved family cat. At least one biotechnology company in the United States has offered cat cloning services for the privileged and bereaved. But don t assume that your cloned kitty will be exactly the same as the one you know and love. An individual is a product of more than its genes the environment plays an important role in shaping personality and many other traits. On December 22, 2001, a kitten named CC made history as the first cat and the first domestic pet ever to be cloned.
CC and Rainbow, the donor of CC s genetic material, are pictured at the right. But do you notice something odd about this picture? If CC is a clone of Rainbow an exact genetic copy then why are they different colors? The answer lies in the X chromosome. In cats, a gene that helps determine coat color resides on this chromosome. Both CC and Rainbow, being females, have two X chromosomes. (Males have one X and one Y chromosome. ) Since the two cats have the exact same X chromosomes, they have the same two coat color genes, one specifying black and the other specifying orange. Very early in her development, each of Rainbow s cells turned off oneentire X chromosome, thereby turning off either the black or the orange color gene. This process, called X-inactivation, happens normally in females, in order to prevent them from having twice as much X-chromosome activity as males.
It also happens randomly, meaning that different cells turn off different X chromosomes. So like all female mammals, Rainbow developed as a mosaic. Each cell that underwent X-inactivation gave rise to a patch of cells that had oneor the other coat color gene inactivated. Some patches specified black,other patches specified orange, and still others specified white, due to more complex genetic events. This is how all calico cats, like Rainbow, get their markings. CC looks different because she was made from a somatic cell from Rainbow in which the X-chromosome with the orange gene had been inactivated; only the black gene was active. What s interesting is that, as CC developed, her cells did not change the inactivation pattern. Therefore, unlike Rainbow, CC developed without any cells that specified orange coat color. The result is CC s black and white tiger-tabby coat.
- Autor: ArturElin
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