Each gene codes for a different protein.
LESSON 8
TEXT 1
Read the text, define its main idea.
GENES BASICS.
Genes are tiny strings of chemicals inside our body cells which contain the coded information that enables our body to grow, develop and function. We inherit half our genes from our father and half from our mother.
We all have about 35,000 inherited genes and there’s a complete copy of them in the nucleus of almost every cell in our body. Genes are made of a substance called DNA which looks like a long twisting ladder - the famous double helix. There are two metres of DNA in each cell nucleus. Most of it is outside our genes, but it is the DNA inside our genes that controls heredity. DNA is composed of four different types of a chemical unit known as bases, or letters. These bases are A, T, G and C (adenine, thymine, guanine and cytosine). They are arranged like steps on a ladder. Each step consists of a linked pair (called a base pair) of amino acids.
Base pairs appear over and over again in varying sequences. A always links with T and C always links with G – so, for example, a tiny part of a sequence might go: AT AT TA AT CG TA CG GC CG TA. The sequence and combination of these pairs on the DNA ladder is the code that our genes use to do their job - which is to give instructions for the manufacture of the many proteins needed for the formation and functioning of our body throughout our lives. Biological machinery in the cell reads the gene’s genetic code and carries out its orders.
Genes are strung together into chromosomes.
All our genes are packed into thread-like structures in the cell nucleus called chromosomes. These normally come in pairs.
In humans, for example, the full set of 46 chromosomes is made up of 23 same-shaped pairs. Each chromosome pair is made up of one chromosome from the mother and one from the father. Because chromosomes come in pairs, the genes the chromosomes carry also come in pairs. So each cell carries two copies of almost every one of that individual's genes, one copy on each of the paired chromosomes, which means there are usually two separate sets of instructions for that gene’s function. The only exception to this is the so-called sex chromosomes, where — if you are male — the two chromosomes are different from one another and so there is only one copy of some genes.
Although most of the cells in the human body - skin cells, muscle cells and so on - contain 46 chromosomes inside the nucleus, there is one important exception: the sex cells, or gametes. These cells - sperm in males and eggs in females - contain only 23 chromosomes, half the normal number.
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To understand why, think what would happen if a sperm and egg each contained the usual 46 chromosomes. When the sperm and egg fused at fertilization, the total number of chromosomes in the first cell of a new life would be 46 + 46 = 92. Every generation the number of chromosomes would double and soon there would be no room in the cell for anything but chromosomes. Halving the number of chromosomes means that when sperm and egg join, the original number of 46 chromosomes is restored.
Junk DNA as well as gene DNA.
Most of the DNA in our chromosomes actually lies outside our genes. Quite what all this surplus DNA - sometimes known as - does is not yet known, though it could be 'dead genes' - old inactive DNA, including DNA inserted during virus infections, that has been automatically copied and passed down from generation to generation.
This junk DNA (which is being mapped by the together with the gene DNA) varies much more between individuals than our genes do. So it is a very reliable way of tracing family relationships and identifying people. There may be as many as a million differences in the DNA sequences of two unrelated (???)
Almost every cell carries all the genes.
Just about every one of the billions of cells that make up your body carries an identical copy of your own unique genetic recipe. Although ail cells have the same basic design, cells are specialised to perform particular roles within the body. The appearance and function of skin cells, for example, is quite different to that of muscle cells. Given that all the cells have an identical genetic recipe, how is it that they end up doing different jobs?
The human genetic recipe contains all the information necessary for the tens of thousands of proteins needed by the body. But any one type of cell will only use a fraction of the total recipe.
Cells only produce the proteins appropriate to their function. Skin cells, for example, produce lots of keratin, a protein that gives toughness and protection where it is most needed - on the outside of the body. But they don’t produce hemoglobin, the protein used by red blood cells to carry oxygen around the body. Since skin cells do not need to transport oxygen around the body, there is little point in them producing this protein.
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Each gene codes for a different protein.
A typical gene consists of several thousand base pairs and the entire human genome (which includes all our genes as well as a lot of ’junk’ information in-between them - in other words all our DNA) contains some three billion base pairs.
Each gene contains the DNA code for a different protein in the body and, together, our genes form the instruction book for every newly developing embryo. They determine not only the way we look, like our height, build and eye and hair colour, but also how our body works - its strengths, weaknesses and susceptibility to disease. Aspects of our personality, such as intelligence or talent, may also be influenced by our genes.
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