Human body
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"Genes and DNA." Life, Human body, Q-files Encyclopedia, 10 Feb. 2022.
https://www.q-files.com/life/human-body/genes-and-dna.
Accessed 19 Mar. 2024.
Genes and DNA 2022. Life, Human body. Retrieved 19 March 2024, from
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Life, Human body, s.v. "Genes and DNA," accessed March 19, 2024.
https://www.q-files.com/life/human-body/genes-and-dna
Genes and DNA
Every cell in your body carries your own, unique set of coded instructions—unless you are an identical twin, in which case you share this code with your twin. The instructions, called genes, are found in two sets of 23 chromosomes located inside the nucleus of body cells. Chromosomes are made primarily of a substance called DNA. The genes are "written" in the chemical structure of the long DNA molecule. They contain all the information a living thing needs to develop, grow and maintain itself through life. Your body has about 23,000 genes: the human genome. They determine your hair colour, your height, any tendency to develop certain diseases, and so on.
Chromosomes
You inherited one set of 23 chromosomes from your mother and one set from your father. Together they make up the 46 paired chromosomes inside your cells. Each set contains the full range of around 23,000 genes. You might inherit an eye colour gene for blue eyes from your mother, and a matching gene, but for brown eyes, from your father. One pair of chromosomes, the sex chromosomes, determine whether you are female or male. Females have the same kind of sex chromosome, called XX, while males have two distinct sex chromosomes, called XY.
DNA
A molecule of DNA (which stands for DeoxyriboNucleic Acid) is enormously long and thin, like a twisted ladder. All the DNA in one cell nucleus joined together would stretch nearly two metres (about six-and-a-half feet).
Each chromosome, which consists of both DNA and proteins, is spread out like an unwound length of rope inside the cell nucleus. But when the cell is about to divide and form two cells, each chromosome twists itself into a tight coil that, in turn, twists itself into a super-coil. As a result, the chromosomes become short, super-coiled bundles of DNA and protein.
The structure of DNA
The instructions in genes are encoded in the structure of DNA. Each DNA molecule consists of two strands that spiral around each other to form a twisted-ladder shape called a double helix. The ladder's "rungs" or cross-links are made of molecules called bases. There are four bases: A, T, G and C (which stand for adenine, thymine, guanine and cytosine). A always forms a cross-link with T, and G always with C. The exact order of A, T, G and C along the DNA provides the coded instructions, and that order is unique to every individual (unless you are an identical twin). The instructions are used by your body like recipes in a cookery book to make the many different proteins that are required to build, run and maintain a cell.
How DNA copies itself
Every minute the body makes about 300 million new cells to replace those that naturally wear out and die. These replacements are produced by the process of cell division. Before dividing, the chromosomes inside a "parent" cell duplicate themselves so that the two "offspring" cells receive full, identical sets of those chromosomes.
During duplication, the DNA molecule in a chromosome copies itself by "unzipping" the cross-links that hold together the two strands. The bases line up and attach to bases (A with T and G with C) in the separated strands so that a new double helix is formed from each strand. The two new DNA molecules separate as the cell division takes place, with each forming a new chromosome.
Identical twins
If two brothers and two sisters have identical genes, they look the same: they are identical twins. They both develop from the same fertilized egg cell which splits in two soon after fertilization. Identical twins are always the same sex and share identical features, such as eye colour. Non-identical, or fraternal, twins are produced when two eggs are fertilized by separate sperm. They are not necessarily the same sex.
Consultant: Richard Walker