The unnecessary genes are switched off. A change in a gene can occur spontaneously no known cause or it can be inherited. Changes in the coding that makes a gene function can lead to a wide range of conditions. Humans typically have 46 chromosomes in each cell of their body, made up of 22 paired chromosomes and two sex chromosomes.
These chromosomes contain between 20, and 25, genes. New genes are being identified all the time. The paired chromosomes are numbered from 1 to 22 according to size. Chromosome number 1 is the biggest. These non-sex chromosomes are called autosomes.
People usually have two copies of each chromosome. One copy is inherited from their mother via the egg and the other from their father via the sperm. A sperm and an egg each contain one set of 23 chromosomes. When the sperm fertilises the egg, two copies of each chromosome are present and therefore two copies of each gene , and so an embryo forms.
The chromosomes that determine the sex of the baby X and Y chromosomes are called sex chromosomes. A person with an XX pairing of sex chromosomes is biologically female, while a person with an XY pairing is biologically male. As well as determining sex, the sex chromosomes carry genes that control other body functions.
There are many genes located on the X chromosome, but only a few on the Y chromosome. Genes that are on the X chromosome are said to be X-linked. Genes that are on the Y chromosome are said to be Y-linked. Parents pass on traits or characteristics, such as eye colour and blood type, to their children through their genes. Some health conditions and diseases can be passed on genetically too. Sometimes, one characteristic has many different forms. Changes or variations in the gene for that characteristic cause these different forms.
These two copies of the gene contained in your chromosomes influence the way your cells work. The two alleles in a gene pair are inherited, one from each parent. Alleles interact with each other in different ways. These are called inheritance patterns.
Examples of inheritance patterns include:. An allele of a gene is said to be dominant when it effectively overrules the other recessive allele. Chromosomes come in matching sets of two or pairs and there are hundreds — sometimes thousands — of genes in just one chromosome. Most cells have one nucleus say: NOO-clee-us. The nucleus is a small egg-shaped structure inside the cell which acts like the brain of the cell. It tells every part of the cell what to do.
But, how does the nucleus know so much? It contains our chromosomes and genes. As tiny as it is, the nucleus has more information in it than the biggest dictionary you've ever seen. In humans, a cell nucleus contains 46 individual chromosomes or 23 pairs of chromosomes chromosomes come in pairs, remember? Half of these chromosomes come from one parent and half come from the other parent. Under the microscope, we can see that chromosomes come in different lengths and striping patterns.
When they are lined up by size and similar striping pattern, the first twenty two of the pairs these are called autosomes; the final pair of chromosomes are called sex chromosomes, X and Y. The sex chromosomes determine whether you're a boy or a girl: females have two X chromosomes while males have one X and one Y. But not every living thing has 46 chromosomes inside of its cells.
For instance, a fruit fly cell only has four chromosomes! Each gene has a special job to do. The DNA in a gene spells out specific instructions—much like in a cookbook recipe — for making proteins say: PRO-teens in the cell. Proteins are the building blocks for everything in your body. Bones and teeth, hair and earlobes, muscles and blood, are all made up of proteins. At least biotechnology-based products are currently in clinical trials. In , the HapMap, a catalog of common genetic variation or haplotypes in the human genome, was created.
This data has helped to speed up the search for the genes involved in common human diseases. In the body, DNA holds the instructions for building proteins, and these proteins are responsible for a number of functions in a cell.
The epigenome is made up of chemical compounds and proteins that can attach to DNA and direct a variety of actions. These actions include turning genes on and off. This can control the production of proteins in particular cells. Recently, scientists have discovered genetic switches that increase the lifespan and boost fitness in worms.
They believe these could be linked to an increased lifespan in mammals. The genetic switches that they have discovered involve enzymes that are ramped up after mild stress during early development. This could lead to a breakthrough in the goal to develop drugs that can flip these switches to improve human metabolic function and increase longevity. The marks can be passed on from cell to cell as they divide, and they can even be passed from one generation to the next.
Specialized cells can control many functions in the body. For example, specialized cells in red blood cells make proteins that carry oxygen from air to the rest of the body. The epigenome controls many of these changes within the genome. Lifestyle and environmental factors such as smoking, diet and infectious diseases can bring about changes in the epigenome. They can expose a person to pressures that prompt chemical responses. These responses can lead to direct changes in the epigenome, and some of these changes can be damaging.
Cancer can result from changes in the genome, the epigenome or both. Changes in the epigenome can switch on or off the genes that are involved in cell growth or the immune response.
These changes can cause uncontrolled growth, a feature of cancer, or a failure of the immune system to destroy tumors. Researchers in The Cancer Genome Atlas TCGA network are comparing the genomes and epigenomes of normal cells with those of cancer cells in the hope of compiling a current and complete list of possible epigenomic changes that can lead to cancer. Researchers in epigenomics are focused on trying to chart the locations and understand the functions of all the chemical tags that mark the genome.
This information may lead to a better understanding of the human body and knowledge of ways to improve human health. Gene therapy uses sections of DNA to treat or prevent disease.
This science is still in its early stages, but there has been some success. For example, in , scientists reported that they had managed to improve the eyesight of 3 adult patients with congenital blindness by using gene therapy.
In , a reproductive endocrinologist, named John Zhang, and a team at the New Hope Fertility Center in New York used a technique called mitochondrial replacement therapy in a revolutionary way. They announced the birth of a child to a mother carrying a fatal genetic defect. Researchers combined DNA from two women and one man to bypass the defect. These are packaged into 23 pairs per cell.
That makes 46 chromosomes in total. Together, the 20, genes on our 46 chromosomes are referred to as the human genome. The role of DNA is similar to the role of the alphabet. It has the potential to carry information, but only if the letters are combined in ways that make meaningful words. Stringing words together makes instructions, as in a recipe. So genes are instructions for the cell.
If genes are like a basic recipe, alleles Ah-LEE-uhls are versions of that recipe. We inherit one allele, or gene version, from each of our parents. That means most of our cells contain two alleles, one per chromosome. The reason: Before we inherit them, alleles are shuffled like a deck of cards. This happens when the body makes egg and sperm cells. They are the only cells with just one version of each gene instead of two , packaged into 23 chromosomes.
Egg and sperm cells will fuse in a process known as fertilization. This starts the development of a new person. By combining two sets of 23 chromosomes — one set from the egg, one set from the sperm cell — that new person ends up with the usual two alleles and 46 chromosomes. And her unique combination of alleles will never arise in the exact same way again. To multiply, a cell splits into two identical copies. The cell uses the instructions on its DNA and the chemicals in the cell to produce an identical DNA copy for the new cell.
Then the process repeats itself many times as one cell copies to become two. And two copy to become four.
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