Scientists have numbered the chromosome pairs from 1 to 22, with the 23rd pair labeled as X or Y, depending on the structure. The first 22 pairs of chromosomes are called autosomes. The 23rd pair of chromosomes are known as the sex chromosomes, because they decide if you will be born male or female. Because they retain their telomeres, such cells generally live longer than other cells. Telomeres also play a role in cancer. The chromosomes of malignant cells usually do not lose their telomeres, helping to fuel the uncontrolled growth that makes cancer so devastating.
In fact, each species of plants and animals has a set number of chromosomes. A fruit fly, for example, has four pairs of chromosomes, while a rice plant has 12 and a dog, In humans and most other complex organisms, one copy of each chromosome is inherited from the female parent and the other from the male parent.
This explains why children inherit some of their traits from their mother and others from their father. The pattern of inheritance is different for the small circular chromosome found in mitochondria. Only egg cells - and not sperm cells - keep their mitochondria during fertilization.
So, mitochondrial DNA is always inherited from the female parent. In humans, a few conditions, including some forms of hearing impairment and diabetes, have been associated with DNA found in the mitochondria. Yes, they differ in a pair of chromosomes known as the sex chromosomes. Females have two X chromosomes in their cells, while males have one X and one Y chromosome.
Inheriting too many or not enough copies of sex chromosomes can lead to serious problems. For example, females who have extra copies of the X chromosome are usually taller than average and some have mental retardation. Males with more than one X chromosome have Klinefelter syndrome, which is a condition characterized by tall stature and, often, impaired fertility. Another syndrome caused by imbalance in the number of sex chromosomes is Turner syndrome.
Women with Turner have one X chromosome only. DNA contains the code, or blueprint, used to synthesize a protein. Genes vary in size, depending on the sizes of the proteins for which they code. Each DNA molecule is a long double helix that resembles a spiral staircase containing millions of steps. The steps of the staircase consist of pairs of four types of molecules called bases nucleotides. In each step, the base adenine A is paired with the base thymine T , or the base guanine G is paired with the base cytosine C.
Each extremely long DNA molecule is coiled up inside one of the chromosomes Chromosomes Genes are segments of deoxyribonucleic acid DNA that contain the code for a specific protein that functions in one or more types of cells in the body. Chromosomes are structures within cells Except for certain cells for example, sperm and egg cells and red blood cells , the cell nucleus contains 23 pairs of chromosomes.
A chromosome contains many genes. A gene is a segment of DNA that provides the code to construct a protein. The DNA molecule is a long, coiled double helix that resembles a spiral staircase. In it, two strands, composed of sugar deoxyribose and phosphate molecules, are connected by pairs of four molecules called bases, which form the steps of the staircase.
In the steps, adenine is paired with thymine and guanine is paired with cytosine. Each pair of bases is held together by a hydrogen bond. A gene consists of a sequence of bases.
Sequences of three bases code for an amino acid amino acids are the building blocks of proteins or other information. Proteins are composed of a long chain of amino acids linked together one after another. There are 20 different amino acids that can be used in protein synthesis—some must come from the diet essential amino acids , and some are made by enzymes in the body.
As a chain of amino acids is put together, it folds upon itself to create a complex three-dimensional structure. It is the shape of the folded structure that determines its function in the body. Because the folding is determined by the precise sequence of amino acids, each different sequence results in a different protein.
Some proteins such as hemoglobin contain several different folded chains. Instructions for synthesizing proteins are coded within the DNA. The code is written in triplets. That is, the bases are arranged in groups of three. Particular sequences of three bases in DNA code for specific instructions, such as the addition of one amino acid to a chain.
For example, GCT guanine, cytosine, thymine codes for the addition of the amino acid alanine, and GTT guanine, thymine, thymine codes for the addition of the amino acid valine. Thus, the sequence of amino acids in a protein is determined by the order of triplet base pairs in the gene for that protein on the DNA molecule.
The process of turning coded genetic information into a protein involves transcription and translation. When transcription is initiated, part of the DNA double helix opens and unwinds. The mRNA separates from the DNA, leaves the nucleus, and travels into the cell cytoplasm the part of the cell outside the nucleus—Home.
Inside a Cell Inside a Cell Often thought of as the smallest unit of a living organism, a cell is made up of many even smaller parts, each with its own function. Human cells vary in size, but all are quite small. There, the mRNA attaches to a ribosome, which is a tiny structure in the cell where protein synthesis occurs. Each molecule of tRNA brings one amino acid to be incorporated into the growing chain of protein, which is folded into a complex three-dimensional structure under the influence of nearby molecules called chaperone molecules.
These cells look and act differently and produce very different chemical substances. However, every cell is the descendant of a single fertilized egg cell and as such contains essentially the same DNA. Cells acquire their very different appearances and functions because different genes are expressed in different cells and at different times in the same cell.
The information about when a gene should be expressed is also coded in the DNA. Gene expression depends on the type of tissue, the age of the person, the presence of specific chemical signals, and numerous other factors and mechanisms. Knowledge of these other factors and mechanisms that control gene expression is growing rapidly, but many of these factors and mechanisms are still poorly understood.
The mechanisms by which genes control each other are very complicated. Genes have chemical markers to indicate where transcription should begin and end. Various chemical substances such as histones in and around the DNA block or permit transcription. Cells reproduce by dividing in two. Because each new cell requires a complete set of DNA molecules, the DNA molecules in the original cell must reproduce replicate themselves during cell division.
Genes are small sections of DNA within the genome that code for proteins. They contain the instructions for our individual characteristics — like eye and hair colour. If you have any other comments or suggestions, please let us know at comment yourgenome.
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