One set of chromosomes comes from the mother, while the other comes from the father. The twenty-third pair is the sex chromosomes , while the rest of the 22 pairs are called autosomes. Typically, biologically female individuals have two X chromosomes XX while those who are biologically male have one X and one Y chromosome XY. However, there are exceptions to this rule.
Biologically female people inherit an X chromosome from their father, and the other X chromosome from their mother. Biologically male people always inherit their X chromosome from their mother. The X chromosome is about three times larger than the Y chromosome, containing about genes, while the Y chromosome has about 55 genes.
Female mammals have two X chromosomes in every cell. James Klaunig, a scientist at IU Bloomington, predicts that the work will yield a more realistic and scientifically-based risk evaluation.
The scientists learned that of all sequenced invertebrate genomes so far, Daphnia shares the most genes with humans.
Daphnia 's gene expression patterns change depending on its environment, and the patterns indicate what state its cells are in. A water flea bobbing in water containing a chemical pollutant will tune-up or tune-down a suite of genes differently than its sisters accustomed to water without the pollutant, for example.
The health effects of most industrially produced compounds in the environment are unknown, because current testing procedures are too slow, too costly, and unable to indicate the causes for their effects on animals, including humans. Over the course of the project, the Daphnia Genomics Consortium has grown from a handful of founding members to more than investigators around the globe. Colbourne agreed, adding, "New model systems rarely arrive on the scene with such clear and important roles to play in advancing a new field of science.
The work also received support from the U. Department of Energy, Lilly Endowment Inc. National Institutes of Health, the U. Daphnia , or water flea, with a clonal brood of offspring.
Credit and Larger Version. A defensive helmet helps protect Daphnia against predators. Daphnia with mild defensive neckteeth; the teeth are used against predators. Juvenile Daphnia with and without defensive neckteeth. The researchers' findings are described in the Feb.
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. Replication happens in a manner similar to transcription, except that the entire double-strand DNA molecule unwinds and splits in two.
After splitting, bases on each strand bind to complementary bases A with T, and G with C floating nearby. When this process is complete, two identical double-strand DNA molecules exist. There are also chemical mechanisms to repair DNA that was not copied properly. However, because of the billions of base pairs involved in, and the complexity of, the protein synthesis process, mistakes may happen. Such mistakes may occur for numerous reasons including exposure to radiation, drugs, or viruses or for no apparent reason.
Minor variations in DNA are very common and occur in most people. Most variations do not affect subsequent copies of the gene. Mistakes that are duplicated in subsequent copies are called mutations. Inherited mutations are those that may be passed on to offspring. Mutations can be inherited only when they affect the reproductive cells sperm or egg.
Mutations that do not affect reproductive cells affect the descendants of the mutated cell for example, becoming a cancer but are not passed on to offspring. Mutations may be unique to an individual or family, and most harmful mutations are rare. Mutations may involve small or large segments of DNA. Depending on its size and location, the mutation may have no apparent effect or it may alter the amino acid sequence in a protein or decrease the amount of protein produced.
If the protein has a different amino acid sequence, it may function differently or not at all. An absent or nonfunctioning protein is often harmful or fatal. For example, in phenylketonuria Phenylketonuria PKU Phenylketonuria is a disorder of amino acid metabolism that occurs in infants born without the ability to normally break down an amino acid called phenylalanine.
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