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.
Phenylalanine, which is toxic This deficiency allows the amino acid phenylalanine absorbed from the diet to accumulate in the body, ultimately causing severe intellectual disability. In rare cases, a mutation introduces a change that is advantageous. For example, in the case of the sickle cell gene, when a person inherits two copies of the abnormal gene, the person will develop sickle cell disease Sickle Cell Disease Sickle cell disease is an inherited genetic abnormality of hemoglobin the oxygen-carrying protein found in red blood cells characterized by sickle crescent -shaped red blood cells and chronic However, when a person inherits only one copy of the sickle cell gene called a carrier , the person develops some protection against malaria Malaria Malaria is infection of red blood cells with one of five species of Plasmodium, a protozoan.
Malaria causes fever, chills, sweating, a general feeling of illness malaise , and sometimes diarrhea Although the protection against malaria can help a carrier survive, sickle cell disease in a person who has two copies of the gene causes symptoms and complications that may shorten life span. Natural selection refers to the concept that mutations that impair survival in a given environment are less likely to be passed on to offspring and thus become less common in the population , whereas mutations that improve survival progressively become more common.
Thus, beneficial mutations, although initially rare, eventually become common. The slow changes that occur over time caused by mutations and natural selection in an interbreeding population collectively are called evolution.
Not all gene abnormalities are harmful. For example, the gene that causes sickle cell disease also provides protection against malaria. A chromosome is made of a very long strand of DNA and contains many genes Genes 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. The genes on each chromosome are arranged in a particular sequence, and each gene has a particular location on the chromosome called its locus.
In addition to DNA, chromosomes contain other chemical components that influence gene function. This XY sex-determination system is found in most mammals as well as some reptiles and plants. Whether a person has XX or XY chromosomes is determined when a sperm fertilizes an egg. Unlike the body's other cells, the cells in the egg and sperm — called gametes or sex cells — possess only one chromosome.
Gametes are produced by meiosis cell division, which results in the divided cells having half the number of chromosomes as the parent, or progenitor, cells. In the case of humans, this means that parent cells have two chromosomes and gametes have one. All of the gametes in the mother's eggs possess X chromosomes. The father's sperm contains about half X and half Y chromosomes. The sperm are the variable factor in determining the sex of the baby. If the sperm carries an X chromosome, it will combine with the egg's X chromosome to form a female zygote.
If the sperm carries a Y chromosome , it will result in a male. During fertilization, gametes from the sperm combine with gametes from the egg to form a zygote.
The zygote contains two sets of 23 chromosomes, for the required There are some variations, though. Recent research has found that a person can have a variety of different combinations of sex chromosomes and genes, particularly those who identify as LGBT. For example, a certain X chromosome called Xq28 and a gene on chromosome 8 seem to be found in higher prevalence in men who are gay, according to a study in the journal Psychological Medicine.
A few births out of a thousand of babies are born with a single sex chromosome 45X or 45Y and are referred to as sex monosomies. Clearly, there are not only females who are XX and males who are XY, but rather, there is a range of chromosome complements, hormone balances, and phenotypic variations that determine sex. This cell then divides and its successors divide numerous times, eventually producing a mature individual with a full set of paired chromosomes in virtually all of its cells.
Besides the linear chromosomes found in the nucleus, the cells of humans and other complex organisms carry a much smaller type of chromosome similar to those seen in bacteria. This circular chromosome is found in mitochondria, which are structures located outside the nucleus that serve as the cell's powerhouses.
Scientists think that, in the past, mitochondria were free-living bacteria with the ability to convert oxygen into energy. When these bacteria invaded cells lacking the power to tap into oxygen's power, the cells retained them, and, over time, the bacteria evolved into modern-day mitochondria. The constricted region of linear chromosomes is known as the centromere. Although this constriction is called the centromere, it usually is not located exactly in the center of the chromosome and, in some cases, is located almost at the chromosome's end.
The regions on either side of the centromere are referred to as the chromosome's arms. Centromeres help to keep chromosomes properly aligned during the complex process of cell division. As chromosomes are copied in preparation for production of a new cell, the centromere serves as an attachment site for the two halves of each replicated chromosome, known as sister chromatids.
Telomeres are repetitive stretches of DNA located at the ends of linear chromosomes. They protect the ends of chromosomes in a manner similar to the way the tips of shoelaces keep them from unraveling.
In many types of cells, telomeres lose a bit of their DNA every time a cell divides. Eventually, when all of the telomere DNA is gone, the cell cannot replicate and dies. White blood cells and other cell types with the capacity to divide very frequently have a special enzyme that prevents their chromosomes from losing their telomeres. 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.
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