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. These subtle differences enable cell biologists to distinguish different chromosomes from one another, much as field biologists learn to distinguish members of a pod of whales by the marks and scars on their fins. The largest chromosome of an organism is generally referred to as chromosome 1, the next largest as chromosome 2, and so on. Different chromosomes contain different genes. That is, each chromosome contains a specific chunk of the genome.
For example, in humans the gene for alpha globin, a part of the hemoglobin protein that carries oxygen in red blood cells, is found on chromosome The gene for beta globin, the other part of the hemoglobin protein, is found on chromosome Stained chromosomes can be photographed and arranged in order of size to produce a karyotype, a chart scientists use to study chromosomes.
A karyotype isn't detailed enough to tell you about the individual genes on a chromosome, but it can tell you whether the chromosomes as a whole are in working order and it may help doctors diagnose and understand diseases. For example, people with Down's syndrome have too many chromosomes, and chromosome rearrangements when a part of a chromosome breaks off and reattaches to a different chromosome are associated with certain cancers.
A quick glance at any karyotype will tell you one of the most important facts about chromosomes: They come in pairs.
The members of a pair are the same size and shape, and they have the same banding patterns. In other words, each person actually possesses two copies of chromosome 1, two copies of chromosome 2, and so on. Human cells contain 23 pairs of chromosomes.
Literature records very few live births, but with death soon after. Hydatidiform moles are usually polyploid. These cell populations, however, come from 1, and only 1, zygote When recording, a mosaic is denoted by a slash between the various clones observed, e.
Numerical anomaly is usually due to a mitotic non-disjunction: 1 daughter cell will get both chromatids of one of the homologues, the other none; so the former will be trisomic, the latter monosomic. Note: Viability of the two daughter cells may differ. In the above-mentioned trisomy 21 example, the clone monosomic for 21 is non-viable and has disappeared.
The phenotype of surviving individuals is more or less affected, according to the proportion of the various clones. Variability of clone proportions is affected by various factors: The precocity of the event e. If 46, XX cells are the most numerous, the anomaly must have occurred late in development; if it occurred at the cell, or cell stage, all, none or part of the embryo could be affected, since by this stage, the cells destined for the primitive streak, and hence the embryo proper have been segregated, and the aberration might be confined to the membranes or placenta.
The distribution of the cell populations during embryogenesis. In this case, the proportions of the various clones will vary from one organ to another. A mosaic must not be confused with a chimaera. In a chimaera, the cells originate from two or more zygotes. They are produced by: mixture, or exchange of cells, from different zygotes e.
Note: Mosaicism is frequent in malignancies, either because normal cells can still be karyotyped, or because the malignant clone produces sub-clones with additional anomalies clonal evolution. Initial breaks are thought to be at the level of the DNA, and are probably frequent events.
DNA repair then occurs. For various reasons, DNA repair is insufficient in chromosome instability syndromes. Most often, the break occurs in a non-coding sequence, and does not result in a mutation. Initial breaks can occur anywhere, short arms of acrocentics included. Ultimately, what is important for the individual, is to retain 2 normal copies of each gene, no more, no less.
This is particularly true for the embryo, where a full balanced genetic complement is vital for normal development. Embryos with unbalanced constitutional anomalies have 1 or 3 copies of a whole set of genes, and abnormal development results.
Note: a full balanced complement is not absolutely necessary for the functioning of many differentiated tissue cells, particularly if they are not called upon to divide. Nevertheless, relatively small imbalances can have dire consequences, even in somatic cells. A good example is the case of the Rb gene, implicated in the formation of retinoblastoma. Normal individuals carry 2 functional copies, but one of these can be inactivated by mutation or removal loss of heterozygosity and the cell continues normal function through the normal allele which is now acting as a tumour suppressor gene.
Loss of the second allele by removal or mutation leads to the formation of the tumour. Note: Many of the structural aberrations formed are cell lethal, and are soon eliminated from the cell population. Of those that survive and are transmitted, the most frequent are translocations, small inversions and deletions. Note: Rearranged chromosomes that are transmitted are called derivative chromosomes der and they are numbered according to the centromere they carry.
Thus a reciprocal translocation between chromosome 7 and chromosome 14 will result in a der 7 and a der B - Main structural anomalies Figure 1 - Reciprocal translocation A mutual exchange between terminal segments from the arms of 2 chromosomes. Provided that there is no loss or alteration at the points of exchange, the new arrangement is genetically balanced, and called a: Balanced rearrangement. Recorded as t, followed by a bracket with the numerals of the 2 chromosomes, and a second bracket indicating the presumptive breakpoints e.
Transmission to descendants constitutional anomalies At meiosis, where there is pairing of homologous chromosome segments normal chromosomes form a bivalent , followed by crossing-over, translocations may form a quadrivalent tetravalent, in Greek and this leads to segregation problems. At meiosis anaphase I, chromosomes separate without centromere separation; this separation occurs at anaphase 2. Segregation of chromatids in the case of a quadrivalent Figure can be according the following: alternate type , which produces normal gametes, or gametes with the parental balanced translocation.
The baby will have a normal phenotype unless cryptic imbalance is present. It gives rise to "duplication-deficiency": there is an excess of some bits and a lack of other bits. In either case, this will result in zygotes with 47 or 45 chromosomes. Characteristics: Reciprocal translocations are, in most cases, balanced rearrangements and the carrier has a normal phenotype. At meiosis, they enhance malsegregations especially when an acrocentric is involved in the translocation : Adjacent 1, adjacent 2, or types lead to miscarriages, or to the birth of a malformed child.
The more unbalanced a zygote is, the less the probability that the child will reach birth. Breakpoints can occur at the centromeres, leading to whole arm exchanges. Complex translocations: Three, or more breaks and more than two chromosomes can participate in exchange, leading to some very complicated rearrangements. The surviving, balanced forms are seen usually as cyclical translocations. The recent introduction of FISH-painting indicates that such complex translocations are much more frequent than we have realised.
Note There will be no mechanical transmission problems at mitosis. Note: Reciprocal and Complex translocations can also occur in somatic cells at any time after birth; they are particularly frequent in cancer processes.
In this activity, dominant forms of a ge ne appear in capital letters while recessive forms of a gene a ppear in lower case letters. Since you get one gene from your mother and one from your father for each trait, you may have a combination of dominant and r ecessive genes for each trait. When both forms of a gene are the same either both dom inant or both recessive you are said to be homozygous for that trait. If you have one dom inant gene and one recessive gene, you are said to be heterozygous for that trait.
One final thing before you begin the activit y. As you will see in the activity, when you receive the dominant form of a gene whether homozygous or heterozygous, you will express the dominant form of the gene.
Y ou will only express the recessive form of the gene if you receive the recessive form from both of your parents, thus being homozygous for the recessive form. Finally, this information should provide you with the basics of how appearance is determined by DNA. If you are a bit confused, follow the steps of the activity and many concepts above will be seen.
By performing the activity, you will be able to see exactly what is meant by some of the terms me ntioned above. Good Luck creating your offspring! This activity requires the use of sharp scissors to cut out the chromosomes. Use caution when using scissors. Ask an adult to help you if necessary. After this activity, you should be able to understand how DNA determines your appearance.
Remember DNA is condensed into chromosomes. You have 23 pairs of chromosomes, 23 from your mother and 23 from your father. Within these chromosomes, there are sections called genes that control specific characteristics or traits. These genes have both a dominant and recessive form. If you have two dominant or two recessive genes for a given trait, you are said to be homozygous for that trait. If you have one dominant and one recessive form of a gene, you are said to be heterozygous for that trait.
The dominant form of a gene will always be expressed while the recessive form of a gene will be expressed only if you have two recessive forms. These are the general rules of how traits are inherited from your parents.
However, there are many exceptions to this rule, which are still being explored by scientists today!
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