rusanjo.com · web viewmutations can also be caused by exposure to specific chemicals or...
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How does mutation introduce new alleles into a population?
● explain how a range of mutagens operate, including but not limited to:
– electromagnetic radiation sources
– chemicals
– naturally occurring mutagens
● compare the causes, processes and effects of different types of mutation, including but not limited to:
– point mutation
– chromosomal mutation
● distinguish between somatic mutations and germ-line mutations and their effect on an organism
● assess the significance of ‘coding’ and ‘non-coding’ DNA segments in the process of mutation
● investigate the causes of genetic variation relating to the processes of fertilisation, meiosis and mutation
● evaluate the effect of mutation, gene flow and genetic drift on the gene pool of populations
Mutations happen for several reasons.
DNA fails to copy accuratelyMost of the mutations that we think matter to evolution are "naturally-occurring." For example, when a cell divides, it makes a copy of its DNA — and sometimes the copy is not quite perfect. That small difference from the original DNA sequence is a mutation.
External influences can create mutationsMutations can also be caused by exposure to specific chemicals or radiation. These agents cause the DNA to break down. This is not necessarily unnatural — even in the most isolated and pristine environments, DNA breaks down. Nevertheless, when the cell repairs the DNA, it might not do a perfect job of the repair. So the cell would end up with DNA slightly different than the original DNA and hence, a mutation.
Mutagens are chemical compounds or forms of radiation (such as ultraviolet (UV) light or X-rays) that cause irreversible and heritable changes (mutations) in the cellular genetic material, deoxyribonucleic acid (DNA)
Three kinds of mutagens- physical agents, chemical agents and biological agents.
Physical agents: Heat and radiationChemical agents Base analogues Alkylating agents Metal ions Biological agents Viruses Bacteria
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Radiation: The radiations are the first mutagenic agent reported in 1920. X-ray radiation: The X-rays are one of the most common types of ionizing radiation used in many medical practices for various purposes. However, the dose for that is very low. It is even used in the sterilizing practises for destroying microorganism. UV-rays: The UV-light is a non-ionizing type of radiation having less energy in it, used in the sterilization and decontamination process during the cell culture and microbiological experiments. The major causes of UV-radiation are- base deletion, strand breakage, cross-linking and generation of nucleotide dimers. The UV-light can be classified into three different categories: UV-A: nearly visible range (320nm) causes pyrimidine dimers.UV-B: (290-320nm) emitted by the sunlight and one of the major mutagen cause changes in DNA that are highly lethal. UV-C: (180-290nm) one of the most energy-consuming form of the UV that is extremely lethal.The UV induced mutations are dimer formation, thymine- thymine dimer and thymine- cytosine dimers are commonly formed as these lesions block replication as well as transcription.
Heat:Heat is another mutagen that causes mutation in our DNA. when we heat the DNA, over a certain degree (>95°C), the DNA becomes denatured- two single-stranded DNA generated from the dsDNA. Also, extreme heat also damages DNA and breaks the phosphodiester bonds too
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Naturally Occurring MutagensThere are a number of mutagens in our environment and they can be classified into two major groups: those of biological origin and those of non-biological origin. The latter group includes metals, such as mercury and cadmium, and cooked foods such as the charred surface of fish or hamburger, etc.
Naturally occurring mutagens are those originating from microbes, plants and animals. Among them the most important and those causing the greatest concern are the products of fungi that are collectively called mycotoxins.
A second group which concerns us consists of several substances produced by green plants such as pyrrolizidine alkaloids, allyl isothiocyanate, cycasin, etc. Many of these have been discovered either from a sudden outbreak of toxicosis in livestock and poultry, or from the frequent occurrence of hepatomas or cancers among inhabitants of certain districts.
Mutagens of animal origin exhibit somewhat different characteristics from mycotoxins and those occurring in plants. They are produced in the animal’s body and may be mutagenic to the animal itself. A good example is dimethylnitrosamine, which is produced in an animal’s stomach when it
ingests foodstuffs containing nitrous acid or nitrite (ham or sausage) together with secondary amines (fish, meat, etc.).
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A POINT MUTATION is a mutation that only affects a single nucleotide of nucleic acid. Point mutations most commonly involve the substitution of one base for another (which changes the complementary base as well in DNA). Point mutations rarely cause a huge change in the underlying organism, which is fortunate as they are by far the most common type of mutation.
Single base substitutions can be divided into transitions and transversions based on whether or not they change the nucleobase's structure. Transitions, which change a nucleotide from a purine to purine or pyrimidine to pyrimidine, occur far more often than transversions.
When a point mutation does occur in a coding region, it can be assigned to one of the following categories, according to the consequence of the mutation (see the figure below for an illustration of different mutations applied to TTCTTC):
Silent mutation : does not change the corresponding amino acid, and thus has no measurable effect on the genome.
Missense mutation: changes the codon causing the amino acid change. If the properties of amino acid (polarity, charge etc.) remain the same, then the mutation is called conservative, in which case it may not affect the protein's function. Otherwise, the mutation is called non-conservative, and it can lead to the loss of protein function and result in disease.
Nonsense mutation: exchanges a normal amino acid codon for a stop codon, which results in the protein's truncation. Usually this shortening also leads to the loss of the protein's function.
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Chromosome mutations are alterations occurring in chromosomes that typically result from errors during nuclear division or from mutagens.
Chromosome mutations result in changes in chromosome structure or in cellular chromosome numbers.
Examples of structural chromosome mutations include translocations, deletions, duplications, inversions, and isochromosomes.
Abnormal chromosome numbers result from nondisjunction, or the failure of chromosomes to separate correctly during cell division.
Examples of conditions that result from abnormal chromosome numbers are Down syndrome and Turner syndrome.
Sex chromosome mutations occur on either the X or Y sex chromosomes.
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Germline MutationGermline mutations are hereditary in nature, since they occur in the gametes which participate in fertilization. Thus, the death of the parent organism where the mutation originally occurred will not erase the mutation, since it has already been passed down to future generations. These mutations occur in the sex cells or gametes, which unite during the act of fertilization to form the zygote. Each gamete donates half of its genetic material to the zygote, so any mutation in one of the gametes will surely be passed to the offspring. The zygote then forms the embryo by cell division, which eventually grows into the organism’s body. Thus, the genetic material of one cell (zygote) is distributed in all the adult organism’s cells. As such, the mutation is found in both, germline and somatic cells. Moreover, the gametes produced by the offspring will also carry the mutated gene, which is passed to the future generations.
Somatic MutationSomatic mutations cannot be transferred to succeeding generations, since such mutations don’t occur in the cells involved in fertilization, i.e., the sperm and ovum. However, these mutations are transferred to daughter cells (cells formed after division). These mutations exist until the death of the parent cell where they originally occurred, or that of the daughter cells if any, or the death of the organism itself. The reason why such mutations are passed only to daughter cells and not to the offspring is because, somatic cells undergo direct cell division to form new cells. Thus, the new cells receive the mutated gene from their parent cell. However, since somatic cells do not participate in the process of fertilization, such mutations are not hereditary.
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Noncoding DNA
Noncoding DNA makes up about 98.5% of the total DNA. While it was previously thought to have no function, newer information is beginning to shed light on the many functions of this mass of DNA. It is involved in the cutting and splicing of large amounts of DNA, is involved in transposon reassembly, genome rearrangements and the production of small RNAs.
It is also possible that noncoding DNA was used as a source of new genes needed for adaptation or for functions during evolution
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More on Germline and Somatic mutation
If a mutation occurs in a germ cell, any resulting gametes produced will also carry the mutation. If the affected gamete is involved in fertilisation, the resulting zygote will carry the mutation in all of its cells. However, if the mutation occurs in a somatic (body) cell during mitosis, then only a small portion of the individual will be affected. Tumours are cells that have undergone a somatic mutation and lost the ability to regulate their growth.
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Mutations and meiosis
Mutations can result from unequal crossing-over during meiosis. In addition, some areas of
the genome simply seem to be more prone to mutation than others. These "hot spots" are
often a result of the DNA sequence itself being more accessible to mutagens. Hot spots
include areas of the genome with highly repetitive sequences, such as trinucleotide repeats,
in which a sequence of three nucleotides is repeated many times. During DNA replication,
these repeat regions are often altered because the polymerase can "slip" as it disassociates
and reassociates with the DNA strand. To better understand a polymerase slip, imagine you
are reading a page of text that is a repeat of a simple sequence. Say that the whole page is
just copies of the word "And" ("And And And..."). Now, imagine that while reading the
page, you briefly glance away and then look back at the text. It's quite likely that you will
have lost your place. As a result, you may read the wrong number of copies from the page.
Similarly, DNA polymerase sometimes slips and makes mistakes when reading repeats.
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Genetic Drift is a change in the allele frequency that is brought about by random sampling. Allele frequency is the proportion of individuals carrying a particular allele in a population. In other words, it is a change in the composition of the gene pool of a population that is brought about by random chance.
Example: A mother with blue eyes and a father with brown eyes can have children with brown or blue eyes. If brown is the dominant allele, even though there is a 50% chance of having blue eyes, they might have all children with brown eyes by chance
Gene Flow is a change in the allele frequency brought about by the transfer of alleles or gametes from one population to another. When individuals migrate from one population to another, new alleles are introduced into the gene pool of that population, leading to a change in the allele frequency of that population.
Example: When American soldiers went to Vietnam during the Vietnam War, they had children with the Vietnamese women there, introducing their genes into the gene pool of the Vietnamese population.
Natural Selection is a process through which a particular allele of a physical characteristic becomes more or less common in a population over a few generations. The physical characteristic that provide an adaptive advantage are selected and become more common in the population over generations.
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What kinds of gene mutations are possible?The DNA sequence of a gene can be altered in a number of ways. Gene mutations have varying effects on health, depending on where they occur and whether they alter the function of essential proteins. The types of mutations include:
Missense mutationThis type of mutation is a change in one DNA base pair that results in the substitution of one amino acid for another in the protein made by a gene.
Nonsense mutationA nonsense mutation is also a change in one DNA base pair. Instead of substituting one amino acid for another, however, the altered DNA sequence prematurely signals the cell to stop building a protein. This type of mutation results in a shortened protein that may function improperly or not at all.
InsertionAn insertion changes the number of DNA bases in a gene by adding a piece of DNA. As a result, the protein made by the gene may not function properly.
DeletionA deletion changes the number of DNA bases by removing a piece of DNA. Small deletions may remove one or a few base pairs within a gene, while larger deletions can remove an entire gene or several neighbouring genes. The deleted DNA may alter the function of the resulting protein(s).
DuplicationA duplication consists of a piece of DNA that is abnormally copied one or more times. This type of mutation may alter the function of the resulting protein.
Frameshift mutationThis type of mutation occurs when the addition or loss of DNA bases changes a gene's reading frame. A reading frame consists of groups of 3 bases that each code for one amino acid. A frameshift mutation shifts the grouping of these bases and changes the code for amino acids. The resulting protein is usually nonfunctional. Insertions, deletions, and duplications can all be frameshift mutations.
Repeat expansionNucleotide repeats are short DNA sequences that are repeated a number of times in a row. For example, a trinucleotide repeat is made up of 3-base-pair sequences, and a tetranucleotide repeat is made up of 4-base-pair sequences. A repeat expansion is a mutation that increases the number of times that the short DNA sequence is repeated. This type of mutation can cause the resulting protein to function improperly.
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