Mutation Animation

Mutations are changes to the base pair sequence of genetic material (either DNA or RNA). Mutations can be caused by copying errors in the genetic material during cell division and by exposure to ultraviolet or ionizing radiation, chemical mutagens, or viruses, or can occur deliberately under cellular control during processes such as meiosis or hypermutation. In multicellular organisms, mutations can be subdivided into germline mutations, which can be passed on to descendants, and somatic mutations. The somatic mutations cannot be transmitted to descendants in animals. Plants sometimes can transmit somatic mutations to their descendants asexually or sexually (in case when flower buds develop in somatically mutated part of plant).

Mutations create variation in the gene pool, and the less favorable (or deleterious) mutations are removed from the gene pool by natural selection, while more favorable (beneficial or advantageous) ones tend to accumulate, resulting in evolutionary change. For example, a butterfly may develop offspring with a new mutation caused say by ultraviolet light from the sun. In most cases, this mutation is not good, since obviously there was no 'purpose' for such change at the molecular level. However, sometimes a mutation may change, say, the butterfly's color, making it harder for predators to see it; this is an advantage and the chances of this butterfly surviving and producing its own offspring are a little better, and over time the number of butterflies with this mutation may form a large percentage of the species. Neutral mutations are defined as mutations whose effects do not influence the fitness of either the species or the individuals who make up the species. These can accumulate over time due to genetic drift. The overwhelming majority of mutations have no significant effect, since DNA repair is able to mend most changes before they become permanent mutations, and many organisms have mechanisms for eliminating otherwise permanently mutated somatic cells.




Point mutations, often caused by chemicals or malfunction of DNA replication, exchange a single nucleotide for another. Most common is the transition that exchanges a purine for a purine (A ↔ G) or a pyrimidine for a pyrimidine, (C ↔ T). A transition can be caused by nitrous acid, base mispairing, or mutagenic base analogs such as 5-bromo-2-deoxyuridine (BrdU). Less common is a transversion, which exchanges a purine for a pyrimidine or a pyrimidine for a purine (C/T ↔ A/G). A point mutation can be reversed by another point mutation, in which the nucleotide is changed back to its original state (true reversion) or by second-site reversion (a complementary mutation elsewhere that results in regained gene functionality). These changes are classified as transitions or transversions. An example of a transversion is adenine (A) being converted into a cytosine (C). There are also many other examples that can be found. Point mutations that occur within the protein coding region of a gene may be classified into three kinds, depending upon what the erroneous codon codes for:
Silent mutations: which code for the same amino acid.
Missense mutations: which code for a different amino acid.
Nonsense mutations: which code for a stop and can truncate the protein.
Insertions add one or more extra nucleotides into the DNA. They are usually caused by transposable elements, or errors during replication of repeating elements (e.g. AT repeats). Insertions in the coding region of a gene may alter splicing of the mRNA (splice site mutation), or cause a shift in the reading frame (frameshift), both of which can significantly alter the gene product. Insertions can be reverted by excision of the transposable element.
Deletions remove one or more nucleotides from the DNA. Like insertions, these mutations can alter the reading frame of the gene. They are irreversible.




Check Out This Video http://www.youtube.com/watch?v=WHFRCrPj0SQ

Causes of mutation

Two classes of mutations are spontaneous mutations (molecular decay) and induced mutations caused by mutagens.

Spontaneous mutations on the molecular level include:

Tautomerism - A base is changed by the repositioning of a hydrogen atom.

Depurination - Loss of a purine base (A or G).

Deamination - Changes a normal base to an atypical base; C → U, (which can be corrected by DNA repair mechanisms), or spontaneous deamination of 5-methycytosine (irreparable), or A → HX (hypoxanthine).

Transition - A purine changes to another purine, or a pyrimidine to a pyrimidine.

Transversion - A purine becomes a pyrimidine, or vice versa.

Benzopyrene, the major mutagen in tobacco smoke, in an adduct to DNA. Produced from PDB 1JDG.

Induced mutations on the molecular level can be caused by:

Chemicals
Nitrosoguanidine (NTG)
Hydroxyamine NH3OH
Base analogs (e.g. BrdU)
Simple chemicals (e.g. acids)
Alkylating agents (e.g. N-ethyl-N-nitrosourea (ENU)) These agents can mutate both replicating and non-replicating DNA. In contrast, a base analog can only mutate the DNA when the analog is incorporated in replicating the DNA. Each of these classes of chemical mutagens has certain effects that then lead to transitions, transversions, or deletions.
Methylating agents (e.g. ethyl methanesulfonate (EMS))
Polycyclic hydrocarbons (e.g. benzopyrenes found in internal combustion engine exhaust)
DNA intercalating agents (e.g. ethidium bromide)
DNA crosslinker (e.g. platinum)
Oxidative damage caused by oxygen(O)] radicals
Radiation
Ultraviolet radiation (nonionizing radiation) - excites electrons to a higher energy level. DNA absorbs one form, ultraviolet light. Two nucleotide bases in DNA - cytosine and thymine-are most vulnerable to excitation that can change base-pairing properties. UV light can induce adjacent thymine bases in a DNA strand to pair with each other, as a bulky dimer.
Ionizing radiation

DNA has so-called hotspots, where mutations occur up to 100 times more frequently than the normal mutation rate. A hotspot can be at an unusual base, e.g., 5-methylcytosine.

Mutation rates also vary across species. Evolutionary biologists have theorized that higher mutation rates are beneficial in some situations, because they allow organisms to evolve and therefore adapt more quickly to their environments. For example, repeated exposure of bacteria to antibiotics, and selection of resistant mutants, can result in the selection of bacteria that have a much higher mutation rate than the original population (mutator strains).


Harmful mutations

Changes in DNA caused by mutation can cause errors in protein sequence, creating partially or completely non-functional proteins. To function correctly, each cell depends on thousands of proteins to function in the right places at the right times. When a mutation alters a protein that plays a critical role in the body, a medical condition can result. A condition caused by mutations in one or more genes is called a genetic disorder. However, only a small percentage of mutations cause genetic disorders; most have no impact on health. For example, some mutations alter a gene's DNA base sequence but don’t change the function of the protein made by the gene.

If a mutation is present in a germ cell, it can give rise to offspring that carries the mutation in all of its cells. This is the case in hereditary diseases. On the other hand, a mutation can occur in a somatic cell of an organism. Such mutations will be present in all descendants of this cell, and certain mutations can cause the cell to become malignant, and thus cause cancer.

Often, gene mutations that could cause a genetic disorder are repaired by the DNA repair system of the cell. Each cell has a number of pathways through which enzymes recognize and repair mistakes in DNA. Because DNA can be damaged or mutated in many ways, the process of DNA repair is an important way in which the body protects itself from disease.


Beneficial mutations

A very small percentage of all mutations actually have a positive effect. These mutations lead to new versions of proteins that help an organism and its future generations better adapt to changes in their environment. For example, a specific 32 base pair deletion in human CCR5 (CCR5-32) confers HIV resistance to homozygotes and delays AIDS onset in heterozygotes.[1] The CCR5 mutation is more common in those of European descent. One theory for the etiology of the relatively high frequency of CCR5-32 in the European population is that it conferred resistance to the bubonic plague in mid-14th century Europe.






Hope You Like This Post, Let me know what you feel about this blog.
Email me : help.me.ishan@gmail.com

Natural Selection Leture

Natural selection is the process by which favorable heritable traits become more common in sucessiove genrations of a population of reproducing organisms,and unfavorable heritable traits become less common, due to differntial reproduction of genotypes,Natural selection acts on the phenotype, or the observable characteristics of an organism, such that individuals with favorable phenotypes are more likely to survive and reproduce than those with less favorable phenotypes. The phenotype's genetic basis, genotype associated with the favorable phenotype, will increase in frequency over the following generations. Over time, this process may result in adaptations that specialize organisms for particular ecological niches and may eventually result in the emergence of new species. In other words, natural selection is the mechanism by which evolution may take place in a population of a specific organism.The term was introduced by Charles Darwin in his groundbreaking 1859 book The Origin of Species in which natural selection was described by analogy to artificial selection, a process by which animals with traits considered desirable by human breeders are systematically favored for reproduction. The concept of natural selection was originally developed in the absence of a valid theory of inheritance; at the time of Darwin's writing, nothing was known of modern genetics. Although Gregor Mendel, the father of modern genetics, was a contemporary of Darwin's, his work would lie in obscurity until the early 20th century. The union of traditional Darwinian evolution with subsequent discoveries in classical and molecular genetics is termed the modern evolutionary synthesis. Although other mechanisms of molecular evolution, such as the neutral theory advanced by Motoo Kimura, have been identified as important causes of genetic diversity, natural selection remains the single primary explanation for adaptive evolution.


Natural selection part 1





Natural selection part 2







Natural selection part 3

Genes and Chromosomes lecture

In today's lecture what I want to do is to look at a little bit of detail at how the connection between Genes and chromosomes was forged.

Chromosomes had not been described when Mendel lived and worked, it was only later with the advent of improved microscopes in the second part of the 19th century of biologists began to describe the structure of cells in some detail, and one of the things they noticed was the formation of dark bodies in the nucleus of cells that would appear just before cells divide and furthermore they notice that as cells were dividing these dark bodies would fall peculiar movement. What they were seeing were chromosomes and movements of these chromosomes in mitosis and meiosis.

Part 1






The significance of the movements of these chromosomes wasn't appreciated at first but then in the early 20th century two-cell biologist independently had an insight Walter Sutton and colleague observed that the highly choreographed movements of chromosomes during meiosis reduced by half number of chromosomes that would be found in gametes. When gametes joined the total number of chromosomes would come back up to its full complement. They realize that this reduction in chromosome number in gametes formation and subsequent restoration zygote formation could explain the patterns of trait transmission that Mendel had described with his laws of segregation and independent assortment.


Part 2





Part 3






In 1903 Sutton and Bovary both independently publish their ideas, which become generally known as the chromosomal theory of inheritance. in that sense they provided a hypothetical mechanism whereby chromosome movements could completely explain these two fundamental principles that Mendel had suggested.

The chromosomal theory of inheritance should seem obvious to us at this point in the course because we are to know so much about DNA learn that before but it wasn't clear then that you could establish this relationship specifically it wasn't clear how during the early part of the 20th century you could prove that genes were on chromosomes and thus prove the chromosome theory of inheritance, sutten & Bovary had suggested this connection but it was just a hypothesis. Confirmation of this hypothesis actually can be attributed to one particular scientist and a remarkable lab group and also to the particular organism, the scientist was Thomas Hunt Morgan who was a embryologist studying patterns of development working at Columbia University.










Like most biologist at the time Morgan to became interested in mechanisms of inheritance that as people began to talk again about Mendel's work .Now Morgan was particularly interested as he was studying development in mutations, and he was interested in how new mutations arose in organisms. Many geneticists at the time had begun working on organisms that had more complex patterns of trait transmission than for example the garden peas Mendel worked on including for example small mammals such as guinea pigs and mice because the way that her color patterns of these mammals would be transmitted from parent to offspring sometimes corresponded to what Mendel observed that also led to a lot of interesting exceptions that these geneticists wanted to understand .so Morgan when he got interested in genetics he set out to work on the genetics of coat color in mammals but mammals are expensive.Morgan couldn't actually raise the money to do this work Morgan's inability to get funded to work on coat color in mammals was probably one of the most fortunate grants turndowns in the history of science because it led Morgan by necessity to start working on a different model organism the Fruit fly a small little fly its scientific name is Drosophila Melagoster and commonly known as Drosophila.

As it turns out Drosophila very quickly became and remains to this day the single most important model organism used in both classical and molecular studies of genetics.From Morgan's point of view there are a lot of advantages to working on fruit flies (i) first of all their cheap and (ii)Fruit flies are also very easy to raise in the laboratory . most important in one of the reasons that supplies remain such an important model organism today(iii) they have a very short generation time adult fruit flies will develop from eggs in only a matter of days and what this means is that it's possible to observe the results of genetic crosses in a very short period time you can do a lot of process he didn't have to wait for those garden peas to growup over a matter of months within a few days you know the answer.

There were some serious problems working with fruit flies that fruit flies that you collect from the wild don't have obvious phenotypic variance. if you put out your pineapple and collect fruit flies, to a first approximation they all look the same ,that is the fruit fly didn't offer traits that Morgan could use in particular establish crosses. This seems like the major problem How you going to understand the genetics of trait transmission if there are obvious traits that you can follow in her crosses. but remember that Morgan was interested in mutation so his first goal really when he started working with Drosophila was to see if and how a mutant phenotype might emerge in a natural wild population .

Morgan and his students and the legion of people who followed him studying fruit flies refer to the phenotypes of these fruit flies in particular ways. they referred to the characteristic that you would observe in a wild fruit fly as being the wild type phenotype. because wild fruit flies don't have a lot of visible variation. it means that basically all fruit flies that you collect our basically just composed of wild type phenotype for any particular characteristic you might be interested. now if they observed an unusual phenotype specifically phenotype that they thought was the mutation they call it a mutant phenotype. we have wild type and mutant phenotypes that are what we're really looking at when we look at fruit flies and the assumption here is that the mutant phenotype somehow must be the result of a mutation in allele for the gene responsible for the trait .another detail is that Morgan and the people who have followed up on fruit flies uses slightly different convention for labeling their alleles,the way it is Morgan designated or labeled essentially the kind of mutant alleles and genotypes he was working with was by him labeling the allele according to the phenotypic characteristic of the mutation of the mutant phenotype don't let me make this clear with an example of a well-known mutation in Drosophila which involved a reduction in the size of wings and these guys are just tiny little flies they have wings but one mutation occurs causes those wings do not develop properly that the wings are also small and scrunched up this mutation has been labeled the vestigial wing mutation or just simply vestigial wings we would label the allele responsible for this mutation VG. the interesting thing is that the mutation in are named after the mutant phenotype not the wild type phenotype.the mutant allele would be referred to as VG and the wild type allele for that same gene we would call VG + .

for any particular mutation that Morgan was studying we can safely assume that the typical wild type Drosophila the one that Morgan would just collect out on his pineapple is homozygous for the wild type allele if we were interested in the vestigial wing of trait if we just caught a wild type individual we would assume it's homozygous for VG + VG + that would be a phenotype for that particular trait.

we observe a mutant that mutant must have at least one mutant allele by for example in that case it's got to be at least VG + VG but actually more often than not the mutant alleles that we find in Drosophila are recessive alles. if we find mutation if we find a mutant phenotype of vestigial wing fly then we can be pretty sure that it's homozygous for the recessive mutant allele in other words it would be VG VG a genotype that particular trait.