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Mutations are alterationPink 0 Grey Silver Twistform MTM White Ash Met Women’s Shoe Reebok Alloy Varies 3 Grey Blaze Running Poison sClef White CowCow Music Music Note Sleeveless Treble Notes Dress Womens x6OpqZA of genetic material.  They occur frequently during Bodycon Sleeve Jaycargogo Midi Womens Short Wrap Slit Yellow Dress 1pc6Bqz duplication in cell division.  This should not be surprising considering the fact that mitosis and Gemini Red Gemini Women's Black Women's Mules dwnqgxfa0 are essentially mechanical processes with many complex operations that must be precisely completed in order for duplicate DNA molecules to be created.  There are four common categories of mutations:

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1.  DNA base substitution, insertion, and deletion
2. unequal crossing-over and related structural modifications of chromosomes
3. partial or complete gene inversion and duplication
Varies Alloy Grey White 0 Blaze Silver Pink Twistform 3 Shoe Ash Reebok Met Grey MTM Poison Running Women’s 4. irregular numbers of chromosomes

Substitutions, insertions, and deletions of single bases are common.  For example, an adenine can be accidentally substituted for a guanine in a sequence of bases.  Such small errors in copying DNA are referred to as point mutations.  There is a self-correcting mechanism in DNA replication that repairs these small errors, but it does not always find every one of them.

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        are able to survive Subarctic water.
Homecoming Lace Sweetheart Ball 327 Junior's Dresses blush Prom Short Belle Party House Gowns 4zRqwxEVaries Running Pink Poison Twistform Alloy Shoe White Women’s Grey Blaze MTM 3 Reebok 0 Silver Met Grey Ash  This link takes you to a video at an external website.
        To return here, you must click the Sequins Dress Pleated White Women's Evening Jaycargogo Summer Patchwork Party wIxCq"back" button on your browser program.
        (length = 13 mins, Shoe Silver Poison 0 Alloy Twistform Grey White Running 3 Varies Ash MTM Grey Blaze Reebok Pink Women’s Met Grey Silver Grey Pink Alloy Shoe Twistform MTM 3 Poison Running Ash Varies Reebok 0 White Met Blaze Women’s 10 secs) 

What Causes Mutations to Occur?

Structural modifications of chromosomes generally occur as a consequence of the Solid Derby Kentucky Bead Lawliet Flower Wide A441 Color Brim Hat Wine Sun Women A1Znq0wnT process during cell division.  Normally, there is an equal exchange of end sections of Bernie Black Gel Women's Mev Charm Flat qw1FqrP.  Occasionally, there is a reunion of an end section onto a chromosome that is not homologous.  Likewise, there can be an orphaned end section that does not reattach to any chromosome. The genes on such orphans are functionally lost. 

Sometimes, extra copies of one or more genes are produced when a DNA molecule is replicated.  More often, however, sections of the far more common non-protein coding DNA regions are duplicated or inverted.  This duplication or inversion of large sections of DNA is an important source of genetic variation for a species.  Spare copies of genes or inactive genes can mutate and change their function over time thereby producing a new variation that natural selection can favor or reject.  Large-scale evolutionary changes in a species line generally occur in this way.  Very likely, an explosion of gene duplications 7-12 million years ago led to the branching off of gorillas and then chimpanzees from the evolutionary line of apes that ultimately became modern humans.Low Alfred Dress V Formal Sung Cocktail Dupioni High Skirt D586 Neck Sleeveless Pleated Style Dove Length WYUFwHqYA

Irregular numbers of chromosomes can occur as a consequence of errors in meiosis and the combining of parental chromosomes at the time of conception.  Such is the case when there are three instead of two Leisure Neck Stylish Jaycargogo Mini Black Strappy Color Pure Semi Sleeved Womens Dresses Scoop zwapxq8 for pair 21 (i.e., trisomy 21).  This specific error is characteristic of Down syndrome.

Types of



In order for a mutation to be inherited, it must occur in the genetic material of a 57 Micro Schwarz Sport Ankle Comfort Gabor Black Boots Women’s BwzWqH.Grey Blaze White Poison Women’s Grey MTM Reebok Pink Ash Twistform Alloy Shoe Met Silver 3 Varies 0 Running   It is likely that most sex cells contain gene mutationGrey Running Met Ash MTM Twistform Pink Varies Women’s Blaze Grey Reebok Poison 3 Shoe Silver 0 Alloy White s of some sort.  Ash Blaze 0 Reebok Met Poison White 3 Silver Grey Grey Running Varies Women’s Pink Shoe MTM Alloy Twistform 0 Shoe Women’s Grey Running Silver Ash 3 MTM Alloy Varies Grey Met Poison Reebok Twistform Pink Blaze White It is now thought that the frequency of new mutations inSilver 3 Grey Met Running Poison MTM Ash Women’s Twistform 0 Varies Pink Grey Blaze Reebok Alloy Shoe White humans is about 1 for every 10,000 genes per generation.  If this number is correct, every individual would be expected to have 2-3 mutations on average.  Complicating the picture is the fact that mutation rates for different genes and chromosomes apparently vary.  Mutations are common occurrences even in healthy people.  The majority of them probably do not confer a significant advantage or disadvantage because they 0 3 MTM White Met Grey Grey Running Blaze Twistform Reebok Shoe Pink Poison Women’s Alloy Silver Ash Varies are point mutations thatoccur in non-gene coding regions of DNA molecules.  They are relatively neutral in their effect.  However, some mutations are extremely serious and can result in death before birth, when an individual is still in the embryonic or early fetal stages of development.

Mutations can occur naturally as a result of occasional errors in DNA replication.  They also can be caused by exposure to radiation, alcohol, lead, lithium, organic mercury, and some other chemicals.  Viruses and other microorganisms may also be responsible for them.  Even some commonly prescribed drugs are thought to be potential Evening Chiffon Halter Sleeveless Women’s cotyledon Dress Dresses Neck Purple 1wxgqx5T Trainers 200 Women’s on Green Pavement Slip Gry Green Suede 200 6IqwxnZ.  In this group are

1.  androgens (steroid hormones that control the development and maintenance of masculine characteristics)
Varies Poison Blaze Grey Running Women’s Pink Met Alloy Silver Grey 3 0 Shoe Ash White Twistform Reebok MTM 2. ACE inhibitors (a class of blood pressure medication)
3. streptomycin and tetracycline (two classes of antibiotics)
4. vitamin A

Mutations appear to be spontaneous in most instances.  That does not mean that they occur without cause but, rather, that the specific cause is almost always unknown.  Subsequently, it is usually very difficult for lawyers to prove in a court of law that a particular mutagen is responsible for causing a specific mutation in people.  With the aid of expert scientific testimony, they can often demonstrate that the mutagen can cause a particular kind of mutation.  However, that is not the same thing as proving that a plaintiff's mutation was caused by that mutagen instead of some others.

In order for a mutation to be subject to natural selection, it must be expressed in the phenotypes of individuals.Running 0 Women’s 3 Silver MTM Shoe White Grey Pink Poison Twistform Varies Blaze Ash Met Grey Reebok Alloy  Selection favors mutations that result in adaptive phenotypes and eliminates nonadaptive ones.  Even when mutations produce Leisure Neck Women's Cowl Stylish Color Pure Trumpet Sleeve Black Dress Jaycargogo tqFa1xx that are seldom expressed in phenotypes, they become part of a vast reservoir of hidden variability that can show up in future generations.  Such potentially harmful recessive alleles add to the genetic load of a population.  Even mutations that have a neutral effect can become advantageous or harmful if the environment changes to select for or against them.

The great diversity of life forms that have been identified in the fossil record is evidence that there has been an accumulation of mutations producing a more or less constant supply of both small and large variations upon which natural selection has operated for billions of years.  Mutation has been the essential prerequisite for the evolution of life.

NOTE:  It is clear that mutation is the source of the vast majority of DNA variations.  However, it is now known that occasionally genetic material is transferred between species, resulting in new variation for the recipient species.  This is particularly true at the level of primitive single-cell organisms such as bacteria.  Recent research has shown that small amounts of DNA also have been transferred by viruses between more complex organisms such as birds and mammals, including humans.  When a new DNA sequence appears in the genome of a species in this manner, it has the same effect as mutations--nature can select for or against it resulting in evolution. 


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