Mutation
A mutation is a permanent change in a DNA sequence. Generally, the word mutation has negative connotations. However, the majority of mutations are actually good or have no effect on the organism. Mutations are essential to evolution, human adaption over hundreds of years has occurred due to the alterations in human DNA over a period of thousand of years. Often, mutations are passed down on to offspring as evident in the study of pedigree slide.
An example is the gradual transition from primitive animals to humans over tens of thousands of years, humans have had to adapt to an ever evolving world and this has occurred due to positive mutations in DNA. Positive mutation also promotes genetic diversity in the human species. The change in an organism's DNA sequence can influence it's physical appearance, its behaviour and its physiology. |
Types of Mutations
Mutations are influenced by the different ways DNA can be changed. There are three main types of mutations:
- Somatic mutations: A somatic mutation is a change in DNA that occurs in all body cells besides germ cells. These mutations cannot be passed on to offspring
- Germ line mutations: A germ line mutation is a DNA change that occurs in sex cells, these mutations can be passed on to offspring
- Point mutations: Point mutations are regarded as being the simplest type of mutation as they only involve a change in a single base pair
- Somatic mutations: A somatic mutation is a change in DNA that occurs in all body cells besides germ cells. These mutations cannot be passed on to offspring
- Germ line mutations: A germ line mutation is a DNA change that occurs in sex cells, these mutations can be passed on to offspring
- Point mutations: Point mutations are regarded as being the simplest type of mutation as they only involve a change in a single base pair
Chromosomal rearrangements
To make it easier to understand the concept of how mutations occur, we will examine chromosomal rearrangements, the process that causes the mutation of DNA.
Say we had a sentence that represents a sequence of nucleotides in a fragment of DNA. There are five chromosomal rearrangements that can occur, two of which are displayed below:
Analogy
THECATATETHEMAT
This sequence is read in codons (3 letter segments)
THE CAT ATE THE MAT
We can now use this sequence to display how the various chromosomal rearrangements take place:
1) Deletion:
THE CAT ATE THE MAT
Deletion is the removal of a random nucleotide in the sequence resulting in the nucleotides after it becoming mixed up
THE CAT AT THE MAT
The nucleotide "E" was removed from the codon
THE CAT ATT HEM AT
Since the nucleotides must be rearranged back into codons, the codons after the letter "E" are now mixed up and are considered corrupt
2) Duplication
THE CAT ATE THE MAT
Duplication is a process in which a part of the DNA sequence is copied at random and arranged into the original sequence causing a repetition of information.
THECATATEATETHETHEMAT
The codons "ATE" and "THE" were duplicated and arranged back into the sequence.
THE CAT ATE ATE THE THE MAT
The end result is a DNA sequence that has genetic repetition
Say we had a sentence that represents a sequence of nucleotides in a fragment of DNA. There are five chromosomal rearrangements that can occur, two of which are displayed below:
Analogy
THECATATETHEMAT
This sequence is read in codons (3 letter segments)
THE CAT ATE THE MAT
We can now use this sequence to display how the various chromosomal rearrangements take place:
1) Deletion:
THE CAT ATE THE MAT
Deletion is the removal of a random nucleotide in the sequence resulting in the nucleotides after it becoming mixed up
THE CAT AT THE MAT
The nucleotide "E" was removed from the codon
THE CAT ATT HEM AT
Since the nucleotides must be rearranged back into codons, the codons after the letter "E" are now mixed up and are considered corrupt
2) Duplication
THE CAT ATE THE MAT
Duplication is a process in which a part of the DNA sequence is copied at random and arranged into the original sequence causing a repetition of information.
THECATATEATETHETHEMAT
The codons "ATE" and "THE" were duplicated and arranged back into the sequence.
THE CAT ATE ATE THE THE MAT
The end result is a DNA sequence that has genetic repetition
Down Syndrome
Down syndrome or trisomy 21, is a chromosomal defect that is especially common in children with older mothers. Down syndrome is most common chromosomal disorder in the world with an estimated one in every 700 babies born will have the syndrome. The syndrome is a condition referred to as trisomy. Trisomy is a condition in which an individual inherits an extra copy of a chromosome. Down syndrome is caused by an individual receiving an extra copy of chromosome 21, hence why it is sometimes referred to as trisomy 21. The syndrome causes physical abnormalities, below average intellectual function, abnormal facial appearances and develop challenges. As a sufferer of down syndrome possesses one additional chromosome 21, in total that have 47 chromosomes instead of 46. Down syndrome is an example of non-disjunction. Non-disjunction occurs when homologous pairs fail to separate and migrate towards the poles during a meiotic division. This results in an uneven number of chromosomes in some daughter in the four daughter cells. In regards to downs syndrome, an extra copy of chromosome 21 is unevenly distributed into one of the gametes.
Compare the karyotype of a person with down syndrome (figure 3) with the karyotype of a healthy person (figure 1)
There is no cure for down syndrome, nor does it go away. Down syndrome occurs at contraception but scientists are yet do discover why or how the offspring receive an additional chromosome. Down syndrome is usually diagnosed at birth through means of a blood test but can also be determined before the baby is born with the use of an ultrasound scan.
More information about down syndrome can be found on the official Australian website below
Compare the karyotype of a person with down syndrome (figure 3) with the karyotype of a healthy person (figure 1)
There is no cure for down syndrome, nor does it go away. Down syndrome occurs at contraception but scientists are yet do discover why or how the offspring receive an additional chromosome. Down syndrome is usually diagnosed at birth through means of a blood test but can also be determined before the baby is born with the use of an ultrasound scan.
More information about down syndrome can be found on the official Australian website below
Turner's Syndrome
Turner's syndrome, another chromosomal disorder, affects females only. Turner's syndrome is a sex chromosome mutation with symptoms that include: short stature, infertility and the inability to menstruate. As evident in figure 4, a female usually posses two "X" chromosomes. However, in females with turner's syndrome, only one "X" chromosome is present, or the second "X" chromosome is abnormal. This is seen in figure 5. It is for this reason that the syndrome is often termed monosomy X. Turner's syndrome affects approximately one in every 2,000 female babies born. In addition, it is estimated that only 1% of females diagnosed with turner's syndrome before birth survive, resulting in miscarriages. Turners syndrome can be diagnosed through the use of an ultrasound scan to determine before birth by looking for any physical abnormalities. Turner's syndrome can be verified if the female fails to display the characteristics of a female undergoing puberty during her teenage years. Treatment aims to correct physical defects as well as bringing out puberty
The above video is women living with Turner's Syndrome and how it impacts their lives