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9.7 Option - Genetics: The Code Broken? : 6. Mechanisms of genetic change
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6. Mechanisms of genetic change
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Students learn to:
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Students: |
Extract from Biology Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit:16Sep05]
Prior learning : Stages 4 and 5 Science, 5.8.2 c) and d)
HSC 9.3.3 and 4
Background information
The term mutation refers to any permanent change in the base sequence of DNA in a cell. This change can be chromosomal and affect a large portion of a chromosome or whole chromosomes. A mutation could also affect just a small piece of DNA (or gene) and this is known as a genic or point mutation.
A mutant is an organism that has a mutation or mutations. A mutant is usually detected either by its phenotype or by its functioning abilities.
process and analyse information from secondary sources to describe the effect of one named and described genetic mutation on human health
- The human inheritable disease sickle-cell anaemia is caused by a recessive base substitution mutation. Red blood cells in sufferers of this disease are sickle or crescent shaped and their haemoglobin molecules differ from normal by one amino acid in a polypeptide chain. This mutation causes red blood cells to block capillaries and cells and tissues may suffer from lack of oxygen.
- Downs syndrome and Klinefelter's syndrome are other examples of genetic mutations that have effects on human health.
distinguish between mutations of chromosomes, including
- rearrangements
- changes in chromosome
- number, including trisomy, and polyploidy and mutations of genes, including
- base substitution
- frameshift
Useful information
Mutagenesis is the process of producing a mutation.
A mutagen is a physical factor or a chemical substance that causes mutations. Gene mutations are caused by mistakes during DNA replication. The rate of these mistakes occurring is increased due to mutagens. Mutagens include:
- High energy radiation eg. Gamma rays, X-rays, ultraviolet (UV) rays.
- Chemicals such as mustard gas, Agent Orange, hydrocarbons in cigarette smoke and many industrial chemicals.
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Changes in chromosomal number
Chromosomal mutations occur during meiosis as a result of homologous pairs of chromosomes not separating during the first division. This is called non-disjunction and results in some gametes with extra chromosomes and others with less than the normal number.- Trisomy An example of this in
humans is Down syndrome. During meiosis, the two number
21 chromosomes (usually in the mother) do not separate
and the resulting ovum contains two number 21
chromosomes instead of just one, with a total of 24
instead of the normal haploid number of 23. When this
ovum is fertilized by a normal haploid sperm with one
number 21 chromosome (total 23), the resulting zygote
is a trisomy and will have three
number 21 chromosomes (total of 47 instead of the
normal diploid number of 46). Down syndrome is also
called trisomy-21 as there are three number 21
homologous chromosomes.
Another example of trisomy in humans is Klinefelter’s syndrome (XXY) where the ovum has two x chromosomes and is fertilized by a normal sperm with one Y.
- Polyploidy is the result of a
diploid (2N) gamete being fertilized by a haploid (N)
gamete to produce a triploid (3N) zygote, or even two
diploid gametes producing a tetraploid (4N) zygote.
These types of chromosomal non-disjunctions are the
result of all homologous chromosomes not separating
during meiosis with one gamete having all (2N)
chromosomes and the others having none.
Polyploidy is common in plants such as modern hybrid bread wheat (Triticum aestivum) which has 6N number of chromosomes. In the weed Datura stramonium, the trisomic effect can be detected by the phenotypes of the seed capsule such as rolled, elongate, globe. Many garden flowering plants such as tulips and daffodils are polyploids and they mainly reproduce asexually by bulbs.
- Trisomy An example of this in
humans is Down syndrome. During meiosis, the two number
21 chromosomes (usually in the mother) do not separate
and the resulting ovum contains two number 21
chromosomes instead of just one, with a total of 24
instead of the normal haploid number of 23. When this
ovum is fertilized by a normal haploid sperm with one
number 21 chromosome (total 23), the resulting zygote
is a trisomy and will have three
number 21 chromosomes (total of 47 instead of the
normal diploid number of 46). Down syndrome is also
called trisomy-21 as there are three number 21
homologous chromosomes.
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Rearrangements
Genes rearrangement on chromosomes result in mutations. There are several ways in which the genes can be rearranged:- Deletion - where part of a chromosome breaks off and a gene is effectively lost from the chromosome eg. Crit-du-Chat syndrome (a deletion occurs in a chromosome from pair 5);
- Duplication - where the same section of a chromosome is copied or repeated and occurs twice. In some cases a piece of a chromosome may break off and join onto other chromosome of the homologous pair;
- Inversion - where the order of the genes is reversed. This can occur due to part of a chromosome breaking off and then rejoining in the opposite direction;
- Translocation - where a part of one chromosome breaks off and migrates to another chromosome where it becomes attached eg. breakages in chromosomes 15 and 21 can result in Down syndrome.
Genic mutations occur when the sequence of bases in a section of the DNA molecule is changed. There are two types of genic mutations:
- Base substitution
Where one base in the DNA is substituted for another. This results in a change in the DNA template to make RNA. This alteration is then transferred to the one resulting codon on the mRNA and the subsequent amino acid sequence in polypeptide production. eg. Sickle-cell anaemia
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Example
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Original DNA :
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CAG TAG GTA |
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Substitute copy :
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CAG AAG GTA
(A has substituted the original T) |
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Original mRNA :
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GUC AUC CAU | |
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Original amino acids :
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valine isoleucine histidine | |
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Substitute mRNA :
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GUC UUC CAU | |
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Substitute amino acids:
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valine phenylalanine histidine |
- Frameshift
Where one base is added or deleted to a DNA molecule. This results in a change in all of the sequence after the addition or deletion. This then affects every RNA codon after the change and so affects every amino acid coded for after the change. This can affect a large proportion of the polypeptide to be made. In some cases, the polypeptide is interrupted or not produced at all if one of the resulting codons is a “stop” codon in the middle of the polypeptide. eg. Muscular dystrophy
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Example
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Original DNA :
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CAG TAG GTA |
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Inserted copy :
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CAG TTA GGT A
(T has been added or inserted) |
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Original mRNA :
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GUC AUC CAU | |
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Original amino acids :
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valine isoleucine histidine | |
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Inserted mRNA :
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GUC AAU CCAU | |
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Inserted amino acids :
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valine asparagine praline |
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Example
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Original DNA :
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CAG TAG GTA |
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Deleted copy :
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CAG TGG TA
(A has been deleted) |
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Original mRNA :
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GUC AUC CAU | |
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Original amino acids:
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valine isoleucine histidine | |
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Deleted mRNA :
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GUC ACC AU | |
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Deleted amino acids :
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valine threonine no amino acid coded for. |
outline the ability of DNA to repair itself
- DNA replicates every time a cell undergoes mitosis for growth and repair or meiosis for gamete production. This continues to occur throughout the life of the organism. Mutations or mistakes in the copying of the DNA are common and cells contain mechanisms to repair these mistakes. Copying errors can be repaired by enzymes such as DNA polymerase that can use the undamaged strand of DNA in the double helix as a template to fix and replace the incorrect damaged base sequence.
describe the way in which transposable genetic elements operate and discuss their impact on the genome
- Tranposable genetic elements are also called transposons or jumping genes. These genes are sections of DNA that are not fixed and can move around among chromosomes.
In bacteria, these genes within plasmids containing DNA can move between cells. The genes that are transferred may be resistant to antibiotics among the bacteria. When the bacteria reproduce, they also reproduce this transferred gene and all of the individuals within a population, will now contain this new gene. This may be how populations of bacteria develop resistance to antibiotics so rapidly. There is therefore a great impact on the bacterial genome.
In more complex organisms, the role of transposons is not yet defined.
distinguish between germ line and somatic mutations in terms of their effect on species
- Cells that produce gametes (eg.
in ovaries and testes) are called
germ-line cells. If mutations occur in
these cells, they can be inherited if fertilization takes
place. The offspring's cells will contain the mutation.
Because of this, mutations in germ-line cells contribute to
the species gene pool and can influence whole populations
of organisms and their evolution.
- Somatic mutations occur in cells in the body other than sex organs such as ovaries and testes. Somatic cells that undergo mutations cannot pass these on to offspring and so have no effect on species and their evolution. An example of this a mutation in skin cells due to exposure to UV rays to cause skin cancer. The mutation simply stays in the individual within a population and does not affect the population as a whole.
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