SC.912.L.16.4: Explain how mutations in the DNA sequence may or may not result in phenotypic change. Explain how mutations in gametes may result in phenotypic changes in offspring.
Mutations: Any change in the original sequence of nucleotides of an organisms genome. This can occur due to natural causes or it can be induced due to environmental changes or exposure to certain chemicals. In order for any change in an organisms genome to be passed down it must occur in the gametes or the gamete producing cells of that organism. Gametes are sex cells like sperm and eggs. Every cells derived from the first cell (zygote) of the organism will contain the new mutations.
Mutations that end up resulting in a change in protein structure may result in an observable change in the expression of a trait. The observable expression of a trait is known as phenotype. The sequence of nucleotides that make up the genes of the organism is known as the genotype.
[insert picture of phenotype] [insert picture of genotype]
Causes of mutations
Mutations can have a number of causes ranging from natural to unnatural causes. During the DNA replication process, and transcription it is possible that the enzymes responsible for these processes can accidentally utilize the wrong nucleotide. If DNA polymerase places an Adenine “A” where a Cytosine “C” should be that would be considered a mutation. Increased stress, lack of sleep, and poor nutrition or diet can lead to increased rates of errors of the DNA enzymes.
Mutations can also be induced due to high energy waves like radiation can cause mutations. An everyday example of radiation that we are all exposed to is the presence of UV (ultra violet) B and C rays. UV rays are so good at causing mutations high levels of UV rays make life almost impossible, thankfully the Earths ozone layer helps to mitigate the majority of the lethal UV rays before they ever reach the surface of the Earth.
Chemicals that cause changes in nucleotide sequences are known as Mutagens. Many everyday chemicals can cause mutations when organisms are exposed to them. Some everyday examples of mutagens are; chemicals found in cigarettes, arsenic, benzene (found in gasoline). This is one reason why some chemicals come with warnings to avoid prolonged or repeated exposure to the skin.
Types of Mutations
Single Point Mutations
Single Point Mutations occur when only a single nucleotide within a designated section of DNA or RNA is changed. If a single nucleotide is changed from “A” to “C” this is an example of a single point mutation. A change in nucleotide sequence could result in a change in protein when you look at translation. A change in protein could result in profound changes to the behavior and physical form of an organism.
Missense: When a single point mutation results in a completely different amino acid being produced during translation.
Nonsense: When a single point mutation results in a “STOP” codon being produced instead of an amino acid. A “STOP” codon will also result in a protein being shorter than expected. This could result in a protein behaving completely differently or not functioning at all!
Silent: When a single point mutation results in the same exact amino acid being produced as the original sequence. This will result in no significant change in the phenotype and the only way to confirm this has occurred would be to sequence that specific section of DNA to confirm a change in “genotype“. These types of mutations will never result in a change in phenotype.
3 Single Point Mutations Compared
Frameshift Mutations
Frameshift mutations occur when nucleotides are inserted/deleted within a gene which changes the “reading frame”. This is similar to taking a simple sentence like “the big cat sat” and you delete the letter “i” from the second word (codon 2) the sentence would read “the bgc ats at”. This simple change has a profound effect on the meaning of the sentence.
Chromosomal Mutations
A chromosomal mutation is when a large section of a chromosome has a mutation or significant structural changes. An example of a chromosomal mutation is if an entire region of the chromosome is deleted or breaks off.
Human ploidy is often assessed to determine if an unborn child will have down-syndrome. Ploidy is number of sets of chromosomes in the cell of an organism. This analysis of human ploidy is called karyotyping.
Down-syndrome is a genetic disorder in which a child will have 1 extra copy of chromosome number 21. A single extra copy of chromosome 21 is also known as trisomy 21. People with trisomy 21 often have distinguishable facial features, physical and developmental delays.
