*Your ConquerHSC Notes are consistently being revised throughout the 2019 HSC Year to ensure quality
Week 6 Content
Overview of Week 6 Inquiry Question – How does mutation introduce new alleles in a population?
Learning Objective #1 – Explain how a range of mutagens operate, including:
- Electromagnetic radiation sources
- Naturally occurring mutagens
Learning Objective #2 – Compare the causes, processes and effects of different types of mutations including:
- Point mutation
- Chromosomal mutation
Learning Objective #3 – Distinguish between somatic mutations and germ-line mutations and their effect on an organism
Learning Objective #4 – Assess the significance of ‘coding’ and ‘non-coding’ DNA segments in the process of mutation
Learning Objective #5 – Investigate the causes of genetic variation relating to the processes of fertilisation, meiosis and mutation
Learning Objective #6 – Evaluate the effects of mutation, gene flow and genetic drift on the gene pool of populations
NEW HSC Biology Syllabus Video – Mutation
Week 6 Homework Set (Essential for Band 6!)
Week 6 Curveball Questions (Moving from Band 5 to Band 6!)
Week 6 Extension Questions
Solutions to Week 6 Questions
Overview of Week 6 Inquiry Question
Welcome back to Week 6 HSC Biology Syllabus Notes!
In this week notes, we will be exploring the importance of mutation in creating new alleles, thus increasing genetic variation and thus affecting evolution.
Mutation is the event where there is a permanent change in the DNA structure resulting in the genetic sequence of a gene (or genes) to be altered.
By altering the genetic sequence of a gene (thus creating a new allele), the amino acid sequence of the polypeptide chain to form a protein may be modified. The function and structure of the protein may be altered so that the protein’s effectiveness to perform its function is lowered or removed completely.
- Recall from Module 5 that if you alter the nucleotide sequence of a DNA of a gene, you will create a new allele. This is because an allele is just an different or alternative version of the same gene consisting of a different base sequence at the same chromosome locus (position).
The relevant definition of evolution here is the changes in allele frequencies in populations over time.
There are two main classes of mutation. These are spontaneous mutation and induced mutation.
Spontaneous mutation are mutation that occur normally without the effect of an environment agent (mutagen). So, these will include DNA replication errors during Interphase mitosis and meiosis that are not corrected by repair enzymes, natural chemical degradation of DNA, unequal crossing over, etc which we will explore later.
Spontaneous mutation (as well as induced mutation) is important in introducing and increasing genetic variation in populations.
Obviously, this would mean that induced mutation is mutation that is caused an environment agent which is also known as a mutagen. There are many types of mutagens which we will explore in the first learning objective.
Under both spontaneous and induced mutations, there are many types of mutations including point and chromosomal mutation. Point and chromosomal mutation can occur in somatic or germ cells.
The DNA sequence that is altered due to point and chromosomal mutation can be located in the coding or non-coding regions of DNA. The effect of mutated DNA’s location on the individual’s genome are different which we will explore.
We will also explore the effects of mutation, gene glow and genetic drift in affecting the gene pool of populations’ and thus their effect on evolution as a whole.
Without further ado, let’s start learning the practical materials!
Learning Objective #1 - Explain how a range of mutagens operate, including but not limited to:
- Electromagnetic radiation sources
- Naturally occurring mutagens
A mutagen is a synthetic or natural agent that cause mutation which greatly enhances the spontaneous rate of mutation.
Similarly, a mutant is an organism or cell that is affected by a mutation, usually with an altered phenotype.
Electromagnetic radiation sources
In Junior science, you would have learnt that there are many types of electromagnetic waves or radiation in the electromagnetic spectrum.
Out of these different types of electromagnetic waves, X-rays (X-radiation) and Ultraviolet radiation are two common examples of electromagnetic radiation to induces mutation as they have high ionising power.
Electromagnetic radiation or any other forms of high ionising radiation are known as physical mutagens.
The relationship between electromagnetic radiation and the rate of mutation is that they are proportional to each other. As you increase the dose of radiation, you will also increase the rate of mutation.
X - Rays
In Beadle and Tatum’s experiment to investigate the relationship between gene and protein, X-rays were used to mutate spores produced from the fungus Neurospora crassa. These mutated spores were then placed in a closed system consisting of an environment with minimal but sufficient nutrition for wild, non-mutated Neurospora crassa spores to develop into fungi and release new spores.
However, the experiment revealed that these mutated spores cannot develop new spores whereas non-mutated spores placed in the same environment could. However, when the amino acid arginine was added into the environment consisting of minimal but sufficient nutrition, the mutated spores could then develop new spores! Beadle and Tatum proposed that the reason for the results is that X-ray altered the nitrogenous base sequence of a gene in DNA that coded for an enzyme required to produce the arginine amino acid.
Therefore, X-rays prevented the enzyme to be produced via protein synthesis by the mutated spores. After adding in arginine amino acid, the mutated spores can be develop into fungus and produce spores.
In the end, Beadle and Tatum proposed the ‘one gene – one enzyme hypothesis’also known as the ‘one gene – one protein hypothesis’ as it was thought since an enzyme is a protein, they essentially mean the same thing.
However, later discoveries revealed that some proteins requires more than one polypeptide. For example, each haemoglobin protein in a red blood cell that has four polypeptide chains. Therefore, Beadle and Tatum’s hypothesis was modified to be known as the ‘one gene – one polypeptide’ hypothesis.
How does X-Ray act as a mutagen?
Keep it relevant for HSC Biology purposes, we do not need to go into too much detail here.
The information that you should know that X-ray radiation can alter the nitrogenous base sequence of DNA . X-ray has high ionising energy (short wavelength) allowing it to either ionising DNA directly or ionise the cellular water that is surrounding the DNA which then give rise to reactive radicals that then reacts with DNA. Either way, the nitrogenous base sequence of the DNA can be altered resulting in insertion or translocation mutations.
Insertion mutation, also known as frameshift mutation, is a type of mutation where one or more extra nitrogenous base is incorporated into the normal DNA sequence. This therefore alters the mRNA codon sequence and a different amino acid can be specified as a result.
Translocation mutation is a type of mutation where chromosomes can swap segments of their DNA sequence, resulting in one chromosome having a segment of another chromosome and vice versa.
How does ultraviolet radiation act as a mutagen?
Ultraviolet radiation can act as a mutagen by causing two adjacent base pairs to form covalent bonds with each other, resulting in a dimer structure.
Radicals cannot be formed using ultraviolet radiation as it is lower ionising strength than X-rays (less capable of removing electrons). So, instead, dimers may be produced due to effect of ultraviolet radiation as a result.
The most common type of dimer structure formed is called thymine dimer. In this case, ultraviolet radiation results in two adjacent thymine bases on the same DNA strand form covalent bonds with each other.
- Yes, sometimes dimers can be formed between base pairs on opposite DNA strands.
As a result, the DNA double helix structure is distorted at the position of which the dimer is located. The formation of the dimer molecule prevents both of the thymine bases from bonding with adenine bases on the opposite strand as the hydrogen bonds between A=T are weakened.
This dimer molecule is not replicated by DNA polymerase during DNA replication and thus no nucleotides are paired with the covalently bonded thymine nitrogenous bases. This therefore alters the mRNA sequence that specifies the amino acid sequence of the polypeptide chain used to produce the required protein.
This may result in the reduction in the protein’s activity or render it being unable to function completely. However, fortunately, we do have enzymes that can repair such cross links between adjacent bases.
In some cases such as excessive exposure to ultraviolet radiation, it causes thymine dimers to be formed amongst adjacent nitrogenous bases in critical genes in DNA. This may result in uncontrollable cell growth or development, leading to skin cancer.
These critical genes are:
- Oncogenes – responsible coding for proteins that help signalling cell growth and development.
- Tumour suppressor genes – responsible for coding proteins that stop/control cell growth & development.
- Stability genes – responsible for specifying for proteins in maintaining the rate of mutation of oncogenes and tumour suppressor genes
Sunscreen has organic molecules that are able to absorb the ultraviolet radiation and, thus, prevents ultraviolet radiation from interacting with your skin cells.
This is why you should wear sunscreen bois and gals!
Chemical mutagens are chemical substances that, when exposed to them, they have the capacity to give rise to mutation.
Chemical mutagens can alter the nitrogenous base sequence of DNA.
Alternatively, some chemical mutagens may insert themselves between nitrogenous bases in the middle of a DNA double helix whereby during DNA replication. This effectively distorts the double helix at the position where the position mutagen is inserted. This results in wrong nitrogenous bases being complementary paired to template strand and the chemical substance may be mistaken as a nitrogenous base during the replication process.
- Carcinogens are chemicals that are capable of causing cancer.
- Teratogens are chemical that are capable of causing birth defects in an offspring such as tetracycline.
Some examples chemical mutagens are aflatoxin B, nitrous acid, naphthyl amine and hydrocarbons present in cigarette smoke (e.g. benzopyrene)
Nitrous acid is a chemical mutagen has the capable to interact with DNA and swap a nitrogenous base with another.
- For example, cytosine may be swapped with uracil such that, during DNA replication, adenine is bonded to uracil instead of guanine. Also, in this case, uracil is now present in the DNA sequence which normally it is not.
- This effectively has alters the mRNA sequence during protein synthesis which may lead to the specification of a different amino acid sequence in the resultant polypeptide chain.
- The reason why I used the word ‘may’ is because sometimes, there are silent mutation. This is because in silent mutations, although the mRNA sequence is altered, the amino acid sequence is unchanged due to the codon degeneracy nature of the translation table.
- We have talked about codon degeneracy briefly in SNPs and protein synthesis learning objectives in previous weeks where there can be multiple codons that specifics for the same amino acid.
Benzopyrene is a chemical mutagen and a hydrocarbon present in cigarette smoke that is capable of being inserted between nitrogenous bases, effectively distorting the DNA double helix at the position where benzopyrene is added. This causes wrong nitrogenous bases being complementary paired to template strands during DNA replication.
- This effectively alters the mRNA sequence during protein synthesis which may lead to the specification of a different amino acid sequence in the resultant polypeptide chain.
Naturally occurring mutagens
Naturally occurring mutagens can be biological (‘living’) or non-biological (‘non-living’) in nature. In both case, these mutagens are classified as naturally occurring because they exist at natural environments.
Some biological mutagens can be substances that are formed as by-products of natural metabolic chemical reactions. For example, cycasin is a naturally occurring biological mutagen that is produced and present in the Australian Macrozamia plant.
Another biological mutagen is dimethylnitrosamine which is produced as a by-product in many animals from the chemical reaction between food containing nitrite and amines.
For example, if you cook sausage (containing sodium nitrite) with fish (containing amines), the combination of the nitriate and amine in the stomach’s natural metabolic processes can form the dimethylnitrosamine mutagen.
Non-biological mutagens include those that are not living or not produced by living organisms. Chromium is a metal and a non-biological mutagen that can be found naturally in sedimentary rocks. Water can dissolve chromium in waterways which end up in potable water. However, in our potable water, chromium exist in trace quantities (0.1 milligram per litre) that is unable to cause any biological harm.
- Some other metals include arsenic, cadmium and nickel.
Learning Objective #2 - Compare the causes, processes and effects of different types of mutation, including but not limited to:
- Point mutation
- Chromosomal mutation
Point mutation is when a nitrogenous base is changed at a particular locus on a chromosome. This effectively means that , similar to SNP, point mutation only affects one gene.
Some examples of point mutation includes the removal, addition and duplication of a nitrogenous base at a chromosome’s locus.
Recall that SNP is similar to point mutation in the sense that a nitrogenous base is altered in an organism’s DNA sequence.
However, as we have mentioned in Module 5’s notes, an event is only classified as a SNP if the modified nitrogenous base at a locus on a chromosome exist in more than 1% of the species’s population.
If less than 1% of the species’s population have the same modified nitrogenous base at the same locus of the chromosome then it is classified as a mutation event such as point mutation.
It is common in industry to say that SNP arise due to point mutation in the population but have persisted within the population. Comparatively, point mutation, by itself, can be an one time event that only happens to one organism or less than 1% of the species’s in the population.
Causes of point mutation
Point mutation can arise due to spontaneously (natural) or be induced (unnatural).
Spontaneous mutation deals with errors in normal DNA replication that are not repaired by enzymes resulting in insertion or deletion of a nitrogenous base from DNA sequence.
Induced mutation can be due to the exposure to environment agents such as ionising radiation, chemicals or naturally occurring substances.
We have already discussed both types of mutation in the previous learning objective.
Effect of point mutation
The effects of point mutation are identical to that of single nucleotide polymorphism that we have discussed in Module 5 notes.
Because they are identical, we will not go into detail in explanation and we will only do a brief recap.
- Please re-visit Module 5’s note on SNP for the extra detail on synonymous, non-synonymous, mis-sense and non-sense modification of a single nucleotide at a chromosome’s locus.
- We will introduce one new type of non-synonymous mutation here though, it is called frameshift mutation.
Before we talk about frameshift mutation, we will briefly re-visit the familiar terms that we talked about in SNPs in Module 5’s notes.
So, there are two broad categories of point mutation being coding point mutation and non coding point mutation.
Coding point mutation occurs within the coding region of DNA that specifies for a protein whereas the latter occurs outside the coding region of DNA.
Therefore, non-coding point mutation does not affect the amino acid sequence of polypeptide chains in protein synthesis whereas coding point mutation does.
There are two types of coding point mutation which are synonymous and non-synonymous mutations.
- For HSC purposes, synonymous mutation is also known as silent mutation which we have introduced in the previous learning objective.
Similar to synonymous SNPs, synonymous (neutral) mutation alters DNA sequence by substitution where a different nitrogenous base is added, effectively replacing of the original DNA base. Thus, the mRNA sequence is altered which leads to the different mRNA codon but still specifying for same the amino acid (due to codon degeneracy). The structure and function of the protein are not changed (negligible).
- NOTE: Again, since we have talked about synonymous and non synonymous SNPs in Module 5’s notes, we suggest you to revisit Module 5’s notes on SNPs for revision. We are not going into detail here as the effects of synonymous and non-synonymous SNPs & mutations are pretty much identical.
Similar to SNPs, non-synonymous mutations can be further divided into mis-sense and non-sense mutations.
- In mis-sense mutations, a different amino acid is specified due to the modified DNA sequence by substitution and thus modifying the mRNA codon sequence. A mutated form of the protein will be formed. For example, haemoglobin round shape is changed to a rod shape due to mis-sense mutation. This results in sickle red blood cells where its shape can block narrow blood vessels which not only lowers the oxygen carried to cells but can cause pain at different parts around the body. This condition is called sickle cell anaemia.
- In non-sense mutations, the different mRNA sequence leads to the specification of a stop codon rather than an amino acid. The resulting polypeptide and, thus protein, will be shortened with a lower operational efficiency or unable to function completely.
Lastly, there is one more type of non-synonymous mutation other than mis-sense and non-sense mutations. It is called frameshift mutation.
Frameshift mutation is a type of non-synonymous mutation where a nitrogenous base is inserted or deleted from the DNA sequence at a particular locus of a chromosome (gene), resulting a shift in the reading frame of the ribosome on the mRNA during translation.
If there is an insertion of one nitrogenous base, the reading frame of ribosome in translation of the mRNA codon sequence will be shifted one nitrogenous base forward. Vice versa, if there is a deletion of one nitrogenous base, the reading frame of ribosome on the the mRNA codon sequence will be shifted one nitrogenous backwards.
As mentioned already, point mutation where a nitrogenous base is changed at a particular locus on a chromosome, thereby only affecting one gene.
Comparatively, chromosomal mutation affects DNA on a chromosomal level meaning that a section of chromosome is changed rather than a single nitrogenous base. In chromosomal mutation, the section of chromosome that is affected contains more than one gene which means that it affects more than one gene.
Cause of chromosomal mutation
The cause of chromosomal mutation is same as point mutation, so see above to recap.
The only difference is that if chromosomal mutation occurs spontaneously, it won’t be occurring during DNA replication stage of mitosis and meiosis. Instead, it will occur during crossing over events in mitosis and meiosis.
High heat energy and viruses can also cause chromosomal mutation, encouraging the abnormal exchange of chromosomal sections.
Effect of chromosomal mutation
The effect of chromosomal mutation can take in four common forms: Deletion, Duplication, Inversion, Translocation. Each of these affects a section of chromosome which usually contains more than one gene.
Deletion chromosomal mutation involves the loss of a chromosome section. This means that any genes (usually more than one) that are located in that loss section is now lost.
The effect of the gamete that inherits such mutated DNA sequence will have some genes that are absent. Therefore, it cannot code for some proteins that are responsible determining its physical, physiological and behavioural traits.
Duplication chromosomal mutation is when a chromosome section is copied. Therefore the gamete may inherit an extra copy of the genes that are located in that copied chromosome section.
Inversion chromosomal mutation involves a chromosome section breaking off and recombining, in a reversed order, with the original chromosome. The effect is that the inversion of chromosome section can prevent crossing over from occurring. This would therefore lower genetic variation of the offspring that is formed from the gametes that inherited the mutated DNA sequence.
Lastly, translocation chromosomal mutation results in one chromosome section separating from a chromosome and combining with a non-homologous chromosome. The effect of this is similar to either deletion or duplication as one chromosome will have less genes whereas the other chromosome will have extra genes. A gamete can inherit either one of the DNA mutated sequence which means that the offspring can either have less genes than normal or a duplicated copy of the genes.
Rather than changing chromosome structure (as outlined above), sometimes chromosomal mutation can lead to changes in chromosome number present in the gamete and, thus, offspring that is derived from the gamete with the mutated chromosomes. This occurs when the mutated chromosomes are not able to be separated during meiosis.
Down syndrome disease is one disease that is caused by chromosomal mutation where offspring has one extra copy of the chromosome 21. This is because, due to chromosomal mutation, the mutated chromosome 21 cannot be separated during meiosis so offspring has an extra chromosome 21 (total of 3 copies). Normal humans are diploid so we only have two copies of each chromosomes.
- Fun Fact: There is fifth form of chromosomal mutation where chromosome is lost during fertilisation of gametes or development of offspring. But, that’s much rarer than the four above that is mentioned.
Learning Objective #3 - Distinguish between somatic mutations and germ-line mutations and their effect on an organism
Somatic mutations are mutations that occur in the DNA of an organism’s somatic cell(s).
Since somatic cells are produced via mitosis, this means that somatic mutation are only passed on via mitosis.
Effect of somatic mutation on an organism and its offspring(s)
Mutation in somatic cells will affect the organism as the mutated DNA sequence contained in a somatic cell is replicated to other cells during mitosis. As a result of this, the organism may develop a change in its characteristic. Recall in Module 5, we mentioned that an organism’s characteristic can be physical, physiological or behavioural.
In this case, somatic mutations can result in a change in the organism’s phenotype in affected area(s) such its body, arms, legs, etc. For example, somatic mutation may lead to cancer which may result in the shape of specific part of the body to change due to the uncontrollable growth of unspecialised cells (cancer cells) in that particular body area.
- Example: Skin cancer.
Alternatively, the somatic mutation may also result in a change in the organism’s physiological characteristics such as rendering the organism unable to produce a protein leading to the death of brain cells, known as Tay Sachs disease.
Mutations that occur in the DNA of somatic cells ARE NOT passed onto an organism’s offspring(s) as meiosis and fertilisation does not involve somatic cells. Therefore, the offspring will not inherit the mutation of its parent.
Germ-line mutation are mutations that occur in the DNA of an organism’s germ or germinal cells.
Since germ cells produce gametes via meiosis, this means that germ-line mutations are only passed on via meiosis.
Effect of germ-line mutation on organism and its offspring(s)
In most cases, germ-line mutations are silent meaning that it does not affect the physical (phenotype) or physiological or behavioural traits of the organism. This is because the organism’s somatic cells are not affected which makes up the bulk of the individual.
Mutations that occur in the DNA of germ cells ARE passed onto an organism’s offspring (s) as meiosis involves a germ cell and the DNA contained within the cell.
The germ cell undergoes meiosis to produce gametes where mutated DNA sequences of the germ cell can be passed onto gametes. When the gamete that inherited the mutated DNA sequence undergoes fertilisation, the resulting offspring will inherit the mutated DNA sequence.
As the zygote develops into an embryo via mitosis, the mutated DNA in the zygote cell will be replicated to form new cells such that the mutation will affect the overall embryo and, thus, the resulting offspring. More specifically, some of these new cells will differentiate into germ cells whereas others differentiate into somatic cells. The somatic cells will contain mutated DNA but it’s the mutated DNA in germ cells can then be be pass onto the gametes produced via meiosis to affected another new generation of offspring. This is because gametes can give rise to mutated offsprings of the next generation when fertilised.
Example: Sickle cell anaemia.
Learning Objective #4 - Assess the significance of 'coding' and 'non-coding' DNA segments in the process of mutation
So, in Module 5’s notes, we have already talked about non-coding and coding DNA regions or segments in SNPs. Moreover, we have also mentioned coding and non-coding DNA regions in the previous learning objective.
Now, we are going to assess the significance of these two different DNA regions in the event of mutation(s).
However, before we go that, I want to point out a fun fact that only approximately 3% of human’s total DNA belong in the coding region (i.e. DNA segments that do specify for proteins).
The rest of the DNA segments or sequences are in the non-coding region, i.e. these DNA sequences do not specify for the production of proteins. However, these DNA segments have an effect on gene expression which we will explore shortly!
Significance of DNA sequences in CODING regions of DNA
DNA sequences located in the coding regions of DNA are significant as they specify for the protein that is to be produced through protein synthesis. We have already talked about this during protein synthesis and protein structure & function in Module 5.
- So basically the DNA sequence in the coding region of DNA is responsible for the mRNA sequence during transcription. Then, the mRNA codons each specify for an amino acid, resulting in an amino acid sequence of a polypeptide chain during translation.
- One or more polypeptide chains can be folded to make a protein and determining its structure and function.
Therefore, mutation in the DNA sequence located in the coding region DNA will therefore affect the mRNA sequence, amino acid sequence and, thus, the structure & function of the resulting protein.
The resulting protein may therefore lose some OR all of its efficiency in performing its function due to changes in its structure.
- Again, the word ‘may’ is used here because some mutation can be silent or neutral as we have learnt in the learning objective #2.
In prokaryotic organisms are the opposite to eukaryotes in the sense that most of its DNA sequences are in the coding region of DNA. Most of these DNA sequences are responsible for the produce of enzymes with the function to repair DNA in the event of mutation.
Significance of DNA sequences in NON-CODING regions of DNA
The DNA sequences that are in the non-coding region of DNA are called regulatory sequences. These sequences in the non-coding region of DNA controls gene expression.
More specifically, they control where and when genes are expressed, the location where splicing to remove introns (non-coding regions of mRNA) and the location where ribosomes will bind to the mRNA.
- For example, if a mutation occurs in a DNA segment located in non-coding region of DNA, the production of small nuclear RNA responsible for splicing (removal of introns) prior to translation may be affected. This may render the small nuclear RNA to partially lose some of its function or completely lose its function. As a result, some introns will not be spliced away or removed in the mRNA before translation. This intron that is not spliced away could signal for a stop codon during the translation which affects the amino acid sequence of the resulting polypeptide chain. This would therefore affect the structure and function of the resulting protein.
- In other cases, the intron that is not spliced away could lead to some exons (coding regions of mRNA) to be skipped so that an amino acid may not be specified as a result.
If you recall, we have talked about late stage Alzheimer’s disease under the SNPs learning objective in Module 5. We mentioned that SNPs can increase the likelihood of an organism developing late stage Alzheimer’s. Similarly, it is found that mutation in an organism’s DNA sequence located in the non-coding region of DNA can also increase the likelihood of the organism in developing diseases including Alzheimer’s disease and other non-infectious diseases such as heart disease.
Learning Objective #5 - Investigate the causes of genetic variation relating to the processes of fertilisation, meiosis and mutation
We have already explored in detail of how fertilisation and meiosis can increase genetic variation in Module 5. More specifically, to summarise, this would include:
- Fertilisation involves the combination of gametes where the alleles for each gene in the two gametes are independently assorted and segregated in meiosis.
- Crossing Over
- Independent assortment
- Random segregation
However, since we have now talked about spontaneous and induced mutation can increase genetic variability, we should make a new category for it.
There are two forms of mutation, which we talked about, that can increase genetic variability. These are:
- DNA replication error during Interphase I meiosis that are NOT repaired or corrected by repair enzymes
- Unequal crossing over during Prophase I of meiosis*
- Transposon (Jumping genes)*
- Spontaneous chemical degradation of DNA*
- Electromagnetic radiation (ionising and non-ionising)**
- Exposure to chemical mutagens**
- Exposure to naturally occurring mutagens**
** We have already talked about these processes, as part of induced mutation, and their effect on the resulting protein in learning objective #1.
- Obviously, only whenthese induced mutation occur in the DNA of a germ cell, then the resulting offspring inherit the mutation of the parent and thus the population will have increased variation (by inheriting the new, mutated DNA sequence or allele).
Unequal crossing over during Prophase I of meiosis
Unequal crossing over can occur if the two homologous chromosomes that are undergoing crossing over are not lined up exactly side-by-side.
This means that one chromosome of the homologous pair could be positioned slightly towards the north pole of the cell whereas the other chromosome of the homologous pair is positioned slightly towards the south pole of the cell.
When crossing over occur, the result is that different lengths chromosome is exchanged between the two non-sister chromatids. This results in two chromosomes of different and abnormal lengths at the end of the crossing over.
One chromosome will have more genes whereas the other chromosome will have fewer genes. This will mean that the gamete that is fertilised to produce the offspring can have less or more genes than normal which will result in consequences in producing the required protein.
Transposon (Jumping Genes)
Transposons is a type of transposable element that is responsible for sponanteous
Transposable elements are sequences of DNA that is capable of moving around the genome. A type of transposable element is called transposons. These are sequences of DNA that is capable of moving from one location of the genome to another.
Fun Fact: They can move around and recombine at a different area in the genome by coding for an enzyme called transposase
- By chance, they may insert (or translocate) themselves in the middle of a gene. As a result of this, it may result in a frameshift mutation which we have talked about. As mentioned already, frameshift mutation causes the ribosome’s reading frame of the mRNA codon sequence to change. This means that the amino acid sequence may be altered and affect the structure and function of the protein which we have discussed already.
Spontaneous chemical degradation of DNA
Sometimes there are chemical reactions involving DNA resulting in the loss of functional groups that are located on the nitrogenous base.
For example, a chemical reaction can lead to the loss of an amino group (process known as deamination) that is located on cytosine. The result is that so that cytosine will be converted into uracil that should not be in DNA. This means that, during DNA replication, adenine will bind with the mutated uracil rather than guanine as cytosine is no longer present.
This will affect the mRNA sequence which may affect amino acid sequence and, thus, the structure and function of the resulting protein. By affecting the structure and function of the protein, the organism’s physical, physiological and behavioural traits may be affected.
Also, if the mutated DNA sequence occurs inside a germ cell, such mutation is passed onto gametes during meiosis which can result in the mutated parent’s offspring to inherit such mutation if the offspring is derived from the gamete that inherited the mutated DNA.
Similar to all other types of mutations that we have mentioned previously, DNA repair enzymes can correct such mutations. However, if DNA repair enzymes fails to correct such mutation then it will persist in the DNA and affect the outcome of DNA replication, transcription, translation, etc.
Learning Objective #6 - Evaluate the effect of mutation, gene flow and genetic drift on the gene pool of populations
In Module 5, we explored population genetics which deals with the study of the changes in the frequency of alleles or genotype in a population over time.
Gene pool essentially refers to all the genes present in all interbreeding organisms in population.
Of course that natural selection affects the genetic variation of a population’s gene pool by constantly selecting organisms with favourable characteristics to tolerate against the changing ambient environment’s selective pressures.
However, mutation, gene flow and genetic drift also affect the genetic variation of populations’ gene pool. We are going to examine them in this learning objective.
But how do biologists assess the change or distribution of genetic variation throughout a population? Well, we measure it in terms of allele frequency.
Keep in mind that although mutation, gene flow and genetic drift affect the allele frequencies of a populations, it is ultimately natural selection that selects the characteristics which are favourable against the ambient environment’s selective pressures.
Effect of mutation on populations' gene pool
As the rate of mutation of DNA in germ cells decreases, the frequency of new alleles passed onto offspring and thus, being introduced into the population, will decrease. Therefore, the genetic variation of a population’s gene pool will decrease.
Vice versa, as the rate of mutation of DNA in germ cells increases, the frequency of new alleles passed onto offspring and thus, being introduced into the population, will increase. Therefore, the genetic variation of a population’s gene pool will increase.
Effect of gene flow on populations' gene pool
Most populations are in fact not isolated as there generally subpopulations within a population. Each subpopulation consists of species that are close relatives to each other.
In terms of heredity, this means that not all genes passed onto offsprings in the next generation is not from one population but can also be due to the mating of species belonging to different subpopulations within the total population.
The term migration also refers to gene flow. Therefore, immigration of species’s from one subpopulations to another subpopulation will allow gene flow amongst species’s in each population. Sometimes, species from a population can also immigrate into a different population in which the mating of the species of the two population allows gene flow between the two populations.
As the species’s from the two populations mate with each other and exchange genetic material, the exchange of alleles will increase allele frequency in the population. Thus, it increases the genetic variation in the population which the one species migrated towards and performed mating activities.
This would also mean that the population where species emigrated (left to go to new population) had experienced a decrease in allele frequency. This means that the population where the species emigrated from have decreased in genetic variation.
The third situation is when there is continuous, high gene flow between the two populations where species from both originally genetically isolated populations immigrate to each other. This will result in the gene pool of the two populations to increase in genetic similarity over time due to gene flow. This would also mean that the genetic variation of both populations, that were originally genetically isolated, to increase due to increase in allele frequencies in species’ of both populations. Furthermore, this would obviously mean that the differences in the gene pools of two population due to mutation will be minimised as they become more genetically similar.
Effect of genetic drift on populations' gene pool
Genetic drift is the event where one or more alleles are lost in a population due to random events that happen by chance. As a result of genetic drift events, the allele frequency of the original population’s gene pool will decrease.
There are two explanations for genetic drift occurring within a population which are called:
- The bottleneck effect
- The founder effect
- Both effects explains genetic drift using events that happen by chance. Therefore, genetic drift is random and happen by chance.
The founder effect essentially refers to the separation (can be in the form of migration) of some species’ from the original population to another location. A barrier (e.g. geographical barrier) can separate or isolate the species’ of the original population from the newly migrated population.
The species that are separated from the original population are known as the founders of the new population that is established in the new location, isolated from original population. By random chance, the new population may have different allele frequencies compared to the old population and, thus, lower genetic variation in the gene pool compared to the old population. This is because, by chance, the founders may not carry all the genetic variation of the old population. The offsprings of the founders will therefore not inherit all the genetic variation present in the original population as the frequency of some alleles are effectively reduced to zero in newly founded population.
Similarly, the original population will experience a decline in the genetic variation due to isolation or migration of a portion of its population to form the new population. As a result, the original population will have lost some of its allele frequency which is measured as a decline in genetic variation in the original population’s gene pool.
- NOTE: Genetic drift has much greater effect on small population compared to large population as the % decrease in allele frequency is greater in smaller populations.
NEW HSC Biology Syllabus Video - Mutation
[Coming this Summer!]
Week 6 Homework Set (Essential for Band 5!)
Week 6 Homework Question #1: Distinguish between point and chromosomal mutation and account for their implications on the affected individual. [6 marks]
Week 6 Homework Question #2: Explain how point and chromosomal mutation can affect offsprings in the new generation and provide examples of such implications. [6 marks]
Week 6 Homework Question #3: List three causes of point and chromosomal mutation and account for the causes. [3 marks]
Week 6 Homework Question #4: Distinguish between somatic mutation and germ-line mutation. [3 marks]
Week 6 Homework Question #5: Explain the significance of mutation of genes in the coding and non-coding regions of DNA. [8 marks]
Week 6 Homework Question #6: Explain the concept of genetic drift and its effect a population’s gene pool. [7 marks]
Week 6 Homework Question #7: Explain the concept of gene flow and its effect on a population’s gene pool. [7 marks]
Week 6 Curveball Questions (Moving from Band 5 to Band 6!)
Week 6 Curveball Question #1: Define the term ‘mutation’ [2 marks]
Week 6 Curveball Question #2: Explain the concept of mutation and its effect on a population’s gene pool [6 marks]
Week 6 Curveball Question #3: Explain how mutation results in new alleles being introduced into a population. [5 marks]
Week 6 Curveball Question #4: Explain how mutation can be caused via electromagnetic radiation mutagens, chemical mutagens and naturally-occurring mutagens. [7 marks]
Week 6 Extension Questions
Solutions to Week 6 Questions