DNA, Mutations and Natural Selection

Structure of DNA

• Arranged as a double-helix

• Two strands that run in opposite directions

• Each strand consists of a chain of nucleotides

• Nucleotides consist of deoxyribose sugar, phosphate group and a nitrogenous base

• Nitrogenous bases on opposite strands are complementary

• Adenine pairs with thymine and cytosine pairs with guanine

• Complementary bases held together by hydrogen bonds

Function of DNA

A molecule in the nucleus of a cell that determines the types of protein a cell can make and codes for these proteins.

Describe how DNA replicates

• Occurs in the nucleus of cells during interphase before mitosis

• The enzyme Helicase unzips the DNA strand

• Free DNA nucleotides pair up using DNA as template

• Due to complementary base pairing

• DNA polymerase joins the nucleotides forming a sugar-phosphate backbone

• Ligase joins the nucleotides to the sugar phosphate backbone,

• Two identical copies are formed

Nuclear DNA vs Mitochondrial DNA

• Most of DNA molecules found in the nucleus of each cell, known as nuclear DNA

• Small amount of DNA found in the mitochondria, known as mitochondrial DNA (mtDNA)

• Makes up less than 1% of total DNA in humans

• Important for the functioning of the mitochondria and therefore the cell

Evolution definition

The change in characteristics of a species over time.

Phenotype definition

Observable characteristics of the individual - the way the genotype is expressed.

Genotype definition

Genetic information carried by an individual.

Allele definition

Different forms of the same gene.

Population definition

A group of organisms of the same species living together in a particular place at a particular time.

Gene pool definition

Sum of all the alleles in a given population.

Allele frequency

How often each allele of a gene occurs in the gene pool for that population.

 

Give an example of how gene pools can be used to compare populations at different locations or times.

• When studying a population, geneticists are interested in how often each allele of a gene occurs in the gene pool for that population. These are called the allele frequencies for the population.

• Populations that differ in the characteristics they possess are likely to have different frequencies of the various alleles of a gene in their respective gene pools. Therefore, any two populations having differing characteristics are likely to have different gene pools.

Mutation definition

Mutations are changes in the DNA resulting in variation in the associated trait. They may occur spontaneously, but are often due to exposure to a mutagen.

Mutagen definition

Agent which is known to increase the rate at which mutations occur.

Mutant defintion

An organism which a characteristic resulting from a mutation is called a mutant.

List mutagens

• Formaldehyde

• Mustard Gas

• Sulfur dioxide

• Antibiotics

• Cigarette smoke

• Pesticides

• Ionising Radiation

  • Ultraviolet light X-ray

  • Cosmic rays

  • Fallout from atomic and nuclear explosions

  • Radiation from radioactive waste

Induced mutations defintion

Increased chance of a mutation occurring due to the presence of a mutagen (mutagenic agent)

Spontaneous mutations definition

A mutation which occurs due to a random error in a biological process such as mitosis or meiosis.

Gene mutation defintion

Changes in a single gene - traits usually produced by that gene are changed or destroyed. Example: sickle cell anaemia

Chromosome mutation defintion

 All or part of the chromosome is affected, therefore a number of genes are affected. Often causing abnormalities so severe that miscarriage can occur in early pregnancy. Example: Trisomy 21 (Down syndrome)

Somatic mutation

• A persons body cells (somatic cells) are involved

• Only the individual with the somatic mutation is affected

• Mutation is passed onto daughter cells each time the body cell divides

• Reproductive cells are not affected - once the individual dies the mutation is lost

• Example: cancerous growths that may be the result of a mutagenic agent

 

Germinal/Germline Mutations

• Reproductive cells are affected (germline cells)

• The individual is not typically affected but produces gamete with changed DNA

• Mutations can be passed on to the next and subsequent generations

• An affected gamete and subsequent embryo can lead to a miscarriage during pregnancy

• Example: PKU phenylketonuria

Point mutation definition

A point mutation is a type of genetic mutation that involves changes in a single nucleotide within the DNA sequence. This means that only one base is changed.

3 different ways mutations can be classified

Heritability of the mutation, effect of the mutation, extent of the mutation.

Heritability of the mutation

Somatic and Germinal/Germline Mutations

Effect of mutation

Missense Mutations, Nonsense Mutations, Neutral Mutations, Silent Mutations

Missense mutations definition

Cause a change in the amino acid, and therefore in the protein produced.

Nonsense mutations definition

Change the base sequence to the code to STOP. This means that the synthesis of the protein will stop, and so a shorter protein is produced that is unlikely to be able to fulfill its function.

Neutral mutation definition

Cause a change in an amino acid; however, the amino acid id of the same type and does not change the structure of the protein enough to change its function.

Silent mutation definition

Do not cause any change in the amino acid, and therefore in the protein produced. This is possible, as most amino acids are coded for by more than one base sequence.

3 types of point mutations

• Inserted - a new nucleotide is added to the DNA strand

• Substituted - an existing nucleotide is replaced with another one, with a different base

• Deleted - a nucleotide is removed from the DNA strand

Frameshift definition

• Occurs when bases have been added or removed

• Results in the series of three bases that code form an amino acid starting at a different base

• Frameshift mutations affect the outcome for all the DNA from that point on

List 5 types of chromosomal mutations

Duplication, Insertion, Deletion, Inversion, Translocation

Duplication mutation definition

A portion of the chromosome is copied twice and the gene sequence is repeated. This could cause the overproduction of a protein which could have a negative effect on the organism.

Insertion mutation definition

A segment of one chromosome is inserted into another chromosome. This can disrupt the genes at the insertion site and lead to various genetic disorders.

Deletion mutation definition

This occurs when the chromosome breaks and a fragment of the DNA, a gene is lost. This means that the chromosome is missing part of the genetic code, which can be lethal if it contained the code for an essential protein such as an enzyme.

Inversion mutation definition

A deletion occurs but this time the fragment reattaches in an inverted (upside down) position. The correct genes are present but in the wrong order so code for an incorrect protein.

Translocation mutation definition

A fragment of the chromosome becomes deleted and rejoins at a different position on the same chromosome. 

Non-Disjunction mutation definition

• During meiosis, a chromosome pair does not separate and so one daughter cell has an extra chromosome and one daughter cell has one less than the normal number.

• These are sometimes referred to not as 'mutations', but as aneuploidy - a change in the chromosome number.

Aneuploidy definition

Aneuploidy refers to a change in the chromosome number. It is a term used to describe situations where a cell has too many or too few chromosomes, rather than a typical gene mutation.

Trisomy definition

Trisomy is a result of non-disjunction, failure of one or more chromatids to separate in the second division of meiosis. The eggs or sperm formed when non-disjunction occurs have one chromosome too many, or one chromosome is missing.

Monosomy defintion

Monosomy is where an individual is missing a chromosome. If an autosome is completely missing, monosomy usually results in severe malformations and miscarriage. If only part of a chromosome is missing, it is referred to as partial monosomy. Part of the chromosome has two copies, but part has only one copy.

Karyotype defintion

A karyotype is a visual representation of an individual's chromosomes, showing their number, structure, and size. It's a genetic test used to detect chromosomal abnormalities that can cause various health problems, including infertility, genetic disorders, and birth defects. 

Explain what is meant by a lethal recessive and the impact these can have on a gene pool.

• Most gene mutations produce a recessive allele because they prevent the gene from producing a protein that will be able to function in the body.

• A person could therefore have large numbers of mutations in the genes and be totally unaware of them.

• If the person reproduces with a partner who has the same recessive mutation, the recessive condition could appear in their offspring.

• Some recessive mutations are lethal if they are not masked by a dominant normal allele.

• These lethal recessives cause the death of the embryo or foetus by a miscarriage or spontaneous abortion, or the early death of the child.

• Tay-Sachs disease (TSD) is a disorder of lipid metabolism that is inherited in an autosomal recessive pattern. It is a lethal recessive condition caused by a mutation in the HEXA gene that codes for the enzyme beta-hexosaminidase.

 

Explain the significance of mutations in the process of evolution – they are the ultimate source of variation introducing new alleles into a population: new alleles may be favourable or unfavourable to survive.

Mutations are fundamentally significant to evolution as they are the ultimate source of variation within a species, introducing entirely new alleles into a population's gene pool. These changes in DNA, which can occur spontaneously or be induced by mutagens, lead to different genotypes and subsequently a variety of phenotypes (observable characteristics, body functioning, and behaviour). Environmental factors, known as selective agents, act differently on these diverse phenotypes, influencing survival rates. While many mutations are unfavourable, such as those causing albinism, Duchenne muscular dystrophy, or cystic fibrosis, some can be neutral, and a few may even be favourable, providing a "survival advantage". For instance, the sickle-cell allele, a point mutation, offers a heterozygote advantage by providing resistance to malaria. This variation, generated by mutations, is the raw material upon which natural selection acts, leading to the "survival of the fittest" and a gradual increase in favourable alleles within the gene pool over generations, thereby driving the process of evolution.

Define gene flow

The movement of genetic material from one population to another.

Distinguish between gene flow and migration

Gene flow is the movement of genetic material from one population to another where as migration is the movement of individuals from one population to another. When individuals move between populations, they enable gene flow. This movement is known as migration.

Describe how migration (gene flow) can cause changes in allele frequencies in a gene pool using an example.

• IB allele thought to have not existed in Europe.

• Much higher frequency in inhabitants of East Asia.

• Mongols invaded Europe multiple times in 12th and 13th century and introduced into the population. 

Barriers

• Isolation results in the formation of separate gene pools.

• As no two environments are identical, environmental pressures may lead to

• selection of different characteristics in different populations.

• Any barrier to gene flow (isolation) between populations will result in

• different allele frequencies.

Geographical barriers examples

• Oceans

• Mountain ranges

• Large lake systems

• Deserts

• Expansive ice sheets

Sociocultural barriers

• Economic status

• Educational background

• Social positions

• Religious beliefs

• Language

Geographical barrier definition

A physical obstacle that separates populations of species, leading to the formation of distinct groups that may eventually evolve into new species due to limited gene flow.

Sociocultural barrier definition

Obstacles to communication, access, and understanding that arise from differences in social and cultural norms, values, and beliefs.

Define natural selection

Natural selection is the change in allele frequency in population as a result of a selective agent.

Explain the three observations that led Darwin to develop his theory of evolution.

1. Variation: all members of a species vary and variation could be passed onto next generation.

2. Birth rate: all living organisms reproduce at rate greater than food supply and other resources increase.

3. Nature’s balance: although high birth rate, species numbers relatively constant.

Explain the factors that are responsible the struggle for existence.

Excessive Birth Rate: Darwin observed that all living organisms reproduce at a rate far greater than the rate at which their food supply and other resources increase. This high birth rate would normally lead to overcrowding.

Limited Resources: Because the birth rate is high and resources like food supply are limited, there isn't enough for all offspring to survive. This scarcity of resources creates a situation where organisms must compete to survive.

Explain how different genotypes (therefore phenotypes) can affect the rate of survival.

Different genotypes within a population lead to a variety of phenotypes, which in turn are acted on differently by factors in the environment, resulting in varying rates of survival. This process is central to natural selection, where certain alleles become more or less frequent in a population's gene pool over time.

• Introduction of Variation by Mutations: The ultimate source of this variation is mutations, which are changes in the DNA that introduce new alleles into a population. These new alleles can be favourable, unfavourable, or neutral for survival. When the DNA of a particular gene is altered by a mutation, the protein it codes for could be missing or abnormal, which can profoundly affect the entire body.

• Impact on Phenotypes: The new alleles give rise to different genotypes (the genetic makeup), which then express themselves as a range of phenotypes (the observable physical characteristics, body functioning, and behaviour). These distinct phenotypes interact with the environment, leading to different survival rates.

• Selective Agents and Survival: Environmental factors, known as selective agents, exert pressure on these phenotypes. Individuals whose phenotypes are better suited to their environment are more likely to survive and reproduce, passing on their advantageous alleles. Conversely, those with less favourable characteristics are more likely to perish before reproducing, and their traits are not passed on.

Explain the relationship between ‘survival of the fittest’ and ‘natural selection’.

'Survival of the fittest' describes the outcome of the "struggle for existence," where individuals with characteristics best suited to their environment are more likely to survive and reproduce. This phenomenon is a key part of natural selection, which is the change in allele frequency within a population driven by environmental factors known as selective agents. Over time, this process leads to an increase in favourable alleles in the gene pool, making the species better adapted to its environment.

Explain the term selective agent.

A selective agent is an environmental factor that acts on a population. This factor influences which individuals are more likely to survive and reproduce within the "struggle for existence". E.g. food source, predation, temperature, pesticides

Describe the process of natural selection in terms of alleles frequencies in the gene pool of a population.

1. Variation: look for the example of variation within the question. Would one allele have an advantage - explain this. This variation has occurred due to mutations.

2. Overpopulation: birth rate exceeds death rate. More offspring are produced than can possibly survive to maturity.

3. Because of this there is a 'struggle for existence" or competition or there is a selective pressure that exists or has just happened (need to identify the selection pressure in the exam question).

4.  Organisms which are better suited to withstand this selection pressure will survive / have more chance of survival and will survive to sexual maturity.

5. This is called survival of the fittest. This is also due to the fact that the organism has a specific allele which makes it "fitter" and able to withstand the selective pressure.

6. Those that do not have this allele will not survive and die.

7. The organisms which survive to maturity will breed with each other and pass these alleles to the next generation.

8. In the gene pool the number of organisms with this characteristic (identify what this is) will increase.

9. This does not happen in the first generation, this will occur over time / several generations.

10. And the frequency of this allele causing a specific characteristic will also increase in the gene pool.

Explain the survival advantage that people of short stature have in cold climates and people with long limbs and short trunks have in hot environments.