Comprehensive Notes on DNA, Mutations, and Evolution

DNA and Mutations

Key Concepts

• DNA: Review of structure, function, and location.
• Genes and Chromosomes: Relationship between DNA, genes, and chromosomes.
• Replication of DNA
• Mutations and Gene Pools: Types of mutations, causes and effects of mutations, gene pools, allele frequencies.
• Evolutionary Mechanisms: Natural selection, random genetic drift, migration, barriers to gene flow, genetic diseases.

Mutations

• Mutations in genes and chromosomes can result from errors in DNA replication, cell division, or from damage caused by mutagens.
• Different genotypes produce a variety of phenotypes, which are acted on differently by factors in the environment, producing different rates of survival.
• Mutations are the ultimate source of variation, introducing new alleles into a population; new alleles may be favorable or unfavorable to survival.

Gene Pools

• Populations can be represented as gene pools that reflect the frequency of alleles of a particular gene; gene pools can be used to compare populations at different times or locations.
• Gene pools are dynamic, with changes in allele frequency caused by:
  • Mutations
  • Differing selection pressures
  • Random genetic drift, including the founder effect
  • Changes in gene flow between adjoining groups
• The incidence of genetic diseases in particular populations illustrates the effects of different factors on the dynamics of gene pools, including the incidence of Tay-Sachs disease, thalassemia (\alpha and \beta), and sickle-cell anemia.
• Natural selection occurs when factors in the environment confer a selective advantage on specific phenotypes to enhance survival and reproduction.
• The mechanisms underpinning the theory of evolution by natural selection include inherited variation, struggle for existence, isolation, and differential selection, producing changes to gene pools to such an extent that speciation occurs.

DNA

• Describe the structure and function of DNA, Describe the relationship between DNA, genes and chromosomes. Describe how DNA replicates.
• Distinguish between nuclear DNA (nDNA) and mitochondrial DNA (mDNA).
• Define evolution, phenotype, genotype, allele, population, gene pool and allele frequency.
• Give an example of how gene pools can be used to compare populations at different locations or times.

Mutations (Definitions)

• Define mutation, mutagen (mutagenic agent), and mutant. List some known mutagens.
• Distinguish between induced mutations and spontaneous mutations.
• State that mutations in genes and chromosomes can result in errors in DNA replication, cell division or from damage caused by mutagens.
• Understand that all not all mutations are harmful; some contribute to human survival.
• Distinguish between the following types of mutations: gene mutation and a chromosome mutation and somatic mutation and a germinal (germline) mutation.
• Define what is meant by a point mutation
• Describe the different ways that mutations can be classified: heritability of the mutation, effect of the mutation, extent of the mutation.
• List and describe the 3 possible types of point mutations.
• Explain what a frameshift is and when it may occurs.
• List and describe the 5 possible types of chromosome mutations.
• Describe the following gene mutations in humans: albinism, Duchenne muscular dystrophy and cystic fibrosis.
• List and describe the 5 possible types of chromosome mutations.
• Define: aneuploidy, trisomy, monosomy and karyotype
• Explain the cause and symptoms of the following chromosomal mutations: Down syndrome, Klinefelter’s syndrome and Turner’s syndrome.
• Explain what is meant by a lethal recessive and the impact these can have on a gene pool.
• Describe Tay-Sachs disease – what it is, the cause, the prognosis.
• Explain how Tay-Sachs disease can lead to changes in allele frequencies in a population.
• 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.

Natural Selection

• Define gene flow. Distinguish between gene flow and migration.
• Describe how migration (gene flow) can cause changes in allele frequencies in a gene pool using an example.
• Distinguish between geographical barriers and sociocultural barriers. Give some examples of both types of barriers.
• Define natural selection.
• Explain the three observations that led Darwin to develop his theory of evolution.
• Explain the factors that are responsible the struggle for existence. Explain how different genotypes (therefore phenotypes) can affect the rate of survival.
• Explain the relationship between ‘survival of the fittest’ and ‘natural selection’. Explain the term selective agent.
• Describe the process of natural selection in terms of alleles frequencies in the gene pool of a population.
• 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.
• Describe what is meant by a selectively advantageous mutation.
• Use the incidence of sickle-cell anaemia to illustrate how natural selection operates in a population.
• Describe what is meant by a heterozygote advantage.
• Distinguish between alpha (\alpha) thalassemia and beta (\beta) thalassemia.
• Describe how the incidence of genetic diseases (Tay-Sachs, a and b thalassemia and sickle-cell anaemia) in particular populations illustrates the effect of different factors on the dynamics of gene pools.
• Give some examples of random genetic drift in human populations (Australian Aboriginal people on Bentinck Islands and the Dunkers in America).
• Describe how the founder effect can lead to changes in allele frequencies in small populations.
• List some examples of events that can cause a genetic bottleneck. (These events create a sudden drop in population size.) Understand that the founder effect and the bottleneck effect are extreme examples of RGD.
• Define species and speciation.
• Describe how a new species can be formed through inherited variation, struggle for existence, isolation, and differential selection.

Evolution

• Evolution is the change in characteristics of a species over time, a gradual change over generations, reflecting changes in allele frequency in populations.

Gene Pool

• A population is a group of organisms of the same species living together in a particular place at a particular time.
• The gene pool is the sum of all the alleles in a given population.
• Allele frequencies describe how often each allele of a gene occurs in the gene pool.
• Populations that differ in characteristics likely have different allele frequencies and different gene pools.

Mutations (Types)

• Offspring variations can arise from new alleles formed by mutations, which can be harmful.
• A mutant is an organism with a characteristic resulting from a mutation.
  • Gene mutations: changes in a single gene.
  • Chromosomal mutations: affect all or part of a chromosome.

Causes of Mutations

• Mutations occur spontaneously, but mutagens increase the rate.
  • Examples of mutagens: mustard gas, formaldehyde, sulfur dioxide, some antibiotics, ionizing radiation (UV light, X-rays, cosmic rays, radioactive waste).
  • X-rays during pregnancy can cause intellectual disability, skeletal malformations, or microcephaly in the child.

Understanding Gene Mutations

• Changes in DNA bases can alter amino acids and proteins, potentially having no effect, altering the protein, or preventing its production.
• Albinism, marked by an absence of pigment, results from one missing protein.
• Mutations are classified by cause, heritability, effect, and extent.

Cause of Mutation

• Induced mutations: caused by mutagens.
• Spontaneous mutations: random errors in biological processes like mitosis or meiosis.

Heritability of Mutation

• Somatic mutations: affect body cells, only impacting the individual.
• Germinal/Germline mutations: affect reproductive cells (gametes), passed on to future generations.

Effect of Mutation

• Missense mutations: change the amino acid, altering the protein.
• Nonsense mutations: change the base sequence to STOP, producing a shorter, non-functional protein.
• Neutral mutations: change an amino acid without significantly altering protein function.
• Silent mutations: no change in amino acid due to multiple base sequences coding for the same amino acid.

Extent of Mutation

• Gene mutation: affects a single gene.
• Chromosomal mutation: affects multiple genes, altering chromosome structure or number, often causing severe abnormalities and miscarriage.

Change in the DNA

• Point mutations: changes in a single nucleotide.
  • Insertion: adding a nucleotide.
  • Substitution: replacing a nucleotide.
  • Deletion: removing a nucleotide.
• Frameshift mutations: occur when bases are added or removed, altering the reading frame and affecting all subsequent DNA coding (unless three bases are added or deleted).

Other Mutations

• Duplication (or insertion): a section of chromosome occurs twice
• Deletion: a piece of DNA is removed
• Inversion: breaks occur in a chromosome and the broken piece joins back in, but the wrong way around
• Translocation: part of a chromosome breaks off and is rejoined to the wrong chromosome
• Non-disjunction (Aneuploidy): 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.

Conditions Due to Mutations

Gene Mutations

• Duchenne muscular dystrophy: may arise from a mutation in the mother or in a male zygote, leading to muscle wasting and respiratory failure.
• Cystic fibrosis: caused by a mutation on chromosome 7, affecting a protein that regulates chloride ion passage across the cell membrane, leading to various symptoms.
  • The mutant allele is recessive.

Chromosomal Mutations

• Trisomy: result of non-disjunction, failure of one or more chromatids to separate in the second division of meiosis.
• Down syndrome (trisomy 21): three copies of chromosome 21, leading to characteristic facial expression, intellectual disability, weak muscles, heart defects, and digestive abnormalities.
  • Partial trisomy: part of an extra copy of chromosome 21 is attached to one of the other chromosomes.
• Patau syndrome (trisomy 13): extra chromosome 13, causing intellectual disability, microcephaly, extra fingers, cleft palate/lip, and malformations of ears and eyes.
• Klinefelter syndrome (XXY): males with an extra X chromosome.

Monosomy

• Monosomy: missing a chromosome.
  • If an autosome is completely missing, monosomy usually results in severe malformations and miscarriage.
  • Partial monosomy: part of a chromosome is missing, such as in Cri-du-chat syndrome (missing portion of chromosome 5).
• Turner syndrome (monosomy X): individuals with only one X chromosome.

Lethal Recessives

• Most gene mutations produce a recessive allele.
• Lethal recessives: cause death of the embryo/fetus or early death of the child if not masked by a dominant normal allele.
• Tay-Sachs disease (TSD): a lethal recessive condition caused by a mutation in the HEXA gene that codes for the enzyme beta-hexosaminidase.

Migration

• Changes in allele frequencies in a gene pool can also be due to gene flow brought about by migration.
• Gene flow: the movement of genetic material from one population to another.
• Migration: movement of individuals between populations.
• Example: The change in the frequency of the I^B allele across Europe and Asia due to Mongol invasions.

Barriers to Gene Flow

• Barriers inhibit interbreeding between populations, leading to separate gene pools.
  • Geographical barriers: include oceans, mountain ranges, large lake systems, deserts and expansive ice sheets.
  • Sociocultural barriers: economic status, educational background, social position, religion, and language.

Development of the Theory of Evolution

• Evolution is a gradual change in the characteristics of a species.
• Darwin's theory of natural selection was based on observations of variation, birth rate, and nature's balance.
• Darwin observed the differences and similarities between animals separated by:
  • geography – those living on the mainland of South America and those on the various islands
  • time – animals recently extinct and species still alive.

Darwin’s Theory of Natural Selection

• Variation: all members of a species vary, and these variations are passed on.
• Birth rate: organisms reproduce at a rate greater than the increase in food supply and resources.
• Nature’s balance: species numbers tend to remain constant.
• Struggle for existence: due to excessive birth rate and limited resources.
• Survival of the fittest: organisms with favorable characteristics survive and reproduce, passing on their traits.
• Natural selection: the selection of alleles in a population that give an organism a greater survival advantage.
• The environmental factor acting on the population is known as the selective agent.
  • There is variation of characteristics within a species
  • More offspring of a species are produced than can possibly survive to maturity.
  • Because of excessive birth rate and limited resources, there is a struggle for existence or competition for survival.
  • Individuals with characteristics best suited to the environment have more chance of surviving and reproducing. This is known as survival of the fittest.
  • Favourable characteristics are passed on to the next generation.
  • In the gene pool, the proportion of alleles that produce favourable characteristics gradually increases

Examples of Natural Selection

• Sickle-cell anaemia

  • Sickle-cell disease results when a person is homozygous for a particular recessive allele. The different base sequence means that the amino acid valine is added instead of glutamic acid.
  • People who are heterozygotes normally show no ill effects unless oxygen is in short supply. When this occurs, their red blood cells show mild sickling.
  • These individuals are carriers and are said to have sickle-cell trait.
    • Its now generally accepted that individuals heterozygous for the sickle-cell allele have a survival advantage in areas where malaria is prevalent. This is known as a heterozygote advantage.

• Tay-Sachs disease

  • Tay-Sachs disease is a recessive condition found only in individuals who are homozygous recessive.
  • People who are heterozygous have a reduced amount of beta-hexosaminidase. It appears that these individuals have some protection from tuberculosis.

Genetic Drift

• Genetic drift: random, non-directional change in allele frequency between generations, significant in small populations.
  • Genetic drift is also known as random genetic drift or the Sewall Wright effect.

Founder Effect

• Founder effect: a small group establishes a new population in a new area, potentially leading to different allele frequencies and decreased genetic variation.

Bottleneck Effect

• Bottleneck effect: a natural disaster reduces population size, potentially leading to different allele frequencies by chance.
• The chance of survival is by chance and not due to a specific trait

Speciation

• Species: a group of individuals that share many characteristics and are able to interbreed under natural conditions to produce fertile offspring.
• Reproductive isolation leads to separate gene pools and, over time, the development of distinct species (speciation).