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Peter Preethlall 1
INTEGRATING MOLECULAR STUDIES AND EVOLUTION
Presented by
Peter PreethlallDCES:TLS-FET, Umlazi District
FEBRUARY 2011
Peter Preethlall 2
INTEGRATING MOLECULAR STUDIES AND EVOLUTION
Outcomes:Educators will understand
an overview of the above contenthow some aspects of the above may be integratedThe concepts of gene mutation and chromosomal aberrationCauses and effects of mutation
3Peter Preethlall
INTEGRATING MOLECULAR STUDIES AND EVOLUTION
Format of presentation:1. Evolutionary science and
society2. Relationship among: variation,
natural selection and speciation
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BIODIVERSITY, CHANGE & CONTINUITYEvolutionary science and society:1. Conservation / preservation1. Selective breeding2. Health applications:
3.1 Pathogens evolve – drug resistance ?3.2 Identification of pathogens – now and future by phylogenetic analysis3.3 Vaccine development and use3.4 Origins of emerging disease3.5 Population diversity and the evolution of antibiotic resistance 3.6 Discovering new drugs3.7 Predict disease outbreaks and charaterize, trace the origins of and fight diseases3.8 Better understand human physiology, dietary needs, adaptations to health stressors3.9 Identify organisms and metabolic processes for bioremediation
“Evolutionary biology is medicines missing basic science”
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Explanation of Evolution in terms of Current Knowledge
NATURAL
SELECTION
VARIATION
EVOLUTION
Sources of variation
1. meiosis:
* crossing-over
*random arrangement of chromosomes
2. chance fertilization
3. Mutations
-adaptation to the environment
-’survival of the fittest’
-micro-evolution – speciation
- macro-evolution
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Explanation of Evolution in terms of Current Knowledge
Current theories accept Darwin’s ideas on natural
selection but :
Explain the sources of variation
Distinguish between micro-evolution, speciation
and macro- evolution
Provide possible explanations for mass extinctions
Provide “evidence” for evolution
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WHAT CAUSES VARIATION?
Members of a population vary from one anotherVariation is the raw material for evolutionary changeThis is controlled by genesArises by recombination; gene mutation and chromosomal mutationOnly gene mutations result in new allelesChromosomal mutations and recombination – contribute greatly to the production of variant genotypes and phenotypes
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Explanation of Evolution in terms of Current Knowledge
Sources of phenotypic variation: current viewsPhenotypic variations are due to variations in the genetic constitution (genotype)The genotype might be different because ..
Meiosis brings about the recombination of chromosomes and alleles which results in the formation of unlike gametes.
* crossing-over between non-sister chromatids * independent assortment of chromosomes Chance/Random fertilisation of egg cells
by sperm cells Mutations
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Sources of genetic variation :
Meiosis Gametes produced by
meiosis are different because of :Crossing-over during first prophaserandom arrangement of chromosomes during first metaphase
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Sources of genetic variation: Chance / Random Fertilisation
– Usually more than one egg cell and sperm cell is produced
– Fertilisation is a chance process
– If there were just 4 egg cells and 4 sperm cells there are 16 possible genotypes of the offspring
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Sources of genetic variation: Chance / Random Fertilisation
The entire genotype and NOT individual alleles is subjected to the natural selection processE.g. In a population of snails stripes & brown colour combined might make them less visible in a woodland habitat,If stripes are controlled by one allele and brown colour by another allele, it is a combination of the two alleles that will be selected for.Recombination may at some time bring the two alleles together so that the combined phenotype can be subjected to natural selection.
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MUTATIONS
Have you ever copied a phone no. incorrectly?
• What are some of the possible consequences of this?
• Mistakes in the DNA code can produce similar results
• Sometimes – no effect on organisms, but often causes serious consequences for individual organisms
Newcastle Hospital
035 20910
3
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MUTATION – A CHANGE IN DNA
A change or mistake in the DNA sequence is called a mutation.
Generally occurs during the cell processes that copy genetic material and pass it from one generation to next
These processes are usually accurate to ensure genetic continuity in both new cells and offspring
However, sometimes mistakes can occur
Changes in the DNA base sequence is referred to as gene mutations
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Sources of genetic variation: Mutations
Mutations are sudden, random changes in the genetic code of an organism
There could be gene mutations and chromosomal mutations
Gene mutations
Gene mutations provide new alleles, and are therefore the ultimate source of variation. A gene mutation is an alteration in the DNA nucleotide sequence of an allele.
Mutation rates are very small in nucleic DNA(1 in 100 000 to 1 in 10 000 000) but rather high in mt DNA.
If the human genome has 50 000 genes, it means that half the egg cells and half the sperm cells will have mutations
Which means that all of us have at least one mutant gene!
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GENE MUTATIONS
NormalmRNA
Protein
Point mutation
mRNA
Protein
The base G was replaced with A. Resulted in insertion of serine instead of glycine into the growing aa chain – creating another protein. Sometimes these errors do not interfere with protein function, but often the effect is disastrous.
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GENE MUTATIONS
Point Mutation-Is a change in a single base pair in DNA
Effects of point mutationConsider the ffg. analogy:THE DOG BIT THE CAT
THE DOG BIT THE CAR
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GENE MUTATIONS
mRNAProtein
NormalmRNA
Protein
Frameshift
mutationProteins produced through fm seldom function properly. Why? Adding or deleting one base of DNA molecule will change every amino acid in the protein after the addition or deletion.
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GENE MUTATIONS
Frameshift mutations- A mutation in which a single base is
added or deleted from DNA
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GENE MUTATIONS
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Sickle cell anaemia – missense mutation
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GENE MUTATIONS
Bending impairsits function
Part ofprotein isremoved
New sectionof amino acidsintroduced
LDLR gene causing FH – different mutations can cause the same disease
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GENE MUTATIONS
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Chromosomal AberrationsThe somatic (2n) and gametic (n) chromosome numbers of a species ordinarily remain constant. This is due to the extremely precise mitotic and meiotic cell division. Somatic cells of a diploid species contain two copies of each chromosome, which are called homologous chromosome. Their gametes, therefore contain only one copy of each chromosome, that is they contain one chromosome complement or genome. Each chromosome of a genome contains a definite numbers and kinds of genes, which are arranged in a definite sequence.
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Chromosomal Aberrations
Sometime due to mutation or spontaneous (without any known causal factors), variation in chromosomal number or structure do arise in nature. - Chromosomal aberrations.Chromosomal aberration may be grouped into two broad classes: 1. Structural and 2. Numerical
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Structural Chromosomal Aberrations
- These are changes at the level of chromosomes
- May occur in a variety of ways* parts of chromosomes are broken off and lost during mitosis or meiosis* Chromosomes may break and rejoin incorrectly* Sometimes the parts join backwards or join to the wrong chromosomes
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Structural Chromosomal Aberrations
There are four common type of structural aberrations:
1. Deletion or Deficiency 2. Insertion /Duplication or Repeat3. Inversion, and 4. Translocation.
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Structural Chromosomal Aberrations
Deletion
Occurs when part of a chromosome is left out
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Structural Chromosomal AberrationsDeletion generally produce striking genetic and physiological effects. When homozygous, most deletions are lethal, because most genes are necessary for life and a homozygous deletion would have zero copies of some genes. When heterozygous, the genes on the normal homologue are hemizygous: there is only 1 copy of those genes.Crossing over is absent in deleted region of a chromosome since this region is present in only one copy in deletion heterozygotes.
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Structural Chromosomal Aberrations
Deletion in Humans:
Chromosome deletions are usually lethal even as heterozygotes, resulting in zygotic loss, stillbirths, or infant death. Sometimes, infants with small chromosome deficiencies however, survive long enough to permit the abnormal phenotype they express.
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Structural Chromosomal Aberrations
INSERTION / DUPLICATION
Occurs when part of a chromatid breaks off and attaches to its sister chromatid. The result is a duplication of genes on the same chromosome
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Structural Chromosomal Aberrations
INVERSIONS
Occur when part of one chromosome breaks out and is inserted backwards
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Structural Chromosomal Aberrations
TRANSLOCATIONS
Occur when part of one chromosome breaks off and is added to a different chromosome
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Non-Disjunction
Generally during gametogenesis the homologous chromosomes of each pair separate out (disjunction) and are equally distributed in the daughter cells. But sometime there is an unequal distribution of chromosomes in the daughter cells. The failure of separation of homologous chromosome is called non-disjunction.This can occur either during mitosis or meiosis or embryogenesis.
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Mitotic non-disjunctionMitotic non-disjunction: The failure of : The failure of separation of homologous chromosomes separation of homologous chromosomes during mitosis is called mitotic non-during mitosis is called mitotic non-disjunction. disjunction.
It occurs after fertilization.It occurs after fertilization.
May happen during first or second May happen during first or second cleavage. cleavage.
Here, one blastomere will receive 45 Here, one blastomere will receive 45 chromosomes, while other will receive 47. chromosomes, while other will receive 47.
Meiotic non-disjunctionMeiotic non-disjunction: The failure of : The failure of separation of homologous chromosomes separation of homologous chromosomes during meiosis is called meiotic non-during meiosis is called meiotic non-disjunctiondisjunction
Occurs during gametogensisOccurs during gametogensis
Here, one type contain 22 chromosome, Here, one type contain 22 chromosome, while other will be 24.while other will be 24.
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Variation in chromosome number
Organism with one complete set of chromosomes is said to be euploid (applies to haploid and diploid organisms).
Aneuploidy - variation in the number of individual chromosomes (but not the total number of sets of chromosomes).
The discovery of aneuploidy dates back to 1916 when Bridges discovered XO male and XXY female Drosophila, which had 7 and 9 chromosomes respectively, instead of normal 8.
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• Nullisomy - loss of one homologous chromosome pair. (e.g., Oat )
• Monosomy – loss of a single chromosome (Maize).
• Trisomy - one extra chromosome. (Datura)
• Tetrasomy - one extra chromosome pair.
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Uses of Aneuploidy
They have been used to determine the phenotypic effect of loss or gain of different chromosomeUsed to produce chromosome substitution lines. Such lines yield information on the effects of different chromosomes of a variety in the same genetic background.They are also used to produce alien addition and alien substitution lines. These are useful in gene transfer from one species to another. Aneuploidy permits the location of a gene as well as of a linkage group onto a specific chromosome.
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Trisomy in Humans
Down SyndromeThe best known and most common chromosome related syndrome. Formerly known as “Mongolism”1866, when a physician named John Langdon Down published an essay in England in which he described a set of children with common features who were distinct from other children with mental retardation he referred to as “Mongoloids.”One child in every 800-1000 births has Down syndrome250,000 in US has Down syndrome.The cost and maintaining Down syndrome case in US is estimated at $ 1 billion per year.
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Trisomy in Humans
Patients having Down syndrome will Short in stature (four feet tall) and had an epicanthal fold, broad short skulls, wild nostrils, large tongue, stubby handsSome babies may have short necks, small hands, and short fingers.They are characterized as low in mentality.Down syndrome results if the extra chromosome is number 21.The risk for mothers less than 25 years of age to have the trisomy is about 1 in 1500 births.At 40 years of age, 1 in 100 birthsAt 45 years 1 in 40 births.
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NATURAL SELECTION PROVIDES A MECHANISM FOR EVOLUTION
Natural selection brings about adaptation to the environmentBut it has no particular goal Because the environment is constantly changingTherefore perfect adaptation is not a probable outcome of natural selectionNatural selection is a process in which preconditions 1 – 3 may result in certain consequences (A & B) – (table on next slide)
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NATURAL SELECTION PROVIDES A MECHANISM FOR EVOLUTION – Contd…
PRECONDITIONS1. The members of a
population have heritable variations
2. In a population, many more individuals are produced in each generation than can survive and reproduce
3. Some individuals have adaptative characteristicsthat enable them to survive and reproduce better than do other individuals
CONSEQUENCES
A. An increasing proportion of individuals in succeeding generations have the adaptive characteristics
B. The result of natural selection is a population adapted to its local environment
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NATURAL SELECTION PROVIDES A MECHANISM FOR EVOLUTION – Contd…
Organisms have variations- Members of a population
vary in their functional, physical and behavioural characteristcs
- Variations are essential to the natural selection process
- Occurrence of variation is completely random
- The variations that make adaptation to the environment possible are passed on from gen. to gen.
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NATURAL SELECTION PROVIDES A MECHANISM FOR EVOLUTION – Contd…
Organisms struggle to exist- Death & famine inevitable since population size
increases faster than supply of food- i.e. availability of resources – low – always competitionOrganisms differ in fitness- fitness is the ability of an organism to survive and
reproduce in its local environment- The fittest will survive and obtain a disproportionate
amount of resources, and convert this into viable offspring
Organisms become adapted- adjust to be more suited to its environment
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NATURAL SELECTION PROVIDES A MECHANISM FOR EVOLUTION – COMPARE WITH ARTIFICIAL SELECTION / SELECTIVE BREEDING
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NATURAL SELECTION PROVIDES A MECHANISM FOR EVOLUTION – COMPARE WITH ARTIFICIAL SELECTION / SELECTIVE BREEDING
E.g. of a useful mutation:A lamb born with short, bent legs that prevented it from jumping fences.
Used in breeding to establish short-legged sheep.
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NATURAL SELECTION PROVIDES A MECHANISM FOR EVOLUTION – COMPARE WITH ARTIFICIAL SELECTION / SELECTIVE BREEDING
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MICROEVOLUTION BY NATURAL SELECTIONe.g. Peppered moths of Manchester
Example : Peppered moths of ManchesterIn the early 19th century, both dark-coloured and light-coloured moths lived in ManchesterThe light-coloured moths were in greater numbersWhen Manchester became industrialised black smoke from the factories collected as soot on the tree trunks
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MICROEVOLUTION BY NATURAL SELECTION e.g. Peppered moths of Manchester
Birds easily spotted the light-coloured moths and ate themThe dark-coloured moths were not easy to see and survived in greater numbers i.e. nature selected them because they were better adapted to the environmentThe dark-coloured moths reproduced and produced more dark-coloured mothsToday most of the moths of this species in Manchester are dark-coloured
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MICROEVOLUTION BY NATURAL SELECTION e.g. Peppered moths of Manchester
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GENETIC DRIFT
In large populations where random mating occur and mutation does not take place, the genetic constitution of the population does not change.
This is known as the Hardy - Weinberg principle
In small populations which have become isolated from the larger group, there may be rapid change in the gene frequency of the different alleles. This is known as genetic drift
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Consequences of micro-evolution
Both natural selection and genetic drift results in populations where the frequency of particular genes is higher/lower than that in the population as a whole
These populations may therefore look different, behave differently and have different physiologies (metabolism)
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SPECIATION
Species: a group of organisms that have a large number of similar characteristics and are able to interbreed to produce fertile offspring
Population : organisms of the same species occupying the same habitat at the same time
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SPECIATION
Individuals of a population showing a great deal of variation may
become separated by a geographic barrier
The 2 populations no longer mix
The 2 populations each reproduce, undergo natural selection (become adapted to their environments) and become genetically different
These genetic differences may lead to reproductive isolating barriers, which keep them as distinct species
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TYPES OF SPECIATION