Transcript
VARIATION & MUTATION
Variation:
describes the differences in characteristics shown by organisms of the same species
can be: discontinuous continuous
Discontinuous Variation:
Produces:individuals with clear-cut differences
with no intermediates between them
examples: blood group in humans wing length in Drosophila sex tongue rolling
Discontinuous Variation: Characteristics are usually controlled by:
1-2 major genes which have: two or more allelic forms
their phenotypic expression is relatively unaffected by environmental conditions
Continuous Variation:
many characteristics in a population show a complete gradation from one extreme to the other without a break
Continuous Variation: Examples:
Mass Height Intelligence Colour of organs & organisms
Continuous Variation:
Characteristics are produced by the combined effects of:
many polygenes and the environment
Influence of the environment
Sources of variation inasexually reproducing organisms :
replication of DNA is nearly perfect - i.e. little possibility of variation in
genotype
any apparent variation between organisms is thus almost certainly the result of environmental influences
Sources of Variation: Meiosis
Allows for new combinations of genes
Creates variation within a population
Four Sources of variation in:
sexually reproducing organisms:
1. Crossing over 2. Random orientation of bivalents
Four Sources of variation in:
sexually reproducing organisms:
4. Random
fertilisation
3. Random orientation of sister chromatids
Metaphase plate
Metaphase plate
OR
DO NOT:Crossing over
generate the major changes in genotype necessary to give rise to new species
Random orientation
Random fertilisation
MUTATIONSgenerate changes in genotype
necessary to give rise to new species
MUTATIONS
A Mutation is:
a change in the amount, arrangement or structure of the DNA of an organism
A mutation produces a change in the genotype & is passed on: when a cell nucleus divides by:
mitosis or meiosis from the mutant cell
Mutant daughter cellsMutant daughter cells
Mutant cell
Mutant cell
Which type of mutation can be inherited by the offspring?
germinal
somatic Occur in somatic cells:
are NOT passed on the offspring
Occur in gamete cells:
are passed on to the offspring
Normal progeny can be produced if mutations occur in gamete cells because:
the effect produced by defective allele is masked by the dominant allele
DOMINANT RECESSIVE
A Mutation may result in:
the change in appearance of a characteristic of a population
e.g. red eyes in Drosophila appeared in 1909
e.g. dark-coloured moth appeared in 1848
The "typica" form of the moth.
The "carbonaria" form.
Mutations can:
occur in: any gene at any timebe:
1. SPONTANEOUS
2. INDUCED
Spontaneous Mutations:
are permanent changes in the genome that occur without any outside influence
occur because the machinery of the cell is imperfect
Spontaneous Mutations arise because:
a) DNA polymerase makes errors in replication
b) meiosis is not perfect:
– non-disjunction
- translocationscan occur
One chromatid goes to each
daughter cell.
Both chromatids are sent to one daughter cell, the other gets
none. TranslocationChromosome 21 fuses
with 14.
Induced Mutations:
occur when some outside agent causes a permanent change in DNA
mutagens: anything that causes a mutation examples:
• Asbestos
• Tar from tobacco
• UV radiation
• Pesticides
• Caffeine UV light causes adjacent
thymines to cross link
Mutations can be:
1. Chromosomal mutations
2. Gene mutations or point mutations
MUTATION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of
the DNA
GENE/POINT MUTATIONSDescribe a change in the
structure of DNA at a single locus
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME
STRUCTURE
ANEUPLOIDY EUPLOIDY / POLYPLOIDY
AUTOPOLYPLOIDY
ALLOPOLYPLOIDY
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
INSERTION
INVERSION
DELETION
SUBSTITUTION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of the DNA
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME STRUCTURE
ANEUPLOIDYLoss or gain of a
single chromosome
EUPLOIDY/POLYPLOIDYThe increase in entire
haploid sets of chromosomes
AUTOPOLYPLOIDYResults from an increase in
the number of chromosomes within the same species
ALLOPOLYPLOIDYChromosome number within
a sterile hybrid doubles, producing fertile hybrids
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
Changes in Chromosome structure [not in syllabus]
MUTATION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of
the DNA
GENE/POINT MUTATIONSDescribe a change in the
structure of DNA at a single locus
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME
STRUCTURE
ANEUPLOIDY EUPLOIDY / POLYPLOIDY
AUTOPOLYPLOIDY
ALLOPOLYPLOIDY
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
INSERTION
INVERSION
DELETION
SUBSTITUTION
Aneuploidy:
loss or gain of a single chromosomeresults from non-disjunction
Down Syndrome: A meiotic error
Cause: non-disjunction at either:
anaphase I anaphase II
anaphase I anaphase II
Homologous chromosomes fail to
separate
Sister chromatids fail
to separate
Eggcell
Spermcell
n + 1
n (normal)
Zygote2n + 1
Fertilisation after non-disjunction in the mother results in a zygote with an extra
chromosome
Down’s Syndrome is a common form of chromosomal mutation in humans:
resulting from non-disjunction
Why is Down’s syndrome also called
trisomy 21?
Trisomy 21
Three copies of Chromosome 21
Causes of Down Syndrome:
1. 96% of the cases: non-disjunction of
chromosome 21 during anaphase of meiosis
2. 3-4% of the cases: translocation (movement) of:
chromosome 21 to chromosome 14, or less commonly to chromosome 22
Relationship Between Age and Aneuploidy
Older mothers more likely to produce aneuploid eggs
Sex Chromosome AbnormalitiesGenotype Gender Syndrome Physical Traits
XXY, XXYY, XXXY maleKlinefelter syndrome
sterility, small testicles, breast
enlargement
XYY male XYY syndrome normal male traits
Klinefelter syndrome
Sex Chromosome AbnormalitiesGenotype Gender Syndrome Physical Traits
XO femaleTurner
syndrome
sex organs do not mature , sterility,
short stature
XXX female Trisomy Xtall stature, learning disabilities, limited
fertility
Turner syndrome
Question: [SEP, 2009]
Suggest explanations for each of the following observations.
1. Meiosis generates biological diversity in eukaryotes. (2)
New combinations of alleles arise through crossing-over and random orientation of both bivalents and sister chromatids.
Question: [SEP, 2009]
Suggest explanations for each of the following observations.
2. Klinefelter’s Syndrome, where individuals would be characterised by a XXY combination of sex chromosomes, is caused by an anomalous event during meiosis. (2)
Due to non-disjunction of sex chromosomes, one cell would result in having two copies of the X chromosome rather than one. This can happen at either anaphase I or II.
MUTATION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of
the DNA
GENE/POINT MUTATIONSDescribe a change in the
structure of DNA at a single locus
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME
STRUCTURE
ANEUPLOIDY EUPLOIDY / POLYPLOIDY
AUTOPOLYPLOIDY
ALLOPOLYPLOIDY
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
INSERTION
INVERSION
DELETION
SUBSTITUTION
Euploidy (Polyploidy)
polyploids are gamete and somatic cells containing multiples of the haploid number of chromosomes
2n = 6 = 2x (diploid)
2n = 9 = 3x (triploid)
2n = 12 = 4x (tetraploid)
Homologous chromosomes
One extra set
Two extra sets
Plants commonly exhibit polyploidy: 95% of ferns 70% of angiosperms many of the fruits & grain are polyploid plants
are polyploid
Polyploidy is often associated with:
advantageous features such as
increased:- Size- Hardiness- Resistance to
disease
having advantageous features is called hybrid vigour
DiploidPolyploid
Hybrid vigour in corn
Polyploids with odd numbered chromosome sets are usually sterile
as they produce mostly aneuploid gametes
Somatic cell:Full set of chromosomes = 9
Gamete:Will contain the INCORRECT number of chromosomes
Benefit of Odd Ploidy-Induced Sterility
Seedless fruit watermelons and bananas
Not all watermelons are red. This triploid, seedless variety has sweet, yellow flesh.
Polyploidy is more common in:
plants than animals
Reason:
the increased number of chromosomes makes normal gamete formation during meiosis much more prone to errors
Why are polyploid plants still capable of reproduction?
As most plants are capable of vegetative propagation.
MUTATION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of
the DNA
GENE/POINT MUTATIONSDescribe a change in the
structure of DNA at a single locus
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME
STRUCTURE
ANEUPLOIDY EUPLOIDY / POLYPLOIDY
AUTOPOLYPLOIDY
ALLOPOLYPLOIDY
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
INSERTION
INVERSION
DELETION
SUBSTITUTION
Autopolyploidy:
• a condition that may arise naturally or artificially
• results from an increase in the number of chromosomes within the same species
2n = 64n = 12
2n
4n
Failure of cell divisionin a cell of a growing
diploid plant afterchromosome duplicationgives rise to a tetraploidbranch or other tissue.
Gametes produced by flowers on this
branch will be diploid.
Offspring with tetraploid karyotypes may be viable
and fertile—a new biological species.
Autopolyploidy can be induced by:
the drug colchicine – spindle formation is inhibited and chromatids fail to separate during anaphase
How is a 4n cell produced from a 2n one?
DNA replicates but cytoplasm does not split
Allopolyploidy
chromosome number within a sterile hybrid doubles,
producing a fertile hybrid
Let’s explain ‘Allopolyploidy’ using Spartina as an example
Spartina alterniflora
Spartina (cord grass): an example of allopolyploidy
Spartina alterniflora2n = 62
Spartina maritima2n = 60
1n 1870 in Townsend harbor in southern England:
a sterile hybrid of two Spartina species was found and was called Spartina townsendii
Spartina townsendii2n = 61
Gametes: n = 30 Gametes: n = 31 STERILE HYBRID
X
Spartina townsdendii went through a process of genome duplication
22 years later, a new vigorous and fertile form was recorded – it was named S. anglica S. maritima
2n = 60
S. alterniflora
2n = 62
n = 30 n = 31Gametes:
S. townsendii
2n = 61
4n = 122
S. anglica
Sterile hybrid:
Allopolyploidy
A NEW speciesS. anglica
Fertile hybrid :
Allopolyploidy has been described as ‘instant evolution’. Explain.
A new species is produced in a short time – over one generation
S. maritima
2n = 60
S. alterniflora
2n = 62
n = 30 n = 31Gametes:
S. townsendii
2n = 61
4n = 122
S. anglica
Sterile hybrid:
Allopolyploidy
How can S. townsendii reproduce, if it is sterile?
Vegetatitve propagation by producing:
Why is S. townsendii sterile?
Meiosis cannot occur properly as chromosomes are NOT
homologous in S. townsendii
Consider this example:
No homologous chromosomes in sterile hybrid:
Species 1 Species 2
2n = 4 2n = 6
Sterile hybrid: 2n = 5
Gametes:
X
X
n = 2 n = 3
BUT what if chromosome
number doubles in a sterile hybrid?
Now homologous chromosomes are present!!
Sterile hybrid: 2n = 5
Chromosome doubling:
Allopolyploidy
4n = 10
Can this new organism produce viable gametes?
YES
Why is the new species fertile?
Sterile hybrid:
2n = 5
Fertile new species:
2n = 10
Meiosis can occur properly as homologous chromosomes are
present
Individual becomes fertile after chromosome doubling
Gametes:
n = 5
2n = 10
X
X
a new species is produced which is fertile with polyploids like itself but infertile with parental species
2n = 10
Meiotic error in anther during pollen grain formation
Sterile hybrid
AB
Meiotic error
AABB
X
ABD
Meiotic error
T. turgidum(Emmer wheat)
(2n = 28)
X
Sterile hybrid
Spot where allopolyploidy
occurred during the
development of T. aestivum.
a b c d e
Primitive wheat (a) crossed withwild grass (b) to produce an infertile hybrid.
Chromosome doubling produces a fertile hybrid (c)
which is crossed with wild grass(d) to produce an infertile hybrid
Chromosome doubling results in fertile hybrid (e)
This hybrid is a cultivated wheat used for flour production
Many crop plants result from doubling of chromosome number
Question: [SEP, 1999]
1. What is polyploidy? (2)
Gamete and somatic cells containing multiples of the haploid number of chromosomes.
2. Suggest TWO mechanisms that may result in polyploidy. (4)
Allopolyploidy – chromosomes double in a sterile F1 hybrid.
Autopolyploidy – duplication of chromosomes of a single species after fertilisation.
Question: [SEP, 2003]
Two closely-related plants, A and B, have chromosome sets AA and BB respectively. When Plant A is crossed with Plant B a hybrid, possessing chromosome set AB, is produced. Such hybrids are sterile.
a) Why are hybrid plants sterile? (3)
As there are no homologous chromosomes which is a prerequisite for meiosis to occur.
Question: [SEP, 2003]b) Fertile polyploidy hybrids might develop from
these sterile plants. Describe a sequence of events that would enable this to occur. (3)
Chromosomes replicate but cytokinesis does not occur. This results in a cell having homologous chromosomes and so meiosis can occur.
c) Given the results observed in these crosses, would you conclude that Plant A and Plant B belong to the same species or to different species? Justify your answer. (3)
Belong to different species. Offspring produced are sterile.
MUTATION
CHROMOSOME MUTATIONResult from a change in the amount, or arrangement of
the DNA
GENE/POINT MUTATIONSDescribe a change in the
structure of DNA at a single locus
CHANGES IN CHROMOSOME
NUMBER
CHANGES IN CHROMOSOME
STRUCTURE
ANEUPLOIDY EUPLOIDY / POLYPLOIDY
AUTOPOLYPLOIDY
ALLOPOLYPLOIDY
DELETION
DUPLICATION
INVERSION
TRANSLOCATION
INSERTION
INVERSION
DELETION
SUBSTITUTION
MUTATIONChange in the amount, structure or arrangement of the DNA of
an organism
GENE/POINT MUTATIONSDescribe a change in the structure of DNA at a single locus
INSERTION
INVERSION
DELETION
SUBSTITUTION
Fig. 6 Gene or point mutation
1) INSERTION: the addition of an extra nucleotide
A GT G C A TA TT G A C A G
2) DELETION: involves the loss of a nucleotide
A GT G C A T A TT C A G
Fig. 6 Gene or point mutation
4) SUBSTITUTION:
a particular base is substituted by another (e.g. sickle-cell anaemia)
A GT G C A T A TT G T A G
3) INVERSION: two nucleotides become arranged in the wrong order
A GT G C A T T TA G C A G
What is a:
Silent mutation:
an alteration in a DNA sequence that does not result in an amino acid change in a polypeptide
Frameshift mutation:
is caused by insertions or deletions of nucleotides which shifts the codon triplets of the genetic code of mRNA and causes a misreading during translation
A silent mutation: no effect on amino acid sequence
A frameshift mutation:
Sickle Cell Anaemia:
in humans is an example of base substitution
affects a base in one of the genes involved in producing haemoglobin
at position 14 in the DNA: the base thymine is
replaced by adenine
Amino acid sequence
Sickle Cell Anaemia:
at low oxygen tensions, haemoglobin S crystallises in the red cells distorting them into a sickle shape
Sickle Cell Anaemia:
many sickle cells are destroyed in the circulation
Result: oxygen-carrying capacity of the blood is lowered
incidence is very high in Africa and Asia
Sickle Cell Trait is the heterozygous condition
heterozygotes are resistant to some forms of malaria
Incidence of Malaria Incidence of Sickle cell anaemia
Inheritance of sickle cell anaemia:
Carrier parents : HbAHbS x HbAHbS
F1 phenotypes: 25% normal : 50% carriers : 25% sufferers
F1 genotypes: HbAHbA HbSHbA HbSHbA HbSHbS
Gametes:
HbAHbAHbS
HbSx
Essay Titles
1. Variation among organisms and between individuals of the same species is purely the result of genetic differences. Discuss this statement. [SEP, 2003]
2. “Random mutations are the driving force of evolutionary change”. Discuss. [SEP, 2009]
3. Compare and contrast the roles of genetic mutations and meiosis in generating genetic diversity. [MAY, 2010]
END OF SECTION
top related