Transcript change in `ploidy`

Heredity, Gene Regulation, and Development
Mutation
A. Overview
Mutation
A. Overview
1) A mutation is a change in the genome of a cell.
Mutation
A. Overview
1) A mutation is a change in the genome of a cell.
2) Somatic Mutations:
- can occur during DNA replication prior to mitosis
- can occur during DNA repair
- can be caused by exposure to a mutagen
- if uncorrected, can be passed to daughter cells.
- typically not the source of heritable mutations
Mutation
A. Overview
1) A mutation is a change in the genome of a cell.
2) Somatic Mutations:
- can occur during DNA replication prior to mitosis
- can occur during DNA repair
- can be caused by exposure to a mutagen
- if uncorrected, can be passed to daughter cells.
- typically not the source of heritable mutations
3) Germ-line Mutations:
- occur in germ-line cells (tissues that produce gametes or spores)
- occur so early in development, before germ-line cells have
differentiated, that they affect germ-line cells.
- occurs in DNA replication or meiosis, producing mutant gametes/spores
VI. Mutation
A. Overview
3) These “changes in a genome” can occur at four scales of genetic organization:
- Change in the number of sets of chromosomes ( change in ‘ploidy’)
- Change in the number of chromosomes in a set (‘aneuploidy’)
- Change in the number and arrangement of genes on a chromosome
(gene duplications, deletions, inversions, translocations)
- Change in the nitrogenous base sequence within a gene
(point mutations)
Typically, the larger the change, the more
dramatic (and negative) the result
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
Triploidy occurs in 2-3% of all
human pregnancies, but almost
always results in spontaneous
abortion of the embryo.
Some triploid babies are born
alive, but die shortly after.
Syndactyly (fused fingers),
cardiac, digestive tract, and
genital abnormalities occur.
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
- if meiosis fails, reduction does not occur and a diploid gamete is produced. This can occur
because of failure of homologs OR sister chromatids to separate in Meiosis I or II, respectively.
Failure of Meiosis I
2n = 4
Gametes:
2n = 4
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
- if meiosis fails, reduction does not occur and a diploid gamete is produced. This can occur
because of failure of homologs OR sister chromatids to separate in Meiosis I or II, respectively.
Failure of Meiosis II
2n = 4
Normal gamete formation is on the bottom, with 1n=2 gametes. The error occurred
up top, with both sister chromatids of both chromosomes going to one pole,
creating a gametes that is 2n = 4.
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
- if meiosis fails, reduction does not occur and a diploid gamete is produced. This can occur
because of failure of homologs OR sister chromatids to separate in Meiosis I or II, respectively.
- this results in a single diploid gamete, which will probably fertilize a normal haploid gamete,
resulting in a triploid offspring.
-
negative consequences of Triploidy:
1) quantitative changes in protein production and developmental regulation.
2) can’t reproduce sexually; can’t produce gametes if you are 3n.
1) quantitative changes in protein production and regulation.
2) can’t reproduce sexually; can’t produce gametes if you are 3n.
3)
some organisms can survive, and reproduce parthenogenetically (eggs by
mitosis… offspring are clones).
Aspidoscelis uniparens is a
species that consists of 3n
females that reproduce clonally –
laying 3n eggs that divide without
fertilization. It evolved from the
diploid species, A. inornata
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
2. Mechanism #2: Failure of Mitosis in Gamete-producing Tissue
2n
1) Consider a bud cell in
the flower bud of a plant.
2n
1) Consider a bud cell in
the flower bud of a plant.
4n
2) It replicates it’s DNA
but fails to divide... Now
it is a tetraploid bud cell.
2n
1) Consider a bud cell in
the flower bud of a plant.
3) A tetraploid flower develops
from this tetraploid cell; eventually
producing 2n SPERM and 2n EGG
4n
2) It replicates it’s DNA
but fails to divide... Now
it is a tetraploid bud cell.
2n
1) Consider a bud cell in
the flower bud of a plant.
4n
2) It replicates it’s DNA
but fails to divide... Now
it is a tetraploid bud cell.
3) A tetraploid flower develops
from this tetraploid cell; eventually
producing 2n SPERM and 2n EGG
4) If it is self-compatible, it can mate
with itself, producing 4n zygotes
that develop into a new 4n species.
Why is it a new species?
How do we define ‘species’?
“A group of organisms that reproduce with one another and are
reproductively isolated from other such groups”
(E. Mayr – ‘biological species concept’)
How do we define ‘species’?
Here, the tetraploid population is even reproductively isolated from its
own parent species…So speciation can be an instantaneous genetic event…
2n
4n
4n
1n
2n
2n
3n
Zygote
1n
2n
Gametes
Triploid is a dead-end…
so species are separate
Zygote
Gametes
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1. Mechanism #1: Complete failure of Meiosis
2. Mechanism #2: Complete failure of Mitosis
3. Mechanism #3: Allopolyploidy - hybridization
Black Mustard
2n = 16
gametes
Polyploidy occurs here;
creating a cell with
homologous sets
n=8
n = 17
2n = 18
Cabbage
n=9
Fertilization
produces a cell
with nonhomologous
chromosomes
2n = 34
New
Species
Spartina alterniflora from
NA colonized Europe
X
Spartina maritima native
to Europe
Sterile hybrid – Spartina x townsendii
Allopolyploidy – 1890’s
Spartina anglica – an
allopolyploid and a
worldwide invasive
outcompeting native
species
VI. Mutation
A. Overview
B. Changes in Ploidy
- These are the most dramatic changes, adding a whole SET of chromosomes
1.
2.
3.
4.
Mechanism #1: Complete failure of Meiosis
Mechanism #2: Complete failure of Mitosis
Mechanism #3: Allopolyploidy - hybridization
The Frequency of Polyploidy
For reasons we just saw, we might expect polyploidy to occur more frequently in
hermaphroditic species, because the chances of ‘jumping’ the triploidy barrier to
reproductive tetraploidy are more likely. Over 50% of all flowering plants are
polyploid species; many having arisen by this duplication of chromosome number
within a lineage.
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
2. Human Examples
a. trisomies
Trisomy 21 – “Downs’ Syndrome”
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
2. Human Examples
a. trisomies
Trisomy 21 – “Downs’ Syndrome”
Trisomy 18 – Edward’s Syndrome
Trisomy 13 – Patau Syndrome
Some survive to birth
Trisomy 9
Trisomy 8
Trisomy 22
Trisomy 16 – most common – 1% of pregnancies – always aborted
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
Extreme effects listed below;
2. Human Examples
most show a phenotype within
a. trisomies
the typical range for XY males
47, XXY – “Klinefelter’s Syndrome”
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
2. Human Examples
a. trisomies
47, XXX – “Triple-X Syndrome”
No dramatic effects on the
phenotype; may be taller.
In XX females, one X shuts
down anyway, in each cell
(Barr body).
In triple-X females, 2 X’s shut
down.
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
2. Human Examples
a. trisomies
47, XYY – “Super-Y Syndrome”
Often taller, with scarring
acne, but within the
phenotypic range for XY males
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘aneuploidy’ (changes in chromosome number)
1. Mechanism: Non-disjunction (failure of a homologous pair or
sister chromatids to separate)
2. Human Examples
b. monosomies
45, XO– “Turner’s Syndrome” (the only human monosomy to survive to birth)
VI. Mutation
A.
B.
C.
D.
Overview
Changes in Ploidy
Changes in ‘Aneuploidy’ (changes in chromosome number)
Change in Gene Number/Arrangement
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
a. process:
If homologs line up askew:
A
B
a
b
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
a. process:
If homologs line up askew
And a cross-over occurs
A
B
a
b
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
a. process:
If homologs line up askew
And a cross-over occurs
Unequal pieces of DNA will be exchanged… the A locus has been duplicated on the
lower chromosome and deleted from the upper chromosome
B
A
a
b
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
a. process:
b. effects:
- can be bad:
deletions are usually bad – reveal deleterious recessives
additions can be bad – change protein concentration
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
a. process:
b. effects:
- can be bad:
deletions are usually bad – reveal deleterious recessives
additions can be bad – change protein concentration
- can be good:
more of a single protein could be advantageous
(r-RNA genes, melanin genes, etc.)
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
a. process:
b. effects:
- can be bad:
deletions are usually bad – reveal deleterious recessives
additions can be bad – change protein concentration
- can be good:
more of a single protein could be advantageous
(r-RNA genes, melanin genes, etc.)
source of evolutionary novelty (Ohno hypothesis - 1970)
where do new genes (new genetic information) come from?
Gene A
Duplicated A
generations
Mutation – may even render the protein
non-functional
But this organism is not selected against, relative to others in the
population that lack the duplication, because it still has the
original, functional, gene.
Gene A
Duplicated A
generations
Mutation – may even render the protein
non-functional
Mutation – other mutations may render the
protein functional in a new way
So, now we have a genome that can do all the ‘old stuff’
(with the original gene), but it can now do something NEW.
Selection may favor these organisms.
If so, then we’d expect many different neighboring genes to have
similar sequences. And non-functional pseudogenes (duplicates that
had been turned off by mutation).
These occur – Gene Families
And, if we can measure the rate of mutation in these genes, then we can
determine how much time must have elapsed since the duplication event…
Gene family trees…
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
2. Mechanism #2: Inversion (changes the order of genes on a chromosome)
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
2. Mechanism #2: Inversion (changes the order of genes on a chromosome)
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
2. Mechanism #2: Inversion (changes the order of genes on a chromosome)
Chromosomes are no longer homologous along entire length
B-C-D on top
d-c-b on bottom
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
2. Mechanism #2: Inversion (changes the order of genes on a chromosome)
Chromosomes are no longer homologous along entire length
ONE “loops” to get
genes across from
each other…
And if a crossover occurs….
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
2. Mechanism #2: Inversion (changes the order of genes on a chromosome)
The cross-over
products are nonfunctional, with
deletions AND
duplications
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
2. Mechanism #2: Inversion (changes the order of genes on a chromosome)
The only functional
gametes are those that
DID NOT cross over –
and preserve the
parental combination of
alleles
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
2. Mechanism #2: Inversion (changes the order of genes on a chromosome)
Net effect: stabilizes sets of
genes. This allows selection
to work on groups of alleles…
those that work well
TOGETHER are selected for
and can be inherited as a ‘coadapted gene complex’
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
1. Mechanism #1: Unequal Crossing-Over
2. Mechanism #2: Inversion (changes the order of genes on a chromosome)
3. Mechanism #3: Translocation (gene or genes move to another homologous set)
Translocation Downs.
Transfer of a 21
chromosome to a 14
chromosome
Can produce normal, carrier,
and Down’s child.
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
E. Change in Gene Structure
1. Mechanism #1: Exon Shuffling
Crossing over WITHIN a gene, in introns, can recombine exons within a gene, producing
new alleles.
EXON 1a
EXON 2a
EXON 3a
Allele “a”
EXON 1A
EXON 2A
EXON 3A
Allele “A”
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
E. Change in Gene Structure
1. Mechanism #1: Exon Shuffling
Crossing over WITHIN a gene, in introns, can recombine exons within a gene, producing
new alleles.
EXON 1a
EXON 2a
EXON 3a
Allele “a”
EXON 1A
EXON 2A
EXON 3A
Allele “A”
EXON 1A
EXON 2a
EXON 3a
Allele “α”
EXON 1a
EXON 2A
EXON 3A
Allele “ά”
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
E. Change in Gene Structure
1. Mechanism #1: Exon Shuffling
2. Mechanism #2: Point Mutations
a. addition/deletion: “frameshift” mutations
Normal
Mutant: A inserted
…T C C G T A C G T ….
…A G G C A U G C A …
ARG
HIS
ALA
DNA
m-RNA
…T C C A G T A C G T ….
…A G G U C A U G C A …
ARG
SER
CYS
Throws off every 3-base codon from mutation point onward
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
E. Change in Gene Structure
1. Mechanism #1: Exon Shuffling
2. Mechanism #2: Point Mutations
a. addition/deletion: “frameshift” mutations
b. substitution
Normal
Mutant: A for G
… T C C G T A C G T ….
…A G G C A U G C A …
ARG
HIS
ALA
DNA
m-RNA
…T C C A T A C G T ….
…A G G U A U G C A …
ARG
TYR
ALA
At most, only changes one AA (and may not change it…)
VI. Mutation
A. Overview
B. Changes in Ploidy
C. Changes in ‘Aneuploidy’ (changes in chromosome number)
D. Change in Gene Number/Arrangement
E. Change in Gene Structure
F. Summary
MUTATION:
-New Genes:
point mutation
exon shuffling
RECOMBINATION:
- New Genes:
crossing over
-New Genotypes:
crossing over
independent assortment
Causes of Evolutionary Change
V A R I A T I O N
Sources of Variation
Natural Selection
Mutation (polyploidy can make new
species)