Chapter 13 – An Introduction to Heredity: Meiosis.
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Transcript of Chapter 13 – An Introduction to Heredity: Meiosis.
Chapter 13 –
An Introduction to Heredity: Meiosis
Heredity: The transmission of traits from one generation to the next
GenesGenes are DNA are DNA segmentssegments
A A locuslocus is a gene’s is a gene’s specific location specific location along the length of a along the length of a chromosomechromosome
Offspring acquire Offspring acquire genes from parents genes from parents by inheriting by inheriting chromosomeschromosomes
Characteristics of Asexual ReproductionAsexual reproduction = form of cell division; A system of cloning and creating exact genetic replicas so there exists no genetic variation
Single-celled organisms can reproduce to create new organisms
Budding is a form of asexual reproduction in multi-cellular organisms
Characteristics of Sexual Reproduction
2 parents must give rise to offspring; each parent giving a haploid set of chromosomes
Offspring has greater genetic variation
Two haploid gametes (reproductive cells) must combine to create a new diploid organism.Each cell of a diploid
organism must have 2 sets of chromosomes. One set donated from the mom, the other set donated from dad.
Homologous Chromosomes: a pair of chromosomes containing the same linear gene sequences, each derived from one parent.
Human cells have a diploid number of 46
2N = 46
This means each cell has 23 homologous pairs
There is one exception…
The sex chromosomes (pair # 23), is homologous in females (XX)
but is only hemizygous in males (XY)
Karyotypes (such as these) are pictures of an individual’s chromosomes.
Autosomes are non-sex chromosomes (pairs 1- 22 in humans)
Meiosis = A form of nuclear division that results in the reduction of chromosome number by half (from diploid cell to a haploid cell)
Let’s review mitosis = a form of nuclear division, whereby the chromosome number and genetic content in the organism remains the same in both parent and daughter cells.
How does mitosis and meiosis compare?
How does meiosis increase genetic diversity?
Crossing over during prophase I
Independent Assortment or random alignment of homologous chromosomes along metaphase plate during metaphase I
Segregation (separation) of homologous chromosomes in Anaphase I and segregation of sister chromatids in Anaphase II
Random Fertilization
Genetic diversity in a species increases the species’ chances of survival and prevents extinction.
In a diverse gene pool, there is a higher probability of a gene that increases fitness during times of instability or stress.
Varieties in Sexual Life Cycles
Type 1: Dominant Stage (stage it spends most of its life in) = Diploid Multicellular (Ex. Humans)
2N
2N
Diploid Multicellular
Meiosis/cytokinesis
N
N
Haploid unicellularFertilization
Mitosis/Cytokinesis
Varieties in Sexual Life Cycles
Type 2: Dominant Stage = Haploid Multicellular (Ex. Fungi and Algae)
N
N
Haploid multicellular
One cell from multicellular haploid cluster is designated a haploid gamete
fertilization
2N
Diploid Single Cell
Meiosis/Cytokinesis
N N
N N
Mitosis/cytokinesis
Sordaria (fungi) – lab #3 Example of Type 2 Life Cycle
Gene that codes for ascospore color
Black (+) = wildtype black spore Tan (tn) = mutant brown spore
Sordaria is a haploid organism, so fungi’s phenotype for spore color is dependent on the single gene that it inherits
Example of Type 2 Life Cycle
Black sporesTan sporesAreas where both haploid strands can fuse, become diploid, and undergo immediate meiosis/cross over.
Both haploid strands grown on an agar plate
Example of Type 2 Life Cycle
Two haploid genes fuses to become
diploid
Cross over might occur
Completion of meiosis
mitosis
Ascus containing 8 ascospores
Ascopore pattern w/o crossover
tntn
++
DNA replication
tntn
++
Meiosis I
Meiosis II
Meiosis II
tn
tn
+
+
Mitosis
Mitosis
Ascospore with 4X4 pattern
tn
tn
+
+
tn
tn
+
+
Gene expression
tn
tn
+
+
tn
tn
+
+
Ascopore pattern #1 w/ crossover
tntn
++
DNA replication
tn
++
Meiosis I
Meiosis II
Meiosis II
tn
+
+
Mitosis
Mitosis
Ascospore with 2X2x2x2 pattern
tn
+
tn
+
tn
+
+ tn
+
Cross-over
+
tn
tn
+
+
tn
tn
+
+
tn
Ascopore pattern #2 w/ crossover
tntn
++
DNA replication
tn
++
Meiosis I
Meiosis II
Meiosis II
tn
+
+
Mitosis
Mitosis
Ascospore with 2x4x2 pattern
tn
+
tn
+
tn
+
+ tn
+
Cross-over
+
tn
tn
+
+
tn
tn
+
+
tn
Varieties in Sexual Life CyclesType 3: Alternation of Generation = Dominant haploid AND diploid multicellular stages
Diploid multicellular stage = Sporophyte
2N
2N Meiosis/cyt
Meiosis/cyt
N
N
4 Haploid spores
Mitosis/cyt
Mitosis/cyt
Haploid multicellular stage = Gametophyte
2N
2N
One cell from multicellular haploid cluster is designated a haploid gamete
fertilization
Mitosis/Cytokinesis
N
N
What can go wrong in meiosis?
No
Non-disjunction Disorders
Meiosis I - Failure to separate
Meiosis II Failure to separate
Abnormal Gametes
Definition: When members of homologous chromosomes fail to separate during Meiosis I – or – when sister chromatids fail to separate during Meiosis II.
Examples: Down Syndrome, Turner’s syndrome, Klinefelter’s syndrome
Normal Gametes
Polyploidy
Polyploidy is a term that describes the inheritance of more that two sets of chromosomes due to non-disjunction during meiosis
Interpret these karyotypes
Klinefelter’s syndrome
Interpret these karyotypes
Down Syndrome
Try these on-line activities
• http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping2.html
X-inactivation (in mammals)
In female mammals, one of the X-chromosomes turns “off” and condenses into a compact barr body.
The barr body is reactivated in the ovary cells only during meiosis for reproduction.
Two X-chromosomes (autosomes not shown)
Barr body due to X-inactivation
Other non-disjuntion disorders…- XO individual (missing 1-X chromosome) –
physically female (same phenotype as normal female who has barr body), but sterile since there is no barr body to reactivate during puberty
- XXX female – sterile - XYY male – abnormally tall- XXY male – one X becomes a barr body,
so phenotypically male, but is sterile when barr body is reativated. He has abnormally small testes
What is a mutation?
Mutations are changes in the DNA. During meiosis, there are 4 different types of chromosomal mutations that can occur.
A
A
A
A
B
B
B
B
A B
A B
A B
C
C
C
C
C
C
D
D
D D
E E
EDB CE
E E
F
F
F
F
F
FG H
G H
G H
G H
G H
G H
1. Deletion: A segment of the chromosome is removed (not justone nuclotide)
C D E F G H
A B
C D E F G HM N O
M N O P Q R P Q R
3. Inversion: A segment within a chromosome is reversed
2. Duplication: A segment of the chromosome is repeated
4. Translocation: A segment from one chromosome moves to another, non-homologous one