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Chapter 13 - Meiosis
A. BergeronAP BiologyPCHS
Why Sex?
Why Sex? Why Does Sexual Reproduction Occur?
Curtis Lively and colleagues studied a species of snail that is native to New Zealand. This snail is parasitized by over a dozen species of worms. Infected snails cannot reproduce (You don’t want to know why…). Snail parasites are common in some habitats but rare in others.
Individuals within the species reproduce only sexually while others only reproduce asexually
Question: Is the frequency of sexually-reproducing snails higher in areas where parasites are common compared to areaswhere parasites are rare?
Task - Use the scientific method to answer the question Can you propose an hypothesis that you could test with anexperiment?
Describe the experiment that you would conduct. ! -Which variable will you test (IV)? ! -What will you measure (DV)? ! -What will you try to keep constant?
What results will you collect?
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Sexually Reproducing Populations Asexually Reproducing Populuations
Rate of Parasitism in a Species of Sexually and Asexually Reproducing New Zealand SnailPa
rasi
tism
Rat
e (P
erce
nt o
f ind
ivid
uals
infe
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)
Would You Believe that Sex Should NOT Have Evolved in the First Place?
2. Changing Environment Hypothesis
Okay, okay…So Maybe Sex is Beneficial after All…
3. The Case of the Infertile Fruit - We Have No Bananas!
1. Evolution of Disease-causing Organisms
Would You Believe that Sex Should NOT Have Evolved in the First Place?
John Maynard Smith Hypothesis
Assume that each organism in a population will produce 4 offspring
All conditions being equal, asexual reproduction should be favored over sexual reproduction
Generation 1
Generation 2
Generation 3
“We Have No Bananas” Class Discussion1. What is a monoculture?
2. Illustrate how the modern species of Cavendish banana (a triploid) could be produced from two diploid parent plants.
4. Identify three (3) ways in which the Tropical Race 4 (TR4) fungus can spread.
5. How are scientists inserting foreign genes into Cavendish cell nuclei?
6. Are the Australian scientists using TR4 in their attempt to genetically engineer a resistant strain of Cavendish?
7. Why do you think that the general public would choose not to consume a genetically engineered banana?
3. Why is the modern Cavendish sterile (i.e. infertile)?
"Acts of God have not been wholly unsolicited."
mt = Metric Ton
Standard Fruit = Dole
United Fruit = Chiquita
mt = Metric Ton
Standard Fruit = Dole
United Fruit = Chiquita
A Novel Form of Genetic Engineering
Using what you know about mitosis, meiosis, and chromosome number, explain whether or not the “Cat-Dog”offspring shown in the picture below are biologically possible. Would you predict that the “Cat-Dog” offspring are fertile or infertile?
Felis catus (cat): n = 19; 2n = 38Canis familiaris (dog): n= 36; 2n = 72
Do Now - Cat-Dog?!
Human Sexual Life Cycle
Haploid (n) -Possess 1/2 the usual number of c’somes (n=23)-Gametes (sperm and egg) are haploid
Diploid (2n)-Possess a full complement of chromosomes (2n=46)
The full set of 46 c’somesis inherited during fertilization (i.e. formationof a zygote)
Sexual Life Cycles in Various OrganismsPlease study these life cycles on your ownFigure (c) illustrates the alternation of generations that isutilized by some organisms
Homologous Chromosomes
Chromosomes that are similarin size and shape
Homologous chromosomescarry the same genes atthe same loci
Locus (s.)/loci (pl.) Location on a chromosome where a gene is located
Gene - DNA instructions that influence one or more hereditary traits
Allele - One version of a particular gene
5 µmPair of homologouschromosomes
Sisterchromatids
Centromere
Key
Maternal set ofchromosomes (n = 3)
2n = 6Paternal set ofchromosomes (n = 3)
Two sister chromatidsof one replicatedchromosome
Two nonsister chromatids in a homologous pair
Pair of homologouschromosomes(one from each set)
Centromere
InterphaseDNA replicationPreparation for meiosis
Meiosis I -1st cell divisionHomologous chromosomes separate
Meiosis II - 2nd cell divisionSister chromatids separateIdentical to a “haploid mitosis”
Meiosis OverviewThe “Reduction Division”
Meiosis Overview
Do Now - Diagramming the Stages of Meiosis
Using a cell with a diploid chromosome number of six (2n=6),create an illustration of your assigned stage of meiosis.
List any/all key events that take place during this stage
Meiotic Events
Overview of Meiosis IThe “Reduction Division”
Interphase Prophase I Metaphase I Anaphase I
Telophase I & Cytokinesis
Overview of Meiosis IIA Haploid Version of Mitosis
Prophase II Metaphase II Anaphase IITelophase II &
Cytokinesis
Meiosis I -1st Cell (Nuclear) DivisionSection 11-4DNA replicated during Interphase IHomologous chromosomes form tetrads during prophase I -Crossing over occurs between homologous chromosomes
Homologous chromosomes separate during anaphase I
Chromosomes can align in multiple ways along metaphase plateduring metaphase I (i.e. independent assortment)
Section 11-4
Figure 11-15 MeiosisMeiosis I - 1st Cell (nuclear) Division
Section 11-4
Figure 11-15 MeiosisMeiosis I - 1st Cell (nuclear) Division
Section 11-4
Figure 11-15 MeiosisMeiosis I - 1st Cell (nuclear) Division
Section 11-4
Figure 11-15 MeiosisMeiosis I - 1st Cell (nuclear) DivisionResult: Chromosome number “cut” in 1/2 at end of meiosis I Diploid parent cell --> 2 haploid daughter cells
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Section 11-4
Figure 11-17 Meiosis II
Section 11-4Meiosis II - 2nd Cell (nuclear) DivisionMeiosis II is virtually identical to mitosisFour haploid (n) daughter cells produced at end of meiosis
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Section 11-4
Figure 11-17 Meiosis II
Meiosis II - 2nd Cell (nuclear) Division
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Section 11-4Figure 11-17 Meiosis II
Meiosis II - 2nd Cell (nuclear) Division
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.
Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up in a similar way to the metaphase stage of mitosis.
The sister chromatids separate and move toward opposite ends of the cell.
Meiosis II results in four haploid (N) daughter cells.
Section 11-4
Figure 11-17 Meiosis II
Meiosis II - 2nd Cell (nuclear) DivisionDue to crossing over and independent assortment of homologous chromosomes, haploid gametes can possess diverse genetic information
Meiosis! (In All of its Glory…)
Meiosis II
Focusing on the Meiosis I --> Meiosis II Transition
Do Now – Mitosis vs. Meiosis
1. How is anaphase I of meiosis different from the mitotic anaphase?
2. (True/False) Crossing over occurs between sister chromatids.
3. At what point during meiosis is the chromosome number reduced to a haploid state?
4. Will genes that are close together or far apart on a chromosome be more likely to have a crossover event between them?
5. What are the sources of genetic variation in mitosis and meiosis, respectively?
Comparing Mitosis and Meiosis
A Comparison of Mitosis and Meiosis• Mitosis conserves the number of chromosome sets,
producing cells that are genetically identical to the parent cell
• Meiosis reduces the number of chromosomes sets from two (diploid) to one (haploid), producing cells that differ genetically from each other and from the parent cell
• The mechanism for separating sister chromatids is virtually identical in meiosis II and mitosis
• Three events are unique to meiosis, and all three occur in meiosis l:– Synapsis and crossing over in prophase I: Homologous
chromosomes physically connect and exchange genetic information
– At the metaphase plate, there are paired homologous chromosomes (tetrads), instead of individual replicated chromosomes
– At anaphase I, it is homologous chromosomes, instead of sister chromatids, that separate and are carried to opposite poles of the cell
A Comparison of Mitosis and Meiosis
Section 11-4Crossing OverOccurs during prophase I of meiosis --> Formation of tetrads
Increases the genetic variability of the gametes
Genes located farther apart on homologous chromosomes are more likely to have a cross over event between them than aregenes located close together on a chromosome
Crossing Over During Prophase ISynapsis
Pairing of homologous chromosomes to form a tetrad
Chiasmata
Crossing over involves the exchange of regions of non-sister
chromatids producing chromosomesthat possess both paternal and
maternal genetic information
Crossing Over Animation
Crossing over takes place between non-sister chromatids onhomologous chromosomes during prophase I
Segments of chromosomes “break off” and switch places witha complementary chromosomal segment on a homologous chromosome
Results of Crossing OverGametes formed at the end of meiosis II may (and often do) possess different genetic information
These differences are due partially to the exchange of segments of maternal and paternal chromosomes duringprophase I
Independent Assortment of Chromosomes During Meiosis I
Independent Assortment
Different alignment possibilities exist when chromosomes“line up” during metaphase I
A diploid organism can produce 2n combinations of maternaland paternal chromosomes in their gametes
Humans can produce approximately 8.4 million different varieties of gametes through independent assortment ALONE!
Movement of Genes (Alleles) on Different Chromosomes During Meiosis
Allele - Alternative version of a gene that is located on onehomologous chromosome
Because most alleles are located on separate chromosomes,they segregate independently during meiosis
Spermatogenesis vs. Oogenesis
Diploidprecursor
cell
SpermatogenesisFormation of male gametes (sex cells) during meiosis
OogenesisProduction of female gametes during meiosis
1st polar body
2nd polar body
Polar body - “Dumping ground” for chromosomes
Only one egg (oocyte) produced during oogenesis
Oogenesis (A Different Look…)
MitosisMeiosis I
Meiosis II
No Seriously…Why Sex?
Okay, okay…So maybe sex is beneficial after all…
Sources of Genetic Variation
1. Crossing over
2. Independent assortment
3. Random fertilization
Human Karyotypes and Homologous C’somes
A karyotype allows you to see chromosomes that have been removed from a cell’s nucleus
Every human body cell (except sperm and egg cells) has 46 chromosomes
The chromosomes come in 23 pairs
Each member of the pair are similar or homologous
You inherit one chromosome in the pair from your mother and one chromosome from your father
AmniocentesisProcess of removing amniotic fluid from the womb for the purposes of biochemical/genetic analysis
Preparing a Karyotype
What type of information can be learned from a karyotype?
Online Karyotyping Activity
Down’s Syndrome (Trisomy 21; 47 XY/XX +21))
• Characterized by mild to severe mental retardation• Short, stocky body type• Enlarged tongue that leads to speech difficulties
Klinefelter’s Syndrome (47, XXY)
• Are genetically male• Have an extra X
chromosome• Develop female
characteristics at puberty because of the extra X chromosome
Turner Syndrome (45, X)
• Have one X chromosome instead of the normal two
• Are female• Shorter than normal• May suffer from mild mental
retardation• Usually infertile
Patau Syndrome (Trisomy 13; 47, XX/XY +13)
Characterized by cleft palate, and polydactyly of one or more of the extremities
Individuals are rarely born healthy and, if so, do not survive past one year of age
What Causes Someone to Have Too Many or Too Few
Chromosomes?
Nondisjunction produces gametes with an abnormal number of chromosomes
Homologous chromosomes fail to separate
Meiosis I:Nondisjunction
Meiosis II
Nondisjunction During Meiosis
Pre-term (Before birth) Genetic Diagnosis (PGD)
Pre-implantation Genetic (PIG) Diagnosisvs.
Should prospective parents be able to use one or both ofthese techniques to screen the genetic make-up of their child?
In the case of pre-implantation genetic diagnosis, what shouldhappen to the embryos that are judged unworthy of implantation?
Can you think of another way to “screen” a child for geneticabnormalities that would not harm the embryo?
If an abnormality is found in the genetic material of the embryowhat (if anything) should be done about it?
Chemicals in Our Food - Bisphenol A (BPA)Used in the manufacture of various plastics
An estimated 8 billion pounds of BPA are used by manufacturers yearly
Plastics containing BPA are used to make baby and water bottles, sports equipment, dental fillings, food and beverage cans, and thermal paper (paper used to print receipts)
BPA is an endocrine disruptor and behaves like estrogen in animals
Scientists are unsure how long-term exposure to BPA (even at low concentrations) might affect humans
Chemicals in Our Food - Bisphenol A (BPA)
Centrosome – Molecular “Girders” for the Cytoskeleton
Microtubule organizingcenter (MTOC;
e.g. centrosome]
+
-
– Contains a pair of centrioles– Centrioles are only found in animal cells
Centrosome
Microtubule
Centrioles0.25 µm
Longitudinal sectionof one centriole
Microtubules Cross sectionof the other centrioleFigure 6.22
10 µm
Column of tubulin dimers
Tubulin dimerα β
25 nm
Actin subunit
10 µm
7 nm
5 µm
Keratin proteinsFibrous subunit (keratinscoiled together)
8–12 nm
Cytoskeletal Proteins are Polymers Constructed from Monomers
Single subunits (e.g. actin, tubulin monomers) will self-assemble into short polymers (i.e. oligomers) but these structures are unstable and tend to depolymerize quicklyFollowing the formation of the cytoskeletal “nucleus” polymerization can occur quickly and the length of the filament increases
The Cytoskeleton and Changes in Cell Shape
The Mitotic Spindle – A Closer Look
Microtubules Chromosomes
Sisterchromatids
AsterCentrosome
Metaphaseplate
Kineto-chores
Kinetochoremicrotubules
0.5 µm
Overlappingnonkinetochoremicrotubules
1 µmCentrosome
The Mitotic Spindle – A Closer Look
• Nonkinetochore microtubules from opposite poles overlap and push against each other, elongating the cell
• In telophase, genetically identical daughter nuclei form at opposite ends of the cell
The Mitotic Spindle – A Closer Look
• In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell
• The microtubules shorten by depolymerizing at their kinetochore ends
The Mitotic Spindle – A Closer Look
Chromosomemovement
Microtubule Motorprotein
Chromosome
Kinetochore
Tubulinsubunits
Dyenin Moves Chromosomes During Mitosis
1) Spindle microtubules slide past each other carrying the chromosomes with them.
Two hypotheses for chromosomal translocation during mitosis
2) Spindle microtubules shorten at the centromere and the chromosomes move using a specialized motor protein.
Dyenin Moves Chromosomes During Mitosis
Blue stain = DNAGreen = TubulinRed = Dynein
Remember chromosome structureduring mitosis:
1. Chromatin has condensed into chromosomes
2. Sister chromatids are attached at the centromere
3. Kinetochore has attached duplicated chromosomes to the spindle
Dyenin Moves Chromosomes During Mitosis
Green = DNARed = Tubulin
p50 = Dyenininhibitor