Chapter 13 - Meiosis 13 Notes.pdf · Chapter 13 - Meiosis A. Bergeron AP Biology PCHS. ... illustrates the alternation of generations that is ... • Three events are unique to meiosis,

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?

0

2.0

4.0

6.0

8.0

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

cted

)

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


Section 11-4


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





Section 11-4


Meiosis II - 2nd Cell (nuclear) Division





Section 11-4Figure 11-17 Meiosis II

Meiosis II - 2nd Cell (nuclear) Division





Section 11-4


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


• 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


• 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


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.


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


Green = DNARed = Tubulin

p50 = Dyenininhibitor

Documents

Chapter 13 - Meiosis 13 Notes.pdf · Chapter 13 - Meiosis A. Bergeron AP Biology PCHS. ... illustrates the alternation of generations that is ... • Three events are unique to meiosis,