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Announcements• Problem sets due this week at the

beginning of lab.• Show your work!• Write out solutions, helps to think things

through.• Review study CD that came with text for lab

this week (especially mitosis and meiosis).

Objectives• Understand inheritance of blood type in humans.• Know how

– organisms vary in chromosome number among lifestages and among species.

– to tell chromosomes apart– to figure out which chromosomes are homologues.

• Understand the role of mitosis and meiosis inasexual and sexual reproduction.

• Distinguish between stages of cell division.• Recognize how chromosome movement during

meiosis results in Mendel's laws of Segregationand Independent Assortment.

Human blood type example

• One locus determines blood type• Three alleles are common• Two alleles are dominant to the third

and codominant with each other

Blood type phenotypes• Four phenotypes: A, B, AB, O• A, B or no substance coats blood cells• Blood type determined by whether

antibodies react to substance

Blood type genotypes• Three alleles present- Ia, Ib, i

– Ia and Ib are codominant– i is recessive

• Possible genotypes– IaIa homozygote, Iai heterozygote,– IbIb homozygote, Ibi heterozygote,– ii homozygote, IaIb heterozygote

Eukaryote chromosome structure• Chromosomes contain DNA & proteins• made of

– centromere- region to which spindle fibersattach during mitosis and meiosis

– arms• region that extends from centromere• contain genes

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Figure 12.3 Chromosome duplication and distribution during mitosis Figure 13.x4 Human male chromosomes

Figure 13.x5 Chromosomes differ in length and position of centromere

Life cycle stages differ in ploidy

• Ploidy = number of copies of each homologue• Common ploidy levels

– Haploid- one copy (1N)– Diploid- two copies (2N)– Polyploid- multiple (>2) copies (3N, 4N, etc)

Figure 13.5 Three sexual life cycles differing in the timing of meiosis andfertilization (syngamy) How do organisms differ in

chromosome number?• Ploidy- how does organism spend most of life?

– Haploid (protists, algae & fungi, moss)– Diploid (ferns, flowering plants, insects, vertebrates)– Polyploid (many plants, few animals)

• ‘haploid’ chromosome number– Humans: 23– Fruit flies: 4– Ferns: thousands

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Reproduction in eukaryotes• asexual

– budding or vegetative reproduction– offspring genetically identical to parent

• sexual– fusion of two haploid gametes– different combinations of genes than in

parents

Figure 13.1 The asexual reproduction of a hydra

Figure 33.7 The life cycle of the hydrozoan Obelia (Layer 3)

Cell cycles in eukaryotes• Interphase

– 90% of cell life– cell growth occurs– chromosomes are copied but not visible

• Mitotic phase– nucleus 'dissolves'– chromosomes condense– cell divides– two daughter cells genetically identical to parent

Figure 12.4 The cell cycle Figure 12-09x Mitosis in an onion root

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Mitosis• Occurs throughout life of a multicellular

organism– development– growth– maintenance

• Involved with asexual reproduction• We define stages (prophase, metaphase,

anaphase, telophase)

Prophase• Nuclear membrane disappears• Mitotic spindle forms

– Microtubule organizing centers(centrosomes) form and migrate to oppositeends of cell

– Microtubules attach to centromere atkinetochore

• Duplicated chromosomes (sisterchromatids) move toward center of cell

Figure 12.5 The stages of mitotic cell division: G2 phase; prophase Figure 12.5 The stages of mitotic cell division : prophase; prometaphase

Metaphase

• Chromosomes align along metaphaseplate

• Centromeres aligned with each other• Kinetochores face centrosomes away

from center of the cell

Figure 12.6 The mitotic spindle at metaphase

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Anaphase

Sister chromatids are pulledtoward opposite ends of cell

Figure 12.5 stages of mitotic cell division: metaphase; anaphase

Telophase

• Nuclear membrane in each daughtercell starts to form

• Chromosomes elongate• Cell division occurs

Stages of mitotic cell division : anaphase; telophase and cytokinesis

Significance of Meiosis• Gamete cells formed with one copy of

each chromosome• Meiotic events cause Mendel's laws

– Segregation- Homologues separate– Independent assortment- independent

orientation of chromosomes• Recombination occurs (independent

orientation, crossing over)

Prophase I (longest phase)• pairing of homologous chromosomes• paired homologues consist of 4

chromatids (tetrads)• X-shaped configurations form (chiasmata)• Crossing over occurs• chiasmata move to end of chromosome

arms

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What happens during crossing over?

• exchange ofsegments atidenticalpositions alonghomologues

• no loss oraddition ofgenetic material

Metaphase & Anaphase I• tetrads moved to metaphase plate• homologues pulled apart into different

daughter cells• sister chromatids remain attached at

centromeres• chromosome number is now reduced

Figure 13.7 The stages of meiotic cell division: Meiosis I

Telophase I and Prophase II

• Chromosomes may uncoil as celldivision occurs

• No chromosome duplication occurs

Figure 13.7 The stages of meiotic cell division: Meiosis II

Metaphase II, Anaphase II,Telophase II

• mitotic division repeated, but nowsister chromatids separate

• genetic material reduced, but notchromosome number