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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