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Objectives Announcements • Understand inheritance of blood type in . • Problem sets due this week at the • Know how beginning of lab. – vary in number among life stages and among species. • Show your work! – to tell apart • Write out solutions, helps to think things – to figure out which chromosomes are homologues. through. • Understand the role of and in asexual and sexual . • Review study CD that came with text for lab • Distinguish between stages of division. this week (especially mitosis and meiosis). • Recognize how chromosome movement during meiosis results in Mendel's laws of Segregation and Independent Assortment.

Human blood type example Blood type phenotypes • Four phenotypes: A, B, AB, O • One determines blood type • A, B or no substance coats blood cells • Three are common • Blood type determined by whether • Two alleles are dominant to the third antibodies react to substance and codominant with each other

Blood type chromosome structure • Three alleles present- Ia, Ib, i • Chromosomes contain DNA & – Ia and Ib are codominant • made of – i is recessive – - region to which spindle fibers • Possible genotypes attach during mitosis and meiosis – arms – IaIa homozygote, Iai heterozygote, • region that extends from centromere b b b – I I homozygote, I i heterozygote, • contain – ii homozygote, IaIb heterozygote

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

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

• 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 and fertilization (syngamy) How do organisms differ in chromosome number? • Ploidy- how does spend most of life? – Haploid (, & fungi, ) – Diploid (, flowering , , ) – Polyploid (many plants, few ) • ‘haploid’ chromosome number – Humans: 23 – Fruit flies: 4 – Ferns: thousands

2 Figure 13.1 The of a hydra Reproduction in • asexual – budding or – offspring genetically identical to parent • sexual – fusion of two haploid – different combinations of genes than in parents

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

Cell cycles in eukaryotes • – 90% of cell life – 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 Figure 12-09x Mitosis in an root

3 Mitosis • Occurs throughout life of a multicellular • Nuclear membrane disappears organism • Mitotic spindle forms – development – organizing centers – growth () form and migrate to opposite ends of cell – maintenance – attach to centromere at • Involved with asexual reproduction • We define stages (prophase, , • Duplicated chromosomes (sister , ) ) move toward center of cell

Figure 12.5 The stages of mitotic : ; prophase Figure 12.5 The stages of mitotic cell division : prophase;

Figure 12.6 The mitotic spindle at metaphase

Metaphase

• Chromosomes align along metaphase plate • aligned with each other • face centrosomes away from center of the cell

4 Figure 12.5 stages of mitotic cell division: metaphase; anaphase

Anaphase

Sister chromatids are pulled toward opposite ends of cell

Stages of mitotic cell division : anaphase; telophase and Telophase

• Nuclear membrane in each daughter cell starts to form • Chromosomes elongate • Cell division occurs

Significance of Meiosis • cells formed with one copy of Prophase I (longest phase) each chromosome • pairing of homologous chromosomes • Meiotic events cause Mendel's laws • paired homologues consist of 4 – Segregation- Homologues separate chromatids (tetrads) – Independent assortment- independent orientation of chromosomes • X-shaped configurations form (chiasmata) • Recombination occurs (independent • Crossing over occurs orientation, crossing over) • chiasmata move to end of chromosome arms

5 What happens during crossing over? Metaphase & Anaphase I • exchange of • tetrads moved to metaphase plate segments at identical • homologues pulled apart into different positions along daughter cells homologues • no loss or • remain attached at addition of centromeres genetic material • 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 cell division 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 now sister chromatids separate • genetic material reduced, but not chromosome number

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