23. A Replicated Chromosome Gene X Homologs separate in meiosis I and therefore different alleles separate. copyright cmassengale Homologs (same genes, different alleles) Sister Chromatids (same genes, same alleles)
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25. Meiosis: Two Part Cell Division Homologs separate Sister chromatids separate Diploid Diploid Haploid copyright cmassengale Meiosis I Meiosis II
26. Meiosis I: Reduction Division Early Prophase I (Chromosome number doubled ) Late Prophase I Metaphase I Anaphase I Telophase I (diploid) copyright cmassengale Nucleus Spindle fibers Nuclear envelope
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28. Tetrads Form in Prophase I Homologous chromosomes (each with sister chromatids) Join to form a TETRAD Called Synapsis copyright cmassengale
32. Metaphase I Homologous pairs of chromosomes align along the equator of the cell copyright cmassengale
33. Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres . copyright cmassengale
34. Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two. copyright cmassengale
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36. Meiosis II: Reducing Chromosome Number Prophase II Metaphase II Anaphase II Telophase II 4 Genetically Different haploid cells copyright cmassengale
37. Prophase II Nuclear envelope fragments. Spindle forms. copyright cmassengale
39. Anaphase II Sister chromatids separate and move to opposite poles . Equator Pole copyright cmassengale
40. Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two. copyright cmassengale
41. Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome copyright cmassengale
49. Oogenesis copyright cmassengale Oogonium (diploid) Mitosis Primary oocyte (diploid) Meiosis I Secondary oocyte (haploid) Meiosis II (if fertilization occurs) First polar body may divide (haploid) Polar bodies die Ovum (egg) Second polar body (haploid) a A X X a X A X a X a X Mature egg A X A X
51. Comparison of Divisions copyright cmassengale Mitosis Meiosis Number of divisions 1 2 Number of daughter cells 2 4 Genetically identical? Yes No Chromosome # Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction
Replication is the process of duplicating chromosome. The new copy of a chromosome is formed by DNA synthesis during S-phase. The chromosome copies are called sister chromatids. Sister chromatids are held together at the centromere.
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Fusion of gametes to produce zygote In humans this takes place near the top of the oviduct . Hundreds of sperm reach the egg (shown in this photo). When a sperm reaches the ovum cell the two membranes fuse and the sperm nucleus enters the cytoplasm of the ovum. This triggers a series of reactions in the ovum that cause the jelly coat to thicken and harden, preventing any other sperm from entering the ovum. The sperm and egg nuclei then fuse, forming a diploid zygote. In plants fertilisation takes place in the ovary at the base of the carpel. The haploid male nuclei travel down the pollen tube from the pollen grain on the stigma to the ovules in the ovary. In the ovule two fusions between male and female nuclei take place: one forms the zygote (which will become the embryo) while the other forms the endosperm (which will become the food store in the seed). This double fertilisation is unique to flowering plants
