The Eukaryotic Cell Cycle G 0 : nondividing telophase metaphase anaphase cell division interphase S: Synthesis of DNA; chromosomes duplicated G 1 : Growth G 2 : Growth prophase cytokinesis Mitosis
Phases of Mitosis, 1 Interphase : The chromosomes (blue) are in the thin, extended state and appear as a mass in the center of the cell. The microtubules (red) extend outward from the nucleus to all parts of the cell. Metaphase : The chromosomes have moved along the spindle microtubules to the equator of the cell. Late prophase : Chromosomes (blue) have condensed and attached to microtubules of spindle fibers (red). Microtubules have reorganized to form the spindle; chromosomes, now condensed, are clearly visible.
In metaphase, sister chromatids are held together at centromere
At end of metaphase, centromere releases sister chromatids
In anaphase, they move to opposite poles
Phases of Mitosis, 2 Anaphase : Sister chromatids have separated, and one set of chromosomes moves along the spindle microtubule to each pole of the cell. Telophase : The chromosomes have gathered into two clusters, one at the site of each future nucleus. Next interphase : Chromosomes are relaxing again into their extended state. Spindle fibers are disappearing, and the microtubules of the 2 daughter cells rearrange into the interphase pattern.
Mitosis: Prophase - Metaphase Kinetochores align at cell’s equator Nucleolus disappears; Nuclear envelope breaks down Microtubules attach to kinetochores Chromosomes condense and shorten Centrioles begin to move apart; Spindle forms Duplicated chromosomes remain elongated Centrioles have also been duplicated Late Interphase Early Prophase Late Prophase Metaphase
Mitosis Anaphase - Cytokinesis Free spindle fibers push poles apart Chromatids become independent chromosomes One set of chromosomes; Begin unwinding Nuclear envelope re-forms Cytoplasm divided along equator Each daughter gets 1 nucleus & half of cytoplasm Spindle disappears; Nucleolus reappears Anaphase Telophase Cytokinesis Next Interphase
Cytokinesis of a Ciliated Cell Cleavage Furrow Daughter Cells
Cytokinesis in Plants Vesicles fuse to form cell wall and membranes Complete separation of daughter cells
2. Different homologous chromosomes with certain alleles are combined with other homologous chromosomes in a random manner…
3. Two gametes produced by meiosis each contribute their unique allelic combinations to produce a new offspring
Meiosis I Homologous chromosomes pair and cross over Homologous chromosomes exchange DNA & align on equator Homologous chromosomes move to opposite poles Prophase I Metaphase I Anaphase I Telophase I
Meiosis II Prophase II Metaphase II Anaphase II Telophase II Four Haploid Cells Similar to Mitosis
Meiosis vs. Mitosis: Comparison of Spindles Meiosis: Duplicated chromosomes with one kinetochore; Paired homologues go to opposite poles. Mitosis: Duplicated chromosomes with two kinetochores; Unpaired homologs split between sister chromatids, which go to opposite poles.
Genetic variability among organisms is essential in a changing environment
Mutations produce new variation but are relatively rare occurrences
Randomized line up and separation of homologous chromosomes in Meiotic Metaphase I and Anaphase I increase variation
The number of possible combinations is 2 n , where n = number of homologous pairs
Crossing Over Variation also enhanced by genetic recombination Crossing over in Meiotic Prophase I creates chromosomes with new allele combinations Combined with homologue shuffling in Metaphase/Anaphase I, each gamete produced in meiosis is virtually unique
Fusion of Gametes Fusion of games from two individuals further increases possible 2n combinations Gametes from two humans could produce about 64 trillion different 2n combinations Taken together with crossing over, each human individual is absolutely genetically unique