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Cell Reproduction Mitosis Chromosomes • Made of DNA • Wrap around histone proteins to become chromatin Chromosomes • Chromatin wind up further to become chromosomes • Each half of a chromosome is a chromatid • Chromatids are held together in the center at the centromere Chromosomes • Sex chromosomes determine an organism’s sex • Females = XX • Males = XY • Number of sex chromosomes in a human cell is 2 Chromosomes • Autosomes – all the chromosomes except for the sex chromosomes • Number of autosomes in humans = 44 Chromosomes • Homologous chromosomes – every cell in sexual organisms has two copies of each autosome Chromosomes • Humans have 44 autosomes so they have 22 homologous pairs. • Dogs have 76 autosomes so they have 38 homologous pairs. – Goat = 58 – Armidillo = 62 – Chicken = 78 – House fly = 12 – Mosquito = 6 Diploid • Cells with 2 sets of chromosomes (homologous prs.) are diploid. • In humans, the diploid number is 46 (2n) Haploid • Cells with one set of chromosomes are haploid. • In humans, the haploid number is 23 (1n). Cell Division -Prokaryotes • Bacteria do not have a nucleus. • They divided by a process known as binary fission. Binary Fission • Chromosomes replicate • The cell grows to twice its original size • The cell divides into two identical cells Cell Division in Eukaryotes • Eukaryotes have a nucleus in their cells • Mitosis: results in new cells with identical genetic material • Meiosis: results in new cells with half the number of chromosomes Cell Cycle • Set of events that make up the life of a cell Interphase • Takes up most of the cell’s lifespan (up to 90%) Interphase • G1 phase – cell grows to mature size • S phase – DNA replication • G2 phase – cell prepares for division Mitotic Phase • Series of events that allows a cell to divide • Prophase • Metaphase • Anaphase • Telophase Prophase • DNA winds up into chromosomes • Centrioles appear at opposite ends of the nucleus Prophase • Spindle fibers form • Chromosomes attach to spindle fibers • Nuclear membrane and nucleolus disappear Metaphase • Chromosomes move to the center (equator) of the cell Anaphase • Chromatids separate by the centromere • They are now considered to be individual chromosomes Telophase • Spindle fibers disappear • Chromosomes begin to unwind into chromatin • Nuclear membranes begin to form Cytokinesis • Immediately follows mitotic division • The cell membrane is split in half Animal Cytokinesis • The membrane pinches off into two new cells Plant Cytokinesis • A cell plate forms in the center. • The plate separates the two cells.
