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Standards 2a. Know meiosis is an early step in sexual reproduction in which the pairs of chromosomes separate & segregate randomly during cell division to produce gametes containing 1 chromosome each. 2b. Know only diploid cells, spermatogonia & oogonia undergo meiosis. 2c. Know random chromosomal segregation explains the probablitity that a particular allele will be in a gamete. 2d. Know new combos of alleles are generated in a zygote through the fusion of male & female gametes (fertilization). 2e. Know why half an person’s DNA sequence comes from each parent. 2f. Know the role of chromosomes in determining a person’s sex. Cell Cycle Chromosomes Compact, coiled DNA Chromatid One half of the chromosome Centromere Between the two chromatids holds them together Number of Chromosomes Humans have 46 chromosomes 2 are sex chromosomes 44 are autosomes Types of chromosomes Sex chromosomes Chromosomes that determine the sex of an organism Humans have an X or Y chromosome Autosomes All other chromosomes in an organism Homologous Chromosomes There are only 23 different chromosomes To have 46 we have two copies of each chromosome We get one from our mom and one from our dad The pairs of each chromosome are called homologues Homologues carry genes for the same traits Karyotype Show’s the 23 pairs of chromosomes 22 homologous pairs of autosomes Diploid & Haploid Diploid Cells that have 2 sets of chromosomes All cells, other than reproductive cells, are diploid Haploid Cells that have only 1 set of chromosomes Reproductive cells (sperm & eggs) are haploid Cell Division All cells come from preexisting cells Prokaryotes Binary Fission Eukaryotes Mitosis Meiosis Binary Fission The chromosome makes a copy of itself The cell grows until it reaches about 2x it’s size A cell wall forms between the two chromosomes and the cell splits Mitosis How all cells divide EXCEPT reproductive cells The cytoplasm and nucleus both divide Results in two cells that are genetically identical Mitosis – Cell Cycle Interphase Time between cell divisions 3 phases Cell Division Time when the cell is dividing 2 phases INTERPHASE G1 phase Grow to mature size Gap between cell division & DNA replication S phase DNA is copied G2 phase Prepares for cell division Gap between DNA replication & cell division Mitosis - Prophase DNA is shortened and tightened into chromosomes Nucleolus & nuclear membrane break down & disappear Two centrosomes appear Animals also have centrioles Centrosomes move to oppositie poles of cell Spindle fibers radiate from centrosomes Mitosis - Metaphase Spindle fibers move chromosomes to the center of cell Mitosis - Anaphase Chromatids separate at the centromere and are pulled to opposite poles of cell by spindle fibers Mitosis - Telophase Spindle fibers break down Chromosome loosens up Nuclear envelope forms around each set of chromosomes Nucleolus forms Cytokinesis Cell membrane pinches inward creating a cleavage furrow Eventually leads to two cells Cell Plate Only in plants Golgi forms vesicles that fuse at the midline and divide the cell Makes a cell wall