Chapter 8: Cellular Reproduction 1. The Cell Cycle 2. Mitosis 3. Meiosis
2 Types of Cell Division 2n 1n Mitosis: occurs in somatic cells (almost all cells of the body) generates cells identical to original Meiosis: occurs in gamete production (sperm & egg) ½ the normal chromosome content (haploid or 1n )
Chromosome Content somatic cells are diploid (2n) 2 of each chromosome i.e., 1 from each parent gametes are haploid (1n) 1 of each chromosome 23 pairs of human chromosomes
1. The Cell Cycle
The Cell Division Cycle G2 M S G 1 all eukaryotic cells progress through the 4 phases of the cell cycle with each division
Stages of the Cell Cycle G 1 : preparation for DNA replication non-dividing cells are arrested at this stage of the cell cycle (referred to as G 0 ) S phase: replication of genetic material (DNA Synthesis) G 2 : preparation for cell division M phase: cell division (Mitosis or Meiosis) ***G 1, S & G 2 collectively make up Interphase ***
2. Mitosis
4 Stages of Mitosis 1) Prophase 2) Metaphase 3) Anaphase 4) Telophase & Cytokinesis
Interphase G 1, S phase & G 2 all events in preparation for cell division DNA replication duplication of organelles preparation of mitotic apparatus
Prophase 1) duplicated chromosomes condense, remain attached at the centromere 2) break down of nuclear envelope 3) centrosomes move to opposite poles 4) spindle fibers form
Duplicated Chromosomes in prophase, condensed duplicated chromosomes remain attached at the centromere each half of duplicated chromosome referred to as sister chromatids Still considered as 1 chromosome!
Metaphase chromosomes align along the metaphase plate of the cell mediated by spindle fibers connecting each centromere to a centrosome at ea pole of the cell
Anaphase sister chromatids are pulled to opposite poles of the cell separate at the centromere, each chromatid still attached to a spindle fiber spindle fibers shorten, pulling each chromatid to opposite poles
Telophase Opposite of prophase: nuclear envelope reforms around chromosomes at each pole spindle fibers break down chromosomes decondense Cytokinesis begins
Cytokinesis Division of the cytoplasm completes the process: the 2 daughter cells are genetically identical to the original parent cell each daughter cell is now in G 1 (the beginning of interphase)
3. Meiosis
Meiosis Produces 1n Gametes Meiosis requires 2 rounds of cell division: Meiosis I differs from mitosis in prophase, metaphase, anaphase Meiosis II basically the same process as mitosis (chromosome #) 2n 1n 1n (DNA content) 4n 2n
Meiosis I Differs from mitosis in several key ways: prophase I metaphase I anaphase I Prophase I: homologous chromosomes pair, exchange genetic material by crossing over Metaphase I: homologous chromosomes align as pairs Anaphase I: homologous chromosome pairs separate (sister chromatids are NOT separated)
Coat-color genes C Eye-color genes E Crossing Over c e Tetrad (homologous pair of chromosomes in synapsis) in Prophase I 1 Breakage of homologous chromatids C c C 2 E e Joining of homologous chromatids E an important source of new genetic combinations (genetic variability) Chiasma c e 3 Separation of homologous chromosomes at anaphase I C E C e c E c e 4 Separation of chromatids at anaphase II and completion of meiosis C C c c E e E e Parental type of chromosome Recombinant chromosome Recombinant chromosome Parental type of chromosome Gametes of four genetic types
Independent Assortment of Chromosomes due to alignment in Metaphase I an additional source of genetic variability Possibility 1 Possibility 2 Two equally probable arrangements of chromosomes at metaphase I 2 n possibilities (n = # of homologous pairs, e.g. 23) Metaphase II Gametes Combination 1 Combination 2 Combination 3 Combination 4
Meiosis II prophase II metaphase II anaphase II telophase II/cytokinesis essentially the same as mitosis results in four haploid (1n) gametes each is genetically unique due to 1) crossing over and 2) independent assortment of homologous chromosomes
Key Terms for Chapter 8 cell cycle, mitosis, meiosis haploid, diploid, somatic cell, gamete interphase, prophase, metaphase, anaphase telophase, cytokinesis centromere, centrosome, spindle fibers sister chromatids vs non-sister chromatids homologous chromosomes crossing over, independent assortment Relevant Review Questions: 1-6, 8, 9, 13, 17, 20