Chapter 8 - Study Notes
Goals of Cell Division/Cell Cycle
DNA - long chains of nucleotides make up genes
Genes are linked together on chromosomes
What are chromosomes? - chromosomes are long strands of chromatin (DNA and associated proteins). Specific nucleotide sequences called genes are found within the chromatin strands of DNA. There are many genes on each chromosome. Most organisms are diploid (2n), and have 2 homologous copies of each chromosome. Some organisms (prokaryotes especially) are haploid (n) and have only one copy of each chromosome.
Prokaryotic Cell Cycle
Cell division occurs through Binary Fission
Eukaryotic Cell Cycle
Takes place in two stages:
G1 phase-No DNA synthesis; Cell growth and differentiation occurs and the 'decision' is made regarding whether the cell should continue on to mitosis.
S phase - DNA is replicated
G2 phase - Cell growth and protein synthesis occurs in preparation for mitosis
Prophase - Chromosomes condense and spindles (microtubules) begin to form. Nuclear membrane and nucleoli disappear and spindles polarize. Chromosomes attach to microtubules at kinetochore
Metaphase - Chromosomes line up at the equator of the cell, the "metaphase plate"
Anaphase - Centromere divides; chromatids separate. Kinetochore microtubules pull attached chromosomes.
Telophase - Spindle disintegrates, Chromosomes relax and nuclear membrane and nucleolus reform.
Cytokinesis - rings develop around center of cell. Contracts until the 2 daughter cells separate.
Factors that effect whether a cell continues to divide or is halted during interphase: anchorage dependence and density dependence.
**Each daughter cell resulting from mitosis is diploid. If mitosis were the only possible way for cells to reproduce, there would be less genetic variability and sexual reproduction would be impossible (if two diploid cells merged, the result would be 2n + 2n = 4n, or tetraploid cells which would be unable to survive).
Meiosis - "reduction division"
Meiosis is the process by which the homologous chromosomes of a diploid cell are separated. Each of the four resulting haploid daughter cells contains one member of each pair of homologous chromosomes.
Prophase I - Chromosomes thicken and condense. Homologous chromosomes come together in pairs. The nuclear membrane disintegrates and the spindles form.
Metaphase I - Paired homologous chromosomes line up along the equator of the cell. One homolog of each pair faces each pole of the cell, with both chromatids of a given homolog becoming attached to microtubules leading to the same pole.
Anaphase I - Homologues separate, one member of each pair going to each pole of the cell. Sister chromatids do not separate.
Telophase I - Two clusters of chromosomes have formed, each containing one member of each pair of homologues. The daughter nuclei are therefore haploid. Cytokinesis may or may not happen at this stage and there is generally no interphase between meiosis I and meiosis II.
Prophase II - If chromosomes have relaxed, they re-condense. Spindles reform and the microtubules attach to the sister chromatids.
Metaphase II - Chromosomes line up along the equator, with sister chromatids of each chromosome attached to microtubules leading to opposite poles of the cell.
Anaphase II - Chromatids separate into independent daughter chromosomes, one moving toward each pole.
Telophase II - Chromosomes finish moving to opposite poles.
Cytokinesis results in four haploid cells, each containing one member of each pair of homologous chromosomes.
Sometimes problems can occur during meiosis that may lead to extra copies of certain chromosomes being present in gamete cells. The major cause of this malfunction of meiosis is non-disjunction, homologous chromosomes or sister chromatids fail to separate properly. (Examples include Down Syndrome, Klinefelter's Syndrome, Turner's Syndrome)