Cancer As the cell grows, its volume increases much more rapidly than the surface area. If the cell becomes too large, the cell might have difficulty supplying nutrients and expelling enough waste products. Diffusion over large distances is slow and inefficient. Cell size affects the ability of the cell to communicate instructions for cellular functions. Cell Size and Growth Cell Cycle Regulation Cellular Reproduction Different cyclin/CDK combinations signal activities, including DNA replication, protein synthesis, and nuclear division throughout the cell cycle. Cyclin and CDK tell a cell if it is time to divide, grow, or replicate. They signal whether to divide or not. G 1 checkpoint M checkpoint Control system G 2 checkpoint Cells continue dividing until they touch one another = density-dependent inhibition Cells anchor to dish surface and divide. Figure 8.8A When cells have formed a complete single layer, they stop dividing (density-dependent inhibition). If some cells are scraped away, the remaining cells divide to fill the dish with a single layer and then stop (density-dependent inhibition). Growth factors bind to specific receptors on the plasma membrane to trigger and stop cell division Growth factor Figure 8.8B Cell cycle control system Plasma membrane Receptor protein Signal transduction pathway G 1 checkpoint Relay proteins 6 Uncontrolled Mitosis If mitosis is not controlled, unlimited cell division occurs causing cancerous tumors If mitosis is not controlled, unlimited cell division occurs causing cancerous tumors Oncogenes are special proteins increase the chance that a normal cell develops into a tumor cell Oncogenes are special proteins that increase the chance that a normal cell develops into a tumor cell Cancer cells Abnormal Cell Cycle: Cancer Cancer is the uncontrolled growth and division of cells. No CDK or cyclin to turn off the switch to multiply. Cancer cells can kill an organism by crowding out normal cells, resulting in the loss of tissue function. Cell Cycle Regulation Causes of Cancer Various environmental factors can affect the occurrence of cancer cells. Cell Cycle Regulation Smoking, radiation, viruses, cancer-causing chemicals (carcinogens), obesity, hormones, and a lack of exercise. Apoptosis Programmed cell death Cells going through apoptosis actually shrink and shrivel in a controlled process. Does not happen fast enough in cancer cells Cell Cycle Regulation When a cell undergoes apoptosis, white blood cells called macrophages consume cell debris. 10 Meiosis Formation of Gametes (Eggs & Sperm) 11 Why Do we Need Meiosis? It is the fundamental basis of sexual reproduction It is the fundamental basis of sexual reproduction Two haploid (1n) gametes (sperm and egg) are brought together through fertilization to form a diploid (2n) zygote Two haploid (1n) gametes (sperm and egg) are brought together through fertilization to form a diploid (2n) zygote Haploid gametes (1n) 23 chromosomes, sex cells like sperm and eggs Diploid zygote (2n) when two gametes combine they form zygote 12 Fertilization Putting it all together 1n = 26 chromosomes 2n = 43 chromosomes Meiosis is a process that produces gametes. Meiosis When gametes combine in fertilization, the number of chromosomes is restored. 14 Replication of Chromosomes Replication is the process of duplicating a chromosome Replication is the process of duplicating a chromosome Occurs prior to division Occurs prior to division Replicated copies are called sister chromatids Replicated copies are called sister chromatids Held together at centromere Held together at centromere Occurs in Interphase 15 Sister Chromatids form Tetrads Sister Chromatids Sister Chromatids Join to form a TETRAD Called Synapsis Homologous Chromosomes (Tetrads) PaternalMaternal eye color locus eye color locus hair color locus hair color locus Homologous chromosomes are similar but not identical. Each carries the same genes in the same order, but the information for each trait may not be the same. 17 Crossing-Over Homologous chromosomes in a tetrad cross over each other Homologous chromosomes in a tetrad cross over each other Pieces of chromosomes or genes are exchanged Pieces of chromosomes or genes are exchanged Produces Genetic recombination in the offspring Produces Genetic recombination in the offspring 18 Crossing-over multiplies the already huge number of different gamete types produced by independent assortment Crossing-Over Stages of Meiosis Reduces the chromosome number by half through the separation of homologous chromosomes Involves two consecutive cell divisions called meiosis I and meiosis II Produces 4 cells Meiosis 20 Meiosis I: Reduction Division Nucleus Spindlefibers Nuclearenvelope Early Prophase I (Chromosome number doubled) Late Prophase I Metaphase I Anaphase I Telophase I (diploid) 21 Prophase I Early prophase Homologs pair. Homologs pair. Crossing over occurs Crossing over occurs. Late prophase Chromosomes condense. Chromosomes condense. Spindle forms. Spindle forms. Nuclear envelope fragments. Nuclear envelope fragments. 22 Metaphase I Homologous pairs of chromosomes align along the equator of the cell 23 Anaphase I Homologs separate and move to opposite poles. Sister chromatids remain attached at their centromeres attached at their centromeres. 24 Telophase I Nuclear envelopes reassemble. Spindle disappears. Cytokinesis divides cell into two. 25 Meiosis II Only one homolog of each chromosome is present in the cell Only one homolog of each chromosome is present in the cell. Meiosis II produces gametes with one copy of each chromosome and thus one copy of each gene. Sister chromatids carry identical genetic information. Gene X 26 Meiosis II: Reducing Chromosome Number Prophase II Metaphase II Anaphase II Telophase II 4 Identical haploid cells 27 Prophase II Nuclear envelope fragments. Spindle forms. 28 Metaphase II Chromosomes align along equator of cell. 29 Anaphase II Sister chromatids separate and move to opposite poles Sister chromatids separate and move to opposite poles. Equator Pole 30 Telophase II Nuclear envelope assembles. Chromosomes decondense. Spindle disappears. Cytokinesis divides cell into two. 31 Results of Meiosis Gametes (egg & sperm) form Four haploid cells with one copy of each chromosome One allele of each gene Different combinations of alleles for different genes along the chromosome 32 Gametogenesis Oogenesis or Spermatogenesis 33 Spermatogenesis Occurs in the testes Occurs in the testes Two divisions produce 4 spermatids Two divisions produce 4 spermatids Spermatids mature into sperm Spermatids mature into sperm Men produce about 250,000,000 sperm per day Men produce about 250,000,000 sperm per day 34 Oogenesis Occurs in the ovaries Occurs in the ovaries Two divisions produce 3 polar bodies that die and 1 egg Two divisions produce 3 polar bodies that die and 1 egg Polar bodies die because of unequal division of cytoplasm Polar bodies die because of unequal division of cytoplasm Immature egg called oocyte Immature egg called oocyte Starting at puberty, one oocyte matures into an ovum (egg) every 28 days Starting at puberty, one oocyte matures into an ovum (egg) every 28 days 35 MitosisMeiosis Number of divisions 12 Number of daughter cells 24 Genetically identical? YesNo Chromosome # Same as parent Half of parent Where Somatic cells Germ cells When Throughout life At sexual maturity Role Growth and repair Sexual reproduction Comparison of Divisions Human Life Cycle GameteFertilizationZygoteMitosisMeiosisOrganismDIPLOIDHAPLOID