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MCDB 1041 Class 3 Chromosomes and Cell division Webpage: mcdb.colorado.edu/courses/1041/ Learning goals: 1. Diagram the cell cycle 2. Describe the basic principle of mitosis (why do cells undergo mitosis?) 3. Diagram how mitosis proceeds, including the numbers of chromosomes present at each phase of mitosis 4. Distinguish between chromosome, replicated chromosomes, and sister chromosomes Chromosomes, continued then Mitosis We inherit our genetic makeup, or genes from our parents by inheriting a chromosome of each type (number) from each of our parents: a A Chromosome 1 (from Dad) Chromosome 1 (from Mom) These two chromosomes are often referred to as a pair of chromosome 1s : homologous chromosomes These homologous chromosomes, one from your father, and one from your mother have a. The same genes, but not necessarily the same sequence b. The same genes with the same exact sequence c. Different genes homologous chromosomes A Chromosome 1 (from Dad) a Chromosome 1 (from Mom) The A (dominant) and the a (recessive) represent two different versions of the same gene, referred to as alleles True or false: The difference between allele A and allele a could be as small as a single DNA base pair a. True b. False An allele is a simply a variation in a DNA sequence A B In this picture (from a human male), the two chromosomes you see for each number (indicated A and B) are: a. Exact replicas of each other b. Slightly different from each other c. Each were inherited from a different parent d. A and C e. B and C What are A and B called? a. Sister chromatids b. Homologous chromosomes Cell division is fundamentally important to life In an embryo: a single cell divides into many thousands of cells and these cells make all the different parts of our bodies In an adult: certain cells called stem cells keep dividing to replenish cells that die: skin cells, blood cells, bone CANCER: Cell division gone wrong Normal cell division and death (1) of an abnormal cell (2) Cell division starting with a cancerous cell Cancer cells have many defects • Mutations • Allow cell to live when it should die • Allow cell to divide when it shouldn t • Chromosome problems • Wrong number ( too many, too few) • Rearrangements When these cells divide, more problems arise…the checks and balances are gone SO, we need to understand mitosis! The Cell Cycle: when cells are dividing, they go through the following phases: • S phase: DNA Synthesis • G phase: Gap or “Growth” phase • M phase: Mitosis (nuclear division): 4 stages Stem cells keep going through the cell cycle Other cells exit Cancer cells behave like stem cells Chromosomal Separa-on– Mitosis Blue: DNA Green: Spindle Proteins h"p://www.cbp.pi".edu/faculty/yong_wan/images/main_cell_cycle.jpg Look at the handout and work with your group on the activity There are 5 stages shown in your handout. Interphase Here Prophase Anaphase Telophase are their namesMetaphase (not in the correct order!): Telophase, Prophase, Interphase, Anaphase, Metaphase Clicker questions: review the concepts Below is a karyotype of a genetically normal female. The karyotype was made from cells undergoing metaphase of mitosis. The circle contains how many chromatids? A. 1 B. 2 C. 4 D. 6 E. 8 Consider a replicated chromosome as a unit. How many replicated chromosomes are present in each human cell at the end of mitosis, after the two new cells have separated? a. b. c. d. e. 0 2 23 46 92 At the entry to mitosis (prophase) in a human cell: a. There are 46 replicated chromosomes b. There are 46 chromosomes c. There are 46 sets of homologous chromosomes d. I am confused What is the correct order of the phases of the cell cycle? a. G1 S G2 M b. G1 G2 S M c. G1 G2 M S d. S G1 G2 M Why does it matter? • The genome must be replicated before M phase (mitosis) • The other phases (G1 and G2) are important growth phases, and also serve as time points at which the cell is “Checked” to make sure contents are correct Mitosis is critical for making more cells as our bodies grow or repair These cells that divide are called somatic cells Germ cells = gametes = sperm or egg The germ cell precursors do not undergo mitosis but rather MEIOSIS— next week!