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Mitosis and Cytokinesis Chapter 5: Cell Growth and Division Section 2 How do cells divide? Cell Cycle Review • What is the “growth phase”? • When is DNA replicated? • When does the cell undergo checkpoints? • What is the function of the checkpoint? DNA Review • • • • • • What type of Carbon-based molecule is DNA? What is the monomer? What makes up the monomer? Is DNA single stranded or double stranded? What are the nitrogenous bases? Where is it stored? Chromosomes Condense • We have already learned that DNA is replicated during the S phase of the cell cycle. – However, there are a lot of intricacies to that process. • Chromosome – one long, continuous thread of DNA that consists of numerous genes along with regulatory information. – Gene – the sequence of nucleotides forming part of a chromosome; the order of which determines the sequence of amino acids in a protein Human Chromosome Number • Somatic Cells – biological cell forming the body of the cell; any cell not including gametes or stem cells • Diploid – chromosomes arranged in homologous pairs – The human diploid number is 46 – Our somatic cells have a diploid number of 46 – DOES NOT include gametes Human Chromosome Number Human Somatic Cells • Human somatic cells contain 46 chromosomes. – If each chromosome was stretched out, they would measure about 10 feet long!! – How would these fit inside a nucleus? • In order to help fit the DNA inside the nucleus, the DNA wraps around proteins called histones. – Histones help condense and organize the DNA Chromosomes Condense • During interphase, DNA is loosely organized. – Proteins must have access to specific genes to make new proteins or copy the entire DNA sequence. – Therefore, being too tightly organized would restrict access to the DNA. • During mitosis, chromosomes are tightly condensed. – Must condense to be divided between the two nuclei. – If they remained stringy, the chromosomes would become entangled. Histones • At all times during the cell cycle, each of your chromosomes is associated with histones. – DNA wraps around the histones at regular intervals. – Parts of the histones will interact causing the DNA to further condense. • Chromatin – the loose combination of DNA and histones Chromatin • “Loose” is used to describe how the DNA folds on itself (condenses) and is NOT referring to how the DNA is wrapped around the histones. • As the cell progresses to Mitosis, DNA is replicated during the S phase. – Both sets of DNA continue to condense in preparation for cell division – Eventually the chromatin form small, thick rods Chromatid • Chromatid – One half of the duplicated chromosome – Sister Chromatid –the 2 identical chromatids – Centromere – the condensed region joining the sister chromatids together. Chromatid Telomeres • Telomere – The ends of chromosomes – Made of repeating nucleotides that do not form genes. (aka this portion does not code for anything). • Function of Telomere: – Prevents the ends of chromosomes from accidentally attaching to each other – Prevents the loss of genes Mitosis • Mitosis divides the parent cell’s nucleus and nuclear contents into two genetically identical nuclei – Each nuclei contains its own single, full set of DNA • 4 Phases of Mitosis 1. 2. 3. 4. Prophase Metaphase Anaphase Telophase Prophase 1. Chromatin condenses into tightly coiled chromosomes – Each consists of 2 identical chromatids 2. Nuclear envelope breaks down 3. Nucleolus disappears 4. Centrosomes and centrioles begin to migrate to opposite sides of the cell 5. Organized microtubules called spindle fibers grow from centrioles and radiate toward the center of the cell Prophase Prophase Metaphase 1. The spindle fibers attach to a protein structure on the centromere 2. When the spindle fibers attach, this causes the chromosomes to align along the cell equator Metaphase Metaphase Metaphase Anaphase 1. Sister chromatids separate from each other 2. Spindle fibers begin to shorten, pulling sister chromatids away from each other and towards opposite side of the cell. Anaphase Anaphase Telophase 1. A complete set of identical chromosomes is positoned at each pole of the cell. 2. Nuclear membranes start to form 3. Chromosomes begin to uncoil 4. Spindle fibers fall apart Telophase Telophase Cytokinesis • Cytokinesis – divides the cytoplasm into 2 cells and completes a full stage of the cell cycle • At the completion of cytokinesis, there will be 2 genetically identical daughter cells *Animal Cells and Plant Cells have differing events during cytokinesis Animal Cell Cytokinesis 1. The cell membrane forms a furrow, or trench. 2. The trench is pulled inward by tiny filaments 3. Gradually the membrane pinches closed. Plant Cell Cytokinesis • The membrane cannot pinch inward due to the cell wall. 1. Cell plate forms between the 2 nuclei – Cell plate is made by golgi apparatus – New cell wall grows as cellulose is laid down Cytokinesis • Typically the cytoplasm is divided evenly between the two new daughter cells. • Single-celled organisms use division in order to reproduce. • Multi-cellular organisms use division for growth, development, and repair.