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Cell Division: Mitosis and Meiosis
Cell Division in Eukaryotes
• Cell Cycle
– The cell cycle is the repeating set of events in
the life of a cell.
– The cell cycle consists of cell division and
interphase.
– Cell division in eukaryotes includes nuclear
division, called mitosis, and the division of
cytoplasm, called cytokinesis.
Cell Cycle
Cell Cycle and Mitosis
• Interphase
– Interphase consists of growth (G1), DNA replication,
and preparation for cell division (G2).
• Mitosis is divided into stages: prophase,
metaphase, anaphase, and telophase.
• Mitosis results in two offspring cells that are
genetically identical to the original cell.
Phases of Mitosis
Phases of Mitosis
• Prophase
– DNA coils into chromosomes
– Nuclear envelope dissolves
• Metaphase
– Chromosomes line up along center of cell
• Anaphase
– Centromeres dissolve and chromatids separate
• Telophase
– Nuclear envelopes reform around separated
chromatids
• Cytokinesis
Control of Cell Division
• Cell division in eukaryotes is controlled by many
proteins.
• Control occurs at three main checkpoints.
– G1: Does cell need to/ is it ready to divide?
– G2: Cell mature and DNA copied successfully?
– Metaphase: Chromatids paired and lined up?
• Cancer may result if cells do not respond to
control mechanisms
Cell Division and Cancer
• Mutations of proto-oncogenes, which regulate
cell growth, or tumor-suppressor genes, which
prevent cell division from occurring too often,may
lead to cancer.
– Proto-oncogenes are like the accelerator
– Tumor-suppressor genes are like the brake pedal
• Cancer is the uncontrolled growth of abnormal
cells.
Cell Division and Cancer
Cell Division and Cancer
• Gene Expression in Cancer
– Unlike normal cells, cancer cells continue to divide
indefinitely, even if they become densely packed.
– Cancer cells will also continue dividing even if they
are no longer attached to other cells.
• Causes of Cancer
– A carcinogen is any substance that can induce or
promote cancer.
– Most carcinogens are mutagens, substances that
cause mutations.
Meiosis
• Meiosis is a process of nuclear division that
reduces the number of chromosomes in new cells
to half the number in the original cell.
• Meiosis leads to four haploid cells (gametes)
rather than two diploid cells as in mitosis.
Stages of Meiosis
Stages of Meiosis
Development of Gametes
• Spermatogenesis is the process by which sperm
cells are produced.
• Oogenesis is the process that produces mature
egg cells.
Meiosis in Male and Female Animals
Sexual Reproduction
• Sexual reproduction is the formation of offspring
through meiosis and the union of a sperm and an
egg.
• Offspring produced by sexual reproduction are
genetically different from the parents.
When Meiosis Goes Awry
• Nondisjunction: the failure of homologous chromosomes to
separate during Meiosis I, or the failure of sister chromatids
to separate during Meiosis II
• Results in gametes with an abnormal number of
chromosomes
– Aneuploidy—cell having an abnormal number of
chromosomes
– Trisomy—3 copies of a chromosome
– Monosomy—1 copy of a chromosome
• http://www.biostudio.com/d_%20Meiotic%20Nondisjunction
%20Meiosis%20I.htm
Disorders Caused by Nondisjunction
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Trisomy 21—Down Syndrome
Trisomy 13 Syndrome (Patau Syndrome)
XXY—Klinefelter’s Syndrome
XYY—Jacobs Syndrome
XXX—Triple X Syndrome
X0—Turner’s Syndrome
XXXXX syndrome
Karyotypes
• A micrograph taken of an organism’s
chromosomes during metaphase; arranged
in homologous pairs for analysis
Trisomy 21 Karyotype