Chromosomes, Chromatids, Mitosis!

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Chromosomes, Chromatids, Mitosis!
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Genomes, Chromosomes, DNA, Genes

• Eukaryotic genomes are made up of multiple
  chromosomes.
• Each chromosome contains one double-stranded DNA
  molecule.
• Each DNA molecule contains many genes.
• Expression of each gene is controlled by its DNA
  sequence and chromosomal configuration.

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Chromosome Number

• Haploid (N) number of chromosomes in a gamete
  (egg or sperm) cell
• Diploid (2N) number of chromosomes in a zygote
  (produce of fertilization) and cells derived from
  the zygote
• Value for N varies by species.
• Drosophila 4
• maize 10
• humans 23

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Life Cycle of Sexual Organism
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Two Types of Cell Division

• Mitosis
• cells divide to produce genetically identical
  cells (exact copies)
• chromosome number in daughter cells is same as in
  original cell
• Meiosis
• cells divide to produce genetically variable
  cells
• chromosome number in daughter cells is half the
  number in original cell

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Two Types of Cell Division

• Mitosis
• cells divide to produce genetically identical
  cells (exact copies)
• chromosome number in daughter cells is same as in
  original cell
• Meiosis
• cells divide to produce genetically variable
  cells
• chromosome number in daughter cells is half the
  number in original cell

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Mitotic Cell Cycle
Phases M mitosis / cytokinesis G1
growth S DNA replication G2 growth
Pierce, B. 2005. Genetics, a conceptual
aplproach. 2nd Ed. WH Freeman.
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Chromosome Anatomy and S phase
1 DNA molecule per chromosome
2 DNA molecules 1 per chromatid
A chromatid is a chromosome held to another
chromosome at the centromere.
Before S phase DNA replication
After S phase DNA replication
Pierce, B. 2005. Genetics, a conceptual
aplproach. 2nd Ed. WH Freeman.
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Process of Mitosis

• Interphase (before mitosis) DNA replicates to
  form duplicated chromosomes (sister chromatids)
• Prophase chromosomes condense to form X-shaped
  structures nuclear membrane disintegrates
• Metaphase chromosomes align in the middle of
  the cell spindle fibers attach to kinetochores
• Anaphase spindle fibers contract and pull
  chromatids apart at the centromeres chromatids
  (now called chromosomes) move to opposite poles
  of the cell
• Telophase nuclear membranes re-form around the
  two chromosomal clusters new cell membrane forms
  in preparation for cytokinesis

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Mitosis Animations

• http//bcs.whfreeman.com/pierce2e
• http//www.biology.arizona.edu/cell_bio/tutorials/
  cell_cycle/cells3.html

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Result of Mitosis

• Mitosis produces two cells that are genetically
  identical to the original cell.
• The daughter cells have the same number of
  chromosomes as the original cell.
• Diploid (2N) cell divides to produce two diploid
  (2N) cells.

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Mitosis Dance Time!

• Objective
• Really learn mitosis.
• Materials
• Pink and blue ribbons (chromosomes /chromatids)
  with velcro centers (centromeres).
• Numbered necklaces (chromosome labels).
• Silver ribbons (spindle fibers).
• Procedure
• Enact mitosis, starting with a diploid cell with
  4 chromosomes.

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Regulation of Cell Division

• Cell division is tightly regulated.
• Checkpoints
• G1/S. Is the cell big enough? Is the DNA
  replication machinery ready?
• G2/M. Is all DNA duplicated? Is the environment
  favorable?

Pierce, B. 2005. Genetics, a conceptual
aplproach. 2nd Ed. WH Freeman.
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Regulation of Cell Division

• Positive regulation
• Activates cell division when conditions are
  favorable
• Negative regulation
• Inhibits cell division when conditions are
  unfavorable
• Proper cell division is a careful balance of
  positive and negative regulation

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What happens when cell division is not properly
regulated?

• Uncontrolled (too much) cell division
• Formation of tumors, masses of cells that divide
  when they should not
• Tumors can become malignant (cancer)

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Cancer is caused by mutations in genes that
regulate cell division

• Oncogenes -- positive regulators
• normal role is to activate cell division
• Growth factors promote growth
• Growth factor receptors bind growth factors,
  initiate signal transduction pathways
• Protein kinases alter protein activity
• Transcription factors turn on genes involved in
  cell division
• mutants activate too much cell division

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Cancer is caused by mutations in genes that
regulate cell division

• Tumor suppressor genes -- negative regulators
• normal role is to inhibit cell division
• Transcription factors turn on genes that
  inhibit activity of positive regulators of cell
  division
• mutants do not function, so cells divide too much

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Tumors are Clonal in Origin

• Tumor cells arise through successive mitotic
  divisions of one original mutant cell.
• Additional mutations may occur, eventually
  leading to malignancy.

Pierce, B. 2005. Genetics, a conceptual
aplproach. 2nd Ed. WH Freeman.
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Summary

• The cell cycle is divided into interphase and
  mitosis.
• Mitosis gives rise to two daughter cells that are
  genetically identical (clones) to the original
  cell.
• Mitosis and cell division are carefully
  regulated.
• Mis-regulation can lead to too much cell
  division, producing tumors and / or cancer.