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Chromosomes
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By the end of this class you should understand:
• The structure and function of human chromosomes
• The purpose of the different types of cell division
• The pattern of chromosome movements in mitosis and meiosis
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Chromosomes
Remember that chromosomes are the packages that our DNA is organized into
Normally spread out Only condensed
during cell division
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Chromatin & Centromere Chromosomes are made of a
material called chromatin A mixture of DNA and protective
protein The core in the center is
referred to as the centromere Not always in the center!
Sometimes off to one side Divides the chromosome into two
arms which may be uneven (long arm and short arm)
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Why the X? Remember chromosomes
are only compacted during mitosis Mitosis doubles the
chromosome from a I to an X
A visual image of the chromosome is only possible at this stage
This visual stain is called a karyotype
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Blood Karyotype
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Actual Karyotype Result
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Chromosomal duplication Chromosomes are typically
spread out in a huge mass of active DNA
Only when the cell is about to divide are they packaged tightly Imagine those blueprints in a
busy library, then imagine them packed into books before being shipped away
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Telomere The tips of chromosomes
have long repeated sequences of DNA that are noncoding but protect the rest of the DNA DNA replication typically fails
to completely duplicate these telomeres
They function as the aglets on your shoelaces
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Telomere Shortening Every time a cell undergoes
mitosis it loses a bit of its telomeres
Adult cells have shorter telomeres and telomere shortening may be linked to aging Dolly the sheep was cloned
from an adult cell and so may have showed premature aging
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Telomerase Immortal cells such as stem
cells and cancer cells express an enzyme called telomerase Lengthens telomeres and
extends life of daughter cells Research in stem cells and
telomerase may provide clues into aging process
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Cell Replication One cell can become two identical cells through
a process called mitosis One cell can also become four haploid cells
through a process called meiosis These are the only two ways to make more cells in
your body Picture:cytoskeletal stainof cell in mitosis
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Mitosis & Meiosis
MITOSIS: Produces two cells
identical to original Many stem cells in
your body constantly perform mitosis to grow new tissue
This is how you grow
MEIOSIS: Produces four haploid
cells Only certain stem cells
in the gonads perform meiosis
This is how you make sex cells (gametes)
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Mitosis & Cell Cycle Mitosis is actually
only a part of the cell replication cycle
Cells that are amitotic (such as brain cells) are in G0 phase and never self-replicate
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Interphase
Actually several major stages of cell growth G1: “Okay, guess I'll get ready to divide” S: “Gonna need to copy my DNA” G2: “And I'll need spindle fibers to do the job”
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Phases of the Cell Cycle
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Phase control Each phase of the cell cycle has controls
The cell can only move to the next phase when given “permission”
What is the name for when these controls break?
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Steps of Mitosis
A video is worth a thousand words! http://www.youtube.com/watch?v=VlN7K1-
9QB0 Key detail: the microtubule spindles that pull
the chromosomes apart come from the centromere and are a type of cytoskeleton structure
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Mitosis Diagram part 1
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Mitosis Diagram part 2
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Mitosis Summary!
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Meiosis Meiosis is a specialized type
of cell replication It produces gametes (sex
cells) Sperm and eggs in humans
and animals Pollen and seeds in plants Bacteria cannot undergo
meiosis
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Steps of Meiosis
Meiosis is similar to a double mitosis Chromosomes are duplicated 46 x 2 = 92 chromosomes
At the end there are four cells with only one set of chromosomes 23 chromosomes x 4 gametes
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Double Mitosis??
Yes! They even have the same steps: Prophase I, Metaphase I, Anaphase I, Telophase I Prophase II, Metaphase II, Anaphase II, Telophase
II In Meiosis I, the homologues are separated In Meiosis II, the sister chromatids are pulled
apart as in Mitosis
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Meiosis I
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Meiosis II
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To Summarize:
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See you in lab!