Transcript Genes and Chromosomes

Mitosis: nuclear division
• Overview:
• DNA goes from grainylooking chromatin to
highly condensed
chromosomes.
• Chromosomes line up
along cell equator.
• Duplicate chromosomes are
pulled apart, divided up.
• DNA de-condenses.
http://www.micro.utexas.edu/courses/levin/bio304/genetics/mitosis.gif
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Centrioles and spindle
•Spindle: web of microtubules that anchor at centrioles at
poles of cell, attach to chromosomes and direct their
movement.
Nucleus
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Mitosis Begins
Prophase:
spindle forms,
nucleolus disappears,
nuclear membrane
dissolves,
chromosomes begin
condensing.
Cell during
Interphase
Cell During
Prophase
tidepool.st.usm.edu/ crswr/mitosis.html ; members.tripod.com/ mrlewisclassroom/biology.htm
Sister chromatids
• During S phase of the cell cycle, each
chromosome is duplicated; these are not
homologous chromosomes, but are
exact duplicates = sister chromatids.
• Sister chromatids are attached at the
centromere. Within the centromere is
the kinetechore where the spindle is
attached and pulls the chromosomes.
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Spindle aligns chromosomes along equator
If chromatids are not pulled on equally by the spindle so
that they are all lined up, mitosis does not continue.
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Mitosis-more
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Metaphase:
chromosomes completely condense,
line up along equator;
sister chromatids visible,
attached to spindle at centromeres.
“Prometaphase”: describes steps up
to where chromosomes are lined up
along cell equator.
http://www.geosciences.unl.edu/~dbennett/images/mitosis_metaphase.jp
g; www.fed.cuhk.edu.hk/.../ plant_metaphase.htm
Mitosis -more
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Anaphase:
sister chromatids lined up along
equator separate from each
other, moving to opposite poles
of the cell.
Telophase:
spindle disappears,
nuclear membrane reforms, and
chromosomes de-condense.
http://iws.ccccd.edu/jbeck/CellDivision%20web/Anaphase.JPG
http://z.about.com/d/biology/1/0/L/2/nemtodatelo.gif
Cytokinesis
Near the end of mitosis, the
cytoplasm –containing the
rest of the organelles- is
divided up into the two new
daughter cells: cytokinesis.
http://www.ias.unt.edu/~tpp001/telophase_cytokinesis_text.JPG
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Cytokinesis
Contractile ring around cell pulls in, creates cleavage furrow,
eventually pinches cell into 2 cells. Cytoplasm (including
organelles) divided between the two cells.
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Plant cells have
cell walls.
A new cell wall is created
between the two new cells.
At the end of
mitosis, vesicles
containing cell
wall material move
to create the
“cell plate”
between the new
cells.
Mitosis vs. Meiosis
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• Prior to mitosis, S phase produces identical copies of
all the DNA which is distributed to the daughter
cells during mitosis; the resulting cells have
identical DNA, with 2 homologous sets of
chromosomes.
• Gametes, which fuse together to create a new
individual, must have only one set of DNA (1+1=2),
so the cell division process that produces gametes
must be different.
• Meiosis is the process that produces haploid
gametes. Features 2 division steps. (1 cell  4 cells)
Meiosis: a special nuclear division process for
producing haploid gametes
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• Diploid organisms must produce haploid gametes
that fuse to produce a new diploid individual.
• Meiosis occurs with germ line cells, a separate set of
cells that produce gametes.
• Meiosis features two cell divisions, so that one germ
line cell produces 4 gametes (in principle).
• Each cell division step can be divided into prophase,
metaphase, anaphase, and telophase.
• Prophase I is subdivided because stuff happens.
Meiosis begins
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Prophase I: Sister chromatids
condense, become visible. Nuclear
membrane dissolves; spindle forms.
Homologous chromosomes pair up!
Stages of Prophase I:
Leptonema
Zygonema
Pachynema
Diplonema
Diakinesis
Stages of Prophase I:
• Leptonema: threads appear, homology
search begins.
• Zygonema: thickening continues, pairing
occurs (synapsis), “bivalents” visible.
• Pachynema: more thickening, “tetrads”
visible, crossing over starts; sister
chromatids are visible.
• Diplonema: chiasmata are visible as pairs of
sister chromatids begin to separate.
• Diakinesis: chromosomes repel each other,
chiasmata move towards ends of
chromatids.
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Synapsis
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• Synapsis is when homologous chromosomes are
paired up and at first stick to each other along their
entire length.
• Chromosomes cross arms, and stick to each other at
a couple of places.
• Pieces can break and actually swap.
• This is called crossing over.
Crossing over: gene shuffling
Tetrad
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Red: sister chromatids from
one parent (identical).
Blue: sister chromatids from
other parent.
Red and Blue are Homologous
http://www.blc.arizona.edu/courses/181summer/graphics/graphics%20le
ct10/Life7e-Fig-09-16-0%20crossing%20over.jpg
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Meiosis I is completed
Metaphase I
Anaphase I
Metaphase I
Paired homologues
line up along cell
equator.
Anaphase I
Homologues
separate!
NOT sister
chromatids.
Reduction division
Telophase I: may be brief or incomplete, because cell will
proceed to Prophase II.
Meiosis II
This is the cellular basis for
Mendel’s Independent
Assortment.
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In Anaphase II,
sister chromatids
separate, leaving
only 1 of each
type of
chromosome in
the cell.
NOTE that
various mixtures
of pink and green
are possible:
Gamete formation in humans- sperm
Through meiosis, one cell divides to produce 4 sperm cells.
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Gamete formation in humans- ovum
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Cytokinesis is
unequal; in each
division, one
cell gets more
cytoplasm than
the other.
The result is one
ovum and 2
polar bodies
which
degenerate and
disappear.
Reproduction in a flowering plant
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