Download Chromosomes and Cell Division

Chromosomes and Cell
Division
Chromosomes
• Visible only during cell division.
• Most condensed form of DNA.
–DNA double helix tightly coiled
around proteins.
Chromatin = loosely coiled form of DNA
An interphase cell has no visible chromosomes.
Although not visible, every chromosome in a nondividing cell is made up of only 1 chromatid.
Only if a cell is preparing to divide, does a
chromosome become made up of 2 chromatids.
Prior to DNA
replication
After DNA
replication
Human Chromosomes
46 Chromosomes
23 pairs
Diploid Cells
• Diploid = 2n
– n = # of types (sizes) of chromosomes
• n = 23 for humans, so the diploid # for humans
is 46 (2 x 23)
– Diploid cells have pairs of chromosomes
• Homologous chromosomes: chromosomes of
the same shape and size, carry genes for the
same traits.
– Each species has a unique diploid number.
Diploid numbers of some commonly studied organisms
Homo sapiens (human)
Mus musculus (house mouse)
Drosophila melanogaster (fruit fly)
Caenorhabditis elegans (microscopic roundworm)
46
40
8
12
Saccharomyces cerevisiae (budding yeast)
Canis familiaris (domestic dog)
Gallus gallus (chicken)
32
78
78
Zea mays (corn or maize)
Muntiacus reevesi (the Chinese muntjac, a deer)
Myrmecia pilosula (an ant)
20
23
2
Cambarus clarkii (a crayfish)
200
Equisetum arvense (field horsetail, a plant)
216
Homologous Chromosomes
e.g.: eye color
From “Dad”
From “Mom”
Fertilization produces diploid cells
• Example: Humans
– The mother’s egg cell has 23 chromosomes
– The father’s sperm cell has 23 chromosomes
– When these cells fuse, the fertilized egg has
46 chromosomes (23 pairs); it is diploid.
• Mitosis produces trillions of body cells that are all
diploid.
– Each of the 23 pairs of chromosomes is called
a “homologous pair”.
Haploid cells
• Gametes (reproductive cells)
– Egg cells (ova) and sperm cells
– Produced in the reproductive organs,
ovaries or testes
• Haploid = 1n
– n = 23 for humans so human haploid cells
contain 23 chromosomes.
– Haploid cells have one of each
chromosome; they do NOT have pairs of
chromosomes.
Diploid vs Haploid Cells
2 of each type of
chromosome is
present in the
cell.
Only 1 of each
type of
chromosome is
present in the
cell.
Note: the chromosomes in both pictures have 1
chromatid each. The # of chromatids does NOT relate to
diploid/haploid.
Cells must be haploid to maintain the
chromosome # of a species during
reproduction
• If reproductive cells were diploid, then
after fertilization a human zygote (fertilized
egg) would have 96 chromosomes.
• To have the 46 chromosomes of a typical
human cell, each reproductive cell should
only have 23 chromosomes.
Meiosis produces Haploid cells
• Involves 2 cell divisions: Meiosis I and
Meiosis II.
– Each cell division consists of PMAT.
– Results in 4 cells.
For cell division, what needs to
occur within a cell?
• 2 copies of every DNA molecule
– DNA replication during the S phase of interphase
• DNA packaged into easily moveable units
– Prophase: DNA “condenses” (becomes more tightly
coiled around proteins)
• Dissolving of the nuclear membrane to allow
for distribution of genetic material (prophase)
• Structures to attach to, and physically move,
the chromosomes
– Spindle fibers (microtubules)
– Made by centrosomes
• An organized way of distributing the
genetic material equally between two
cells.
– Metaphase and Anaphase
• Separation of the material into distinct
cells.
– Telophase: formation of new nuclear
membranes, cytokinesis (division of
cytoplasm)
Meiosis I
• Prior to the beginning of cell division, DNA is copied (replicated).
• Prophase I:
– Similar to mitosis: dissolving of nuclear membrane,
condensing of chromosomes, formation of spindle
fibers. What is unique is:
– Synapsis: pairing of homologous
chromosomes (formation of tetrads).
– Crossing-over: chromatids of homologous
chromosomes exchange genetic material.
• Results in genetic recombination: a new, unique
combination of genes.
Prophase I
• Letter T = tetrads
– Each red box
surrounds a tetrad.
• Letter C =
centrosomes
– Produce spindle
fibers.
• Crossing-over is
occurring in the
lower tetrad.
Crossing-over
Metaphase I
• Lining up of tetrads (homologous pairs)
along the “equator” of the cell.
– The arrangement of the chromosomes is
random.
– This allows for “independent assortment”.
• The random distribution of genes from different
chromosomes to different gametes.
• Genetic recombination: new, unique combination
of genes will be present in the daughter cells.
Metaphase I
• Chromosomes
arranging like in “A” is
just as likely as
chromosomes
arranging like in “B”.
• Each time cells in the
testes/ovaries go
through meiosis,
unique cells are
produced.
How many arrangements are
possible?
• For our simulation?
• For humans?
How many arrangements are
possible?
• For our simulation:
– 4 possibilities
– 2n = 2 3 = 8
• For humans:
– 2n = 223 = 8,388,608
– Wow!
– And this doesn’t even include recombination
resulting from crossing-over!
Anaphase I
• Separation of homologous
chromosomes to opposite sides of the
cell.
– Sister chromatids do NOT separate.
• The direction they are pulled depends on
their alignment during metaphase I.
• Chromosomes are “independently
assorted”.
Anaphase I
Telophase I:
• Chromosomes are
segregated at
opposite poles, one
group on each side of
the cell. Cytokinesis
begins, forming two
daughter cells.
Each chromosome consists
of two chromatids.
Telophase I:
• Are the new
cells diploid
or haploid?
• HAPLOID
– Only one of
each
chromosome
Meiosis II
• After PMAT I the daughter cells are
haploid. So why round two of PMAT?
– The chromosomes are still in a “duplicated”
state, two DNA molecules per chromosome.
– The normal state of genetic material in a cell
is one DNA molecule per chromosome.
Prophase II
Nucleus dissolves and spindle fibers form again
Metaphase II
Chromosomes line up “single-file” along the
midline of the cell.
Anaphase II
Sister chromatids of each chromosome detach
from each other and move to opposite sides of the
cell.
Telophase II
A nuclear membrane forms around each set of
chromosomes and cytokinesis begins, forming 4
daughter cells.
After Telophase II and Cytokinesis
• A new nuclear
membrane
should be
shown.
• Cells are in
Interphase.
• Review: Separation
of chromosomes
during meiosis.