Transcript Chromosomes

Part 1: Chromosomes
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All living things are made up of one or more cells.
These cells all contain DNA (deoxyribonucleic acid) which
is your genetic material.
DNA can be stored in two forms as either chromatin
(relaxed or unraveled) or as chromosomes (coiled)
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Chromatin coils to form a
chromosome.
Chromosomes are rod
shaped and comprised of
both DNA and proteins
called histones.
Histones are proteins that
help the DNA maintain the
coiled shape of a
chromosome.
DNA needs to be relaxed in
order to be read. However,
it is never completely
uncoiled as it would take up
too much space. And it is
never super coiled into a
chromosome unless a cell is
dividing.
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Most of the time, your
cells are not dividing.
Therefore you have
unduplicated
chromosomes.
A dividing cell will have a
duplicated chromosome
which is made up of
identical chromatids
Constricted area is the
centromere
This is a eukaryotic
chromosome. Prokaryotic
organisms only have one
chromosome and it is
round.
What kind of
chromosome is
this?
 Chromosomes
contain genes
which are areas
of DNA that code
for particular
proteins.
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There are two types
Sex Chromosomes: determine the sex
(gender) of an organism
Autosomes: does not determine
gender
Every cell has two copies of each
autosome – 1 from mother and 1 from
father called homologous
chromosomes or homologues
Homologous chromosomes carry the
same type of genetic information but it
may not be identical
• Every cell has 2 sex chromosomes
that can differ depending on what
they receive from their parents
• XX – female
• XY – male
Does the number of chromosomes
have anything to do with the
complexity of an organism?
Photomicrograph
of the
chromosomes in a
dividing cell
 Human karyotypes
show 22
homologous pairs
of autosomes and
2 sex
chromosomes
 What do you notice
about the
arrangement?
 Is this a male or
female?
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Diploid (di=2) cells have two sets of
chromosomes
• All normal human cells (except
reproductive) are diploid and called
somatic (body) cell
• Represented by 2n
Haploid (one) cells only have one set of
chromosomes
• All reproductive cells (sperm and egg);
called gametes(sex cells)
• Represented by 1n
A haploid cell has the possibility of
joining together with another haploid
cell to form a diploid cell. (fertilization)
Part 2: Cell Cycle
 Binary
fission:
division of a
prokaryotic cell in
two offspring cells
 Three general stages
• Copy DNA
• Grow
• Divide
 Two types in eukaryotes
 Mitosis
• Results in two cells with genetic material identical to the
original cell – used for: repair, growth, and asexual
reproduction
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 Meiosis
• Reduces the chromosome number by half and the genetic
material is not the same
• Produces gametes which are sex cells
• Part of sexual reproduction
 The gametes are haploid cells which retain the ability to
join together and form diploid cells.
 Two
major phases of the cell
are interphase and cell
division
 Interphase is the time
between divisions
 Cell division is the division of
the nucleus (mitosis) and the
cytoplasm (cytokinesis)
 Divided
into three sub-
phases
 G1: where the cell matures
 S: DNA is replicated
 G2: Cell prepares for
division
*Cells do not divide forever*
 G0: cells have exited the
cell cycle and do not copy
DNA or divide any more
 Division
of the nucleus; called the M
phase
 It is divided into four different phases
• Prophase
• Metaphase
• Anaphase
• Telophase
 Shortening
and tightening of
DNA into chromosomes
 The nuclear membrane and
nucleolus disappear
 Centrosomes appear and
migrate toward the poles of
the cell. In animal cells, the
centrosomes contain
centrioles.
 Spindle fibers begin to radiate
form the centrosomes
• Kinetochore fibers
• Polar fibers
 Chromosomes
are easier to
identify - can easily see the
X shape.
 Kinetochore fibers move
the chromosomes to the
center.
 Once there the
chromosomes are held in
place.
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 The
chromatids
separate and move
centromere first to
opposite poles
 Once the chromatids
separate they are
considered to be
individual
chromosomes
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 Once
the chromosomes
reach the poles of the
cell the spindle fibers
disassemble
 The chromosome
returns to its uncoiled
state
 Nuclear membrane and
a nucleolus forms in
each cell
 During
telophase the
cytoplasm begins to divide
 Animals
• Cleavage Furrow:
pinching inward of the
membrane midway of
the poles
 Plants
• Cell Plate: Vesicles
formed by the Golgi
apparatus fuse midway
of the dividing cell
 The
end result of
mitosis is two
identical
daughter cells.
 The cells will go
back to
interphase to
grow and develop
into full cells.
 Cell
growth (G1 )
checkpoint: the cell
determines if it is
ready to start the
process of division
 DNA synthesis (G2)
checkpoint: check
DNA synthesis
 Mitosis checkpoint:
check if mitosis
proceeded correctly
 When control is lost
it can lead to cancer
Chapter 8
Part 3: Meiosis and Reproduction
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Meiosis
• Reduces the
chromosome
number by half and
the genetic material
is not the same
• Produces gametes
which are sex cells
• Part of sexual
reproduction
 The gametes are
haploid cells
which retain the
ability to join
together and
form diploid
cells.
 Very
similar to
mitosis but
produces haploid
cells instead
through 2 divisions:
Meiosis I and
Meiosis II
 None of the cells
have the same
genetic information
 All
of the aspects of
prophase that occur in
mitosis occur here as well
 Synapsis: pairing of
homologous chromosomes
 Tetrad: the pair formed in
synapsis
 Crossing-over: portions of
the chromosomes break off
and exchange with
homologue
 Genetic Recombination:
new genetic material is
made
 Metaphase
I
• The tetrads line up in the middle
of the cell
 Anaphase
I
• Here the homologous
chromosomes separate and go
toward opposite poles
• Since they randomly lined up in
metaphase, they now randomly
separate. This process is called
independent assortment.
• Independent Assortment: the
random separation of
homologues
 Telophase
I and
Cytokinesis
• Here the cell will split into
two new cells
• The end result is two cells
that are haploid cells.
• Each cell is now considered
haploid cells, even though
they have two copies of that
chromosome. This is due to
the fact that it no longer has
a homologue.
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Occurs on the two cells formed during meiosis I
Same procedure as mitosis
End result of meiosis II is four new cells containing half the number of
chromosomes of the original cell
**Some cells will start meiosis after the nuclear membrane completely
reforms; others start at the end of meiosis I
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Sometimes errors can
occur in meiosis
Whether the homologous
chromosomes fail to
separate in anaphase I or
the sister chromatids fail
to separate in anaphase II,
nondisjunction has
occurred.
This will lead to one or
more sex cells that have
too many chromosomes
and one or more sex cells
that are missing a
chromosome.
The result is a genetic
disorder.
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 Meiosis
occurs only in the
reproductive organs in animals
 Spermatogenesis
• Each haploid cell called a
spermatid is form; spermatids
develop into sperm cells
 Oogenesis
• In cytokinesis I and II the
cytoplasm does not split evenly
• One egg (ovum) developed and
three polar bodies
• The polar bodies will eventually
degenerate
 Requires
1 parent - produces
identical offspring
 Types
• Mitosis
• Budding
• Parthenogensis
• Fragmentation
 Benefits
• Quick
• Produce many offspring at one time
• No energy wasted on finding a mate
 Cons
• No genetic variation – could lead to
the extinction of a species.
 Requires
2 parents – leads to
genetically different offspring.
 Benefits
• Offspring are genetically different
than each other and their
parents*** - This allows a species to
adapt to rapidly changing
conditions.
 Cons
• Must find a mate
• Expend energy in producing
gametes that may not be fertilized
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*** With the exception of identical twins!***