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The Cell Cycle
Chapter 8
All cells come from pre-existing cells
Sperm that did not make it in
• One characteristic that
distinguishes living from
non-living is the ability to
reproduce
• Cellular reproduction
allows the continuity of
life, growth and repair
• Cellular reproduction can
be asexual or vegetative,
or sexual
8-cell human embryo
Asexual / Vegetative Reproduction
Plant cuttings will sprout roots, as will a potato, which is a
tuber – a modified, underground stem.
Asexual reproduction - budding
Mold
Yeast
Hydra
Genetically
identical
clones
All somatic cells reproduce mitotically
• Somatic cells are all the cells of the body,
except the gametes (egg and sperm)
• Skin cells, liver cells, cells that line the G.I.
tract, etc. are constantly dividing, to
replace dead cells
• Other cells such as neurons, adipose
cells, muscle cells, etc. never or rarely
divide
The Cell Cycle
• A typical human cell undergoes a division about every
24 hours (there are many exceptions!)
• The cell cycle is basically an alternation of 2 major
phases – Mitosis and Interphase
• Interphase is the phase in which the cell spends 23 of
the 24 hours – the cell grows, carries out its
“housekeeping duties” and its specialized activities
• Mitosis takes about 1 hour
The Cell Cycle
Phases of Interphase
• G1 phase: The period prior to the synthesis of DNA.
In this phase, the cell prepares for cell division
- proteins are synthesized
- the cell increases in mass
• S phase: The period after G1, where all genetic
material (DNA) is synthesized
• G2 phase: The period after DNA synthesis has
occurred but prior to the start of mitosis.
- cell continues to increase in size
- centrosome divides into 2
- In animal cells, each centrosome has 2 centrioles
How DNA Packs into a Metaphase Chromosome
Chromosomes
A human Karyotype
All somatic cells are Diploid (2n)
• The cells have 2 versions of every DNA strand or chromosome
• The 2 versions are called homologous chromosomes
•One homologue comes
from the sperm and the
other from the egg
•Human somatic cells
have 46 chromosomes –
or 23 homologous
chromosome pairs
DNA (chromosome) replication
• The cell has to replicate
(duplicate) all its DNA
• The duplicated DNA is then
organized into distinct bundles
called chromosomes – the
duplicates are connected to each
other at the centromere
• Each duplicated DNA strand is
called a sister chromatid
• Human cells still have 46
chromosomes, and 23
homologous pairs, but 92 sister
chromatids
DNA Replication
(p-arm)
(q-arm)
Duplicated
Chromosome
The duplicated chromosome
Kinetochore
• It is a protein complex found in the
centromere region of a chromosome
• Each sister chromatid must have a
kinetochore
• Spindle fibers attach at the kinetochore
and help pull the sister chromatids apart
The mitotic spindle at Metaphase
(Animal Cells only)
(Animal Cells only)
Centriole duplication
Centrioles have microtubules arranged in 9 triplets
Phases of Mitosis
The cell is in Interphase (~23 hours) before
it enters mitosis (1 hour). These are the
phases of mitosis:
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase (followed immediately by
cytokinesis)
Phases of Mitosis
G2 of Interphase
Prophase
Anaphase
Prometaphase
Telophase & beginning
of cytokinesis
Metaphase
Completion of cytokinesis
Prophase
1. Chromatin condenses into
chromosomes
2. Nucleolus begins to disappear
3. Centrioles duplicate and start to
move apart
4. Centrioles start making spindle
fibers
Prophase, continued
1. Nuclear membrane begins to
disappear
2. This allows spindle fibers to
attach the chromosomes
3. Centrioles are at opposite poles
of the cell
Metaphase
1. The chromosomes are
aligned at the equator
of the cell – called the
metaphase plate
Anaphase
1. Sister chromatids pull apart
and are now considered
daughter chromosomes
2. At the end of anaphase,
each group of
chromosomes is clustered
at opposite poles.
Telophase
1. In animal cells, the
cleavage furrow begins
to form
2. Nuclear membrane
begins to re-form
3. The spindle fibers begin
to disassemble
4. Chromosomes begin to
return to chromatin state
5. Nucleolus begins to
reappear
6. Cytokinesis takes place
Interphase
• Nucleus contains
chromatin
• Only one set of centrioles
(one centrosome)
• Fully formed nuclear
membrane
• Fully formed nucleolus
Centrioles will replicate
once the cell is ready to
divide again
PROPHASE
PROMETAPHASE
METAPHASE
ANAPHASE
TELOPHASE
Cytokinesis in Animal Cells
Cleavage Furrow
Contractile ring made of actin microfilaments, “pinches” the cell into two.
Cytokinesis in Plant Cells
Plant cells cannot be “pinched” into
two new daughter cells, because
of the cell wall.
The Golgi body secretes cell wall
material packaged in transport
vesicles that line up on the
equator. These vesicles fuse to
create a cell plate.
The cell plate divides the cell in
two. The cell plate becomes the
cell wall.
Gametes
• Eggs and sperm are called gametes
• All other cells of the body are called
somatic cells
• Eggs develop in the ovaries and sperm
develop in the testes.
• All eggs and sperm develop from cells
called PGCs or Primordial germ cells
• PGCs are diploid – 46 chromosomes
Diploid vs. Haploid
• A cell that has both members of a
homologous pair of chromosomes in the
nucleus, is considered to be Diploid (2n)
• A cell that has only one homologue of a
chromosome pair (so either the maternal
or paternal homologue is missing) is
considered Haploid (n)
Importance of Meiosis
• Reduces the number of chromosomes by
half – the cell goes from diploid (2n) to
haploid (n) – humans go from 23 pairs to
23 single chromosomes
• Prevents polyploidy (multiple
chromosomes) – and maintains the
chromosome number of each species
• Produces genetic variation and contributes
to evolution
Primordial Germ Cell (PGC)
• Found in ovaries and testes
• These cells are diploid (2n) so they have
23 pairs of chromosomes or 46 total.
The Human
Life Cycle
Prophase I of Meiosis
• Homologous chromosomes pair up with
each other
• The sister chromatids of each pair
CROSSOVER with each other and swap
pieces of DNA
• This creates variation in the chromosomes
of the gametes that will form
THE END