Big Idea 3
Ch. 12 – The Cell Cycle
Essential Knowledge 3.A.2 (a & b)
Review – Chromosome structure
Before we begin the cell
cycle, let’s review
chromosomes
 A chromosome is a tightly
coiled piece of DNA
 DNA is a double helix of
chromatin wrapped
around histone proteins
 Parts:
 One side is a chromatid
 The other side is an
identical sister chromatid
 Centromere (no genes
located here)
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Chromosomes and numbers
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Each of your somatic cells (body
cells) have 46 chromosomes
Each parent gives 23 chromosomes,
making 23 pairs (46 individual
strands)
The chromosomes that make up a
pair (one from each parent) are
called homologous chromosomes
They are the same length and have
the same centromere position
(holding them together) and they
carry genes that control the same
inherited traits
Ex – the gene for earlobe type will be
located at the same position on both
homologous chromosomes); they
would each code for earlobe type,
they may not code for the same
TYPE of earlobe
Size and complexity of an organism
has nothing to do with how many
chromosomes it has
Haploid and diploid
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Gametes are sex cells (like an egg or
sperm) that have half the number of
chromosomes as our regular body cells–
in humans each gamete has 23
chromosomes (instead of 46).
The symbol n can be used to represent
the number of chromosomes in a gamete
A cell with n number of chromosomes is
called a haploid cell (haploos means single
in greek) – they have one set of
chromosomes
When one haploid gamete combines with
another it’s called fertilization – when that
happens, the cell will now contain a total
of 2n chromosomes (n from the female
and n from the male parent)
A cell that has a 2n number of
chromosomes is called a diploid cell – they
have 2 sets of chromosomes
Overview: The Cell Theory
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There are 3 parts to the cell theory:
– 1. A cell is the basic unit of life
– 2. All living things are made of one or more
cells
– 3. Cells make more cells
Overview
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Mitosis is generally used
to mean exact copying of
cells (technically it’s the
division of the nucleus)
Why is it important?
– It’s what helped change
you from a zygote to an
embryo to a fetus and
finally to you (growth*).
– It helps some organisms
reproduce (asexually)
– Used when you form scabs
(repair)
*you start life as a single cell
and end up with millions of
cells with the exact same
DNA inside
The Cell Cycle
This how we go from one
cell to make 2 cells
 It’s basically like a
photocopy machine
 Cells duplicate their genetic
material before they divide,
ensuring that each daughter
cell receives an exact copy
of the genetic
material(DNA)
 Three parts:
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– Interphase
– Mitosis
– Cytokinesis
The Cell Cycle cont. Interphase
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Interphase is the first step
(happens before mitosis starts)
– Consists of 3 phases
– This is where the cell spends most
of its time
Begins with the G1 phase
(Stands for Gap, but think of it
as Grow, because that’s what
happens) – Cell gets bigger
 S phase (stands for synthesis) is
next – this is where DNA is
copied (you double the amount
of DNA)
 G2 is the last phase of
interphase – continues to grow
 Some cells never copy
themselves (like in your central
nervous system – nerves in your
brain) so they stay in a phase
called G0
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LE 12-5
INTERPHASE
G1
S
(DNA synthesis)
G2
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Mitosis is conventionally divided into four basic
phases:
– Prophase
– Metaphase
– Anaphase
– Telophase
– Remember: PMAT (or Please Make Another Taco);
Cytokinesis is well underway by late telophase
Phases of Mitosis
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it’s a smooth transition between each
phase (like a movie)
Prophase – first step of Mitosis
– Chromatin (loose DNA) condenses
and thickens into chromosomes
– Nucleolus disappears
– Centrosomes begin to move to
either side of the cell
– Spindle fibers begin to form
– prometaphase –
 nuclear membrane
disappears,
 chromosomes form two
kinetochores at the
centromere - (kinetochores
are like little pacmans that
are inside the chromosomes
and spindles attach to them)
 Microtubules (which make up
the spindle fibers) attach to
the chromosomes
Metaphase
Meet in the Middle
 All the chromosomes
line up along the
center of the cell
 Guided by the spindle
fibers
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Anaphase
Move apart –
separate to the poles
 All the sister
chromatids separate
simultaneously (we
don’t know how)
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Spindle Apparatus
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The mitotic spindle is an
apparatus of microtubules
(which make up spindle fibers)
that controls chromosome
movement during mitosis
In anaphase, sister chromatids
separate and move along the
kinetochore microtubules
toward opposite ends of the
cell
LE 12-8b
Chromosome
movement
Microtubule
Motor
protein
Chromosome
Kinetochore
Tubulin
subunits
Telophase
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The End
New nuclear envelope
forms
Chromsomes relax back
into chromatin (DNA
strands)
Nucleoli reappear
Each side has an
identical copy of the
DNA
*basically this is a
rewind of what
happened in Prophase
Cytokinesis
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Happens AFTER mitosis is
complete
In animal cells,
cytokinesis occurs by a
process known as
cleavage, forming a
cleavage furrow
Division of the cytoplasm
End with two identical
daughter cells
Cells begin the process
over again (starting with
G1)
Plants
Plants go through the
same process with
just a small difference
 They can’t just divide
in half because of
their cell wall
 They build a cell plate
that forms in the
center of the cell,
which forms a new
cell wall
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The cell cycle is regulated by a
molecular control system
The frequency of cell division varies with
the type of cell
 The cell cycle appears to be driven by
specific chemical signals present in the
cytoplasm
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Control of the Cell Cycle
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For many cells, the G1
checkpoint seems to be the
most important one
If a cell receives a go-ahead
signal at the G1 checkpoint, it
will usually complete the S, G2,
and Mitosis phases and divide
If the cell does not receive the
go-ahead signal, it will exit the
cycle, switching into a
nondividing state called the G0
phase
G1
S
M
G2
The Cell Cycle Clock: Cyclins and
Cyclin-Dependent Kinases
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How does the cell know when to divide?
– Two types of regulatory proteins are
involved in cell cycle control: cyclins and
cyclin-dependent kinases (Cdks)
– The activity of cyclins and Cdks fluctuates
during the cell cycle
There are proteins inside of the cell, and as
they build up throughout the life of the cell you
eventually get a critical number that tells the
cell to advance to the next stage of division
Called cyclins (the number of these fluctuates
depending on where you are in the cell cycle –
so depending on how many you have, it will tell
the cell to move on to the next phase)
CDKs are a cyclin-dependent kinase (a chemical
that can speed up actions in the cell)
A typical cell in your body would have many
CDKs; cyclin builds up over time, and begin to
fit into the CDKs, which activates a “CDK cyclin
complex” (you don’t need to know specific
complex names for the test), which creates
a protein that can do stuff like:
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MPF – mitosis promoting factor- is a
specific type of CDK that break up the
nucleus to begin cell division
– Work on microtubules to help build the
spindle apparatus
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Cancer and other stuff
Cancer is basically cells reproducing out
of control
– Occasionally you get a mutation that
suppresses the cancer suppressing
gene
– They can move to other parts of
your body and activate other cells to
become cancerous
– A group of cells growing out of
control is called a tumor
If abnormal cells remain at the original
site, the lump is called a benign tumor
Malignant tumors invade surrounding
tissues and can metastasize, exporting
cancer cells to other parts of the body,
where they may form secondary tumors
Apoptosis – programmed cell death (the
cells shrink and shrivel in a controlled
process)
– Happens in your hands and feet as
they begin to develop, or when a
leaf falls from a tree
Necrosis – cell death due to injury or
illness of the tissue
Binary Fission
Prokaryotes (bacteria and archaea) reproduce by a type
of cell division called binary fission
 In binary fission, the chromosome replicates (beginning
at the origin of replication), and the two daughter
chromosomes actively move apart
 Since prokaryotes evolved before eukaryotes, mitosis
probably evolved from binary fission
 Certain protists exhibit types of cell division that seem
intermediate between binary fission and mitosis
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