Download Cell Cycle

Human body makes more
than 20 billion new cells
every day. Over the course
of 7 years, all body cells
have been replaced (except
nerve cells).
How does your body make
all these new cells?
Cell Cycle Notes
Cell Division — process by which a
cell divides into 2 new cells
• Why do cells need to divide?
1. Living things grow by producing
more cells, NOT because each
cell increases in size
2. Repair of damaged tissue
3. If cell gets too big, it cannot get
enough nutrients into the cell
and wastes out of the cell to
maintain homeostasis
I. DNA – Deoxyribonucleic acid; located in nucleus
A. Long and thread-like DNA in a non-dividing cell –
chromatin
B. Thick, short, coiled doubled DNA in a dividing cell –
chromosome
1. chromosome
consists of 2 parts:
a. chromatid
b. centromere –
protein tab that
holds 2 sister
chromatids
together
2. chromosomes are divided into segments
called – genes
C. Illustration: chromatin to chromosomes
duplicates
itself
chromatin
coils up into
chromosomes
Why does DNA need to change
from chromatin to chromosome?
More efficient division
II. Cell Cycle – the repeating sequence of growth and
division of a eukaryotic cell
A. The cell spends most of its time in – Interphase
1. cell grows, DNA is loosely wound, and cell
carries out normal functions
2. 3 phases:
a. cell grows and matures – G1 (first growth)
b. chromatin duplicates – S (synthesis)
c. organelles replicate and cell prepares for division –
G2 (second growth)
G1 phase
S phase
G2 phase
B. Division of the nucleus – mitosis
C. Division of the cytoplasm and organelles –
cytokinesis
Occurs quickly
1. In animal cells the
cytoplasm pinches in
2. In plant cells a cell
plate forms
Cell plate
D. After mitosis and cytokinesis, the cell returns to
Interphase
III. Mitosis – division of the nucleus into 2
nuclei, each with the same number and
kind of chromosomes
A. HANDOUT
Prophase
duplicated
chromosomes
spindle fibers
form
nuclear envelope
breaking apart
Metaphase
(middle)
spindle fibers
centriole
Anaphase
(away)
individual
chromosomes
Telophase
(two)
nuclear envelope
reforming
chromosomes
unwind into
chromatin
Cytokinesis
Interphase
centrioles
nuclear
envelope
chromatin
B. 4 phases PMAT
1. chromosomes condense, nuclear envelope
breaks down, spindle fibers form – Prophase
2. chromosomes line up along equator -- Metaphase
3. chromatids separate, centromere divides, and
move to opposite poles – Anaphase
4. chromosomes uncoil, new nuclear envelope
forms spindle fibers break down – Telophase
What Phase Of the Cell Cycle Is It?
A
Anaphase
D
Interphase
B
C
Telophase
Cytokinesis
E
F
Metaphase
Prophase
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C. Mitosis occurs only in body cells, or – somatic cells
D. 2 new cells formed have same number and kind of
chromosomes as original
1. original cell called – parent cell
2. cells formed are known as – daughter cells
2
Daughter
Cells
Parent Cell
E. Many organisms, especially unicellular
organisms, reproduce by means of cell division-called asexual reproduction
ex: bacteria
Summary: Cell Cycle
Interphase
Mitosis (PMAT)
Cytokinesis
** Daughter cells genetically identical to parent
cells
F. Chromosome Number
1. number of chromosomes each organism has in
its cells varies from species to species
example: human = 46
dog = 78
lettuce = 18
gorilla, chimp = 48
2. all cells (except the sex cells) in an organism have the
same number of chromosomes
example: human = 46
human skin cells = 46
human muscle cells = 46
human heart cells = 46
G. Regulating the Cell Cycle – not all cells move through
cell cycle at the same rate
1. in eukaryotic cells,
timing of the cell cycle
is regulated by –
proteins called cyclins
a. cyclins initiate the
various stages of
cell cycle
2. cell cycle checkpoints -- regulate progress of cell cycle
a. ensure that damaged DNA not passed on to daughter cells
b. G1 checkpoint at end of G1 phase -- determines whether
cell is ready to divide or not
c. G2 checkpoint at end of G2 phase – check to be sure DNA
replication is complete
d. M checkpoint at Metaphase –
check for whether chromosomes
properly attached to spindle fibers
3. injuries affect rate of cell division
a. cells at edge of injury stimulated to divide rapidly -producing new cells
b. when healing is completed, rate of cell division -- slows
down and returns to normal
4. when cells come into contact with each other
in culture, they stop dividing
G. Uncontrolled Cell Growth
1. control over cell cycle can break down resulting in –
cancer
2. cancer cells do not respond normally to – signals that
regulate cell division
3. cells divide uncontrollably forming – tumors and can
invade other tissues
4. mutations in DNA leading to
tumors can be caused by:
a. chemicals – smoking or
asbestos (lung cancer)
b. radiation – sun -- UV rays
(skin cancer)
c. viruses – HPV (cervical
cancer) or hepatitis (liver
cancer)
d. immune system disorders –
HIV (many different
cancers)
e. heredity – genetics
3. p53 – “ guardian of the genome”
a. plays role in triggering control mechanisms at
checkpoints -- suppresses tumors to prevent cancer
b. p53 can either – repair faulty DNA
or – kill off cell if it can’t be repaired
(apoptosis)
c. problems with gene p53 -- related to more than
50% of all cancers
Phase
Chromosome
Appearance & Location
Important Events
Interphase
DNA/chromatin copies itself
DNA replication, cell grows
and replicates organelles
Chromosomes coil up
Nuclear envelope
disappears, spindle fibers
form
Chromosomes line up in
the middle
Spindle fibers connect to
chromosomes
Anaphase
Chromosome copies divide
and move apart
Telophase
Chromosomes uncoil back
into chromatin
Cytokinesis
Chromatin
Spindle fibers pull
chromosome copies apart
to opposite poles
Nuclear envelopes reform,
2 new nuclei are formed,
spindle fibers disappear
Division of the rest of the
cell: cytoplasm and
organelles
Prophase
Metaphase