Download Cell Cycle


Cell Cycle
◦ Interphase – normal cell activity
◦ Mitotic phase – cell division
INTERPHASE
INTERPHASE
Growth
G1
G1
S synthesis)
(DNA
(DNA synthesis)
Growth
G2G
2

3 stages
G1 - cell grows and functions normally
- high amount of protein synthesis
occurs
- more organelles are produced
2. S - DNA replication (doubles DNA)
3. G2 - cell growth continues
1.
- prepare for division
Chromosome – DNA packaged around protein
 Diploid – 2 full sets of chromosomes (2n)
 Haploid – 1 set of chromosomes (n)
 Most human cells contain 46 chromosomes
(23 sets)
 Humans, n = 23


2 sister chromatids joined at centromere
Chromosomes
Sister chromatids
 Homologous chromosomes:
• Look the same
• Control the same traits
• Independent origin - each one was
inherited from a different parent
Chromosome Duplication
Non-sister
chromatids
Centromere
Duplication
Sister
chromatids
Two unduplicated
chromosomes
Sister
chromatids
Two duplicated chromosomes
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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A diploid cell has two sets of each of its chromosomes
A human has 46 chromosomes (2n = 46)
In a cell in which DNA synthesis has occurred all the
chromosomes are duplicated and thus each consists of two
identical sister chromatids
Maternal set of
chromosomes (n = 3)
2n = 6
Paternal set of
chromosomes (n = 3)
Two sister chromatids
of one replicated
chromosome
Centromere
Two nonsister
chromatids in
a homologous pair
Pair of homologous
chromosomes
(one from each set)


Chromosomes 1-22 are autosomes
Chromosome 23 is a sex chromosome
INTERPHASE
INTERPHASE
G1
Growth
G1
S
S
(DNA synthesis)
(DNA synthesis)
G2
Growth
G2
100 µm
1. Reproduction.
An amoeba, a single-celled
eukaryote, is dividing into two
cells. Each new cell will be an
individual organism (LM).
200 µm
2. Growth and
development.
This shows a sand dollar
embryo shortly after the
fertilized egg divided, forming
two cells (LM).
20 µm
3. Tissue renewal.
These dividing bone
marrow cells (arrow) will
give rise to new blood
cells (LM).



Second part of the cell cycle
Consists of :
◦ Mitosis – Internal separating cell parts and
DNA
◦ Cytokinesis – cell splits
Results in genetically identical daughter cells
INTERPHASE
G1
S
(DNA synthesis)
G2
1. Prophase (package & poles)
◦ DNA condenses into chromosomes
◦ Centrioles move to opposite ends of cell
◦ Nuclear envelope dissolves
2. Metaphase (middle)
◦ Spindle fibers form and attach to
kinetochore connecting centrioles to the
chromosomes
◦ Centrioles start to pull and chromosomes
line up along the cells equator
3. Anaphase (apart)
◦
◦
Centromeres split
Sister chromatids separate and are pulled to
opposite ends of cell
4. Telophase
◦ Cleavage furrow forms
◦ Reverse prophase: nucleus reforms and
chromosomes unravel

Contractile ring
finally divides cell into
two identical
daughter cells with
the same genetic
information
100 µm
Cleavage furrow
Contractile ring of
microfilaments
Vesicles
forming
cell plate
Wall of
patent cell
1 µm
Cell plate
New cell wall
Daughter cells
Daughter cells
(a) Cleavage of an animal cell (SEM)
(b) Cell plate formation in a plant cell (SEM)
G2 OF INTERPHASE
Centrosomes
(with centriole pairs)
Nucleolus
Chromatin
(duplicated)
Nuclear
Plasma
envelope membrane
PROPHASE
Early mitotic
spindle
Aster
Centromere
Chromosome, consisting
of two sister chromatids
PROMETAPHASE
Fragments
of nuclear
envelope
Kinetochore
Nonkinetochore
microtubules
Kinetochore
microtubule
METAPHASE
ANAPHASE
Metaphase
plate
Spindle
Centrosome at Daughter
one spindle pole chromosomes
TELOPHASE AND CYTOKINESIS
Cleavage
furrow
Nuclear
envelope
forming
Nucleolus
forming

http://www.youtube.com/watch?v=JcZQkmoo
yPk
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
Interphase
◦ G1 - primary growth
◦ S - genome replicated
◦ G2 - secondary growth
Mitotic phase
◦ M – mitosis




Prophase
Metaphase
Anaphase
Telophase
◦ C - cytokinesis
 Meiosis
and Other Factors

Factors that affect cell division
◦ Anchorage
◦ cell density
◦ chemical growth factors

Most animal cells divide only when stimulated,
and some not at all
◦ In laboratory cultures
 Most normal
cells divide only
when attached
to a surface
 They continue
dividing until they
touch
Figure 8.8A
Cells anchor to
dish surface
and divide.
When cells have
formed a complete
single layer, they
stop dividing (densitydependent inhibition).
If some cells are
scraped away, the
remaining cells divide
to fill the dish with a
single layer and then
stop (density-dependent
inhibition).

Growth factors signal the cell cycle control system
◦ A set of proteins within the cell controls the cell cycle

Signals affect critical checkpoints in the cell cycle
◦ Determine whether a cell will go through the complete
cycle and divide
G1 checkpoint
G0
Control
system
G1
M
G2
M checkpoint
G2 checkpoint
S
◦ Growth factors attach to receptors to trigger cell
division
Growth factor
Plasma membrane
Receptor
protein
Signal
transduction
pathway
Relay
proteins
G1 checkpoint
Control
system
G1
M
Figure 8.9B
G2
S

Cancer cells – out of control cell growth
◦ Skip checkpoints
◦ Divide excessively to form masses called tumors
◦ Crowd out normal cells and invade tissue
Lymph
vessels
Tumor
Blood
vessel
Glandular
tissue
A tumor grows from a
single cancer cell.
Cancer cells invade
neighboring tissue.
Cancer cells spread through
lymph and blood vessels to
other parts of the body.

Mitosis – division of somatic (body) cells
◦ Produces diploid cells
◦ 2 Daughter cells identical to parent

Meiosis – division which produces gametes
(sex cells)
◦ Produces haploid cells
◦ 4 Daughter cells have half of parent DNA
Chromosomes
Homologous chromosome: one of a matching
pair of chromosomes, one inherited from each
parent.
Sister chromatids are identical

Chromosomes are matched in homologous
pairs
◦ For example human cells have 46 chromosomes
 23 homologous pairs

Gametes have a single set of chromosomes
◦ Gametes, eggs and sperm, are haploid
 With a single set of chromosomes
◦ Sexual reproduction
 Involve the
alternation of
haploid and
diploid stages
Haploid gametes (n = 23)
n
Egg cell
n
Sperm cell
Meiosis
Fertilization
Multicellular
diploid adults
(2n = 46)
Diploid
zygote
(2n = 46)
Mitosis and
development
Figure 8.13
2n

Meiosis reduces the chromosome
number from diploid to haploid
◦ Meiosis, like mitosis
 Is preceded by chromosome duplication
◦ But in meiosis
 The cell divides twice to form four
daughter cells
◦ The first division, meiosis I
◦ Prophase I
 Starts with synapsis- the pairing of
homologous chromosomes
 In crossing-over - Homologous
chromosomes exchange segments
◦ Meiosis I separates each homologous pair
 And produce two daughter cells, each with
one set of chromosomes
MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with centriole
pairs)
Nuclear
envelope
PROPHASE I
METAPHASE I
Sites of crossing
over
Spindle
Chromatin
Figure 8.14 (Part 1)
Sister
chromatids
Tetrad
Microtubules
Metaphase
attached to
plate
kinetochore
Centromere
(with kinetochore)
ANAPHASE I
Sister chromatids
remain attached
Homologous
chromosomes separate
◦ Meiosis II is essentially the same as mitosis
 The sister chromatids of each chromosome
separate
 The result is a total of four haploid cells
 https://www.youtube.com/watch?v=rqPMp0
U0HOA
MEIOSIS II: Sister chromatids separate
TELOPHASE I
AND CYTOKINESIS
II
PROPHASE II
METAPHASE II
ANAPHASE
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister chromatids
separate
Figure 8.14 (Part 2)
Haploid daughter cells
forming

8.15 Review: A comparison of mitosis and meiosis
Mitosis
Meiosis
Parent cell
(before chromosome replication)
Meiosis i
Prophase I
Prophase
Duplicated
chromosome
(two sister chromatids)
Figure 8.15
2n = 4
Metaphase
Chromosomes
align at the
metaphase plate
Tetrads
align at the
metaphase plate
Anaphase
Telophase
Sister chromatids
separate during
anaphase
Homologous
chromosomes
separate during
anaphase I;
sister
chromatids
remain together
2n
Daughter cells
of mitosis
Tetrad formed
by synapsis of
homologous
chromosomes
Chromosome
replication
Chromosome
replication
2n
No further
chromosomal
replication; sister
chromatids
separate
during
anaphase II
Metaphase I
Anaphase I
Telophase I
Haploid
n=2
Daughter
cells of
meiosis I
Meiosis ii
n
n
n
n
Daughter cells of meiosis II

Genetic variability
1. Crossing-over
2. Independent assortment of chromosomes
3. Random fertilization of eggs by sperm

All of these factors ensure genetic differences
in a population
◦ Independent assortment of chromosome pairs at
metaphase I of meiosis
 Lead to many different combinations of chromosomes in
eggs and sperm
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Figure 8.16
Combination 1Combination 2
Combination 3Combination 4

Homologous chromosomes carry different
versions of genes
◦ Gene – segment of DNA coding for a physical trait
Coat-color
genes
Eye-color
genes
Brown
Black
C
E
Meiosis
c
White
e
Pink
Tetrad in parent cell
(homologous pair of
duplicated chromosomes)
Figure 8.17A
Brown coat (C); black eyes (E)
C
E
C
E
c
e
c
e
Chromosomes of
the four gametes
Figure 8.17B
White coat (C); pink eyes (e)
◦ How crossing
over leads to
genetic variation
Coat-color
genes
Eye-color
genes
E
C
e
c
1
Breakage of homologous
chromatids
C
E
c
e
2
Tetrad
(homologous pair
of chromosomes in
synapsis)
Joining of homologous
chromatids
C
E
c
e
Chiasma
3
C
E
C
e
c
E
e
c
4
Figure 8.18B
Separation of homologous
chromosomes at
anaphase I
Separation of chromatids
at anaphase II and
completion of meiosis
C
E
C
e
c
E
c
e
Parental type of
chromosome
Recombinant chromosome
Recombinant chromosome
Parental type of
Gametes ofchromosome
four genetic
types
1.
Deletion – part of the chromosome is lost
◦ Ex. Cri du Chat Syndrome = “Cry of the Cat”
Duplication –production of extra copies of
parts of the chromosome.
2.
◦
Ex. Cat eye syndrome – severe mental retardation
Inversion - Reverses the direction of parts
of a chromosome.
3.
◦
Least likely to have serious effects.
4.
Translocation - When one part of a
nonhomologous chromosome breaks off and
attaches to another chromosome.
◦ All essential genetic material is present so most
children appear normal
◦ Ex. Cleft palate & Chronic myelogenous leukemia
(CML)

http://www.youtube.com/watch?v=XAGxp9j5
rtc
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5. Non-disjunction - When homologous
chromosomes fail to separate properly
during anaphase 1 of meiosis.
Ex. Trisomy 21 (aka. Down Syndrome)