How Cells Reproduce

1

• How Cells Reproduce

2
Understanding Cell Division

• What instructions are necessary for inheritance?
• How are those instructions duplicated for
  distribution into daughter cells?
• By what mechanisms are instructions parceled out
  to daughter cells?

3
Reproduction

• Parents produce a new generation of cells or
  multicelled individuals like themselves
• Parents must provide daughter cells with
  hereditary instructions, encoded in DNA, and
  enough metabolic machinery to start up their own
  operation

4
Division Mechanisms

• Eukaryotic organisms
• Mitosis
• Meiosis
• Prokaryotic organisms
• Prokaryotic fission

5
Roles of Mitosis

• Multicelled organisms
• Growth
• Cell replacement
• Some protistans, fungi, plants, animals
• Asexual reproduction

6
Chromosome

• A DNA molecule attached proteins
• Duplicated in preparation for mitosis

one chromosome (unduplicated)
one chromosome (duplicated)
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Chromosome
a One chromosome (unduplicated)
one chromatid
two sister chromatids
one chromatid
b One chromosome (duplicated)
Stepped Art
Fig. 9-3a, p.142
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Chromosome
multiple levels of coiling of DNA and proteins
centromere (constricted region)
fiber
beads on a string
DNA double helix
core of histone
nucleosome
Fig. 9-4, p.143
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Chromosome Number

• Sum total of chromosomes in a cell
• Somatic cells
• Chromosome number is diploid (2n)
• Two of each type of chromosome
• Gametes
• Chromosome number is haploid (n)
• One of each chromosome type

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Human Chromosome Number

• Diploid chromosome number (n) 46
• Two sets of 23 chromosomes each
• One set from father
• One set from mother
• Mitosis produces cells with 46 chromosomes--two
  of each type

12
Organization of Chromosomes
DNA
one nucleosome
DNA and proteins arranged as cylindrical fiber
histone
13
Organization of Chromosomes

• Chromosome structural organization

14
The Cell Cycle
interphase
G1
S
telophase
anaphase
Mitosis
G2
metaphase
prophase
Figure 9.5Page 144
15
The Cell Cycle

• The cell cycle

16
Interphase

• Usually longest part of the cycle
• Cell increases in mass
• Number of cytoplasmic components doubles
• DNA is duplicated

17
Mitosis

• Period of nuclear division
• Usually followed by cytoplasmic division
• Four stages
• Prophase
• Metaphase
• Anaphase
• Telophase

18
Control of the Cycle

• Once S begins, the cycle automatically runs
  through G2 and mitosis
• The cycle has a built-in molecular brake in G1
• Cancer involves a loss of control over the cycle,
  malfunction of the brakes

19
Stopping the Cycle

• Some cells normally stop in interphase
• Neurons in human brain
• Arrested cells do not divide
• Adverse conditions can stop cycle
• Nutrient-deprived amoebas get stuck in interphase

20
The Spindle Apparatus

• Consists of two distinct sets of microtubules
• Each set extends from one of the cell poles
• Two sets overlap at spindle equator
• Moves chromosomes during mitosis

21
Spindle Apparatus
one spindle pole
one of the condensed chromosomes
spindle equator
microtubules organized as a spindle apparatus
one spindle pole
22
Maintaining Chromosome Number

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Maintaining Chromosome Number
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
XX (or XY)
Fig. 9-6a, p.145
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Maintaining Chromosome Number
a Two of the chromosomes (unduplicated) in a
parent cell at interphase
b The same two chromosomes, now duplicated, in
that cell at interphase, prior to mitosis
c Two chromosomes (unduplicated) in the parent
cells daughter cells, which both start life in
interphase
Fig. 9-6b, p.145
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Maintaining Chromosome Number
pole
pole
microtubule of bipolar spindle
p.145
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Stages of Mitosis

• Prophase
• Metaphase
• Anaphase
• Telophase

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Early Prophase - Mitosis Begins

• Duplicated chromosomes begin to condense

Figure 9.7 Page 146
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Late Prophase

• New microtubules are assembled
• One centriole pair is moved toward opposite pole
  of spindle
• Nuclear envelope starts to break up

Figure 9.7 Page 146
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Transition to Metaphase

• Spindle forms
• Spindle microtubules become attached to the two
  sister chromatids of each chromosome

Figure 9.7 Page 146
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Metaphase

• All chromosomes are lined up at the spindle
  equator
• Chromosomes are maximally condensed

Figure 9.7 Page 147
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Anaphase

• Sister chromatids of each chromosome are pulled
  apart
• Once separated, each chromatid is a chromosome

Figure 9.7 Page 147
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Telophase

• Chromosomes decondense
• Two nuclear membranes form, one around each set
  of unduplicated chromosomes

Figure 9.7 Page 147
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Results of Mitosis

• Two daughter nuclei
• Each with same chromosome number as parent cell
• Chromosomes in unduplicated form

Figure 9.7 Page 147
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a Cell at Interphase
The cell duplicates its DNA, prepares for nuclear
division
Mitosis
pair of centrioles
nuclear envelop
chromosomes
b EARLY PROPHASE
c LATE PROPHASE
d TRANSITION TO METAPASE
Mitosis begins. The DNA and its associated
proteins have started to condense. The two
chromosomes color-coded purple were inherited
from the female parent. The other two (blue) are
their counterparts., inherited from the male
parent.
Chromosomes continue to condense. New
microtubules become assembled. They move one of
the two pairs of centrioles to the opposite end
of the cell. The nuclear envelope starts to break
up.
Now microtubules penentrate the nuclear region.
Collectively, they form a bipolar spindle
apparatus. Many of the spindle microtubules
become attatched to the two sister chromatids of
each chromosome.
Fig. 9-7a, p.146
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Interphase
Early Prophase
pair of centrioles
nuclear envelope
Stepped Art
Fig. 9-7a, p.146
36
microtubule
e METAPHASE
f ANAPHASE
g TELOPHASE
h INTERPHASE
All chromosomes have become lined up at the
spindle equator. At this stage of mitosis (and of
the cell cycle), they are most tightly
condensed
Attachments between the two sister chromatids of
each chromosome break. The two are separate
chromosomes, which microtubules move to opposite
spindle pores.
There are two clusters of chromosomes, which
decondense. Patches of new membrane fuse to form
a new nuclear envelope. Mitosis is completed.
Now there are two daughter cells. Each is
diploid its nucleus has two of each type of
chromosome, just like the parent cell.
Fig. 9-7b, p.146
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Stepped Art
Fig. 9-7b, p.146
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Mitosis

• Mitosis step-by-step

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Cytoplasmic Division

• Usually occurs between late anaphase and end of
  telophase
• Two mechanisms
• Cell plate formation (plants)
• Cleavage (animals)

40
Animal Cell Division
41
Animal Cell Division

• A ring of microfilaments in the same plane as the
  spindle equator contracts, dividing the animal
  cell

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Animal Cell Division
1 Mitosis is over, and the spindle is now
disassembling.
2 At the former spindle equator, a ring of
micro-filaments attached to the plasma membrane
contracts.
3 As its diameter shrinks, it pulls the cell
surface inward.
4 Contractions continue the cell is pinched in
two.
Fig. 9-8a, p.148
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Animal Cell Division
Stepped Art
Fig. 9-8a, p.148
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Cell Plate Formation
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Cell Plate Formation
cell plane forming
1 As mitosis ends, vesicles cluster at the
spindle equator. They contain materials for anew
primary cell wall.
2 Vesicle membranes fuse. The wall material is
sandwiched between two new membranes that
lengthen along the plane of a newly forming cell
plate.
3 Cellulose is deposited inside the sandwich. In
time, these deposits will form two cell walls.
Others will form the middle lamella between the
walls and cement them together.
4 A cell plate grows at its margins until it
fuses with the parent cell plasma membrane. The
primary wall of growing plant cells is still
thin. New material is deposited on it.
Fig. 9-8b, p.148
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Cell Plate Formation
cell wall
former spindle equator
vesicles converging
cell plate
Stepped Art
Fig. 9-8b, p.148
47
Cell Plate Formation

• Cytoplasmic division

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Cell Division

• Individual cells of a human embryo divide,
  developing from a paddlelike structure into a
  hand

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Cell Division
ring of microfilaments midway between the two
spindle poles, in the same plane as the spindle
equator
Fig. 9-9, p.149
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Cell Division
Fig. 9-10, p.149
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Mitotic Control

• Kinases
• Growth factors
• Checkpoint genes

52
Mitotic Control
Fig. 9-11a, p.150
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Mitotic Control
Fig. 9-11b, p.150
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Tumors

• Sometimes a checkpoint gene mutates and control
  over cell division is lost.
• Cells uncontrollable division forms an abnormal
  mass called a tumor.
• Neoplasms

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Cancer
Fig. 9-12, p.150
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Cancer

• Cancer and metastasis

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Cancer
benign tumor
malignant tumor
1 Cancer cells slip of out their home tissue
2 The metastasizing cells become attached to the
wall of a blood or lymph vessel. They secrete
digestive enzymes onto it. Then they cross the
wall at the breach.
3 Cancer cells creep or tumble along inside
blood vessels, then leave the bloodstream the
same way they got in. They start new tumors in
new tissues.
Fig. 9-13, p.151
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cell at interphase
nucleus
cytoplasm
telophase
prophase
metaphase
anaphase
Fig. 9-15, p.153