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How Cells Reproduce
Chapter 9
Hsueh-Fen Juan
Oct. 16, 2012
Video: Henrietta's immortal cells
Impacts, Issues
Henrietta’s Immortal Cells
 Henrietta Lacks died of cancer at age 31, but her
cells (HeLa cells) are still growing in laboratories
9.1 Overview of
Cell Division Mechanisms
 Individual cells or organisms produce offspring
by the process of reproduction
 When a cell reproduces, each descendent
receives information coded in DNA, and enough
cytoplasm to begin operating
 細胞分裂包含：1.細胞核分裂(mitosis or meiosis)
2.細胞質分裂
Mitosis, Meiosis, and the Prokaryotes
 Eukaryotic cells
• Mitosis copies DNA and divides a nucleus,
producing two identical nuclei
• Meiosis is a nuclear division that produces
haploid gametes for sexual reproduction
• (有絲、減數都是在討論細胞核的分裂，非細胞質)
 Prokaryotic cells reproduce asexually by
prokaryotic fission (原核裂變，和有絲、減數分
裂相比，機制上完全不同)
Comparison of Cell Division Mechanisms
Key Points About Chromosome Structure
 Each species has a characteristic number of
chromosomes that differ in length and shape
• Each consists of one double strand of DNA
• After duplication, each consists of two double
strands (sister chromatids) that remain attached
to each other at a centromere until late in
nuclear division
• Chromatid: 染色分體 (chromosome: 染色體)
Centromere: 著絲點 (centrosome: 中心體)，其上
的 binding sites 稱 kinetochore (著絲粒)
A Chromosome and Sister Chromatids
Key Points About Chromosome Structure
 A chromosome consists of DNA that is wrapped
around proteins (histones 組織蛋白) and
condensed
 Each histone and the DNA wrapped around it
make up a nucleosome (核小體), the smallest
unit of structural organization in eukaryotic
chromosomes
B When a chromosome
is at its most condensed,
the DNA is packed into
tightly coiled coils.
centromere
A Duplicated human
chromosome in its most
condensed form. If this
chromosome were
actually the size shown in
the micrograph, its two
DNA strands would
stretch out about 800
meters (0.5 miles).
multiple levels of
coiling of DNA and
proteins
fiber
beads on
a string
Chromosome
Structure
DNA
double
helix
core of
histones
nucleosome
C When the coiled coils
unwind, a molecule of
chromosomal DNA and
its associated proteins
are organized as a
cylindrical fiber.
D A loosened fiber
shows a “beads-on-astring” organization.
The “string” is the DNA
molecule; each “bead”
is one nucleosome.
E A nucleosome
consists of part of a
DNA molecule looped
twice around a core of
histone proteins.
Stepped Art
Fig. 9-3 (b-e), p. 143
Animation: Chromosome
structural organization
The cell cycle
G1
S
Interval of cell growth and activity
before DNA replication (chromosomes
unduplicated)
Interval of cell growth when
the DNA is replicated (all
chromosomes are duplicating)
Telophase
Anaphase
cytoplasmic division;
each descendant cell
enters interphase
Metaphase
Prophase
G2
Interphase
ends for
parent cell
Interval after DNA
replication; the cell
prepares to divide
(Cells make the
proteins that will
drive mitosis)
Stepped Art
Fig. 9-4, p. 144
Animation: The cell cycle
9.1 Key Concepts:
Chromosomes and Dividing Cells
 Individuals have a characteristic number of
chromosomes in each of their cells
 The chromosomes differ in length and shape,
and they carry different portions of the cell’s
hereditary information
 Division mechanisms parcel out the information
into descendent cells
9.2 Introducing the Cell Cycle
 Cell cycle
• A sequence of three stages (interphase, mitosis,
and cytoplasmic division) through which a cell
passes between one cell division and the next
Interphase
 Interphase consists of three stages, during
which a cell increases in size, doubles the
number of cytoplasmic components, and
duplicates its DNA
• G1: Interval of cell growth and activity，(DNA未
複製)
• S: Interval of DNA replication (synthesis，DNA複
製中)
• G2: Interval when the cell prepares for division.
Cells make the proteins that will drive mitosis.
(DNA已完成複製)
Interphase and the Life of a Cell
 Most cell activities take place during G1
 Control mechanisms work at certain points in the
cell cycle; some can keep cells in G1(例：神經細
胞大多永遠停於G1，因此不分裂)
 Loss of control may cause cell death or cancer
(例：強制使神經細胞跳脫G1會使其死亡，而非細
胞分裂，因為它有控制機制使其在cell cycle失控
時self-destruct)
Mitosis and the Chromosome Number
 Mitosis produces two diploid nuclei with the same
number and kind of chromosomes as the parent
 Chromosome number
• The sum of all chromosomes in a type of cell
• Human cells have 46 chromosomes paired in 23
sets (diploid number，二倍體)
• Pairs have the same shape and information about
the same traits (except sex chromosomes XY)
Mitosis and the Chromosome Number
 Bipolar spindle (雙極紡錘絲)
• A dynamic network of microtubules that forms
during nuclear division
• Grows into the cytoplasm from opposite poles of
the cell and attaches to duplicated chromosomes
• Microtubules from opposite poles attach to
different sister chromatids and separate them
Bipolar Spindle
Separates Sister Chromatids
Mitosis Maintains Chromosome Number
9.2 Key Concepts:
Where Mitosis Fits in the Cell Cycle
 A cell cycle starts when a new cell forms by
division of a parent cell, and ends when the cell
completes its own division
 A typical cell proceeds through intervals of
interphase, mitosis, and cytoplasmic division
9.3 A Closer Look at Mitosis
 When a nucleus divides by mitosis, each new
nucleus has the same chromosome number as
the parent cell
 There are four main stages of mitosis: prophase,
metaphase, anaphase, and telophase
Prophase
 Prophase
• Chromosomes condense
• Microtubules form a bipolar spindle with motor
proteins traveling along them (消耗ATP)
• Nuclear envelope breaks up
• Microtubules attach to the chromosomes
• 中心體在前期之前(early prophase)即完成複製
• 植物細胞無中心體，但有其他構造控制紡錘絲
 Centrosome
• A region near the nucleus that organizes spindle
microtubules; usually includes two centrioles (中
心體通常含兩個中心粒)
Metaphase and Anaphase
 Metaphase
• All duplicated chromosomes line up midway
between the spindle poles by adding and losing
tubulin subunits to grow or shrink microtubules.
 Anaphase
• Microtubules separate the sister chromatids of
each chromosome and pull them to opposite
spindle poles
• 3 cell activities bring this about:
1.連接在染色分體上的紡錘絲微管縮短
2.機動蛋白把染色分體拖往紡錘絲微管縮短的方向
3.在赤道板上重疊的微管也彼此滑動縮短
Telophase
 Telophase
• Two clusters of chromosomes reach the spindle
poles and decondense
• A new nuclear envelope forms around each
cluster
• New plasma membrane may assemble
 Two new nuclei are formed, each with the same
chromosome number as the parent cell
Mitosis
Fig. 9-6 (2), p. 147
Fig. 9-6 (2a), p. 147
Fig. 9-6 (2b), p. 147
Fig. 9-6 (2c), p. 147
Fig. 9-6 (2d), p. 147
Fig. 9-6 (2e), p. 147
Fig. 9-6 (2f), p. 147
Animation: Mitosis-step-by-step
Animation: Mechanisms for chromosome
movement
9.3 Key Concepts:
Stages of Mitosis
 Mitosis divides the nucleus, not the cytoplasm
 Mitosis has four sequential stages: prophase,
metaphase, anaphase, and telophase
 A bipolar spindle forms; it moves the cell’s
duplicated chromosomes into two parcels, which
end up in two genetically identical nuclei
9.4 Cytoplasmic Division Mechanisms
 In most kinds of eukaryotes, the cell cytoplasm
divides between late anaphase and the end of
telophase, but the mechanism of division differs
 Cytokinesis (aka cytoplasmic division)
• The process of cytoplasmic division
Cytoplasmic Division
in Animal and Plant Cells
 Animal cells
• A contractile ring partitions the cytoplasm
• A band of actin filaments rings the cell midsection,
contracts, and pinches the cytoplasm in two (The
indentation is called Cleavage furrow 注意)
 Plant cells
• The plane of division is established by
microtubules and actin filaments that formed
and broke up before mitosis began
• A cell plate forms midway between the spindle
poles; it partitions the cytoplasm when it reaches
and develop into primary cell wall merging with
the parent cell wall
Cytoplasmic Division
in Animal and Plant Cells
Fig. 9-7a, p. 148
Fig. 9-7b, p. 148
Animation: Cytoplasmic division
9.4 Key Concepts:
How the Cytoplasm Divides
 After nuclear division, the cytoplasm divides
 One nucleus ends up in each of two new cells
 In animal cells, the cytoplasm pinches in two
 In plant cells, a cross-wall forms in the
cytoplasm and divides it
9.5 When Control is Lost
 Sometimes, controls over cell division are lost
• Cancer may be the outcome
Cell Cycle Controls
 Checkpoints in the cell cycle allow problems to
be corrected before the cycle advances
 Proteins produced by checkpoint genes interact
to advance, delay, or stop the cell cycle (when
DNA’s not copied completely, damaged, or the
nutrient is not sufficient to support cell growth)
• Kinases can activate other molecules to stop
the cell cycle or cause cells to die
• Growth factors can activate kinases (by
binding to receptors on target cell) to activate
genes that stimulate cells to start mitosis
Protein Products of
Checkpoint Genes in Action
53BP1
BRCA1
Checkpoint Failure and Tumors
 When all checkpoint mechanisms fail, a cell
loses control over its cell cycle and may form a
tumor (abnormal mass) in surrounding tissue
 Usually one or more checkpoint gene products
are missing in tumor cells
• Checkpoint gene products (protein) that inhibit
mitosis are called Tumor suppressors (腫瘤抑
制物)
• Checkpoint genes encoding proteins that
stimulate mitosis are called Protooncogenes (原
癌基因)
Neoplasms
 Neoplasms (腫瘤)
• Abnormal masses of cells that lack control over
how they grow and divide
• Benign (noncancerous 良性) neoplasms, such as
ordinary skin moles, stay in one place and are not
cancerous
• Malignant (cancerous 惡性) neoplasms are
cancerous
Characteristics of Cancer Cells
 Cancers (malignant neoplasms)
1. Cells grow and divide abnormally; capillary
blood supply to the cells may increase
abnormally (微血管暴量，供血異常暴增)
2. Cells may have altered plasma membrane and
cytoplasm: Membrane may be leaky and
missing proteins; Cytoskeleton may be
shrunken or disorganized; Metabolism may
shift toward fermentation from aerobic
respiration (超級瘋狂)
3. Cells have altered recognition proteins and
weakened adhesion; may break away and
invade distant tissues (metastasis 轉移)
Benign and
Malignant
Tumors
benign tumor
malignant tumor
A Cancer cells
break away from
their home tissue.
B The metastasizing
cells become
attached to the wall
of a blood vessel or
lymph vessel. They
release digestive
enzymes onto it. Then
they cross the wall at
the resulting breach.
C 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. 9-11, p. 151
Animation: Cancer and metastasis
Skin Cancers
A basal cell
carcinoma
Squamous
cell carcinoma
Malignant melanoma
ABC video: Blood test for lung cancer
ABC video: Genetically modified cancer
fighters
9.5 Key Concepts:
The Cell Cycle and Cancer
 Built-in mechanisms monitor and control the
timing and rate of cell division
 On rare occasions, the surveillance mechanisms
fail, and cell division become uncontrollable
 Tumor formation and cancer are the outcome