How Cells Reproduce
Dividing HeLa Cells
Eukaryotes and Prokaryotes
 Eukaryotic cells undergo mitosis and/or meiosis
• Separates duplicated chromosomes of parent cell
into two daughter nuclei
• Another mechanism divides cytoplasm
Mitosis and Meiosis
 Mitosis
• Basis of growth, cell replacements, and tissue
repair in multicelled species
• Basis of asexual reproduction in many singlecelled and multicelled species
 Meiosis
• Basis of sexual reproduction
• Precedes formation of gametes or sexual spores
Cell Division Mechanisms
Chromosome Structure
 Eukaryotic chromosome
• Association of DNA, histones, and other proteins
 Nucleosome
• Smallest unit of organization
• Double-stranded DNA looped twice around a
spool of histones
Structure of a Condensed Chromosome
Sister Chromatids
 A duplicated
chromosome consists
of two sister
chromatids
 Sister chromatids
remain attached at
their centromere until
late in mitosis
Introducing the Cell Cycle
 Cell cycle
• Starts when a new cell forms
• Runs through interphase
• Ends when cell reproduces by nuclear and
cytoplasmic division
Interphase
 Most cellular activities occur in interphase
• G1: Cell grows in mass, doubles number of
cytoplasmic components
• S: DNA replication duplicates chromosomes
• G2: Cell prepares for division
G1
Interval of cell growth before
DNA replication (chromosomes
unduplicated)
S
Interval of cell growth when
the DNA is replicated (all
chromosomes duplicated)
cytoplasmic
division; each
daughter cell
enters interphase
G2
Interval after DNA
replication; the cell
prepares to divide
Interphase ends for parent cell
Fig. 8.5, p.128
Chromosome Number
 Sum of all chromosomes in cells of a given type
 In human body cells, chromosome number is 46
 Body cells are diploid (have two of each kind of
chromosome)
Mitosis and Chromosome Number
 Mitosis maintains parental chromosome number
from one generation to the next
• Bipolar spindle divides sister chromatids
A Closer Look at Mitosis
 Mitosis
• A nuclear division mechanism that maintains the
chromosome number
 Mitosis proceeds in four stages:
•
•
•
•
Prophase
Metaphase
Anaphase
Telophase
Prophase
 Duplicated chromosomes become threadlike as
they start to condense
 Microtubules form a bipolar spindle
 Nuclear envelope starts to break apart
Metaphase
 All chromosomes become aligned midway
between the two spindle poles
• Chromosomes in most condensed forms
Anaphase
 Sister chromatids detach from each other
• Spindles move them toward opposite poles
Telophase
 Chromosomes reach opposite spindle poles
 Nuclear envelope forms around each cluster
 Both new nuclei have the parental chromosome
number
Mitosis
Mitosis
Fig. 8.7a, p.130
nuclear
envelope
chromosome
b Early Prophase
Mitosis begins. DNA and its associated
proteins have started to condense. Two
chromosomes (color-coded purple) were
inherited from the female parent. The
other two (blue) are their counterparts,
inherited from the male parent.
Fig. 8.7b, p.130
pair of centrioles
c Late Prophase
The duplicated chromosomes continue
to condense. New microtubules move
one of two pairs of centrioles to the
opposite side of the nucleus. The
nuclear envelope starts to break up.
Fig. 8.7c, p.130
e Metaphase
All of the chromosomes have
become lined up midway
between the spindle poles.
At this stage of mitosis, the
chromosomes are in their
most tightly condensed form.
Fig. 8.7e, p.130
f Anaphase
Sister chromatids separate as
motor proteins moving along
spindle microtubules drag
them to opposite spindle
poles. Other microtubules
push the poles farther apart.
Fig. 8.7f, p.130
g Telophase
There are two clusters of
chromosomes, which now
decondense. Patches of
new membrane fuse to form
a new nuclear envelope.
Mitosis is over.
Fig. 8.7g, p.130
h Two Daughter
Cells at Interphase
After cytoplasmic division,
there are two daughter
cells. Each is diploid: Its
nucleus has two of each
type of chromosome, just
like the parent cell.
Fig. 8.7h, p.130
Cytoplasmic Division Mechanisms
 Mechanisms of cytoplasmic division differ in
plant and animal cells
 In animal cells
• A contractile ring of microfilaments (part of cell
cortex) contracts and pulls the cell surface inward
until the cytoplasm is divided
Cytoplasmic Division in Animal Cells
Cytoplasmic Division in Plant Cells
Results of Cell Division:
A Human Embryo
When Control is Lost
 Mutant checkpoint genes can cause tumors by
disrupting normal controls