Chapter 8
How cells Reproduce
Cell Division Mechanisms
Reproduction
– Produces a generation of individuals like
parents
Cell division
– Bridges two generations
Each daughter cell receives
– A required number of DNA molecules
– Some cytoplasm
Overview of cell division
Eukaryotes and Prokaryotes
Eukaryotic cells undergo mitosis and/or
meiosis
– Separates duplicated chromosomes of
parent cell into two daughter nuclei
– Another mechanism divides cytoplasm
– Prokaryotic cells divide by a different
process
Mitosis and Meiosis
Mitosis
A process of cell division which results in the
production of two daughter cells.
The daughter cells are identical to one another
and to the original parent cell.
Basis of growth, cell replacements, and tissue
repair in multicelled species
Basis of asexual reproduction in many singlecelled and multicelled species
Mitosis and Meiosis
Meiosis
Basis of sexual reproduction
Precedes formation of gametes or sexual
spores
Daughter cell produced have half the
chromosome as the original cell
Chromosome Structure
Eukaryotic chromosome
Association of DNA, histones, and other proteins
Proteins structurally organize the chromosome
and affect access to its genes
Nucleosome
Smallest unit of organization
Double-stranded DNA looped twice around a
spool of histones
Structure of a Condensed
Chromosome
centromere
a One duplicated human chromosome in its most condensed form.
multiple levels of coiling
of DNA and proteins
When a chromosome
is most condensed, the
proteins associated with
it interact in ways that
package loops of DNA,
which are already coiled,
into higher order levels
of coiling.
At a deeper level of structural
organization, the chromosomal
proteins and DNA are
organized as a cylindrical
fiber.
fiber
Immerse a chromosome in
saltwater and it loosens to a
beads-on-a-string organization.
What appears to be a “string” is
one DNA molecule. Each “bead”
is a nucleosome.
beads on
a string
DNA
double
helix
core of
histone
nucleosome
A nucleosome consists
of part of a DNA molecule
looped twice around a core
of histone proteins.
Fig. 8.4d, p.127
Sister Chromatids
A duplicated chromosome consists of two
sister chromatids, each with a kinetochore
Sister chromatids remain attached at their
centromere until late in mitosis(or meiosis)
one
chromatid
one chromosome
(unduplicated)
its sister
chromatids
one chromosome
(duplicated)
Key Concepts:
CHROMOSOMES AND DIVIDING CELLS
Individuals of a species have a characteristic
number of chromosomes in each cell
Chromosomes differ in length and shape, carry
different portions of cell’s hereditary information
Mechanisms divide information between
daughter cells, along with enough cytoplasm for
each cell to operate on its own
Introducing the Cell Cycle
Cell cycle
In eukaryotic cells, a series of events from
the time a cell forms until it reproduces
A cycle consist of interphase, mitosis, and
cytoplasmic division
Starts when a new cell forms
Runs through interphase
Ends when cell reproduces by nuclear and
cytoplasmic division
Interphase
Consist of three stages
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)
G1
cytoplasmic
division; each
daughter cell
enters interphase
S
S
Interval of cell growth when
the DNA is replicated (all
chromosomes duplicated)
G2
G2
Interval after DNA
replication; the cell
prepares to divide
Interphase ends for parent cell
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 maintains parental chromosome
number from one generation to the next
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
First stage of mitosis
Duplicated chromosomes become
threadlike as they start to condense
Microtubules form a bipolar spindle
Nuclear envelope starts to break apart
Mitosis
A dynamic network of microtubules called the
bipolar spindle grows from opposite poles of
the cell
Transition to Metaphase
Microtubules from one spindle pole
harness one chromatid of each
chromosome
– Microtubules from the opposite spindle
pole harness its sister chromatid
Other microtubules extend from both poles
and grow until they overlap at the spindle’s
midpoint
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
Microtubules that overlap at spindle’s
midpoint slide past each other, push poles
farther apart
Motor proteins drive movements
Telophase
Two identical clusters (one chromosome
of each type) reach opposite spindle poles
Nuclear envelope forms around each
cluster
Both new nuclei have the parental
chromosome number
Mitosis
Mitosis
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
Cell plate Formation
Plate grows inward until its edges reach
the plasma membrane
Cell plate matures
By the end each daughter cell will
enclosed by its own plasma membrane
and cell wall
Cytoplasmic Division in Plant
Cells
Key Concepts:
HOW THE CYTOPLASM DIVIDES
After nuclear division, the cytoplasm divides and
typically puts a nucleus in each daughter cell
The cytoplasm of an animal cell is simply
pinched in two
In plant cells, a cross-wall forms in the
cytoplasm and divides it
Controls Over Cell Division
Products of checkpoint genes control cell
cycle
– Kinases
– Growth factors
When Control is Lost
Mutant checkpoint genes can cause
tumors by disrupting normal controls
Neoplasms: Benign and Malignant
Cancers
Altered cells grow and divide abnormally
Malignant cells may metastasize (break
loose and colonize distant tissues)
Cancer cells grow and divide abnormally
Cytoplasm and plasma membrane of
cancer cells become altered
Cancer cells have a weekend capacity for
adhesion