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Biology
A Guide to the Natural World
Chapter 9 • Lecture Outline
The Links in Life’s Chain: Genetics and Cell Division
Fifth Edition
David Krogh
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9.1 An Introduction to Genetics
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An Introduction to Genetics
• DNA is an information-bearing molecule
that plays a critical role in the reproduction,
development, and everyday functioning of
living things.
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DNA
• DNA contains the information for the
production of proteins, which carry out a
wide variety of tasks in living things.
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DNA
• One series of bases contains information for
the production of one protein, while a
different series of bases specifies a different
protein.
• Each series of protein-specifying bases is
known as a gene.
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Genome
• Most of the cells in an organism contain a
complete copy of that organism’s genome,
meaning its collection of genetic
information.
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Genome
• Before cells divide, their genome must first
be copied and the resulting copies
apportioned evenly into what will become
two daughter cells.
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9.2 An Introduction to Cell Division
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Cell Division
• Cell division takes place because:
• Cells die and need to be replaced.
• Cells can only grow so large before they
become dysfunctional.
• There are times in which an organism needs
quantities of new cells above “replacement”
level.
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Cell Division
• Cell division includes:
• The duplication of DNA (replication)
• The apportioning of the copied DNA into two
quantities in a parent cell (mitosis)
• The physical splitting of this parent cell into
two daughter cells (cytokinesis)
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Cell
Division
cell
nucleus
1. Replication
DNA is duplicated.
2. Mitosis
The two quantities of DNA
are moved to opposite sides
of the parent cell.
3. Cytokinesis
The parent cell splits
into two daughter cells.
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Figure 9.4
DNA Replication
• In DNA replication, the two strands of the
double helix unwind, after which each
single strand serves as a template for
construction of a second, complementary
strand of DNA.
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DNA Replication
• The result is a doubling of the original
quantity of DNA.
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DNA
Replication
1. Original DNA
molecule unwinds.
2. New DNA strands
are synthesized
from the two
original strands.
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Figure 9.5
9.3 DNA in Chromosomes
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DNA is Packaged in Chromosomes
• DNA comes packaged in units called
chromosomes.
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Chromosomes
• Chromosomes are composed of DNA and
its associated proteins—a combined
chemical complex called chromatin.
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Chromosomes and DNA
Replication
• Chromosomes exist in an unduplicated state
until such time as DNA replicates, prior to
cell division.
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Chromosomes and DNA
Replication
• DNA replication results in chromosomes
that are in duplicated state, meaning one
chromosome composed of two identical
sister chromatids.
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(a) DNA is packaged in units called chromosomes
DNA wraps around
protein to make
chromatin
Chromatin folds
up to make
chromosomes
duplicated cell
chromosome
DNA
chromatin
(b) DNA replication at two levels
DNA replication . . . . . . has this effect at
the chromosomal level.
unduplicated chromosome
(not actual shape)
duplicated chromosome
sister chromatids
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Figure 9.6
Matched Pairs
• Chromosomes in human beings (and many
other species) come in matched pairs, with
one member of each pair inherited from the
mother, and the other member of each pair
inherited from the father.
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Matched Pairs
• Such homologous chromosomes have
closely matched sets of genes on them,
although many of these genes are not
identical.
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Homologous Chromosomes
• A given paternal chromosome may have
genes that code, for example, for different
hair or skin color than the counterpart genes
on the homologous maternal chromosome.
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Chromosomes
• Human beings have 46 chromosomes.
• 22 matched pairs and either a matched pair
of X chromosomes (in females) or an X and
a Y chromosome (in males).
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Karyotype
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Figure 9.7
The Cell Cycle
• Cell division fits into the larger framework
of the cell cycle, meaning a repeating
pattern of growth, genetic replication, and
cell division.
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The Cell Cycle
• The cell cycle has two main phases:
interphase and mitotic phase.
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The Cell Cycle
• In interphase, the cell carries out its work,
grows, and duplicates its chromosomes in
preparation for division.
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The Cell Cycle
• In mitotic phase, the duplicated
chromosomes separate (mitosis) and the
cell splits in two (cytokinesis).
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4. Mitosis
The cell begins the
process of apportioning
its DNA into two opposite
sides of itself.
5. Cytokinesis
With mitosis nearly complete, the cell begins
the process of splitting into two daughter
cells. Once this is finished, each of the two
cells moves back into G1 of interphase.
cytokinesis
3. Interphase—G2
In this gap-2 phase,
DNA replication has
been completed, and
the cell continues with
its normal functions,
even as it prepares
G2
for mitosis and
cytokinesis.
1. Interphase—G1
In this gap-1 phase of
the cell cycle, the cell is
growing and carrying
out its normal functions.
Cell
cycle
G1
2. Interphase—S
In this synthesis phase,
S
the cell is replicating its
DNA (duplicating its
chromosomes) in preparation
for mitosis and cytokinesis.
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Figure 9.9
9.4 Mitosis and Cytokinesis
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Mitosis and Cytokinesis
• There are four stages in mitosis: prophase,
metaphase, anaphase, and telophase.
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Mitosis
• The essence of the process is that duplicated
chromosomes line up along an equatorial
plane of the parent cell, called the
metaphase plate, with the sister chromatids
that make up each duplicated chromosome
lying on opposite sides of the plate.
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Mitosis
Suggested Media Enhancement:
Mitosis
To access this animation go to folder C_Animations_and_Video_Files
and open the BioFlix folder.
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Mitosis
• Attached to fibers called microtubules, the
sister chromatids are then pulled apart, to
opposite poles of the parent cell.
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Mitosis and Cytokinesis
mitotic spindle
spindle fibers
(microtubules)
metaphase
plate
pair of
centrosomes
chromosomes
(each a pair of
sister chromatids
joined together)
nucleus
replicated,
uncondensed
DNA
End of
interphase
DNA has already duplicated
back in S phase. Centrosome
has doubled.
Prophase
Metaphase
Mitosis begins:
Chromosomes take shape;
the two centrosomes begin
to move toward the cellular
poles, sprouting microtubules
as they go
Attachment and alignment:
Microtubules attach to sister
chromatids and align them at
the metaphase plate.
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Figure 9.10 (1 of 2)
cleavage
furrow
separating
chromatids
spindle fibers
shortening
Anaphase
Separation:
Sister chromatids are
moved to opposite poles
in the cell, each chromatid
now becoming a full-fledged
chromosome.
Telophase and
cytokinesis
Completion
of cytokinesis
Exit from mitosis:
Chromosomes decondense;
nuclear envelopes form
around the two separate
complements of chromosomes. Cleavage furrow
begins to form.
One cell becomes two:
The cell membrane pinches
together completely;
membranes on either side
fuse together, creating two
cells.
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Beginning of
interphase
These two cells now enter
the G1 phase of interphase.
Figure 9.10 (2 of 2)
Mitosis and Cytokinesis
• Once cell division is complete, sister
chromatids that once formed a single
chromosome will reside in separate
daughter cells, with each sister chromatid
now functioning as a full-fledged
chromosome.
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Mitosis and Cytokinesis
• Cytokinesis in animal cells works through a
ring of protein filaments that tightens at the
middle of a dividing cell.
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Cytokinesis in Animals
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Figure 9.11
Cytokinesis
• Membranes on the portions of the cell being
pinched together then fuse, resulting in two
daughter cells.
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9.5 Cell Division in Plants
and Bacteria
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Variations in Cell Division
• Because of their cell walls, plant cells must
carry out cytokinesis differently from
animal cells.
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Plant Cell Division
• The plant’s solution is to grow new cell
walls and plasma membranes near the
metaphase plate, thus dividing the parent
cell into two daughter cells.
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Cytokinesis in Plants
cell wall
two
daughter
cells
vesicles
plasma
membrane
1. Membrane-lined
vesicles accumulate
near the metaphase
plate. The vesicles
contain precursors
to the cell wall.
2. Vesicles fuse
together, forming
a cell plate that
grows toward the
parent cell wall.
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3. The newly formed
plasma membrane and
cell wall fuse with the
parent plasma membrane
and cell wall, forming two
distinct daughter cells.
Figure 9.12
Binary Fission
• Prokaryotes such as bacteria employ a
process called binary fission.
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Binary Fission
• They double their single, circular
chromosome, with the two resulting
chromosomes attaching to different sites on
the plasma membrane.
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Binary Fission
• Then, an outgrowth of plasma membrane
and cell wall, called a septum, begins
growing from opposite sides of the cell, in
between the two chromosomes.
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Binary Fission
• When the two septum extensions join in the
middle, they divide the one cell into two.
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Binary Fission in Bacteria
cell wall
two
daughter
cells
chromosome
cell membrane
parental
bacterial
cell
1. Bacterial cell
starts with a
single, circular
chromosome
attached to its
plasma
membrane.
2. The chromosome
replicates and
the daughter
chromosomes
attach to different
sites on the
plasma membrane.
3. The cell
membrane and
wall grow an
extension between
the attachment
points of the two
chromosomes.
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4. The cell wall and
membrane join
together in the
middle, resulting
in two new cells.
Figure 9.13
Cell Division in Bacteria
Animation 9.1: Cell Division for Bacteria
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