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BIOLOGY
A GUIDE TO THE NATURAL WORLD
FOURTH EDITION
DAVID KROGH
Genetics and Cell Division
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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 an array
of tasks in living things.
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DNA
• The information in DNA is encoded in
chemical substances called bases, which are
laid out along the DNA double helix in four
varieties:
–
–
–
–
adenine (A)
thymine (T)
guanine (G)
cytosine (C)
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DNA
G
A
G
A
T
A
T
Two “handrails” made of
sugar and phosphate.
G
C
A
C
C
T
T
G
Genetic information in the
molecule is contained in
the sequence of “bases”
along one strand of the
double helix. In this
example, the order of
a few of these bases
is CTGA.
G
A
G
C
A
A
G
A
C
C
T
G
A
T
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Figure 9.2
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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Protein Synthesis
• Protein synthesis begins with the information in
a sequence of DNA bases being copied onto a
length of a molecule called messenger RNA
(mRNA).
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Protein Synthesis
• mRNA moves out of the cell’s nucleus to a
structure in the cell’s cytoplasm called a
ribosome.
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Protein Synthesis
• There, the mRNA sequence is brought together
with the building blocks of proteins, amino
acids.
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Protein Synthesis
• The mRNA sequence is “read” within the
ribosome, and as this happens, a chain of amino
acids is linked together in the ribosome in the
order specified by the mRNA sequence.
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Protein Synthesis
• The result is a chain of amino acids that folds
into a protein.
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Protein Synthesis
DNA
1.The information contained in
a length of DNA is transcribed
onto a length of messenger
RNA (mRNA).
mRN A
nucleus
2.The mRNA then exits the cell’s nucleus
and goes to a structure in the cell’s
cytoplasm called a ribosome.
cytosol
amino acids
ribosome
protein
3.Here, the mRNA sequence is “read,” and
a string of amino acids is put together
in the order specified by the sequence.
The result is a protein.
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Figure 9.3
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 is Packaged 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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Chromosomes and DNA Replication
(a) DNA is packaged in units called chromosomes
DNA wraps around
protein to make
chromatin.
Chromatin folds
up to make
chromosomes.
duplicated
chromosome
DNA
(b) DNA replication
at two levels
cell
chromatin
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
The two chromosomes
above each number are a
homologous pair (they
are the same in size and
function).
A human male has one
pair of non-homologous
chromosomes, X and Y
(females have XX).
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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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The Cell Cycle
5.Mitotic Phase—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
4.Mitotic Phase—Mitosis
Now the cell begins the process
of apportioning its DNA into two
opposite sides of itself.
M
G2
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 for mitosis
and cytokinesis.
Cell
cycle
G1
S
1.Interphase—G1
In this gap 1 phase of
the cell cycle, the cell
is growing and carrying
out its normal functions.
2.Interphase—S
In this synthesis phase, 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
Mitosis and cytokinesis
chromosomes (each a
pair of sister chromatids
joined together)
mitotic
spindle
pair of
centrosomes
nucleus
metaphase
plate
replicated,
uncondensed
DNA
spindle fibers
(microtubules)
Prophase
mitosis begins
End of intephase
DNA has already
duplicated back in S
phase. Centrosome has
doubled.
Chromosomes take shape;
the two centrosomes
begin to move toward the
cellular poles, sprouting
microtubules as they go.
Metaphase
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)
Mitosis and Cytokinesis
separating
chromatids
cleavage
furrow
spindle fibers
shortening
Telophase and
cytokinesis
exit from mitosis
Anaphase
separation
Sister chromatids are
moved to opposite poles
in the cell, each chromatid
now becoming a
full-fledged chromosome.
Chromosomes decondense;
nuclear envelopes form around
the two separate complements
of chromosomes. Cleavage
furrow begins to form.
Completion of cytokinesis
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 Variations in Cell Division
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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.
3.The newly formed plasma
membrane and cell wall
fuse with the parent plasma
membrane and cell wall,
forming two distinct
daughter cells.
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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
PLAY
Animation 9.1: Cell Division for Bacteria
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