The Continuity of Life:
Cellular Reproduction
Cell Division
• The cell cycle describes the sequence of
activities that occurs from one cell
division to the next.
• During cell division, one parent cell
divides producing two daughter cells.
Chapter 9
Cell Division
• Cells divide for both
reproduction and growth.
– Reproduction
• Asexual: offspring formed
from a single parent.
• Sexual: offspring formed from
two parent cells.
– Growth and Development
• Body cells grow and renew
through asexual reproduction.
Parent
Cell
Daughter
Cell
Daughter
Cell
Types of Cell Division
• Binary Fission
– Prokaryotic Cell Division
• Mitosis
– Eukaryotic Cell Division that gives rise to
non-sex (non-gamete) cells.
• Meiosis
– Eukaryotic Cell Division that gives rise to
sex (gamete) cells.
Prokaryotic Life Cycle
• Most of the
Prokaryote’s life is
spent replication
its DNA and
growing larger.
• Only a small
fraction of time is
spent during cell
division.
Prokaryotic Cell Division
1. Circular DNA is
attached to the
plasma membrane.
2. DNA is replicated, 2
creating two
identical copies.
3. The plasma
membrane
expands, separating
the DNA copies.
4. The new plasma
membrane grown
inward at the
middle of the cell.
3
5. The parent cell
divides into two
daughter cells.
Binary Fission
1
5
4
Eukaryotic DNA
Organization in the
Nucleus
• In Eukaryotes, DNA is kept in the nucleus.
• The entire sequence of an organism’s DNA
is called its genome.
• Within the nucleus, DNA is organized into
pairs of homologous chromosomes (each
chromosome in the pair is identical).
– Humans have 23 homologous paired
(46 individual) chromosomes.
Eukaryotic DNA
Organization into
Chromosomes
• DNA is
supercoiled
about itself.
Eukaryotic Chromosome Structure
1 Chromosome
Eukaryotic
Chromosome
Structure
Genes: DNA sequence
that contains information.
Telomeres: a repeated
nucleotide sequence
adding stability.
Centromere: Site of
attachment of duplicated
chromosomes during cell
division.
Chromosomes are called
sister chromatids when
attached at the
centromere.
When not preparing for cell division, DNA forms
chromatin and is relatively inaccessible.
Eukaryotic Chromosome Structure
1 Chromosome
1 Sister Chromatid
1 Duplicated
Chromosome
Genes
Centromere
1 Duplicated
Chromosome
1 Pair of Homologous
Chromosomes
Chromosomes During Mitosis
• Each of our cells has all
46 chromosomes (23
homologous pairs).
The Human
Genome
– The genetic information
for all of our genes and
proteins.
• Chromosome pair 23
contains the sex
chromosomes.
– XX or XY.
• The remaining
chromosomes are
autosomes.
– Same size and same
genetic composition
within the pair.
Eukaryotic Cell Cycle
Interphase: phase
where the cells
grows and
duplicates its
chromosomes.
Cell Division: parent
cell divides
chromosomes and
intracellular
components into
two daughter cells.
Eukaryotic Cell Cycle
• During cell division, cells under go
several distinct phases of development.
–
–
–
–
–
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis
Types of Eukaryotic
Cell Division
• Mitosis
– Eukaryotic Cell Division that gives rise to
non-sex (non-gamete) cells.
Early Prophase - Late Prophase - Metaphase
• Meiosis
– Eukaryotic Cell Division that gives rise to
sex (gamete) cells.
Anaphase
During late interphase, duplicated chromosomes
are in their relaxed form, as chromatin, within
the nucleus.
- Telophase - Cytokinesis
In early prophase, chromosomes condense and
shorten.
Spindle microtubules, protein fibers required for
proper movement of chromosomes, begin to form.
By late prophase, the nucleus is being recycled.
Spindle microtubules attach to the kinetochore,
a protein structure at the centromere.
In anaphase, sister chromatids
separate, pulled by microtubules
attached at the kinetochore.
“Free” spindle fibers interact to
lengthen the cell.
During metaphase, spindle fibers lengthen and
shorten to line chromosomes at the equator.
After chromosomes have reached the
poles, during telophase, microtubules are
recycled.
A new nuclear envelope forms around the
chromosomes and cytokinesis occurs.
The details of
cytokinesis differ
according to phylum.
During cytokinesis, the cell divides into
two daughter cells, each containing a
nucleus and half of the existing cytoplasm.
Cytokinesis in Animal Cells
• In animal cells, microfilaments attached to the
plasma membrane form a ring around the equator.
• During cytokinesis, this spindle fiber ring contracts
until the two cells are separated.
Cytokinesis in Plant Cells
• During plant cell cytokinesis, the golgi apparatus
forms carbohydrate vesicles that migrate to the
equator.
• These vesicles fuse to form the new cell wall.
Interphase begins again after cells have
separated.
Spindle microtubules are recycled, the
nuclear membrane is completed, and
chromosomes relax.
Interphase
1
Phases of
Mitosis
2
3
Mitosis
4
1. Early Prophase: chromosomes
condense and microtubules appear.
2. Late Prophase: nucleus breaks down,
spindle fibers attach to kinetochores. 5
3. Metaphase: fibers line up the
chromosomes at the equator.
4. Anaphase: sister chromatids separate 6
and microtubules elongate the cell.
5. Telophase: chromosomes relax,
nuclear envelope starts to form,
microtubules are recycled.
6. Cytokinesis: the cell divides in two.
Interphase
1
Mitosis
in real time
3
Mitosis vs. Meiosis
Both mitosis and meisosis follow the same basic steps:
Interphase
Anaphase
2
Prophase
Metaphase
Anaphase
Telophase
Interphase
Mitosis is used for
asexual reproduction
of cells for growth and
repair.
Metaphase
Transition, metaphase to anaphase
From The American Society for Cell Biology, http://cellimages.ascb.org/
Meiosis is used for
sexual reproduction,
making male and
female sex cells.
Sexual reproduction leads to
mixing of genetic material
(exchanging genes).
Control of Cell Division
Control of Cell Division
A growth factor
molecule binds to
a receptor which
upregulates cyclin
production.
The cell has
checkpoints at
every major step
in cell division.
Cyclins bind to
Cdk proteins to
stimulate DNA
Replication.
Chromosome Variability
• In organisms which
undergo sexual
reproduction, half
of the chromosome
comes from each
parent.
• The parents may each give a different allele, the
gene for a particular physical trait, to the offspring.
Meiosis
• Two rounds of cell division to produce gametes,
sex cells with 1 copy of each chromosome.
• Meiosis I produces diploid cells, possessing 2
copies of each chromosome.
• Meiosis II produces haploid cells, or gametes,
those possessing 1 copy of each chromosome.
– Examples: curly vs. straight hair, blue vs. brown eyes.
• Mixing of alleles leads to genetic diversity and
variability within the chromosome.
Meiosis I
Meiosis II
• Meiotic cell division
begins like mitotic cell
division.
• During prophase I:
chromosomes condense
and pair up, the nuclear
envelope begins to
break down, and spindle
microtubules begin to
appear.
• During metaphase I,
paired chromosomes
align at the equator,
pairs attaching to
spindle fibers from
opposite poles.
• In anaphase I,
microtubules pull
homologous paired
chromosomes to
opposite poles.
• Sister chromatids do
not separate!
• Similar to mitotic telophase,
microtubules stretch the cell
and disappear in telophase I.
• Cytokinesis occurs creating
two haploid cells with one
duplicated chromosome
copy in each.
• The second round fo
meiosis behaves much
like mitosis with half the
number of chromosomes.
• In prophase II, the
chromosome condenses
and spindle fibers form.
• In metaphase II, spindle
fibers attach to the
kinetochore of each sister
chromatid, aligning them
at the equator.
• Like mitosis, in anaphase
of meiosis II, sister
chromatids are pulled
apart by lengthening
spindle fibers.
• Next, in telophase II,
microtubules release
chromatids in the newly
forming nucleus and
cytokinesis (cell division)
occurs.
Meiosis I
• At the end of meiosis, four new
cells are formed.
• Each cell is a haploid, possessing
one copy from each pair of
chromosomes (diploids have two).
A. Meiosis I Early Prophase
Poles appear, chromosomes begin to
separate.
B. Meiosis I Late Prophase
Nuclear envelope disintegrates,
microtubules appear.
C. Meiosis I Pro-Metaphase
Spindle microtubules are assembled.
D. Meiosis I Metaphase
Microtubules attach to chromosomes
which align at the equator.
E. Meiosis I Anaphase
Attached microtubules shorten,
chromosomes are pulled the
poles.
F. Meiosis I Telophase
Free microtubules lengthen and
cytokinesis begins.
Meiosis I in real time
From The American Society for Cell Biology, http://cellimages.ascb.org/
Cell Types formed during
Mitosis and Meiosis
• Diploid cell:
– Formed during Mitosis and Meiosis I.
– Contains a full set of homologously
paired chromosomes (two copies of
each).
• Haploid cell:
– Formed only during Meiosis II.
– Contains only one copy of the
chromosomes (half the number all
other cells have).
– Also called a gamete, a sex cell (egg
and sperm).
Meiosis II in real time
From The American Society for Cell Biology, http://cellimages.ascb.org/
A Comparison:
Mitosis and Meiosis
Chromosome Attachment in Mitosis
• In Mitosis, spindle microtubules attached to
each individual chromatid at the kinetochore,
pulling sister
chromatids apart
during anaphase.
Each new cell two complete copies of the
chromosomes, one from each sister chromatid.
Counting Chromosomes
• If “n” is the number of unique chromosomes in an
organism:
– A diploid cell contains 2n chromosomes, a pair of each
chromosome.
– A haploid cell contains n chromosomes, one copy of each.
Diploid
Haploid Gametes
Parental Cells
Female Egg and
Male Sperm Cell
Diploid
Fertilized Egg
• Humans have 23 unique chromosome pairs.
– A muscle cell contains 46 chromosomes (23 pairs).
– A female egg contains 23 chromosomes (one of each).
Chromosome Attachment in Meiosis
In Meisosis I,
spindle fibers
attached to the
kinetochore separating duplicated
chromosomes, not individual
chromatids of the chromosome, during the first
anaphase.
Meiosis II attachment is the same as mitosis.
Each new cell receives two complete copies of the chromosome
in the form of one of the pair of duplicated chromosomes.
Introducing Genetic Variability
• In organisms that undergo meiosis, genetic
recombination can occur.
– The formation of new combinations of alleles on a
chromosome.
• Sexual organisms receive half of their
chromosomes from the mother and half from
the father when two haploid gametes combine
(egg and sperm).
• During prophase I of meiosis,
pairs of chromosomes align
and crossing over of the
chromosomes occurs.
Chromosome Diversity Through
Crossing Over
• Homologous
chromosomes pair up,
side by side.
• Enzymes join
homologous sections
of the chromosome,
like a zipper.
Chromosome Diversity Through
Crossing Over
• Recombination enzymes bind to the joined
chromosomes.
• The enzymes nick the DNA strands, joining opposite
strands together, creating a chiasma, the site of a
cross-over event.
• New gametes produced during meiosis now possess
a mixture of genes (alleles) from both parents of the
organism.
More in Chapter 12
Introducing More Genetic Variability
Introducing More Genetic Variability
• Genetic variability can also come about
in the mixing of chromosomes.
• Genetic variability can also come about
in the mixing of chromosomes.
• Example: Mosquito, 3 chromosomes
• Example: Mosquito, 3 chromosomes
Maternal Chromosomes
Maternal Chromosomes
Paternal Chromosomes
Paternal Chromosomes
During meiosis, chromosomes are
randomly mixed and separated into
newly formed cells.
Introducing More Genetic Variability
• Genetic variability can also come about
in the mixing of chromosomes.
• Example: Mosquito, 3 chromosomes
There are 2n
possible sets of
chromosomes.
For the mosquito,
n=3, so there are
23=8 chromosome
combinations.
Meiosis I
Possible sets of chromosomes after meiosis.
Eukaryotic Life Cycles
Eukaryotic Cell Cycle
All eukaryotic cells
go through phases
of interphase (cell
growth and DNA
replication) and
cell division
(mitosis and
meiosis).
When do organisms
undergo meiosis
instead of mitosis?
Life Cycle of a Eukaryote
Humans, and most animals, only undergo meiosis for
sexual reproduction: Diploid lifestyle.
Life Cycle of a Eukaryote
Plants spend half of their time as a haploid, half as a
diploid: Alternation of Generations.
Homework
Chapter 9:
Mitosis and Meiosis are essentially identical except
for two stages of cell division.
a. What are those two stages?
b. What happens to the chromosomes in these
two stages (from part a) during the three types of
cell division: Mitosis, Meiosis I, and Meiosis II?