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Cell Cycles & Life Cycles
Cell Cycles, Sex, & Ploidy!
1. 
DNA is the molecule of inheritance.
2. 
A chromosome is one long dsDNA.
  In eukaryotes, the dsDNA molecule is wrapped
with histones & other proteins to form chromatin.
3. 
A gene (a discrete unit of heredity) is a
specific region of DNA on a chromosome.
  Specific gene: instructions for a specific protein
4. 
Each chromosome = 100s–1000s of genes.
 
Growth & Development
–  New cells produced
–  Need the right number of cells
in the right location
•  Either too few or too many is bad.
 
Cell Replacement
–  Lost or damaged cells
replaced
 
 
I. 
II. 
Sexual Reproduction
offspring from union of
egg and sperm (combine
Sea star regenerating
Prokaryotes Divide Asexually!
Cells divide to reproduce!
Asexual Reproduction
offspring from single
parent (daughter cells have
identical DNA as parent cell)!
Asexual Reproduction
–  New organism formed!
 
 
Budding Hydra
Binary Fission “to split in
half”!
A Chromosome (1 DNA
molecule) is duplicated,
separates, and then cell
divides!
some DNA from both parent
cells)!
Sperm cells and egg cell
Heyer
Binary fission of a bacterium
1
Cell Cycles & Life Cycles
Eukaryotic Cells Have Multiple
Chromosomes!
Eukaryotic cells have 5–20x
more DNA per cell than do
bacteria.!
  Divided into linear dsDNA +
histone & other proteins :
chromosomes"
  If straight, typical chromosome
Eukaryotic Cells Have Multiple
Chromosomes!
 
 
would be ~ 4 cm long.!
 
Humans have 46 different
chromosomes!
–  So collectively amounts to ~ 2 m
long dsDNA packed into each cell
nucleus!
(4 m after replication!)!
Chromosomes Must Duplicate Before
Division!
[Semiconservative
Replication]
In division, each new cell needs the correct number and kind of chromosomes !
– Replicate and divide chromosomes in
nucleus!
– Distribution of organelles!
The Cell Cycle!
 
 
Ordered sequence of
events in a cellʼs life"
Interphase (G1/S/G2):
90% of cell cycle!
–  Most cellular activity!
 
Human duplicated chromosome
Chromosome duplication
and separation
The Cell Cycle!
Interphase:!
1.  G1: Growth stage, part 1!
  Cell is metabolically active
and grows larger!
2. 
Mitotic phase (M)!
–  Mitosis = nucleus
divides!
–  Cytokinesis =
cytoplasm & plasma
membrane divides!
All cell reproduction requires
DNA duplication!
 
Each cell must have its own copy of
genetic material!
– DNA replication in S Stage of
Interphase!
 
Then DNA must be separated
(segregation) so that each cell has a
complete copy!
– Mitosis of Mitotic Phase!
S: Synthesis stage!
  DNA is replicated!
3. 
G2: Growth stage, part 2!
  More metabolism &
growth"
The eukaryotic cell cycle
Heyer
The eukaryotic cell cycle
2
Cell Cycles & Life Cycles
Duplicated chromosomes!
Early mitotic phase:
One chromatid
chromatin
condenses into distinct
chromosomes!
Sister chromatid
Centromere!
Duplicated chromosome: two sister
chromatids!
  Sister chromatids contain identical DNA!
  Sister chromatids are held together by
the centromere!
 
chromatin
condenses into distinct
chromosomes!
Mitosis!
Sister chromatids
separate, move to
opposite poles!
  Sister chromatids
are now
“unduplicated
chromosomes”!
  Cell elongates!
 
Chromatin!
Chromosomes!
Separation of sister chromatids!
Late Mitosis!
New nuclear
envelopes form !
  Chromosomes de-condense !
  Cytokinesis begins!
 
Heyer
3
Cell Cycles & Life Cycles
Mitosis & Cytokinesis in animal cells!
Cytokinesis in plant cells!
Mitosis without cytokinesis
⇒ Coenocytic (“shared cell”)!
100–1000
Nuclei per cell!
(fluorescently labeled)
Plasmodial slime mold Red alga, Halydictyon  
Giant cell with
multiple nuclei"
4 cm In contrast, multinuclear cells may
also form by fusion of cells
⇒ Syncytial (“together cell”)!
Cells Arise From Preexisting Cells!
I. 
a.  Mitosis: production of two identical nuclei!
b.  Cytokinesis: physical division of the cell into two"
II. 
 
Heyer
Multinuclear muscle fiber forms by
fusion of many embryonic myoblast cells"
Asexual (Mitotic) Reproduction!
Sexual (Meiotic) Reproduction!
a.  Meiosis: production of four non-identical nuclei!
b.  Cytokinesis: physical divisions of the cell !
c.  Fertilization: fusion of two
sex cells!
d.  Syngamy: fusion of two nuclei!
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Cell Cycles & Life Cycles
Ploidy: how many copies of
the same chromosome?!
Duplicated chromosomes
 
 
 
 
Haploid: one version of
each chromosome (n)
haploid
Diploid: two versions of
each chromosome (2n)
diploid
Polyploid: many versions
of each chromosome
tetraploid
Aneuploid: abnormal
number of chromosomes
aneuploid
Diploid cells have homologous pairs of chromosomes: 1 set from mom, 1 set from dad.!
Homologous chromosomes (homologs):
same set of genes, but maybe different
versions of those genes (alleles)
Cellular Division Vocabulary!
Levels of Doubles —!
 
Each chromatid contains
double-stranded DNA!
–  Two complementary
strands!
 
Each duplicated
chromosome contains
sister chromatids!
–  Two identical sisters!
 
In diploid organisms,
chromosomes are in pairs!
–  Two homologous
chromosomes!
Chromosomes Matched in
Homologous Pairs!
Meiosis: Reductive Division of
Diploid Cells
— Reduces Chromosome Number in Half!
Each human cell
 
 23 pairs
= 46 chromosomes
 Homologs: same size,
shape, band pattern,
position of centromere,
and gene loci !
 
Meiosis has 2 consecutive divisions"
– Meiosis I: Homologs separate from
homologous pairs (diploid→haploid)!
– Meiosis II: Sister chromatids separate
(duplicated → unduplicated chromosomes)!
Human karyotype
Heyer
5
Cell Cycles & Life Cycles
Meiosis II!
Meiosis I!
 
 
 
In both cells from first
meiotic division: !
  Sister chromatids
separate!
  Daughter
chromosomes now
unduplicated!
  Total of 4 haploid
cells produced!
Homologous pairs
separate!
2 new daughter
cells!
Both are now
haploid, but with
duplicated
chromosomes!
Mitosis & Meiosis Compared!
 
Spermatogenesis:
one primary spermatocyte →
but
Oogenesis:
one primary oocyte →
Meiosis
doesn’t
always
create 4
cells.
Mitosis separates chromatids in a single division event
–  If diploid cell→ then diploid daughter cells
–  If haploid cell→ then haploid daughter cells
 
Meiosis I separates homologs, then
Meiosis II separates chromatids
–  Diploid cell→ Haploid daughter cells
One microsporocyte →
but One megasporocyte →
→ 4 sperm
→ 1 ovum + 2 polar bodies
Sex = Meiosis + Syngamy!
Meiosis doesn’t always
create 4 cells.
Remember —
in plants,
meiosis
produces
spores,
not gametes!
Meiosis:
2n  1n
→ 4 microspores
Heyer
→ 1 megaspore + 2 polar bodies
Diploid  haploid
Syngamy:
1n  2n
Haploid  diploid
6
Cell Cycles & Life Cycles
Sexual Reproduction
Produces Genetic Variation!
I.
Independent Assortment!
Variation arises from!
 
I. 
Independent chromosome assortment
in meiosis!
II.  Crossing-over between homologous
chromosomes in meiosis!
III.  Random process of fertilization!
Homologs line up by chance in Meiosis I!
Results in many different possible chromosome
combinations in gametes !
2n possible combinations (223 = >8 mill. for humans)!
 
 
 
III. Random
 
 
 
 
 
Fertilization!
Fertilization!
Fertilization is a random process!
A gamete (sex cell) from one individual unites
with one from another individual!
Given that a man can produce 223 genetically
different sperm, and a woman can produce
223 genetically different ova:!
One mating couple can produce a diploid
zygote with any of >70 trillion combinations
of chromosomes! (223 x 223)!
(Not even counting variation from crossingover!)!
Sexual Life Cycles
 
Two haploid sex cells (gametes) → one diploid cell (zygote)!
1.  Plasmogamy: fusion of gamete cell plasma membranes!
• 
• 
→ Syncytial cell (cell with >1 nuclei resulting from fusion of cells)!
→ Heterokaryotic: non-identical nuclei!
2.  Syngamy or Karyogamy: fusion of 2 haploid nuclei !
• 
 
→ 1 diploid nucleus !
Isogametes: both gametes contribute significant
cytoplasm to the zygote.!
  Mating types: + or –!
 
Heterogametes: one gamete (ova) contributes
most/all of zygoteʼs cytoplasm. The other gamete (sperm) only provides the
second haploid nucleus.!
  Mating types: female or male"
Kelp!
Sporangia
10 cm
Sporophyte
(2n)
Life Cycle of a
Phaeophyte
(Laminaria)
Mature female
gemetophyte
(n)
Diploid life history
(animals)
Heyer
Alternation of Generations
(plants)
Haploid life history
(fungi)
Key
Haploid (n)
Diploid (2n)
MEIOSIS
Zoospore
Female
Developing
sporophyte
Zygote
(2n)
Gametophytes
(n)
Male
Egg
FERTILIZATION
Sperm
Fig. 28.16
7
Cell Cycles & Life Cycles
Gametophyte/Sporophyte relationships in Terrestrial Plant Groups
PLANT GROUP
Mosses and other
nonvascular plants
Ferns and other seedless
vascular plants
Seed plants (gymnosperms and angiosperms)
Reduced, independent
(photosynthetic and
free-living)
Reduced (usually microscopic), dependent on surrounding
sporophyte tissue for nutrition
Gametophyte Dominant
Sporophyte
Reduced, dependent on
Dominant
gametophyte for nutrition
Gymnosperm Ovule!
Dominant
Gymnosperm
Sporophyte
(2n)
Integument
Angiosperm
Microscopic female
gametophytes (n) inside
ovulate cone
Microscopic
female
gametophytes
(n) inside
these parts
of flowers
Sporophyte
(2n)
Gametophyte
(n)
Example
Egg nucleus (n)
Immature
female cone
Male gametophyte
(within a germinated
pollen grain) (n)
Megasporangium
(2n)
Microscopic
male
gametophytes
(n) inside
these parts
of flowers
Sporophyte (2n)
(a) Unfertilized ovule
Food supply
(female
gametophyte
tissue) (n)
Discharged
sperm nucleus (n)
Micropyle
Megaspore (n)
Microscopic male
gametophytes (n)
inside pollen
cone
Seed coat
(derived from
integument)
Female
gametophyte (n)
Spore wall
Embryo (2n)
(new sporophyte)
Pollen grain (n)
(b) Fertilized ovule
(c) Gymnosperm seed
Sporophyte (2n)
Gametophyte
(n)
Fig. 30.2
Key
Haploid (n)
Diploid (2n)
Fig. 30.3
Key
Haploid (n)
Diploid (2n)
Ovule
Ovulate
cone
Pollen
cone
Integument
Megasporangium
(2n)
Pollen
Pollen grain
grains (n) MEIOSIS
MEIOSIS
Ovary
Germinating
seed
Microsporangium (2n)
Surviving
megaspore (n)
Archegonium
Conifer
(Gymnosperm)
Fig. 30.6
Embryo
(2n)
Female
gametophyte
Seeds
Food
reserves
(n)
Seed coat
(2n)
Anthophyte
!
(Angiosperm)
Nucleus of
developing
endosperm
(3n)
Pollen
tube
FERTILIZATION
Fig. 30.10
Unicellular Chlorophyte
 
Plasmodial Slime Mold
 
Cellular Slime Mold
 
Apicomplexan parasite
 
Ciliate"
Microspore
(n)
Generative cell
Tube cell
Male gametophyte
(in pollen grain)
Pollen
(n)
grains
Stigma
Pollen
tube
Megaspore
(n)
Antipodal cells
Female gametophyte
Central cell
(embryo sac)
Synergids
Egg (n)
Sperm
Style
Pollen
tube
Sperm
(n)
FERTILIZATION
Zygote (2n)
Egg nucleus (n)
Egg
nucleus (n)
Discharged sperm nuclei (n)
Life Cycle of a Unicellular
Chlorophyte (Chlamydomonas)
Sexual Variants in the Protista!
 
Embryo (2n)
Endosperm (3n)
Seed
Seed coat (2n)
Life Cycle of an
Sperm
nucleus (n)
MEIOSIS
Megasporangium
(2n)
Microsporangia
Seedling
Life Cycle of a
MEIOSIS
Ovule (2n)
Microsporocytes
(2n)
Mature
sporophyte
(2n)
Microsporangium
Microsporocytes (2n)
Anther
Mature flower on
sporophyte plant
(2n)
Megasporocyte (2n)
Flagella
–
1 µm
Cell wall
+ Gamete
+
(n)
Nucleus
Zoospore
ASEXUAL
REPRODUCTION
Cross
section of
cup-shaped
chloroplast
Key
Mature cell
(n)
–
FERTILIZATION
SEXUAL
REPRODUCTION
Zygote
(2n)
MEIOSIS
Haploid (n)
Diploid (2n)
Fig. 28.22
Heyer
8
Cell Cycles & Life Cycles
Life Cycle of a
4 cm
Plasmodial Slime Mold
FERTILIZATION
FERTILIZATION
Emerging
amoeba
(n)
Feeding
plasmodium
Zygote
(2n)
Fruiting
bodies
(n)
MEIOSIS
Amoebas
(n)
ASEXUAL
REPRODUCTION
Aggregated
amoebas
Mature
sporangium
Germinating
spore
Zygote
(2n)
SEXUAL
REPRODUCTION
Solitary
amoebas
(feeding stage)
(n)
600 µm
Young
sporangium
Amoeboid cells
(n)
Slime Mold
Spores
(n)
Mature
plasmodium
(preparing to fruit)
Flagellated
cells
(n)
Life Cycle of a Cellular
Spores
(n)
Migrating
aggregate
MEIOSIS
1 mm
Stalk
Key
Haploid (n)
Diploid (2n)
Fig. 28.24
Inside mosquito
Inside human
Merozoite
Sporozoites
(n)
Merozoite
Sporozoites
(n)
Liver
Liver
Liver cell
Liver cell
Oocyst
MEIOSIS
Oocyst
Apex
Red blood
cell
Merozoite
(n)
Zygote
(2n)
MEIOSIS
0.5 µm
Apicomplexan
parasite
FERTILIZATION
(Plasmodium [malaria])
0.5 µm
Asexual repro by
schizogony:
- Mutiple rounds of mitosis
→ coenocytic schizocyst.
- Then budding/segmentation
to form progeny
Gametes
Key
Gametocytes
(n)
Gametocytes
(n)
Haploid (n)
Diploid (2n)
Fig. 28.10
Life Cycle of a
Contractile
vacuole
Fig. 28.10
Key
Haploid (n)
Diploid (2n)
Fig. 28.11
Ciliate
50 µm
Red blood
cells
FERTILIZATION
Gametes
(Paramecium)
Apex
http://www.tulane.edu/~wiser/protozoology/notes/api.html Red blood
cell
Merozoite
(n)
Zygote
(2n)
Life Cycle of a
Red blood
cells
Haploid (n)
Diploid (2n)
200
µm
Inside mosquito
Inside human
Key
Amoeboid “slug”
Fig. 28.25
Oral groove
Cell mouth
Cilia
Micronucleus
Food vacuoles
Macronucleus
(a) Feeding, waste removal, and water balance
Another twist on coenocy4c mitosis MEIOSIS
Compatible
mates
Diploid
micronucleus
Haploid
micronucleus
The original
Diploid
macronucleus micronucleus
disintegrates.
MICRONUCLEAR
FUSION
Key
(b) Conjugation and reproduction
ConjugationR
eproduction
Heyer
9