Cell Reproduction:
Mitosis & Meiosis
Chapter 8
(and the beginning of Chapter 10)
Overview
• DNA replication
• Overview of cell division
• Mitosis
• Meiosis
DNA Replication
Occurs during interphase of
cell cycle
1 DNA molecule untwisted
Each parent strand serves
as template for new strand
= 2 new DNA molecules,
each ½ old & ½ new
= semi-conservative
replication
Enzymes break H bonds
between 2 strands
= unwinds & exposes
nucleotide bases
Free nucleotides pair with
exposed bases
Each parent strand has new
one made on it
= twist together to form double
helix
DNA replication in a little more
detail …
Sugar-phosphate backbones of 2
DNA strands run in opposite
directions
5’ end
= Phosphate group on sugar’s C
3’ end
= –OH group on sugar’s C
DNA polymerase adds nucleotides to 3’
ends only
Daughter strand grows in 5’ to 3’ direction
= 1 daughter strand
synthesized continuously
= Other daughter strand
synthesized disjointedly
Replication Enzymes:
Helicases
Catalyze breaking of H bonds so double
helix can unwind
Work with small proteins to prevent
rewinding of parent strands
Replication Enzymes:
DNA Polymerases
Catalyze addition of free nucleotides to
exposed bases on each strand
Also have proofreading abilities
Replication Enzymes:
DNA Ligases
Work on discontinuously-assembled strand
Seal together short stretches of new
nucleotides
Transcription vs. DNA replication
Transcription
DNA replication
Only part of DNA
strand unwound
Whole DNA
molecule unwound
RNA polymerase
adds nucleotides to
growing strand
DNA polymerase
adds nucleotides to
growing strand
Results in 1 free
mRNA strand
Results in 2
double-helix DNA
molecules
Mistakes occur that can be lethal if not caught
e.g. wrong base-pairing
DNA proofreading mechanisms fix most
replication errors & breaks in strands
(proofread & correct mismatches)
Repair enzymes repair some changes by
snipping out damaged sites or mismatches
If mismatch can’t be fixed, replication is stopped
Cell Division: An Overview
Parents reproduce to produce new
generation of cells or multicellular
organism
Offspring inherits all information & metabolic
machinery from parent
Prokaryotic Cell Division
Prokaryotic cells reproduce
asexually
= binary fission
Eukaryotic Cell Division
DNA in eukaryotic cells is in nucleus
Eukaryotic cells can’t divide by fission
Must copy & package DNA into > 1
nucleus before cytoplasm can split
Two Types of Cell Division
Mitosis:
– Produces 2 genetically identical cells
– Happens throughout body
Meiosis:
– Produces 4 genetically different cells
– Cells only have ½ of genetic info
– Happens only in gonads
Mitosis
One part of the cell cycle
Growth, cell replacement, tissue repair
Also used for asexual reproduction
= organisms clone selves
Unique to eukaryotes
The Cell Cycle
The period from one cell division to next
Interphase: The Longest Phase
90% of cell cycle length
Interphase
G1: Gap / Growth Phase
Cell growth
# of cytoplasmic
components doubled
S: Synthesis Phase
DNA duplicated
Chromosome & copy = sister chromatids
Joined at centromere
G2: Gap or Growth Phase II
Makes proteins
necessary for cell
division
Cell prepares to divide
Cells stay in G1 if making
macromolecules
Enter S when DNA & accessory proteins
are copied
Rate of DNA replication is same for all cells
of a species
Same cycle length for same type of cells
Different cycle lengths for different types of
cells
e.g. cells in red bone marrow divide every
second
e.g. nerve cells stay in G1 indefinitely
Rate of cell division is under control
(checkpoints, molecular brakes, etc.)
After G2, cell enters mitosis
Mitosis maintains cell’s chromosome #
Chromosome Number
Humans have 46
chromosomes
= diploid (2n)
2 of each type of
chromosome
= one set from mother, one
from father
During mitosis:
Each 2n parent cell produces two 2n
daughter cells
Each daughter cell has each pair of
chromosomes
= 23 pairs
During mitosis, 2 sister chromatids (duplicated
chromosomes) separate
Each becomes independent chromosome that
ends up in 1 of daughter cells
The Mitotic Spindle
Present in every cell
Made of microtubules
= change length by
addition or removal of
tubulin subunits
Originates from pair of
centrioles
Early in cell division, duplicated
chromosome is condensed =
coils up
DNA winds twice around histones
= nucleosome
Keeps chromosomes organized
during nuclear division
Late Interphase / Pre-Prophase
Outside of nucleus, 2 centrioles
duplicate selves
Early Prophase
Inside nucleus:
Chromosomes begin to
condense
Outside nucleus:
Spindle begins to form
Nuclear envelope begins to
fall apart
Late Prophase
Nuclear envelope completely
falls apart
Spindle fibres from each pole
attach to sister chromatids of
each chromosome
Metaphase
Chromosomes line up halfway
between spindle poles
Anaphase
Sister chromatids of each
chromosome separate & move
to opposite poles
(motor proteins attached to
kinetochores drag chromatids
along microtubules)
Spindle poles pushed apart by
growing microtubules
Telophase
1 of each type of chromosome
reaches each spindle pole
= 2 identical groups of
chromosomes at each cell pole
Chromosomes decondense
Nuclear envelope forms around
each cluster of chromosomes
= two nuclei, each with 2n # of
chromosomes
Cytokinesis
Cytoplasm of cell divides
Results in 2 daughter cells, each
with same number of
chromosomes as parent cell
Cytokinesis in Animal Cells
Contractile ring mechanism
Halfway between cell’s poles,
plasma membrane constricts
= cleavage furrow
(ATP energy causes contraction
of actin filaments)
Cleavage furrow deepens until
cytoplasm split into 2
Cytokinesis in Plant Cells
Cell plate formation
Golgi vesicles move to cell
equator & fuse
Vesicle membranes
become cell membranes
Contents become cellulose
cell wall
Summary of Mitosis
Nuclear & cellular division that maintains
chromosome #
Used for growth, repair, asexual
reproduction
Cell division & DNA replication regulated so
that:
DNA only replicated once before cell division
Cells that never divide do not replicate DNA
Cells don’t try to replicate DNA if lack the
energy & raw materials to complete process
Cellular Controls over Mitosis
Anchorage dependence
Animal cells must be in contact with a solid
surface to divide
Density-dependent inhibition
Crowded cells stop dividing
Growth factors
Required to start & continue dividing
Secreted by other cells
Cell Cycle Checkpoints
Cell cycle has checkpoints:
– Structure of chromosomal DNA monitored
– Completion of phases monitored
– Determines if good time for cell division
Rely on internal & external cues
G1 checkpoint is most important:
If no go-ahead signal, cell will switch to
non-dividing G0 phase
e.g. nerve & muscle cells remain in G0
indefinitely
Cancer & Cell Division
If immune system doesn’t recognize &
destroy a cancerous cell, it may divide
multiple times & form a tumor
Benign
Cells remain localized
Malignant
Spreads to other parts of body & disrupts
function
Why don’t cancer cells follow the
rules?
Don’t exhibit density-dependence
Have defective control systems
Ignore / over-ride checkpoints
Some synthesize own growth factors so
continue dividing
Divide indefinitely
Types of Cancers
Carcinomas
Internal & external coverings of body e.g.
skin
Sarcomas
Supportive tissues e.g. bone & muscle
Leukemias & Lymphomas
Blood-forming tissues e.g. bone marrow,
spleen, lymph nodes
Ways to Treat Cancer
If not severe:
Surgical removal of tumor
Radiation therapy
(damages DNA of cancer cells to greater
degree than normal cells)
If severe:
Chemotherapy
Uses drugs to disrupt cell division
e.g. Paclitaxel freezes the mitotic spindle at
metaphase
e.g. Vinblastin prevents spindle formation
Also affects rapidly-dividing normal cells e.g.
intestinal lining, immune cells, hair follicle cells
Cloning
Donor cells from 1
animal starved so
enter non-dividing
G0 phase
Nucleus removed
from unfertilized
egg cell of another
animal
Donor cell & egg cell placed next to each other
in culture dish & electrically stimulated
Cells fuse & enter mitosis
Cell continues mitotic divisions & forms embryo
Embryo implanted into surrogate mother
(same spp. as egg cell)
Surrogate mother gives birth to genetic twin of
“donor cell” animal
Mitosis vs. Meiosis
Mitosis:
– Occurs in somatic cells
– Results in 2 genetically identical cells
– Growth, cell replacement, tissue repair
= asexual reproduction
Meiosis:
– Occurs in sex cells
– Results in 4 genetically different cells with ½
genetic info of parent cell
= sexual reproduction
Asexual vs. Sexual Reproduction
Asexual reproduction:
Individual makes multiple offspring with identical
DNA
Sexual reproduction:
Allows for variety in heritable traits
Adaptive in changing environments
Meiosis → formation of gametes → fertilization
The Eukaryotic Chromosome
Double-stranded DNA & associated proteins
Chromosomes duplicated during interphase
Unduplicated
Duplicated
Centromere
Sister chromatids
Chromosome Number
Almost every cell in body has 2 complete
sets of chromosomes
One set from mother, one from father
2 sets = diploid (2n)
Each cell has 2 versions of each gene
Homologous chromosomes
Pair of chromosomes that carry genes for same
heritable traits
Except sex chromosomes (X or Y)
Genes
Sequences of chromosomal DNA
Contain heritable information to make
new individuals
Individuals have pairs of genes on pairs
of chromosomes
Each member of pair of gene = allele
Allele
One of the variant forms of a gene at a particular
(locus) location on a chromosome
Different alleles produce variation in inherited
characteristics (e.g. hair & eye colour, etc.)
Basis for evolution: endless combinations of alleles
lead to variations in traits
So What is Meiosis?
Nuclear division that halves chromosome #
Occurs only in sex (reproductive) cells
1st step in formation of gametes (
or
)
Gametes fuse with opposite sex gametes to
form new individual
Humans are diploid (2n) with 46
chromosomes
(23 + 23 homologous chromosomes)
Meiosis halves chromosome number so
daughter cells (gametes) are haploid (n)
with 23 chromosomes
Gametes
Have only 1 set of chromosomes
= haploid (n)
Each gamete has 1 allele for each gene
In humans = eggs or sperm
During meiosis, one cell goes through 2
divisions to end with formation of 4 cells,
all with haploid (n) nuclei
Interphase
Same as in mitosis:
Cell grows & duplicates cytoplasmic
components
DNA is replicated
Prophase I
Chromosomes condense
Crossing-over occurs between
homologous chromosomes
Centrioles move to opposite
sides of nuclear envelope
Nuclear envelope begins to fall
apart
Crossing Over
When chromosomes condense during
prophase, homologous chromosomes
stick very closely together & form a tetrad
Maternal & paternal chromosomes swap genes
= exchange segments of genetic info
Homologous chromosomes become mixture of
maternal & paternal info
chiasma
Metaphase I
Homologues of chromosomes
tethered by microtubules at
opposite spindle poles
Chromosomes line up along
equator of cell
Anaphase I
Chromosomes pulled apart &
move towards respective poles
Poles move further apart
Telophase I
Cytoplasm divides
Results in 2 haploid cells
(only have 1 of each pair of
homologous chromosomes)
Chromosomes still duplicated
Prophase II
New mitotic spindle forms in each cell
Chromatids of each chromosome
become tethered to opposite poles
Metaphase II
Chromosomes line up along
equator of cell
Anaphase II
Chromatids separate & move
towards opposite poles
Spindle poles pushed apart
Telophase II
Nuclear envelope forms around each
chromosome cluster
Cytokinesis
Cytoplasm divides
Results in 4 haploid (n) daughter cells
Chromosomes are unduplicated
Meiosis—things to pay attention to:
1. DNA replication:
a. Occurs only during interphase before Meiosis I
2. Meiosis I
a. Prophase: crossing-over
b. Metaphase: line up in 2 rows
c. Anaphase: separation of homologous chromosomes
3. Meiosis II
a. Similar to mitosis but no interphase precedes it
b. Division results in haploid cells
Meiosis & Trait Variation
Can occur via:
– Crossing over
– Random alignment of chromosomes at
metaphase I
a. Crossing Over
Exchanges of allele-containing segments occurs
between non-sister chromatids (i.e. between
maternal & paternal chromosomes)
Gene-swapping: different versions of heritable
information are swapped
= leads to recombination of genes & variation in traits
b. Metaphase I Alignments
a.k.a random assortment
Duplicated chromosomes randomly tether
to spindle poles
i.e. no set rules for where maternal &
paternal chromosomes should be
positioned
Which half of homologous chromosome pair
ends up at which pole is totally random
223 (8,388,608) possible combos of maternal &
paternal chromosomes!
From Gametes to Offspring
In animals, diploid germ cells become
gametes
Gametes differ from species to species
Male Gamete Formation
Germ cell (spermatogonium) develops into
1° spermatocyte
Enters meiosis
Results in 4 haploid cells (spermatids) that
differentiate into sperm cells
Female Gamete Formation
Germ cell (oogonium) develops into 1°
oocyte (immature egg)
Grows in size
4 daughter cells differ in structure &
function
When 1° oocyte divides after meiosis I, one
daughter cell (2° oocyte) gets most of
cytoplasm
Other cell (1st polar body) is very small
After meiosis II, one of 2° oocyte’s daughter cells
is 2nd polar body (also very small)
Other gets most of cytoplasm and develops into
ovum (egg)
1st polar body’s daughter cells are both polar
bodies
Polar bodies eventually degenerate
Sole function: to ensure ovum is haploid
Ovum gets most of cytoplasm & metabolic
machinery
Is able to support early cell divisions of new
individual after fertilization
Fertilization: When 2 Gametes
Become 1
Male & female gametes unite
Haploid nuclei fuse
Restores diploid nature of cells
(n + n = 2n)
↑ variation among offspring:
– Random gametes fusing
– Millions of possible chromosome combos in
each gamete
Summary of Meiosis
Nuclear division that halves chromosome
number
Results in n male & female gametes that
can fuse during fertilization to produce 2n
offspring
Chromosomal Abnormalities
Abnormal chromosome structure:
Breakage of chromosome leads to
rearrangements that affect genes on that
chromosome
Abnormal chromosome number:
Chance events occur before or after cell
division that result in wrong chromosome #
Changes in Chromosome Structure
Can have neutral to harmful effects,
depending on type of chromosomal
change
4 types of rearrangement:
• Inversion
• Deletion
• Duplication
• Translocation
(a) Inversions
Broken fragment reattaches to original
chromosome but in reverse direction
Genes still present in normal #, so less
harmful than other categories
(b) Deletions
Fragment of chromosome is lost
Cause severe physical & mental problems
e.g. cri du chat
(c) Duplications
Fragment from one chromosome joins to a
sister chromatid or homologous
chromosome
Can have severe effects
(d) Translocations
Fragment of chromosome attaches to
non-homologous chromosome
May or may not be harmful
If chromosomal changes occur in sperm
or egg cells:
= may cause congenital disorders
If chromosomal changes occur in somatic
cells:
= can lead to development of cancer
(which is why cancer is generally not
heritable)
Heritable Changes in
Chromosome #
Chance events occur before or after cell
division that result in wrong chromosome
#
Consequences can be minor or lethal
Most changes in chromosome number
occur because of non-disjunction
= 1 pair of chromosomes do not separate
during mitosis or meiosis
Aneuploidy:
Normal #  1 chromosome
e.g. trisomy 21 (Down Syndrome)
Polyploidy:
3n, 4n, etc.
Normal in many plants & animals
# of sex chromosomes can also be
abnormal
E.g. XO, XXX, XXY, XYY
Will return to this when covering inheritance