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Ch. 8 Cell Reproduction
Why do cells make new cells?
1. Growth
2. Maintenance – cells wear out
3. Repair - injury
4. Regeneration – lost body part
5. Reproduce asexually
8.1 Some organisms reproduce asexually
• Their offspring are genetic copies of the
parent and of each other
Figure 8.1A
– All organisms can reproduce sexually
• Creating variety in the offspring
Figure 8.1B
8.3 Prokaryotes
reproduce by binary
fission
• One chromosome
• Chromosome copies itself
• Cell lengthens,
copies move apart
• Cell membrane
divides cell in two
Eukaryotic nuclei contain many
chromosomes
Each chromosome contains thousands of genes
- must be organized before cell can dividing
Prokaryotic chromosomes
LM 340
Colorized
TEM
32,500
One-celled protists also do fission, but
must do mitosis first to divide
chromosomes evenly
8.5 The cell cycle - two phases
Interphase
– most of cell life
- non-dividing cell
INTERPHASE
G1
S
(DNA synthesis)
G2
Mitotic (M) phase – dividing cell
Figure 8.5
Cell Cycle
INTERPHASE
G1  growth, normal life functions
S  “synthesis” – DNA replicates
G2  final growth; prepares to divide
MITOTIC PHASE
Mitosis – chromosomes condense, organize and divide
- each new cell gets one copy of every chromosome
Cytokinesis – cytoplasm divides
Some cells divide often
embryo
Bone marrow stem cells
Plants and simple organisms
Skin cells
Digestive lining cells
Some divide rarely or not at all
Nerve cells
Muscle cells
Cartilage or tendon cells
BEFORE a cell divides
DNA condensation
•DNA is ALREADY COPIED (replicated)
•coils, packs, condenses  Forms dense “CHROMOSOMES”
•keeps copies organized and intact until cell splits
CHROMOSOMES CONDENSE
BEFORE CELL DIVISION
DNA
DNA wraps
around
histones
These group to
form chromatin
Groups of
histones form
nucleosomes
This coils and
wraps until it all fits
into the nucleus.
One chromosome
(copied and packed
for cell division)
SEM of human
chromosomes
Sister chromatids
identical copies
Centromere
holds chromatids
together
Prokaryotes have a single, circular
chromosome, no histones; no nucleus
Double-stranded
(replicated)
chromosome
Eukaryotic chromosomes
Nucleus in
non-dividing cell
Early mitosis
Chromosome is in
CHROMATIN form
CHROMATIN begins
to condense before a
cell can divide
Ch.10 Chromosomes of eukaryotes
duplicate in cell division
Before cell divides - DNA replicates
a. Helicase “unzips” molecule
b. Original strands are templates
c. Free nucleotides in nucleus
d. Polymerase – base-pairing rules
 two identical molecules
Semi-conservative replication
• Original strands serve as templates
• New molecules have one original strand
and one new strand
PHASES OF MITOSIS
PROPHASE – cell organizes, prepares
•chromatin
•nuclear membrane, nucleolus
•spindle and asters
•centrioles
PROMETAPHASE – chromosomes condensed
•move toward middle
•centrioles to opposite poles
•spindle
METAPHASE – chromosomes in middle
•Centromeres and spindle fibers
LM 250
– The stages of cell division
INTERPHASE
PROPHASE
Centrosomes
(with centriole pairs)
Chromatin
Early mitotic
spindle
PROMETAPHASE
Centrosome
Fragments
of nuclear
envelope
Kinetochore
Nucleolus
Nuclear
envelope
Figure 8.6 (Part 1)
Chromosome, consisting
Plasma
membrane ot two sister chromatids
Centromere
Spindle
microtubules
ANAPHASE -chromosomes separate
• spindle fibers pull
• single copies to opposite poles of cell
TELOPHASE – return to normal
• chromosomes relax/uncoil into chromatin
• nuclear membranes form; nucleoli appear
• spindle fibers disappear
CYTOKINESIS - division of cytoplasm
•
identical daughter cells
Cleavage furrow
ANAPHASE
METAPHASE
Cleavage
furrow
Metaphase
plate
Spindle
Figure 8.6 (Part 2)
TELOPHASE AND CYTOKINESIS
Daughter
chromosomes
Nuclear
envelope
forming
Nucleolus
forming
Cell division differs for plant and animal cells
8.7 Animal cytokinesis
• Microfilaments
SEM 140
• Cleavage furrow
Cleavage
furrow
• Pinch cytoplasm in two
Cleavage furrow
Contracting
ring of
microfilaments
Daughter cells
Figure 8.7A
Cell plate
forming
Wall of
parent cell
Daughter
nucleus
In plants
- no centrioles or asters
TEM 7,500
(has spindle)
- cell plate
- new cell wall grows
Cell wall
New cell wall
along sides of cell plate
Vesicles containing
cell wall material
Figure 8.7B
Cell plate
Daughter cells
Plant cell mitosis
Control of Cell Division
Chemical signals tell a cell when to divide
Some made by cell; some from cell environment
Normal rate: growth, repair
- asexual reproduction in some organisms
Slow rate: some cell types divide rarely (liver)
- aging  slow healing, lose some cells
Fast rate: some cell types (skin, digestive lining)
- embryo, growing zones (bones, root tips)
Uncontrolled  cancer
Growth factors
• Proteins secreted by cells, can start or stop cell division
• When cells crowded, growth factors used up
 stop dividing
After forming a
single layer,
cells have
stopped dividing.
Providing an
additional supply of
growth factors
stimulates
further cell division.
Figure 8.8B
Cyclins
- proteins produced by cell at constant rate
- when reach high level, tell cell to divide
Programmed Cell Death
apoptosis
1. Cells damaged too much to repair
– self-destruct
2. Embryonic development – lose unneeded cells
8.10 Cancer – uncontrolled cell division
• Cancer cells ignore cycle controls
– Repeated and rapid cell divisions; immortal
• tumor – mass of non-functioning cells
– Benign – not cancer; do not spread to nearby tissue
– May get large enough to stop tissue function
• Malignant tumor – cancer
– Will invade and destroy neighboring tissue
• Metastatic tumor – spreads to other body parts
– Starts new cancers
Cumulative DNA damage
Cells begin to divide abnormally
Continue dividing, invade
healthy tissue
(malignant)
Spread through blood or lymph
stream, can start new tumors
(metastatic)
Cells look abnormal – in structure; size of
nucleus :: cytoplasm
• If not treated early, cancer will spread
• kills by destroying organ function
Lymph
vessels
Tumor
Blood
vessel
Glandular
tissue
A tumor grows from a
single cancer cell.
Figure 8.10
Cancer cells invade
neighboring tissue.
Cancer cells spread through
lymph and blood vessels to
other parts of the body.
BE CAREFUL when sunning
• Wear sunscreen, sunglasses
• Avoid brightest part of the day
• Don’t use tanning beds
The most common cancer - skin
Melanoma -deadliest
Melanoma
Basal cell
Squamous Cell
Cancer treatment
Radiation – high-energy, carefully aimed at tumor
Chemotherapy – drugs specific for tumor types
• interfere with cell division
• some normal cells destroyed, too
– Skin (lose hair); digestive lining (nausea)
Surgery – remove tumor and nearby cells
Boost immune system – healthy diet, reduce stress
Asexual Reproduction
New organism from one parent
– offspring identical genetically
Advantages:
1) no mate – good for solitary, sessile organisms
or when population density is low
2) continues a successful gene makeup
3) usually fast, large numbers
Disadvantage: no genetic variation
Asexual Reproduction
One-celled organisms, some simple multicelled
vegetative propagation in plants
LM
10
Hydra with bud
Yeast, sponges
can also bud
Regeneration – replace lost body part
starfish can replace an arm
Lizards can replace a tail
flatworms can regrow a body
Stem cells and differentiation
Differentiate – cells specialize as embryo develops
Stem cell – unspecialized cell
- can differentiate to form many kinds of cells
- depends on chemical signals from nearby cells
Multipotent or totipotent
Stem cells for cloning tissues
Meiosis for Sexual Reproduction
Sexual Reproduction - two parents
a. Offspring are genetic mix of both parents
b. Have a NEW combination of genes
Advantage – genetic variation in offspring
a. Some may have traits that favor survival
b. Can pass these traits on to offspring
c. Darwin’s theory - “ survival of the fittest”
d. Variation in individuals allows a species to evolve
Sexual Reproduction in bacteria and protists
CONJUGATION
a. Recipient cell gets new genes
b. Bacteria and protists
Complex organisms – make special cells
a. gametes – sperm and egg
b. Gametes combine in fertilization
- make a zygote  new organism
Chromosome Number: Diploid and Haploid
Homologous chromosomes
a. matched chromosome pairs
b. one member of pair from each parent
c. carry genes for the same traits
d. 22 autosome pairs; one pair sex chromosomes X, Y
Gene for one trait
Cells with paired chromosomes are diploid
a. 2n (n = number)
b. Human: 2n = 46 (23 pairs)
b. Somatic (body) cells are diploid
2 sets of chromosomes
- 2 of every gene
Locus – location of
gene on a
chromosome
Fruit fly 2n = 8
Meiosis is “Reduction Division”
gametes – sperm and egg
Haploid (n) - one set of chromosomes
- one member from each homologous pair
human: n = 23
Meiosis Reduces the Chromosome Number
2n parent cell
DNA replicates in interphase
First division – pairs separate
Second division – sister
chromatids separate
 4 haploid daughter cells
Homologous pairs separate in MEIOSIS
TWO cell divisions
Diploid cell - Has pairs
(2n=2)
- Daughter cells have ½ parent
chromosome number
Meiosis I - Pairs separate
(n = 1)
Meiosis II - copies separate
(n = 1)
Haploid cells - (n = 1)
Crossing over – only in meiosis
a. In Prophase I
b. Homologous chromatids trade pieces
c. Increases genetic variation
Meiosis I: homologous pairs separate
- makes two daughter cells, but sister chromatids are still attached
MEIOSIS I: Homologous chromosomes separate
INTERPHASE
Centrosomes
(with centriole
pairs)
Nuclear
envelope
PROPHASE I
METAPHASE I
Sites of crossing over
Spindle
Chromatin
2n parent cell
Sister
chromatids
synapsis
Tetrad
ANAPHASE I
Microtubules
Metaphase
attached to
plate
kinetochore
Centromere
(with kinetochore)
pairs line up
Sister chromatids
remain attached
Homologous
chromosomes separate
pairs separate
disjunction
Meiosis II: sister chromatids separate  4 haploid cells
MEIOSIS II: Sister chromatids separate
TELOPHASE I
AND CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND CYTOKINESIS
Cleavage
furrow
Sister chromatids
separate
2n  n
two daughter cells
one chromosome set each
two copies (sisters)
chromatids
separate
disjunction
Haploid daughter cells
forming
4 daughters
one set
single copies
8.15 Review: Comparing mitosis and meiosis
Mitosis
Meiosis
2n
copies
Parent cell
(before chromosome replication)
Prophase I
Prophase
Duplicated
chromosome
(two sister chromatids)
2n
Meiosis i
Tetrad formed
by synapsis of
homologous
chromosomes
Chromosome
replication
Chromosome
replication
2n = 4
Metaphase
Chromosomes
align at the
metaphase plate
Tetrads
align at the
metaphase plate
Anaphase
Telophase
Sister chromatids
separate during
anaphase
Homologous
chromosomes
separate during
anaphase I;
sister
chromatids
remain together
Metaphase I
2n
copies
2n
copies
copies
2n
single
2n
Daughter cells
of mitosis
2n
No further
chromosomal
replication;
sister
chromatids
separate
during
anaphase II
Anaphase I
Telophase I
Haploid
n=2
Daughter
cells of
meiosis I
n
n
n
Daughter cells of meiosis II
copies
n
single
Meiosis ii
n
n
Causes of genetic variation
1. Homologous pairs have different genes
• same traits, but may be different forms
2. Crossing over – homologs trade pieces before
separating  new gene combinations
3. Pairs position in Metaphase I - independent
• n pairs  2n possible combinations
4. Random fertilization of eggs by sperm
• Any egg or sperm is equally likely to be used
5. Gene or chromosome mutation
- Error in replication or cell division
8.16 Chromosomes line up randomly in meiosis
– Many different gene combinations in haploid gametes
Possibility 1
Possibility 2
Two equally probable
arrangements of
chromosomes at
metaphase I
Metaphase II
Gametes
Combination 1
Combination 2
Combination 3
Combination 4
Figure 8.16
Making sperm and egg
Sperm:
2n parent cell
 4 haploid sperm
Ovum:
2n parent cell
 1 haploid egg
+ haploid polar bodies
Ovum needs all the cytoplasm
Ovum and polar body (0.1mm)
Sperm needs only DNA
- and flagellum
- and mitochondria for power
- and acrosome to penetrate ovum
When meiosis goes wrong
Nondisjunction
- do not separate correctly
In mitosis  defective nucleus, cell usually dies
In meiosis  defective gamete
 wrong number of chromosomes in zygote
8.21 Accidents during meiosis can alter chromosome number
Nondisjunction
in meiosis I
Normal
meiosis I
Normal
meiosis
II
Nondisjunctio
n in meiosis II
Gametes
Gametes
n1
n1
n1
n 1
n 1
n 1
n
Number of chromosomes
Number of chromosomes
Nondisjunction in meiosis I
All gametes abnormal
Nondisjunction in meiosis II
Some gametes normal
n
Wrong chromosome number in zygote
 wrong number in every cell in organism
• Fertilization after nondisjunction  trisomy in zygote
Trisomy = 3
Egg cell
n+1
Zygote
2n + 1
Sperm cell
n (normal)
Abnormal chromosome number = aneuploidy
KARYOTYPE
picture of a person’s chromosomes
Photographed during mitosis
- sorted into homologous pairs
- largest-to-smallest
- sex chromosomes last
Abnormalities visible:
- missing or extra
- pieces broken or moved
- pieces added or lost
autosomes
sex chrom.
Trisomy 21
Normal male
karyotype
Normal female
karyotype
Down Syndrome
Trisomy chromosome # 21
8.22 Abnormal number of sex chromosomes
usually do not affect survival in humans
• Nondisjunction of large chromosomes is usually lethal
• Down Syndrome - # 21 is very small, carries few genes
• In sex chromosomes, leads to varying degrees of
malfunction, but usually not lethal
Turner Syndrome XO
Characteristic facial
features
Web of
skin
Constriction
of aorta
Poor breast
development
Under developed
ovaries
Figure 8.22B
Turner Syndrome
Klinefelter Syndrome XXY
Klinefelter Syndrome
Abnormalities of Sex Chromosomes in Humans
8.23 Other chromosome changes can
cause birth defects and cancer
Chromosomes break – pieces lost or rearranged
- in somatic cells  increases cancer risk
- in gametes  genetic disorders
Deletion
Translocation
Translocation
Duplication
Homologous
chromosomes
Inversion
Activates a cancer-causing gene