Transcript Chapter 8 Notes

Chapter 8
Cellular Reproduction: Cells from Cells
PowerPoint® Lectures for
Campbell Essential Biology, Fifth Edition, and
Campbell Essential Biology with Physiology,
Fourth Edition
– Eric J. Simon, Jean L. Dickey, and Jane B. Reece
Lectures by Edward J. Zalisko
© 2013 Pearson Education, Inc.
Biology and Society:
Virgin Birth of a Dragon
• In 2002, zookeepers at the Chester Zoo were
surprised to discover that their Komodo Dragon
laid eggs.
– The female dragon had not been in the company of
a male.
– The eggs developed without fertilization, in a
process called parthenogenesis.
– DNA analysis confirmed that her offspring had
genes only from her.
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Biology and Society:
Virgin Birth of a Dragon
• A second European Komodo dragon is now known
to have reproduced
– asexually, via parthenogenesis, and
– sexually.
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Figure 8.0
WHAT CELL REPRODUCTION
ACCOMPLISHES
• Reproduction
– may result in the birth of new organisms but
– more commonly involves the production of new
cells.
• When a cell undergoes reproduction, or cell
division, two “daughter” cells are produced that
are genetically identical
– to each other and
– to the “parent” cell.
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WHAT CELL REPRODUCTION
ACCOMPLISHES
• Before a parent cell splits into two, it duplicates its
chromosomes, the structures that contain most
of the cell’s DNA.
• During cell division, each daughter cell receives
one identical set of chromosomes from the lone,
original parent cell.
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WHAT CELL REPRODUCTION
ACCOMPLISHES
• Cell division plays important roles in the lives of
organisms.
• Cell division
– replaces damaged or lost cells,
– permits growth, and
– allows for reproduction.
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Figure 8.1a
FUNCTIONS OF CELL DIVISION
Colorized SEM
Growth via Cell Division
LM
Cell Replacement
Human kidney cell
Early human embryo
WHAT CELL REPRODUCTION
ACCOMPLISHES
• In asexual reproduction,
– single-celled organisms reproduce by simple cell
division and
– there is no fertilization of an egg by a sperm.
• Some multicellular organisms, such as sea stars,
can grow new individuals from fragmented pieces.
• Growing a new plant from a clipping is another
example of asexual reproduction.
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Figure 8.1b
FUNCTIONS OF CELL DIVISION
LM
Asexual Reproduction
Division of an amoeba
Regeneration of a sea star
Growth of a clipping
WHAT CELL REPRODUCTION
ACCOMPLISHES
• In asexual reproduction, the lone parent and its
offspring have identical genes.
• Mitosis is the type of cell division responsible for
– asexual reproduction and
– growth and maintenance of multicellular
organisms.
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WHAT CELL REPRODUCTION
ACCOMPLISHES
• Sexual reproduction requires fertilization of an
egg by a sperm using a special type of cell division
called meiosis.
• Thus, sexually reproducing organisms use
– meiosis for reproduction and
– mitosis for growth and maintenance.
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THE CELL CYCLE AND MITOSIS
• In a eukaryotic cell,
– most genes are located on chromosomes in the
cell nucleus and
– a few genes are found in DNA in mitochondria and
chloroplasts.
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Eukaryotic Chromosomes
• Each eukaryotic chromosome contains one very
long DNA molecule, typically bearing thousands of
genes.
• The number of chromosomes in a eukaryotic cell
depends on the species.
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Figure 8.2
Species
Indian muntjac deer
Koala
Opossum
Giraffe
Mouse
Human
Duck-billed platypus
Buffalo
Dog
Red viscacha
rat
Number of chromosomes in body cells
6
16
22
30
40
46
54
60
78
102
Eukaryotic Chromosomes
• Chromosomes are
– made of chromatin, fibers composed of roughly
equal amounts of DNA and protein molecules and
– not visible in a cell until cell division occurs.
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LM
Figure 8.3
Chromosomes
Eukaryotic Chromosomes
• The DNA in a cell is packed into an elaborate,
multilevel system of coiling and folding.
• Histones are proteins used to package DNA in
eukaryotes.
• Nucleosomes consist of DNA wound around
histone molecules.
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Figure 8.4
DNA double helix
Histones
TEM
“Beads
on a
string”
Nucleosome
Tight helical fiber
Duplicated
chromosomes
(sister
chromatids)
TEM
Thick supercoil
Centromere
Eukaryotic Chromosomes
• Before a cell divides, it duplicates all of its
chromosomes, resulting in two copies called sister
chromatids containing identical genes.
• Two sister chromatids are joined together tightly at
a narrow “waist” called the centromere.
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Eukaryotic Chromosomes
• When the cell divides, the sister chromatids of a
duplicated chromosome separate from each other.
• Once separated, each chromatid is
– considered a full-fledged chromosome and
– identical to the original chromosome.
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Figure 8.5
Chromosome
duplication
Sister
chromatids
Chromosome
distribution to
daughter cells
The Cell Cycle
• A cell cycle is the ordered sequence of events that
extend
– from the time a cell is first formed from a dividing
parent cell
– to its own division into two cells.
• The cell cycle consists of two distinct phases:
1. interphase and
2. the mitotic phase.
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Figure 8.6
S phase
(DNA synthesis; chromosome duplication)
Interphase: metabolism and
growth (90% of time)
G1
G2
Mitotic
(M) phase:
cell division
(10% of time)
Cytokinesis
(division of
cytoplasm)
Mitosis
(division
of nucleus)
The Cell Cycle
• Most of a cell cycle is spent in interphase.
• During interphase, a cell
– performs its normal functions,
– doubles everything in its cytoplasm, and
– grows in size.
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The Cell Cycle
• The mitotic (M) phase includes two overlapping
processes:
1. mitosis, in which the nucleus and its contents
divide evenly into two daughter nuclei and
2. cytokinesis, in which the cytoplasm is divided in
two.
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Mitosis and Cytokinesis
• During mitosis the mitotic spindle, a footballshaped structure of microtubules, guides the
separation of two sets of daughter chromosomes.
• Spindle microtubules grow from structures within
the cytoplasm called centrosomes.
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Figure 8.7a
INTERPHASE
Centrosomes
(with centriole
pairs)
Chromatin
PROPHASE
Early mitotic
spindle
Centrosome
Fragments of
nuclear envelope
Centromere
Nuclear
envelope
Plasma
membrane
Chromosome
(two sister chromatids)
Spindle microtubules
Figure 8.7b
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Nuclear
envelope
forming
Spindle
Daughter
chromosomes
Cleavage
furrow
Figure 8.7b
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Nuclear
envelope
forming
Spindle
Daughter
chromosomes
Cleavage
furrow
Mitosis and Cytokinesis
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
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Mitosis and Cytokinesis
• Cytokinesis usually
– begins during telophase,
– divides the cytoplasm, and
– is different in plant and animal cells.
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Mitosis and Cytokinesis
• In animal cells, cytokinesis
– is known as cleavage and
– begins with the appearance of a cleavage furrow,
an indentation at the equator of the cell.
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Animation: Cytokinesis
Right click slide / select “Play”
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SEM
Figure 8.8a
Cleavage
furrow
Cleavage furrow
Contracting ring of
microfilaments
Daughter cells
Mitosis and Cytokinesis
• In plant cells, cytokinesis begins when vesicles
containing cell wall material collect at the middle of
the cell and then fuse, forming a membranous disk
called the cell plate.
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Figure 8.8b
Daughter nucleus
LM
Wall of parent cell
Cell plate
forming
Cell wall
Vesicles containing
cell wall material
Cell plate
New cell wall
Daughter cells
Cancer Cells: Growing Out of Control
• Normal plant and animal cells have a cell cycle
control system that consists of specialized
proteins, which send “stop” and “go-ahead” signals
at certain key points during the cell cycle.
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What Is Cancer?
• Cancer is a disease of the cell cycle.
• Cancer cells do not respond normally to the cell
cycle control system.
• Cancer cells can form tumors, abnormally growing
masses of body cells.
• If the abnormal cells remain at the original site, the
lump is called a benign tumor.
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What Is Cancer?
• The spread of cancer cells beyond their original
site of origin is metastasis.
• Malignant tumors can
– spread to other parts of the body and
– interrupt normal body functions.
• A person with a malignant tumor is said to have
cancer.
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Figure 8.9
Lymph
vessels
Tumor
Blood
vessel
Glandular
tissue
A tumor grows
from a single
cancer cell.
Cancer cells invade
neighboring tissue.
Metastasis: Cancer
cells spread through
lymph and blood
vessels to other parts
of the body.
Cancer Treatment
• Cancer treatment can involve
– radiation therapy, which damages DNA and
disrupts cell division, and
– chemotherapy, the use of drugs to disrupt cell
division.
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Cancer Prevention and Survival
• Certain behaviors can decrease the risk of cancer:
– not smoking,
– exercising adequately,
– avoiding exposure to the sun,
– eating a high-fiber, low-fat diet,
– performing self-exams, and
– regularly visiting a doctor to identify tumors early.
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MEIOSIS, THE BASIS OF SEXUAL
REPRODUCTION
• Sexual reproduction
– depends on meiosis and fertilization and
– produces offspring that contain a unique
combination of genes from the parents.
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Figure 8.10
Homologous Chromosomes
• Different individuals of a single species have the
same
– number and
– types of chromosomes.
• A human somatic cell
– is a typical body cell and
– has 46 chromosomes.
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Homologous Chromosomes
• A karyotype is an image that reveals an orderly
arrangement of chromosomes.
• Homologous chromosomes
– are matching pairs of chromosomes that
– can possess different versions of the same genes.
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Pair of homologous
chromosomes
Centromere
One
duplicated
chromosome
Sister
chromatids
LM
Figure 8.11
Homologous Chromosomes
• Humans have
– two different sex chromosomes, X and Y, and
– 22 pairs of matching chromosomes, called
autosomes.
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Gametes and the Life Cycle of a Sexual Organism
• The life cycle of a multicellular organism is the
sequence of stages leading from the adults of one
generation to the adults of the next.
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Figure 8.12
Haploid gametes (n  23)
n
Egg cell
n
Sperm cell
MEIOSIS
FERTILIZATION
Multicellular
diploid adults
(2n  46)
2n
Diploid
zygote
(2n  46)
MITOSIS
and development
Key
Haploid (n)
Diploid (2n)
Gametes and the Life Cycle of a Sexual Organism
• Humans are diploid organisms with
– body cells containing two sets of chromosomes
and
– haploid gametes that have only one member of
each homologous pair of chromosomes.
• In humans, a haploid sperm fuses with a haploid
egg during fertilization to form a diploid zygote.
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Gametes and the Life Cycle of a Sexual Organism
• Sexual life cycles involve an alternation of diploid
and haploid stages.
• Meiosis produces haploid gametes, which keeps
the chromosome number from doubling every
generation.
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Figure 8.13-3
1
Chromosomes
duplicate.
Pair of
homologous
chromosomes
in diploid
parent cell
2
Duplicated pair
of homologous
chromosomes
INTERPHASE BEFORE MEIOSIS
Homologous
chromosomes
separate.
3
Sister chromatids
separate.
Sister
chromatids
MEIOSIS I
MEIOSIS II
The Process of Meiosis
• In meiosis,
– haploid daughter cells are produced in diploid
organisms,
– interphase is followed by two consecutive
divisions, meiosis I and meiosis II, and
– crossing over occurs.
© 2013 Pearson Education, Inc.
Figure 8.14a
MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE
INTERPHASE
Centrosomes
(with centriole pairs)
PROPHASE I
Sites of
crossing over
Spindle
Nuclear Chromatin
envelope
Chromosomes
duplicate.
Sister
chromatids
Pair of
homologous
chromosomes
Homologous
chromosomes
pair up and
exchange
segments.
METAPHASE I
Microtubules
attached to
chromosome
ANAPHASE I
Sister chromatids
remain attached
Centromere
Pairs of
homologous
chromosomes
line up.
Pairs of
homologous
chromosomes
split up.
Figure 8.14ab
PROPHASE I
Sites of
crossing over
Spindle
Sister
chromatids
Pair of
homologous
chromosomes
METAPHASE I
Microtubules
attached to
chromosome
Centromere
ANAPHASE I
Sister chromatids
remain attached
TELOPHASE I AND
CYTOKINESIS
Cleavage
furrow
Figure 8.14b
MEIOSIS II: SISTER CHROMATIDS SEPARATE
TELOPHASE I AND
CYTOKINESIS
PROPHASE II
METAPHASE II
ANAPHASE II
TELOPHASE II
AND
CYTOKINESIS
Sister
chromatids
separate
Haploid
daughter
cells forming
Cleavage
furrow
Two haploid
cells form;
chromosomes
are still doubled.
During another round of cell division, the sister
chromatids finally separate; four haploid
daughter cells result, containing single
chromosomes.
Figure 8.14ba
PROPHASE II
METAPHASE II
ANAPHASE II
Sister
chromatids
separate
TELOPHASE II
AND
CYTOKINESIS
Haploid
daughter
cells forming
LM
Figure 8.14bc
LM
Meiosis II in
a lily cell
Review: Comparing Mitosis and Meiosis
• In mitosis and meiosis, the chromosomes duplicate
only once, during the preceding interphase.
• The number of cell divisions varies:
– Mitosis uses one division and produces two
diploid cells.
– Meiosis uses two divisions and produces four
haploid cells.
• All the events unique to meiosis occur during
meiosis I.
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Figure 8.15
MITOSIS
MEIOSIS
Prophase I
Prophase
Duplicated
chromosome
MEIOSIS I
Parent cell
Metaphase I
Metaphase
Chromosomes
align.
Sister
chromatids
separate.
Homologous
pairs align.
Anaphase I
Telophase I
Anaphase
Telophase
2n
Site of
crossing
over
2n
MEIOSIS I
Homologous
chromosomes
separate.
Haploid
n2
MEIOSIS II
Sister
chromatids
separate.
n
n
n
n
The Origins of Genetic Variation
• Offspring of sexual reproduction are genetically
different from their parents and one another.
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Independent Assortment of Chromosomes
• When aligned during metaphase I of meiosis, the
side-by-side orientation of each homologous pair
of chromosomes is a matter of chance.
• Every chromosome pair orients independently of
all of the others at metaphase I.
• For any species, the total number of chromosome
combinations that can appear in the gametes due
to independent assortment is
– 2n, where n is the haploid number.
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Independent Assortment of Chromosomes
• For a human,
– n = 23.
– With n = 23, there are 8,388,608 different
chromosome combinations possible in a gamete.
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Animation: Genetic Variation
Right click slide / select “Play”
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Figure 8.16-3
POSSIBILITY 2
POSSIBILITY 1
Two equally probable
arrangements of
chromosomes
at metaphase of
meiosis I
Metaphase
of
meiosis II
Gametes
Combination a Combination b
Combination c Combination d
Because possibilities 1 and 2 are equally likely, the four possible types of
gametes will be made in approximately equal numbers.
Random Fertilization
• A human egg cell is fertilized randomly by one
sperm, leading to genetic variety in the zygote.
• If each gamete represents one of 8,388,608
different chromosome combinations, at fertilization,
humans would have 8,388,608 × 8,388,608, or
more than 70 trillion different possible chromosome
combinations.
• So we see that the random nature of fertilization
adds a huge amount of potential variability to the
offspring of sexual reproduction.
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Colorized LM
Figure 8.17
Crossing Over
• In crossing over,
– nonsister chromatids of homologous chromosomes
exchange corresponding segments and
– genetic recombination, the production of gene
combinations different from those carried by
parental chromosomes, occurs.
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Animation: Crossing Over
Right click slide / select “Play”
© 2013 Pearson Education, Inc.
Figure 8.18
Prophase I of meiosis
Homologous
chromatids exchange
corresponding
segments.
Duplicated pair
of homologous
chromosomes
Chiasma, site of
crossing over
Metaphase I
Sister chromatids
remain joined at their
centromeres.
Spindle
microtubule
Metaphase II
Gametes
Recombinant
chromosomes combine
genetic information
from different parents.
Recombinant chromosomes
The Process of Science:
Do All Animals Have Sex?
• Observation: No scientists have ever found male
bdelloid rotifers, a microscopic freshwater
invertebrate.
• Question: Does this entire class of animals
reproduce solely by asexual means?
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The Process of Science:
Do All Animals Have Sex?
• Hypothesis: Bdelloid rotifers have thrived for
millions of years despite a lack of sexual
reproduction.
• Prediction: Bdelloid rotifers would display much
more variation in their pairs of homologous genes
than most organisms.
• Experiment: Researchers compared sequences of
a particular gene in bdelloid and non-bdelloid
rotifers.
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The Process of Science:
Do All Animals Have Sex?
• Results:
– Non-bdelloid sexually reproducing rotifers had a
nearly identical homologous gene, differing by only
0.5% on average.
– The two versions of the same gene in asexually
reproducing bdelloid rotifers differed by 3.5–54%.
• Conclusion: Bdelloid rotifers have evolved for
millions of years without any sexual reproduction.
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LM
Figure 8.19
When Meiosis Goes Awry
• What happens when errors occur in meiosis?
• Such mistakes can result in genetic abnormalities
that range from mild to fatal.
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How Accidents during Meiosis Can Alter Chromosome
Number
• In nondisjunction,
– the members of a chromosome pair fail to separate
at anaphase,
– producing gametes with an incorrect number of
chromosomes.
• Nondisjunction can occur during meiosis I or II.
© 2013 Pearson Education, Inc.
Figure 8.20-3
NONDISJUNCTION IN MEIOSIS I
NONDISJUNCTION IN MEIOSIS II
Meiosis I
Homologous
chromosomes fail
to separate.
Meiosis II
Sister
chromatids
fail to
separate.
Gametes
n1
n1
n–1
Abnormal
n–1
n1
n–1
Abnormal
n
n
Normal
How Accidents during Meiosis Can Alter Chromosome
Number
• If nondisjunction occurs, and a normal sperm
fertilizes an egg with an extra chromosome, the
result is a zygote with a total of 2n + 1
chromosomes.
• If the organism survives, it will have
– an abnormal karyotype and
– probably a syndrome of disorders caused by the
abnormal number of genes.
© 2013 Pearson Education, Inc.
Figure 8.21
Abnormal egg
cell with extra
chromosome
n1
Normal
sperm cell
n (normal)
Abnormal zygote with
extra chromosome
2n  1
Down Syndrome: An Extra Chromosome 21
• Down syndrome
– is also called trisomy 21,
– is a condition in which an individual has an extra
chromosome 21, and
– affects about one out of every 700 children.
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LM
Figure 8.22
Trisomy 21
Down Syndrome: An Extra Chromosome 21
• The incidence of Down syndrome in the offspring
of normal parents increases markedly with the age
of the mother.
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Figure 8.23
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
25
30
35
40
Age of mother
45
50
Abnormal Numbers of Sex Chromosomes
• Nondisjunction in meiosis
– can lead to abnormal numbers of sex
chromosomes but
– seems to upset the genetic balance less than
unusual numbers of autosomes, perhaps because
the Y chromosome is very small and carries
relatively few genes.
© 2013 Pearson Education, Inc.
Table 8.1
Evolution Connection:
The Advantages of Sex
• Asexual reproduction conveys an evolutionary
advantage when plants are
– sparsely distributed and unlikely to be able to
exchange pollen or
– superbly suited to a stable environment.
• Asexual reproduction also eliminates the need to
expend energy
– forming gametes and
– copulating with a partner.
© 2013 Pearson Education, Inc.
Figure 8.24
Runner
Evolution Connection:
The Advantages of Sex
• Sexual reproduction may convey an evolutionary
advantage by
– speeding adaptation to a changing environment or
– allowing a population to more easily rid itself of
harmful genes.
© 2013 Pearson Education, Inc.
Figure 8.UN01
Duplication
of all
chromosomes
Distribution via
mitosis
Genetically
identical
daughter cells
Figure 8.UN02
Chromosome (one
long piece of DNA)
Centromere
Sister
chromatids
Duplicated chromosome
Figure 8.UN03
S phase
DNA synthesis; chromosome duplication
Interphase
Cell growth and
chromosome duplication
G2
G1
Mitotic
(M) phase
Genetically
identical
“daughter”
cells
Cytokinesis
(division of
cytoplasm)
Mitosis
(division of
nucleus)
Figure 8.UN04
Human Life Cycle
Haploid gametes (n  23)
Key
Haploid (n)
n
Diploid (2n)
Egg cell
n
Sperm cell
FERTILIZATION
MEIOSIS
Male and female
diploid adults
(2n  46)
MITOSIS
and development
2n
Diploid
zygote
(2n  46)
Figure 8.UN05
MITOSIS
MEIOSIS
Parent
cell (2n)
MEIOSIS I
Parent
cell (2n)
Chromosome Chromosome
duplication
duplication
Pairing of
homologous
chromosome
Crossing
over
Daughter
cells
2n
2n
MEIOSIS II
n
n
n
Daughter cells
n
LM
Figure 8.UN06
(a)
(b)
(c)
(d)