08 lecture presentation

WHAT CELL REPRODUCTION
ACCOMPLISHES
• Reproduction:
– May result in the birth of new organisms
– More commonly involves the production of new cells
© 2010 Pearson Education, Inc.

• 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.

• Before a parent cell splits into two, it duplicates its
chromosomes, the structures that contain most of the organism’s
DNA.
• During cell division, each daughter cell receives one set of
chromosomes.

• Cell division plays important roles in the lives of organisms.
• Cell division:
– Replaces damaged or lost cells
– Permits growth
– Allows for reproduction

• In asexual reproduction:
– Single-celled organisms reproduce by simple cell division
– 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.

LM
Asexual Reproduction
Figure 8.1ba

Figure 8.1bb

Figure 8.1bc

• In asexual reproduction, the lone parent and its offspring have
identical genes.
• Mitosis is the type of cell division responsible for:
– Asexual reproduction
– Growth and maintenance of multicellular organisms

• 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
– Mitosis for growth and maintenance

• In a eukaryotic cell:
– Most genes are located on chromosomes in the cell nucleus
THE CELL CYCLE AND MITOSIS

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.

Number of chromosomes
in body cells
Indian muntjac deer
Species
Opossum
Koala
Human
Mouse
Giraffe
Buffalo
Dog
Red viscacha rat
Duck-billed platypus
102
78
60
54
46
40
30
22
16
6
Figure 8.2

• Chromosomes:
– Are made of chromatin, a combination of DNA and protein molecules
– Are not visible in a cell until cell division occurs

• Before a cell divides, it duplicates all of its chromosomes,
resulting in two copies called sister chromatids.
• Sister chromatids are joined together at a narrow “waist” called
the centromere.

• When the cell divides, the sister chromatids separate from each
other.
• Once separated, each chromatid is:
– Considered a full-fledged chromosome
– Identical to the original chromosome

Chromosome
duplication
Sister
chromatids
Chromosome
distribution to
daughter cells
Figure 8.5

The Cell Cycle
• A cell cycle is the orderly 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:
– Interphase
– The mitotic phase

• Most of a cell cycle is spent in interphase.
• During interphase, a cell:
– Performs its normal functions
– Doubles everything in its cytoplasm
– Grows in size
Video: Animal Mitosis

Nuclear
envelope
LM
Plasma
membrane
Chromosome, consisting
of two sister chromatids
Spindle
microtubules
Fragments of
nuclear envelopeCentrosome
Centromere
Early mitotic
spindle
Centrosomes
(with centriole pairs)
Chromatin
PROPHASEINTERPHASE
Figure 8.7.a

• The mitotic (M) phase includes two overlapping processes:
– Mitosis, in which the nucleus and its contents divide evenly into two
daughter nuclei
– Cytokinesis, in which the cytoplasm is divided in two
Video: Sea Urchin (time lapse)

Cytokinesis
(division of
cytoplasm)
Mitosis
(division of nucleus)
Mitotic (M) phase:
cell division
(10% of time)
Interphase: metabolism and
growth (90% of time)
S phase (DNA synthesis;
chromosome duplication)
G1 G2
Figure 8.6

Mitosis and Cytokinesis
• During mitosis the mitotic spindle, a football-shaped structure of
microtubules, guides the separation of two sets of daughter
chromosomes.
• Spindle microtubules grow from two centrosomes, clouds of
cytoplasmic material that in animal cells contain centrioles.

• Mitosis consists of four distinct phases:

– (A) Prophase
– (B) Metaphase
– (C) Anaphase
– (D) Telophase

ANAPHASEMETAPHASE TELOPHASE AND CYTOKINESIS
Spindle Daughter
chromosomes
Cleavage
furrow
Nuclear
envelope
forming
Figure 8.7b

ANAPHASEMETAPHASE TELOPHASE AND CYTOKINESIS
Spindle Daughter
chromosomes
Cleavage
furrow
Nuclear
envelope
forming
Figure 8.7ba

• Cytokinesis typically:
– Occurs during telophase
– Divides the cytoplasm
– Is different in plant and animal cells
Animation: Cytokinesis
Blast Animation: Cytokinesis in Plant Cells

Cleavage furrow
Contracting ring of
microfilaments
Daughter cells
Figure 8.8ab

Daughter cells
New cell wall
Vesicles containing
cell wall material Cell plateCell wall
Wall of
parent cell
Cell plate
forming
Daughter
nucleus
LM
Figure 8.8b

Wall of
parent cell
Cell plate
forming
Daughter
nucleus
LM
Figure 8.8ba

Daughter cells
New cell wall
Vesicles containing
cell wall material Cell plate
Cell wall
Figure 8.8bb

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.

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.
• The spread of cancer cells beyond their original site of origin is
metastasis.
• Malignant tumors can:
– Spread to other parts of the body
– Interrupt normal body functions
• A person with a malignant tumor is said to have cancer.

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.
Glandular
tissue
Blood
vessel
Tumor
Lymph
vessels
Figure 8.9

Cancer Treatment
• Cancer treatment can involve:
– Radiation therapy, which damages DNA and disrupts cell division
– Chemotherapy, which uses drugs that disrupt cell division

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
– Regularly visiting a doctor to identify tumors early

• Sexual reproduction:
– Uses meiosis
– Uses fertilization
– Produces offspring that contain a unique combination of genes from the
parents
Meiosis, the Basis of Sexual Reproduction

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
– Has 46 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.

Pair of homologous
chromosomes
LM
One duplicated
chromosome
Centromere
Sister
chromatids
Figure 8.11

• Humans have:
– Two different sex chromosomes, X and Y
– Twenty-two pairs of matching chromosomes, called autosomes

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.

Multicellular
diploid adults
(2n  46)
MEIOSIS FERTILIZATION
MITOSIS
2n
and development Key
Sperm cell
n
n
Diploid
zygote
(2n  46)
Diploid (2n)
Haploid (n)
Egg cell
Haploid gametes (n  23)
Figure 8.12

• Humans are diploid organisms in which:
– Their somatic cells contain two sets of chromosomes
– Their gametes are haploid, having only one set of chromosomes

• In humans, a haploid sperm fuses with a haploid egg during
fertilization to form a diploid zygote.
• Sexual life cycles involve an alternation of diploid and haploid
stages.

• Meiosis produces haploid gametes.

MEIOSIS I
Sister
chromatids
separate.
MEIOSIS II
Homologous
chromosomes
separate.
INTERPHASE BEFORE MEIOSIS
Sister
chromatids
Duplicated pair of
homologous
chromosomes
Chromosomes
duplicate.
Pair of homologous
chromosomes in
diploid parent cell
Figure 8.13-3

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
– Crossing over occurs

MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE
Sister chromatids
remain attached
Pair of
homologous
chromosomes
INTERPHASE
Sister
chromatids
Homologous
chromosomes
pair up and
exchange
segments.
Chromosomes
duplicate.
Pairs of
homologous
chromosomes
line up.
Pairs of
homologous
chromosomes
split up.
Nuclear
envelope
Chromatin
Centromere
Microtubules
attached
to chromosome
Sites of
crossing over
Spindle
Centrosomes
(with centriole
pairs)
PROPHASE I METAPHASE I ANAPHASE I
Figure 8.14a

TELOPHASE II
AND
CYTOKINESIS
Sister
chromatids
separate
ANAPHASE II
Cleavage
furrow
TELOPHASE I
AND
CYTOKINESIS
Two haploid
cells form;
chromosomes
are still
doubled.
MEIOSIS II: SISTER CHROMATIDS SEPARATE
PROPHASE II METAPHASE II
During another round of cell division, the sister
chromatids finally separate; four haploid
daughter cells result, containing single
chromosomes.
Haploid
daughter
cells forming
Figure 8.14b

TELOPHASE II
AND
CYTOKINESIS
Sister chromatids
separate
ANAPHASE IIPROPHASE II METAPHASE II
Haploid
daughter cells
forming
Figure 8.14ba

PROPHASE II METAPHASE II
Figure 8.14bb

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.

Duplicated chromosome
(two sister chromatids)
MITOSIS
Prophase
Chromosome
duplication
Chromosomes
align at the
middle of the
cell.
Metaphase
Sister
chromatids
separate
during
anaphase.
Anaphase
Telophase
Daughter cells
of mitosis
2n2n
Prophase I
Metaphase I
Anaphase I
Telophase I
MEIOSIS
Chromosome
duplication
Homologous
chromosomes come
together in pairs.
MEIOSIS I
Site of crossing over
between homologous
(nonsister) chromatids
Homologous pairs
align at the middle
of the cell.
Chromosome with two
sister chromatids
Homologous
chromosomes
separate during
anaphase I;
sister chromatids
remain together.
Daughter
cells of meiosis I
Sister chromatids
separate during
anaphase II.
Haploid
n  2
MEIOSIS II
Parent cell
(before chromosome duplication)
2n  4
Daughter cells of meiosis II
n n n n
Figure 8.15

(two sister chromatids)
MITOSIS
Prophase
Chromosome
duplication
Chromosomes
align at the
middle of the
cell.
Metaphase
Prophase I
Metaphase I
MEIOSIS
Chromosome
duplication
Homologous
chromosomes come
together in pairs.
MEIOSIS I
Site of crossing
over between
homologous
(nonsister)
chromatids
Homologous
pairs align at
the middle
of the cell.
Parent cell
(before chromosome duplication)
2n  4
Figure 8.15a

Sister
chromatids
separate
during
anaphase.
Anaphase
Telophase
Daughter cells
of mitosis
2n2n
Anaphase I
Telophase I
Chromosome with
two sister chromatids
Homologous
chromosomes
separate during
anaphase I;
sister chromatids
remain together.
Daughter
cells of meiosis I
Sister chromatids
separate during
anaphase II.
Haploid
n  2
MEIOSIS II
Daughter cells of meiosis II
n n n n
Figure 8.15b

The Origins of Genetic Variation
• Offspring of sexual reproduction are genetically different from
their parents and one another.

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 the others
during meiosis.

• 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.
• For a human:
– n = 23
– 223 = 8,388,608 different chromosome combinations possible in a gamete
Blast Animation: Genetic Variation: Independent Assortment
Animation: Genetic Variation

Metaphase of
meiosis I
POSSIBILITY 1 POSSIBILITY 2
Figure 8.16-1

Metaphase of
meiosis I
Metaphase
of meiosis II
Figure 8.16-2

Metaphase of
meiosis I
Metaphase
of meiosis II
Combination a
Combination b Combination c Combination d
Gametes
Figure 8.16-3

Crossing Over
• In crossing over:
– Homologous chromosomes exchange genetic information
– Genetic recombination, the production of gene combinations different
from those carried by parental chromosomes, occurs
Blast Animation: Genetic Variation: Fusion of Gametes
Animation: Crossing Over

Prophase I
of meiosis
Duplicated pair of
homologous
chromosomes
Figure 8.18-1

Homologous chromatids
exchange corresponding
segments.
Prophase I
of meiosis
Duplicated pair of
homologous
chromosomes
Chiasma, site of
crossing over
Figure 8.18-2

Metaphase I
segments.
Sister chromatids
remain joined at their
centromeres.
Prophase I
of meiosis
Duplicated pair of
homologous
chromosomes
Chiasma, site of
crossing over
Spindle
microtubule
Figure 8.18-3

Metaphase I
Metaphase II
segments.
Sister chromatids
centromeres.
Prophase I
of meiosis
Duplicated pair of
homologous
chromosomes
Chiasma, site of
crossing over
Spindle
microtubule
Figure 8.18-4

Metaphase I
Metaphase II
Recombinant chromosomes
Gametes
Recombinant
chromosomes
combine genetic
information from
different parents.
segments.
Sister chromatids
centromeres.
Prophase I
of meiosis
Duplicated pair of
homologous
chromosomes
Chiasma, site of
crossing over
Spindle
microtubule
Figure 8.18-5

• What happens when errors occur in meiosis?
• Such mistakes can result in genetic abnormalities that range from
mild to fatal.
When Meiosis Goes Awry

How Accidents during Meiosis Can Alter
Chromosome Number
• In nondisjunction, the members of a chromosome pair fail to
separate during anaphase, producing gametes with an incorrect
number of chromosomes.
• Nondisjunction can occur during meiosis I or II.

Meiosis I
Nondisjunction:
Pair of homologous
chromosomes fails
to separate.
NONDISJUNCTION IN MEIOSIS I NONDISJUNCTION IN MEIOSIS II
Figure 8.20-1

Meiosis I
Nondisjunction:
Pair of homologous
chromosomes fails
to separate.
NONDISJUNCTION IN MEIOSIS I
Meiosis II
Nondisjunction:
Pair of sister
chromatids
fails to separate.
NONDISJUNCTION IN MEIOSIS II
Figure 8.20-2

Meiosis I
Abnormal gametes
Gametes
Nondisjunction:
Pair of homologous
chromosomes fails
to separate.
NONDISJUNCTION IN MEIOSIS I
Number of
chromosomes
Meiosis II
Nondisjunction:
Pair of sister
chromatids
fails to separate.
Abnormal gametes Normal gametes
nnn  1n  1 n – 1n  1
NONDISJUNCTION IN MEIOSIS II
n – 1 n – 1
Figure 8.20-3

• 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 number of
genes.

Abnormal egg
cell with extra
chromosome
Normal
sperm cell
n  1
n (normal)
Abnormal zygote
with extra
chromosome 2n  1
Figure 8.21

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
– Affects about one out of every 700 children

• The incidence of Down Syndrome increases with the age of the
mother.

Age of mother
25 35 4520 30 40 50
10
0
20
30
40
50
60
70
80
90InfantswithDownsyndrome
(per1,000births)
Figure 8.23

Abnormal Numbers of Sex Chromosomes
• Nondisjunction can also affect the sex chromosomes.

• Sexual reproduction may convey an evolutionary advantage by:
– Speeding adaptation to a changing environment
– Allowing a population to more easily rid itself of harmful genes
• Asexual reproduction conveys an evolutionary advantage when
plants are:
– Sparsely distributed
– Superbly suited to a stable environment

Duplication
of all
chromosomes
Genetically
identical
daughter
cells
Distribution via
mitosis
Figure 8.UN1

Chromosome (one
long piece of DNA)
Centromere
Sister
chromatids
Figure 8.UN2

Interphase
Cell growth and
chromosome duplication
G2
Mitotic
(M) phase
S phase
DNA synthesis; chromosome duplication
G1
Genetically
identical
“daughter”
cells
Cytokinesis
(division of
cytoplasm)
Mitosis
(division of
nucleus)
Figure 8.UN3

MITOSIS
Male and female
diploid adults
(2n  46)
MEIOSIS
Sperm cell
Human Life Cycle
Key
Haploid (n)
Diploid (2n)
Haploid gametes (n  23)
Egg cell
Diploid
zygote
(2n  46)
and development
FERTILIZATION
2n
n
n
Figure 8.UN4

Daughter
cells
Parent
cell (2n)
MITOSIS
Chromosome
duplication
2n 2n
MEIOSIS
MEIOSIS IParent
cell (2n)
Chromosome
duplication
Daughter cells
n
MEIOSIS II
Pairing of
homologous
chromosome
Crossing over
n nn
Figure 8.UN5

08 lecture presentation

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08 lecture presentation