Download Chromosomes

Chapter 8: Cell Reproduction
Original slide set from: www.laney.edu/wp/laura_coronado/files/2011/08/SVChap8.ppt
WHAT CELL REPRODUCTION
ACCOMPLISHES
 Reproduction:
• May result in the birth of new
organisms
• More commonly involves the
production of new cells
Cell Division
 Cell division plays important roles in the
lives of organisms.
• Replaces damaged or lost cells
• Permits growth
• Allows for reproduction
Human kidney cell
LM
Colorized TEM
FUNCTIONS OF CELL DIVISION
Cell Replacement
Growth via Cell Division
Early human embryo
Figure 8.1a
Asexual Reproduction
• Single-celled organisms reproduce by simple
cell division
• There is no fertilization of an egg by a sperm
• The parent and its offspring have identical
genes.
Asexual Reproduction
Binary Fission
• Prokaryotic cells divide through a
simple form of division called
Binary Fission
• 3 step process
• Single “naked” strand splits and
forms a duplicate of itself.
• The two copies move to opposite
sides of the cell
• Cell “pinches” into two new and
identical cells called "daughter
cells". (Cell wall then forms if
applicable)
Asexual Reproduction
• Mitosis is the type of cell division
responsible for:
 Asexual reproduction
 Growth and maintenance of multicellular organisms
 Some multicellular organisms, such as sea stars, can
grow new individuals from fragmented pieces.
 Growing a new plant from a clipping
LM
FUNCTIONS OF CELL DIVISION
Asexual Reproduction
Amoeba
Sea stars
African Violet
Figure 8.1b
Sexual Reproduction
 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
LM
Chromosomes
Figure 8.3
Chromosomes
 Chromosomes:
• Are made of chromatin, a combination of
DNA and protein molecules
• Are not visible in a cell until cell division
occurs
–Before a parent cell splits into two, it
duplicates its chromosomes
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
Figure 8.2
Eukaryotic Cell Genetic
Information
 Most genes on chromosomes in cell nucleus
 A few genes found in mitochondrial and
chloroplast DNA
 Each chromosome: one very long DNA
molecule, typically with thousands of genes.
 Histones are proteins used to package DNA.
 Nucleosomes consist of DNA wound around
histone molecules.
DNA double helix
Histones
TEM
“Beads on
a string”
Nucleosome
Tight helical fiber
Looped domains
Laura Coronado
TEM
Duplicated chromosomes
(sister chromatids)
Bio 10
Centromere
Chapter 8
Figure 8.4
Chromosome (one
long piece of DNA)
Centromere
Sister
chromatids
Duplicated chromosome
Laura Coronado
Bio 10
Chapter 8
Figure 8.UN2
Chromosomes
 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
Laura Coronado
Bio 10
Chapter 8
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
S phase (DNA synthesis;
chromosome duplication)
Interphase: metabolism and
growth (90% of time)
G1
Mitotic (M) phase:
cell division
(10% of time)
Cytokinesis
(division of
cytoplasm)
G2
Mitosis
(division of nucleus)
Laura Coronado
Bio 10
Chapter 8
Figure 8.6
Interphase
Interphase
 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
Laura Coronado
Bio 10
Chapter 8
Interphase
• Interphase
 3 Stages
•
•
•
•
•
G1 (Gap 1) Phase - Cell performs its normal function (cells which do not
divide stay in this stage for their entire life span)
-cells grow and mature
S (Synthesis) Phase - Here the cell actively duplicates its DNA in
preparation for division
G2 (Gap 2) Phase - Amount of cytoplasm (including organelles) increases
in preparation for division.
Another possibility
§ G0 Phase cells do not prepare for cell division
• Generally straight from G1 phase
• Example: fully developed cells in Central Nervous System never divide again
Mitosis
Mitosis
 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
Mitosis and Cytokinesis
 Mitosis consists of four distinct phases:
• (A) Prophase
• (B) Metaphase
• (C) Anaphase
• (D) Telophase
 Cytokinesis typically:
• Occurs during telophase
• Divides the cytoplasm
• Is different in plant and animal cells
INTERPHASE
Centrosomes
(with centriole pairs)
Chromatin
Plasma
membrane
Early mitotic
spindle
Centrosome
Fragments of
nuclear envelope
Centromere
Chromosome, consisting
of two sister chromatids
Spindle
microtubules
LM
Nuclear
envelope
PROPHASE
Figure 8.7.a
Prophase
• Chromosomes condense
• Nuclear membrane breaks
down
• Centrioles migrate to opposite
poles (in animal cells)
• Microtubules attach to
chromosomes and centrioles
Prophase
METAPHASE
ANAPHASE
TELOPHASE AND CYTOKINESIS
Nuclear
envelope
forming
Spindle
Cleavage
furrow
Daughter
chromosomes
Figure 8.7b
Metaphase
• Chromosomes line up along
the center of the cell
Metaphase
Anaphase
• Microtubules shorten
• Chromatids separate a pull to
opposite sides
Anaphase
Telophase
• Nuclear membrane forms
around each set of
chromosomes
• Chromosomes unwind
Cytokinesis
• Cytoplasm split in two
• Cell membrane
separates the two
daughter cells
Telophase and
Cytokinesis
Animal cell mitosis
SEM
Cleavage
furrow
Cleavage furrow
Contracting ring of
microfilaments
Daughter cells
Figure 8.8a
Plant Cell Mitosis
• Plant cell mitosis is similar to
animal cell mitosis BUT
cytokinesis is different
• In plant, fungi and algae cell,
a cell plate forms in the
middle of the cell to divide
the two cells.
Cell plate
forming
Daughter
nucleus
LM
Wall of
parent cell
Cell wall
Vesicles containing
cell wall material
Cell plate
New cell wall
Daughter cells
Figure 8.8b
Result of Mitosis
• 2 daughter cells that are
identical to each other and
identical to the parent cell
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
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.
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
Meiosis
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.
LM
Pair of homologous
chromosomes
Centromere
Sister
chromatids
One duplicated
chromosome
Figure 8.11
Human Chromosomes
 Humans have:
• Two different sex chromosomes, X and Y
• Twenty-two pairs of matching chromosomes,
called autosomes
 Humans are diploid organisms in which:
• Their somatic cells contain two sets of
chromosomes
• Their gametes are haploid, having only one
set of chromosomes
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.
Haploid gametes (n  23)
Egg cell
n
n
Sperm cell
FERTILIZATION
MEIOSIS
Multicellular
diploid adults
(2n  46)
2n
MITOSIS
and development
Diploid
zygote
(2n  46)
Key
Haploid (n)
Diploid (2n)
Figure 8.12
Meiosis
 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, which keeps
the chromosome number from doubling every
generation.
Chromosomes
duplicate.
Homologous
chromosomes
separate.
Sister
chromatids
separate.
Pair of homologous
chromosomes in
diploid parent cell
Duplicated pair of
homologous
chromosomes
INTERPHASE BEFORE MEIOSIS
Sister
chromatids
MEIOSIS I
MEIOSIS II
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
INTERPHASE
Centrosomes
(with centriole
pairs)
Nuclear
envelope
Chromatin
Chromosomes
duplicate.
PROPHASE I
Sites of
crossing over
Spindle
Sister
chromatids
Pair of
homologous
chromosomes
Homologous
chromosomes
pair up and
exchange
segments.
METAPHASE I
ANAPHASE I
Microtubules
attached
to chromosome
Sister chromatids
remain attached
Centromere
Pairs of
homologous
chromosomes
line up.
Pairs of
homologous
chromosomes
split up.
Figure 8.14a
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 8.14b
LM
Figure 8.14bc
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, while meiosis II is the same as mitosis
since it separates sister chromatids.
MEIOSIS
MITOSIS
Prophase I
Prophase
Chromosome
duplication
Duplicated chromosome
(two sister chromatids)
MEIOSIS I
Chromosome
duplication
Parent cell
(before chromosome duplication)
2n  4
Homologous
chromosomes come
together in pairs.
Site of crossing over
between homologous
(nonsister) chromatids
Metaphase I
Metaphase
Homologous pairs
align at the middle
of the cell.
Chromosomes
align at the
middle of the
cell.
Anaphase I
Telophase I
Anaphase
Telophase
2n
Sister
chromatids
separate
during
anaphase.
Daughter cells
of mitosis
2n
Homologous
chromosomes
separate during
anaphase I;
sister chromatids
remain together.
Chromosome with two
sister chromatids
Daughter
cells of meiosis I
Haploid
n2
MEIOSIS II
Sister chromatids
separate during
anaphase II.
n
n
n
Daughter cells of meiosis II
n
Figure 8.15
Independent Assortment of
Chromosomes
 When aligned during metaphase I of meiosis, the sideby-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
POSSIBILITY 2
POSSIBILITY 1
Metaphase of
meiosis I
Metaphase
of meiosis II
Gametes
Combination a
Combination b
Combination c
Combination d
Figure 8.16-3
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.
Figure 8.17
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
Prophase I
of meiosis
Duplicated pair of
homologous
chromosomes
Homologous chromatids
exchange corresponding
segments.
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
Figure 8.18-5
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.
 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.
NONDISJUNCTION IN MEIOSIS II
NONDISJUNCTION IN MEIOSIS I
Meiosis I
Nondisjunction:
Pair of homologous
chromosomes fails
to separate.
Meiosis II
Nondisjunction:
Pair of sister
chromatids
fails to separate.
Gametes
n1
n1
n–1
Abnormal gametes
n–1
Number of
chromosomes
n1
n–1
Abnormal gametes
n
n
Normal gametes
Figure 8.20-3
Abnormal egg
cell with extra
chromosome
n1
Normal
sperm cell
n (normal)
Abnormal zygote
with extra
chromosome 2n  1
Figure 8.21
Down Syndrome
 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.
LM
Chromosome 21
Figure 8.22
Infants with Down syndrome
(per 1,000 births)
90
80
70
60
50
40
30
20
10
0
20
25
30
35
40
45
50
Age of mother
Figure 8.23
Abnormal Numbers of Sex
Chromosomes
 Nondisjunction can also affect the sex
chromosomes.
Table 8.1
Evolution Connection:
The Advantages of Sex
 Asexual reproduction conveys an
evolutionary advantage when plants are:
• Sparsely distributed
• Superbly suited to a stable environment
 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
Figure 8.24
Duplication
of all
chromosomes
Distribution via
mitosis
Genetically
identical
daughter
cells
Figure 8.UN1
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.UN3
Human Life Cycle
Haploid gametes (n  23)
Key
Haploid (n)
n
Egg cell
Diploid (2n)
n
Sperm cell
MEIOSIS
FERTILIZATION
Male and female
diploid adults
(2n  46)
Diploid
zygote
(2n  46)
MITOSIS
and development
2n
Figure 8.UN4
MEIOSIS
MITOSIS
Parent
cell (2n)
Chromosome
duplication
Parent
cell (2n)
Chromosome
duplication
MEIOSIS I
Pairing of
homologous
chromosome
Crossing over
2n
Daughter
cells
2n
MEIOSIS II
n
n
n
n
Daughter cells
Figure 8.UN5
Differences between Mitosis
and Meiosis
• Meiosis has 2 divisions – two rounds of
chromosome separation.
• Crossing over in meiosis – exchange of
genetic material between homologous
chromosomes – occurs during
synapsis(pairing of homologous
chromosomes in M I)
Differences between Mitosis
and Meiosis
• Mitosis occurs in all cells, meiosis limited
to certain cells
• Mitosis produces 2 identical cells, Meiosis
produce 4 cells which are not identical
• Mitosis : daughter cells of same ploidy as
parent; Meiosis: daughter cells haploid of
parent
LM
(b)
(a)
(c)
(d)
Figure 8.UN6