meiosis
asexual
reproduction
CHAPTER 8
The Cellular Basis of Reproduction
and Inheritance
Sexual
reproduction
mitosis
Sect 8.3
Prokaryotic vs. Eukaryotic
• Binary fission
• Cell divides in half
• Chromosome
replicates & attaches
to plasma membrane
• 2 copies will be
separated by growth
that occurs between
them
• 2 identical cells form
• Genes grouped into
chromosomes (long
thread-like
structures)
• Cell duplicates
chromosomes as well
as genes
• Mitosis occurs
• Duplicated
chromosomes are
evenly distributed
into 2 daughter nuclei
• cytokinesis
Sect 8.4
Chromosomes
• Consists of a long DNA
molecule (represents
thousands of genes)
• Also consists of proteins
(structure, helps control
gene activity)
• This DNA-protein complex is chromatin
• Chromatin is organized into a long, thin fiber
• Fiber is coiled & folded to form the
chromosome
• Gene – a unit of information on a chromosome
- consists of DNA
- can be passed along to offspring
• All cells in the body (somatic) have
the same # of chromosomes (46)
• Except for sperm & eggs (sex cells –
23)
• When chromosomes replicate, they
form 2 identical structures called
sister chromatids (joined by a
centromere)
• When the sister chromatids separate
during mitosis, each goes into a
daughter cell
Sect 8.5
Cell Cycle (summary)
• Cell doubles its amount of
cytoplasm including organelles
• DNA is duplicated
• Nucleus & its contents divide by
mitosis
• Cytoplasm is divided between 2
daughter cells
Cell Cycle (Interphase)
• Lasts 90% of total - S Phase (synthesis)
when DNA
time for cell cycle
synthesized
• Divided into 3
- G2 Phase (second
phases:
gap)
period
after
st
- G1 Phase (1 gap)
DNA
synthesis
period of
begins but before
interphase before
mitosis
begins
DNA synthesis
(proteins
begins (growth in
synthesized
essential
size, proteins,
to cell division)
organelles)
Cell Cycle (M Phase)
• 10% of cell life
• Mitosis – nucleus & contents
divide
• Cytokinesis – cytoplasm divides
which generates 2 daughter
cells
Sect 8.6
Cell Cycle (Phases of Mitosis)
• Prophase
• Prometaphase
• Metaphase
• Anaphase
• Telophase/Cytokinesis
p. 130-131
Prophase
• Chromatin tightens
& thickens
(chromosomes
now visible)
• Each duplicated
chromosome is
seen as 2 identical
sister chromatids
joined by a
centromere
•Nucleus
disappears
•Microtubules form
a radial array called
an aster around
each centriole
•Mitotic spindle
forms
Prometaphase
• Nuclear envelope
disappears &
spindle enters
nucleus
• Bundles of
microtubules
extend from pole
to equator
• Centromeres have
kinetochores to
which kinetochore
fibers are attached
Metaphase
• Centriole pairs are now
at opposite poles
• Chromosomes line up on
the metaphase plate (a
plane between the 2
poles)
Anaphase
• Paired centromere
of each
chromosome moves
apart
-sister chromatids
are now free of
each other
-each chromatid is
now a chromosome
• Spindle moves the
chromosomes to opposite poles
• Poles move farther apart
• Each pole now has a complete
set of chromosomes
Telophase
• Polar fibers
elongate
• Nuclear
envelopes are
formed from
earlier
fragments
from the
parent cell
•Nucleolus
reappears
•Mitosis is complete
Sect 8.7
Cytokinesis
(animals – cleavage)
• Shallow groove in cell surface near
old metaphase plate (called cleavage
furrow)
• Contractile ring of microfilaments is
found on cytoplasm side of furrow
-ring contracts & shrinks in diameter
-Cleavage furrow deepens until parent
cell pinches in two
Cytokinesis
(plants – no cleavage furrow)
• Cell plate forms across midline
of parent cell
• Double membrane is formed
• Fusion of each of these 2
membranes w/plasma
membrane results in the
formation of 2 daughter cells
Sect 8.8
What affects mitosis?
Normal Cell Division – 3 factors:
Anchorage Dependent – cells must be
“anchored” to a solid surface to divide;
keeps cells separated from their normal
surroundings from dividing
inappropriately
Density-Dependent Inhibition – rate of
cell division decreases as cell
population becomes denser
Growth Factors – proteins secreted by
certain body (somatic) cells that
stimulate other cells to divide
p. 133
Cell-Cycle Control Systems
Sect 8.9
•It is a cyclically operating set of proteins
(mostly growth factors) in the cell that
triggers & coordinates major events in the
cell cycle.
•Major checkpoints in G1, G2, and M:
- G1 checkpoint seems to be most critical in
many cells (gives go-ahead to start cellcycle & divide, non-dividing cells are stuck
at G1 checkpoint called Go)
-G2 checkpoint gives go-ahead to start
M phase
-M checkpoint go-ahead signals that
the proper attachment of the spindle
fibers to the chromosomes & prompts
the separation of the sister chromatids
-What is signal transduction?
* Protein that receives signals for
specific activities
Uncontrolled Cell Growth- can produce
excessive growth resulting in an
abnormal mass called a tumor
• Benign = abnormal • Malignant = mass
mass of normal
of cancer cells that
cells that normally
can spread beyond
remain at their
their original site to
original site
neighboring cells
(growth) or other
• can usually be
sites of the body
removed by
(metastasis –
surgery
circulatory system)
Cancer cells do not have a properly
functioning cell-cycle.
• Carcinomas – cancers of the
external & internal linings of body
(skin, linings of intestines)
• Sarcomas – cancers of tissues that
support body (bone, muscle)
• Leukemias & Lymphomas – cancer
of blood-forming tissue (bone
marrow, spleen, lymph nodes)
Treatment for Cancer
• In radiation
therapy, cancer
cells are most
likely dividing
at any given
time & will
destroy cells by
disrupting cell
division w/out
injuring normal
cells
• Chemotherapy
uses the same
strategy as
radiation
therapy but
uses chemicals
instead
Sect 8.11
Sexual & Asexual Reproduction
• When offspring
results from 2
• When an
parents
individual
inherits all its
• Offspring has a
genes from a
unique
SINGLE parent
combination of
genes inherited • Consists of
from BOTH
simple division
parents
or budding
Sect 8.12
Sexual Life Cycle (human)
• Each somatic cell = 46
chromosomes
• Chromosomes differ from each
other in size, position of
centromere, staining pattern
• Matched in pairs (this creates a
karyotype)
•Chromosomes that make up a pair are
called homologous chromosomes
- same size & have same centromere
position
blue
green
- each carries genes controlling same
inherited trait but possibly different versions
-genes for a certain trait are located in a
particular place - locus
•23rd pair of
chromosomes is the
sex chromosomes
- XY for males
- XX for females
•All other chromosomes
are called autosomes
Sect 8.13
Chromosome Numbers
• Humans inherit 2 sets
- maternal = 23
- paternal = 23
• Cells containing 2 sets are
diploid or 2N
• Cells containing 1 set are
haploid or 1N
•Gametes are haploid cells
- they are reproductive cells, either
sperm or egg
•Each sperm or egg has single set of
22 autosomes plus 1 sex chromosome
•Fertilization occurs when
egg & sperm unite &
create a zygote
•Production of gametes requires a
special type of division
- use reduction division called meiosis
- reduces chromosome number by
half
Sect 8.14
Meiotic Cell Division
Meiosis I
- Prophase I
- Metaphase I
- Anaphase I
- Telophase I/
Cytokinesis
Meiosis II
- Prophase II
- Metaphase II
- Anaphase II
- Telophase II/
Cytokinesis
p. 138-139
Meiosis I
• Produces haploid gametes in diploid
organisms
(Prophase I)
•Homologous chromosomes come together
as pairs
- each is made of 4 chromatids
- each forms a tetrad
- 90% of meiotic cell division
- crossing over of genetic material
can occur (segments of non-sister
chromatids can exchange places)
- spindle forms
- nuclear envelope & nucleolus
disappear
(Metaphase I)
•Tetrads line up on the metaphase
plate
(Anaphase I)
•Homologous pairs separate
& go to opposite poles (only
tetrad splits)
(Telophase I/ Cytokinesis)
•Cytoplasm divides making
2 daughter cells
•Interkinesis occurs (rest
time)
Meiosis II
(Prophase II)
Spindle moves
chromosomes
toward the
equator
(Metaphase II)
•Chromosomes line up on the
metaphase plate
(Anaphase II)
•Centromeres of sister chromatids separate
•Individual chromosomes move to poles
(Telophase II/ Cytokinesis)
•Nuclear envelope reforms
•4 daughter cells are made (each has
½ the number of chromosomes of
parent)
Sect 8.15
Comparing Mitosis & Meiosis
p. 140
• Behavior of
• Behavior of
chromosomes is
chromosomes is
same
same
•
4
daughter
cells
• 2 daughter cells
that are each N
that are each
•
Very
different
2N
than mitosis
because of
tetrad
Genetic Variation
• Independent Assortment
• Random Fertilization
• Crossing Over
Sect 8.16
Independent Assortment
• Orientation of homologous pairs
relative to the 2 poles is random
(there are 2 possibilities)
• The first meiotic division results
in independent assortment of
maternal and paternal
chromosomes into daughter
cells
•In humans, the number of
combinations of maternal or paternal
chromosomes is 223 = 8 million
•There are 2N possible combinations
where N is the haploid number
Sect 8.17
Random Fertilization
• An egg cell represents 8 million
different possibilities
• A sperm cell represents 8 million
different possibilities
• A zygote will have 64 trillion diploid
combinations
Sect 8.18
Crossing Over
• Identical regions of
chromatids are
exchanged
- results in a change in
genetic material
- this is known as
genetic recombination
• Produces combinations
different from those
originally inherited