Chapter 8 The Cellular Basis of Reproduction and
Inheritance
Connections Between Cell Division and Reproduction
I. Intro
A. What builds organisms? Why do I look like I do and you
look like you do and plants look like they do, etc…?
B. What are all organisms designed to do?
1. Who builds organisms and where is that information
stored?
2. Genes build organisms with the sole purpose of
preserving themselves through time.
3. In other words, organisms are built by genes with the
sole purpose of reproducing those genes.
C. Life cycle – sequences of events going from the adults
in one generation to the adults in the next.
1. development – fertilized egg to adult
2. reproduction – formation of new individuals from existing
ones.
a) Sexual reproduction – involves passing traits from both
parents to next generation (offspring)
(1) sperm and ovum (egg) = gametes, each have one copy
of the organism’s GENOME (genetic information).
(2) Highly varied individuals due to mixing of genes from
parents resulting in a unique combination of traits
(3) offspring not genetically identical to parents
(4) multicellular and many single-celled organisms
b) Asexual reproduction – involves passing traits from only
one parent to the next generation
(1) offspring genetically identical
(2) single-celled organisms
D. Cell division – one cell divides into two – the heart of
organismal life cycle.
1. We all started as a single cell inside a female… Then
what?
2. links the life cycle: development and reproduction
a) Development – fertilized egg to embryo to adult
b) Reproduction – continuity of life from generation
to generation. Basis for both sperm and egg
development, and asexual reproduction.
PROKARYOTIC CELL DIVISION
II. Prokaryotes (bacteria) reproduce by binary fission
A. What must a cell do before it can divide? Replicate its
DNA of course!
B. Binary fission – “dividing in half” – a form of cell
division in bacteria – (a type of asexual reproduction)
C. Genes carried on one circular DNA molecule with
associated proteins (a single chromosome)
D. Minimal packaging (few proteins) compared to
eukaryotes, attached to cell wall at one point
E. No nucleus, DNA in semifluid in cell center
1. DNA must be replicated (copied)
2. Each chromosome is moved apart
3. new plasma membrane and cell wall grow inward and
divide cell.
The Eukaryotic Cell Cycle
III.Eukaryotic chromosomes
A. Chromosomes (chromo = colored, soma = body) are
generally more complex and larger than prokaryotes
B. Chromosome = DNA plus associated proteins (histones)
1. Genes organized into several separate chromosomes
2. humans
a) 50,000 to 100,000 genes (3 billion base pairs of
DNA – 3m in length)
b) found on 46 chromosomes
c) Each chromosome is between 40,000,000 and
250,000,000 base pairs.
d) Different species may have different numbers of
chromosomes – dog 78, horse 64, donkey 62, mule 63
C. Can see the chromosomes under a light microscope just
prior to and during cell division.
IV. Chromosomes must duplicate and then split in two
before a cell divides
A. The DNA in each chromosome is replicated (copied)
prior to it becoming visible during cell division
B. The copies are called (identical) sister chromatids, and
they are attached to each other at the centromere.
C. Each new daughter cell will get one of the sisters
1. Ex. If the classroom is a cell and each student is a
chromosome, each student will duplicate and now have
an identical twin (sisters). Each new classroom will get
one of the twins and thus we have generated two identical
classrooms and students.
V. The cell cycle (eukaryotes)
A. Why do cells divide?
1. Basis of reproduction for EVERY organism
2. Multicellular organism:
a) grow to an adult
b) Replaces dead or worn out cells
c) Some cells in growing and adult organisms still
divide regularly (once an hour, a day, etc…). Some
have stopped dividing like muscle cells and neurons
(nerve cells).
B. Dividing cells undergo a CELL CYCLE
1. A sequence of repeated steps every division
VI.
THE CELL CYCLE (Interphase + Mitotic (M) phase)
A. Your either in the cycle…or your NOT (G0 = resting cells
– out of the cycle)
B. Interphase – most of the cell cycle spent here (90% or
more) - divided into three subphases
1. G1 subphase – “gap” 1 (growth 1)
a) cell grows in size,
b) increases production of proteins and organelles
(like mitochondria and ribosomes)
c) gather nutrients (ex. do you have enough
nucleotides to replicate the entire genome)
d) can stay here for a very long time if needed
2. S subphase – “synthesis” – DNA synthesis
(replication) – DNA is copied – centrosomes replicate also
a) every chromosome now has a sister
3. G2 subphase – “gap” 2 – metabolic activity, more
protein production in preparation for cell division.
C. The mitotic or “M” phase (divided into two subphases)
1. Mitosis – nuclear division
a) nucleus and contents divide and form two equal
daughter nuclei
2. Cytokinesis – cytoplasm divides into two
a) overlaps mitosis
b) Result is two daughter cells with identical
chromosomes.
D. Very accurate – 1 error in chromosome distribution in
100,000 divisions in yeast!!!
VII. Cell division is a continuum of dynamic changes
A. MTOC – microtubule organizing centers
1. Structures from where microtubules emerge in
eukaryotes
2. Two Types
a) basal bodies –
(1) organize cilia and flagella
(2) a single centriole
b) centrosome –
(1) organization of the mitotic and meiotic spindle
apparatus separating the chromosomes during cell
division.
(2) pair of orthogonal centiroles + associated proteins
B. Mitosis is continuous, but we break it into 4 main
phases
1. Prophase –
a) mitotic spindle forming from centrosomes.
b) centrosomes start moving away from each other
(1) Strangely, destroying centrioles has no effect on
cell division.
c) chromatin condenses into discrete, visible
chromosomes – sister chromatids joined at
centromere.
d) nucleoli disappear
e) Nuclear membrane breaks into fragments (late in
prophase).
f) centrosomes at the poles now
g) kinetochore – protein structure associated with the
centromere
(1) each chromosome has a pair of kinetochores (one
per sister chromatid)
h) Microtubules can now get to the chromosomes
with nuclear membrane gone.
i) Some microtubules attach to kinetochores of
chromosomes
j) Others contact free microtubules from the other
pole.
k) Protein “motors”, similar to our old friend
Kinesin, move the chromosomes toward the center of
the cell! – powered by ATP!!
2. Metaphase
a) Mitotic spindle fully formed
b) Chromosomes aligned on metaphase plate
(1) metaphase plate = imaginary plane between the
poles
c) For each chromosome, the kinetochores of each
sister face opposite poles.
3. Anaphase
a) begins when centromeres of each chromosome
come apart separating sisters
(1) each sister is now called a DAUGHTER
chromosome
b) Motor proteins “walk” the daughter chromosomes
to opposite poles of the cell.
c) Microtubules attached to kinetochores shorten at
the same time.
d) Spindle microtubules not attached to chromosomes
lengthen!!
(1) Thus, poles are moving further apart and cell
elongates.
e) Anaphase ends when chromosomes reach the
poles.
4. Telophase and cytokinesis
a) Telophase
(1) cell elongation continues
(2) The reverse of prophase – chromosomes uncoil,
nuclear envelope of daughter nuclei begin to reform,
nucleoli begin to reappear, mitotic spindle
dissappears.
(3) Mitosis (division of the nucleus) has now ended
b) Cytokinesis, division of the cytoplasm,
(1) Results in two daughter cells
(2) Occurs with telophase
(3) Differs for plants and animal cells
(a) Animals – ring of microfilaments contracts
around periphery of cell forming a cleavage furrow
that will cleave the cytoplasm - cleavage
(b) Plants –
(i) Can’t pinch in like animals – cell wall to
deal with
(ii) Vesicles with cell wall material collect
in the middle of the cell
(iii)The vesicles fuse forming a membrane
enclosed disc = cell plate
(iv) Cell plate grows out as more vesicles
fuse
(v) Cell plate membrane fuses with
cytoplasmic membrane and cell wall
material of cell plate join the parental cell
wall. – each daughter cell has own plasma
membrane and own cell wall.
VIII. Anchorage, cell density, and chemical growth
factors affect cell division
A. Multicellular plants and animals must control the timing
of cell division to grow and develop normally.
B. Anchorage dependence –
1. Most plant and animal cells will not divide unless in
contact with a surface.
a) Most cells are anchored to ECM or to other cells in
a tissue (tight junctions, anchoring junction,
communicating junction)
b) keeps cells that have come loose from dividing
C. Density dependent inhibition (DDI) - Cells stop dividing
when a single layer is formed and they are touching each
other (scrape off some cell and what happens? Fills in like cutting your skin)
1. What causes DDI? What stops the cells from dividing?
a) Growth factor – protein secreted by body cells that
stimulates neighboring cells to grow
b) Inhibition thought to be caused by a decrease in
amount of available growth factor (used up by the
large number of cells)
c) Add growth factor to a sheet of cells, they become
smaller and more numerous (still a single sheet)
IX.
Growth factors signal the cell cycle control system
A. How do cells know when to divide? and when not to…?
B. Scientists once thought it was like falling dominoes
where one event triggered the next. Thus, once it starts, it
just goes to the end.
C. Is the cell ready? Is the outside environment
appropriate?
D. We now know that there are checkpoints. Like dominoes
with stops along the way.
E. There are three major checkpoints (stop signs)
1. cell will stop at each checkpoint by default
2. if everything is good – there are “GO” signals to
proceed.
a) Intracellular GO signals - Signals from within
indicating that everything is ready.
b) Extracellular GO signals – environmental
conditions and signals from other cells
F. The Checkpoints:
1. G1 checkpoint of interphase – most important.
a) If cell crosses G1 checkpoint, it must go all the way.
Why do you think?
b) Cells can arrest (hault) at the G1 checkpoint for
long periods of time or for life entering G0 if there is
no extracellular signal.
2. G2 checkpoint of interphase
3. M checkpoint – metaphase does not just lead into
anaphase, cell cycle control proteins trigger the
separation of sisters chromatids.
G. How are growth factors related to this process?
1. extracellular G1 “GO” signals
H. Critical in the understanding of cancer
X. CANCER
A. claims lives of 1 out 5 in US
B. Cancer cells – cell cycle control system out of order –
cell divides excessively creating a tumor (abnormal mass
of cells)
C. Not all tumors are cancerous
1. Malignant (cancerous) tumor
a) Three properties of cancer
(1) cells grow in an unlimited, aggressive manner
(2) invade surrounding tissues
(3) metastasize - break off and move around body
making new tumors.
2. Benign (non-cancerous) tumor – abnormal mass of
essentially normal cells
a) Lack the three properties of cancer
b) can be a problem in certain places like the brain
and other vital organs
D. Four categories of malignant tumor
1. Carcinoma – cancers originate in the external or
internal coverings of the body (skin, intestine lining,
breast cancer, colon, pancreatic – colon)
2. Sarcoma – arise in tissues that support the body
(bone, cartilage, adipose, blood)
a) Leukemia and lymphoma– cancers of bloodforming tissues (bone marrow, spleen, lymph nodes,
etc)
E. Two types of treatment – both target cell division
1. Chemotherapy
a) antimitotic drugs – target mitotic spindle
(1) taxol – freezes spindle after it forms
(a) discovered in the bark of the Pacific yew from the
NW US.
(b) Inhibits (binds) tubulin and prevents
microtubules from disassembling.
(2) Vinblastin – prevents spindle from forming
(a) obtained from flower (periwinkle) native to rain
forests of Madagascar
2. Radiation therapy
XI. Review of the functions of mitosis: Growth, cell
replacement, and asexual reproduction
A. Mitosis makes it possible for organisms to
1. Grow
2. Regenerate and repair tissues
3. Reproduce asexually
B. Mitosis leads to same number and type of
chromosomes
MEIOSIS and Crossing over
XII. Chromosomes are matched in homologous pairs
A. Somatic cells (soma = body) - body cells
1. A human somatic cell has 46 chromosomes, 2 sets of
23
B. Homologous chromosomes:
1. Each chromosome in a somatic cell has a twin – nearly
identical in length and centromere location
2. Homologous chromosomes carry similar genes
a) if one twin has the gene for hemoglobin in one
location or locus, the other does as well
C. Two general types of chromosomes:
1. Autosomes
a) Pairs 1 through 22
b) found in both males and females
2. Sex chromosomes
a) The 23rd pair
b) XX in females and XY in males (mammals)
c) only small parts of X and Y are homologous, most
genes on X do not have counterparts on Y
d) determines gender
3. One of the homologues is inherited from mom and the
other from dad
XIII. Gametes have a single set of chromosomes
A. Having two sets of chromosomes, one from each parent,
is the key to the human life cycle and all other sexually
reproducing organisms!!
B. The letter “n” = one set of chromosomes
1. 2n = 2 sets (diploid)
2. 3n = 3 sets (triploid)
3. n = 1 set (haploid)
C. Somatic cells are diploid cells
D. Sex cells (gametes) are haploid cells
1. Produced by MEIOSIS
E. Sexual life styles alternate between diploid and haploid
F. Fertilization - The fusion of haploid gametes results in a
single diploid starting cell called a ZYGOTE (fertilized egg).
XIV. Meiosis reduces the chromosome number from
diploid to haploid = Interphase + Meiosis I + Meiosis II
A. Overview:
1. Start with a diploid (2n) cell
2. There are two consecutive divisions – Meiosis I and
Meiosis II
3. The result is four daughter cells, each with one set (n)
of chromosomes
4. Homologous chromosomes are split up in meiosis I
5. Sisters separate in meiosis II
XV. Stages of Meiosis
A. Interphase:
1. Like mitosis, there is a single duplication of the
chromosomes and centrosomes
B. Meiosis I – homologous chromosomes (twins separate)
1. Prophase I – most complicated, occupies 90% of
meiotic division. Chromatin condenses. Synapsis occurs
(homologous chromosomes come together as pairs)
making a tetrad. Crossing over occurs exchanging genes
that may differ between homologous. Nucleoli disappear,
centrosomes move apart, nuclear envelope breaks down,
spindle microtubules attach to kinetochore of tetrads and
motor proteins bring tetrads to the metaphase plate.
2. Metaphase I
3. Anaphase I – starts with splitting of tetrads and
migration of sister chromatids to each pole.
4. Telophase I – chromosomes arrive at poles. Each pole
now has a haploid number.
5. Cytokinesis
6. In some organisms there is an interphase
(chromosomes uncoil, nuclear envelope forms, etc…)
before meiosis I, and in other organisms it just keeps
going.
C. Meiosis II – sisters (clones) separate – same as mitosis
except you start with a haploid number of chromosomes.
1. Prophase II
2. Metaphase II
3. Anaphase II
4. Telophase II and Cytokinesis
XVI. Review : A comparison of mitosis and meiosis
A. Mitosis – produces identical daughter cells – growth,
tissue repair, asexual reproductioin
B. Meiosis – produces haploid daughter cells for sexual
reproduction
C. All events unique to meiosis occur in meiosis I
1. In prophase I, duplicated homologous chromosomes
pair to form tetrads allowing for crossing-over
2. In metaphase I, tetrads are aligned at the metaphase
plate
3. At the end of meiosis I, there are two haploid cells but
each chromosome still has two sister chromatids
D. Meiosis II is virtually identical to mitosis (except cells
are haploid)
E. Mitosis can occur in diploid or haploid cells and results
in identical daughter cells
F. Meiosis can only occur in diploid cells – results in four
haploid daughter cells
XVII. Independent orientation of chromosomes in
meiosis and random fertilization lead to varied offspring
A. The orientation of tetrads in metaphase I of meiosis I is
a matter of chance
B. When they separate in anaphase I, maternally of
paternally inherited genes move independently (randomly)
to both poles – daughter cells are a mix of maternal and
paternal genes.
C. The number of possible combinations of 23 pairs of
chromosomes is 2n or 223 combinations.
D. Combining gametes (fertilization) we get 223 X 223
possible combinations
XVIII.
Homologous chromosomes carry different
versions of genes
A. There can be two or more different flavors of the same
gene
B. Ex. Eye color and coat color in mice
C. C (brown) and c (white) - for different coat color genes
and E (black) and e (pink) for different eye color genes
D. There are two possible outcomes in a gamete (21).
XIX. Crossing over further increases genetic variability
A. Crossing over – the exchange of corresponding
segments between two homologous chromosomes
B. Chiasma – the site of crossing over
C. Synapsis – pairing of two homologous chromosomes
(tetrads)
D. crossing over takes place during synapsis – the only
time the homologues are together.
E. Steps in Crossing over
1. Synapsis
2. Breaking of homologous chromatids
3. Joining of homologous chromatids to new partners
4. Separation of tetrads at Anaphase I
5. Separation of chromatids at Anaphase II
F. Crossing over leads to Genetic recombination, the
chromosomes carrying the shuffled genes are called
recombinants.
G. Why do this? Increases variability – new combinations
of genes – shuffles them up
H. Crossing over can occur several times in variable
locations in each tetrad! Parents could never produce
identical offspring from independent fertilization events.
XX. A karyotype is a photographic inventory of an
individual’s chromosomes
A. Errors can occur in meiosis leading to gametes with
abnormal chromosome number and/or structure (ex.
Down’s Syndrome and Klinefelter’s Syndrome (XXY)).
B. Karyotype – orderly display of magnified images of the
individuals chromosomes from metaphase of mitosis
XXI. An extra copy of chromosome 21 causes Down
Syndrome
A. Trisomy 21 – 3 number 21 chromosomes per cell
B. Gametes with abnormal number of chromosomes
usually abort development (miscarriage).
C. DOWN SYNDROME  characterized by John Langdon
Down in 1866
D. Most common chromosome abnormality (1 out of 700
children born!!!)
E. Characteristics:
1. Unique facial features; notably round face, Flattened
nose bridge, Small irregular teeth, Short stature, Heat
defects, Susceptibility to respiratory infection, Leukemia,
Alzheimer’s disease, Mental retardation
XXII. Accidents during meiosis can alter chromosome
number
A. nondisjunction – when chromosome pairs fail to
separate
1. Can occur in either meiosis I and/or meiosis II
2. Usually results in a miscarriage
XXIII.
Abnormal numbers of sex chromosomes do
not usually affect survival
A.
XXIV.
Alterations of chromosome structure can
cause birth defects and cancer
A. Number of chromosomes can be correct, but structural
changes can be afoot – caused by breakage of
chromosomes
B. Four major types of structural changes:
1. Deletion – part of chromosome lost – lose genes – most serious (cri du
chat or cat cry syndrome – deletion in chromosome 5 - MR, small head, cry
like mewing of a cat, death as infant or early childhood)
2. Duplication – segment of chromosome repeats - extra copies of some
genes
3. Inversion – fragment breaks off and reattaches in the opposite direction
– less harmful, still have all genes in same number
4. Translocation – fragment of one chromosome becomes attached to a
non-homologous chromosome – may or may not be harmful
a) Translocation in somatic cells can lead to cancer
(1) CML – chronic myelogenous leukemia – cancerous white blood
cells – part of chromosome #22 has switched places with part of #9
resulting in the Philadelphia chromosome.