Cytology of Genetics
Eukaryotes
 replication
 mitosis
 meiosis
 recombination
 chromosome variation
 number of sets
 number of chromosomes is a set
 chromosome modifications
- Speciation due to chromosome
modifications
1
Eukaryotes - cells with linear chromosomes in
true nuclei bounded by nuclear envelopes and
that undergo meiosis.
DNA can be in three organelles in eukaryotic
cells:
nucleus - linear DNA, 96-98% of total DNA in
the cell
mitochondria - circular DNA, 1-2% of
cellular DNA
chloroplast - circular DNA, 1-2%of cellular
DNA
Nucleus
1. Nuclear membrane - has openings, ‘pores’,
that allows passage of material such as
nucleotides, enzymes into the nucleus and
mRNA, tRNA, and rRNA out of the
nucleus.
2
2. Nucleolus - dark staining body in the
nucleus, region of high transcription of
rRNA.
3. Chromatin - relaxed form of linear DNA
found at interphase. Chromatin exists in
two forms:
euchromatin - stains lightly at interphase,
regions of potential transcription.
Heterochromatin - stains dark at
interphase, regions of inactive or
condensed DNA.
4. Chromosomes - condensed (tightly wound)
DNA during mitosis and meiosis. The
chromosome is actually a complex of DNA
and proteins. There are two primary types
of proteins:
1. basic  histones
2. acidic
3
Histones - basic proteins rich in lysine and
arginine. The positive charge allows
the proteins to interact with the
phosphate groups in the DNA.
There are 5 types of histones:
H1 - lysine rich
H2A - slightly lysine rich
H2B - slightly lysine rich
H3 - arginine rich
H4 - arginine rich
The histone proteins H2A, H2B, H3, and H4
combine (2 copies of each) to form an
octomer. DNA is coiled twice around an
octomer in the nucleus. This gives the
appearance of beads on a string. Each bead
is called a nucleosome.
4
The nucleosomes facilitate condensing of the
chromatin in interphase and to form
chromosomes for mitosis and meiosis.
Reason for condensing of the chromatin
before cell division - to insure each daughter
cell gets all the genetic information.
First step in condensation is the connecting of
the nucleosomes by histone H1.
The next step is the coiling of the
nucleosomes, further shortening the length of
DNA. This coiled structure can exist during
interphase.
5
The coiled DNA can be attached to proteins
on the nuclear membrane during interphase
forming loops. Prior to the start of either
mitosis or meiosis the loops attach to a
nonhistone (acidic) protein scaffolding. This
further condenses the chromatin.
The scaffolding can also have folds resulting
in a densely packed DNA = chromosomes
6
Condensation of Chromatin to form
Chromosomes
DNA
Diameter
(nm)
2
Length
40 mm
DNA + histones
(nucleosomes)
10 - 11
8 mm
Solenoid fibril
25 - 30
1 mm
Looped domains
250
50 μm
Radial loops +
scaffold
850
4 μm
7
Chromosome shape
1. primary constriction - determines length of
arms of the chromosome. It is also the site
for attachment of the microtubules during
mitosis or meiosis. Other names for the
primary constriction are centromere or
kinetochore.

Primary constriction
Based on the location of the primary
constriction you can describe a chromosome
as metacentric, acrocentric or telocentric.
arms =
1 arm > other arm
only one arm
8
2. Secondary constriction - on a few
chromosomes there can be a second area
that appears to be constricted. This is a
region of late transcribing DNA that codes
for rRNA genes. It is related to the
nucleolus and is called the NOR or
nucleolar organizing region.
secondary constriction
primary constriction
9
Cell Cycle
stages
G1 - growth stage for synthesis of products
needed for DNA replication.
S - DNA synthesis stage
G2 - growth stage of products necessary for
cell division.
M - Mitosis or Meiosis
Control of the cell cycle
10
Timing of the cell cycle is controlled by the
presence and activity of specific genes and
gene products.
cdc genes  cell division cycle genes
MPF  maturation promoting factors
cdc2 protein + cyclin = MPF
cyclin  protein present during interphase
that disappears at mitosis
The presence of MPF triggers mitosis, the
breakdown of the nuclear membrane, and
cyclin degradation.
The absence of cyclin allows mitosis to end.
As mitosis ends cyclin and MPF are low
11
Summary of Control of the Cell Cycle
Mitosis
MPF
cyclin

As interphase progresses cyclin is synthesized
and combines with cdc2 protein to produce
inactive MPF.

interphase
cdc2 + cyclin = MPF
protein

inactive MPF is then modified to produce
active MPF

presence of active MPF induces mitosis

mitosis (start) cyclin starts to be degraded

loss of cyclin deactivates MPF
mitosis (end) absence of cyclin allows
mitosis to end

12
nuclear membrane starts to reform with the
absence of MPF allowing the cell cycle to
start again
Mitosis - nuclear division with the resulting
daughter cells having the same amount of
DNA as the parent cell
Four stages of mitosis
1. prophase
2. metaphase
3. anaphase
4. telophase
13
Meiosis - Two successive nuclear divisions
that result in the reduction of the
chromosome number to half that of the
parental cell
equational
division
N
N
2N
reductional
division
N
N
2N  somatic cell chromosome number
N  gametic cell chromosome number
14
Terminology for Meiosis
 Each chromosome in a cell has a duplicate
based on gene location and order. Together
these chromosomes are called homologous
chromosomes
 What makes the chromosomes homologous
is that they contain the same genes in the
same linear sequence.
 The homologous chromosomes pair during
meiosis.
 Chromosomes that do not pair at meiosis
are called non-homologous chromosomes.
15
Meiosis can be divided into stages just like
mitosis. Because of the events in the first
division, prophase is divided into sub-stages.
Reductional division
1. Prophase I
a) leptotene
b) zygotene
c) pachytene
d) diplotene
e) diakinesis
2. Metaphase I
3. Anaphase I
4. Telophase I
Interkinesis
16
Equational division
1. Prophase II
2. Metaphase II
3. Anaphase II
4. Telophase II
17
Prophase I
a) Leptotene
 chromosomes begin to condense
 chromosome ends (telomeres) are
attached to the nuclear membrane
 nucleolus is present
 sister chromatids are not visible.
b) Zygotene
 chromosomes continue to condense
 pairing (synapsis) of the homologous
chromosomes occurs. Paired
chromosomes called a bivalent.
 synaptonemal complex starts to form
 nucleolus present
18
Prophase I
c) Pachytene
 chromosomes continue to condense
 synaptonemal complex extends the
length of the bivalent
 crossing-over between non-sister
chromatids occurs
 sister chromatids begin to become
visible
 nucleolus present
19
Prophase I
d) Diplotene
 chromosomes continue to condense
 synaptonemal complex disappears
 centromeres become visible
 sites of crossing-over become visible.
Called chiasmata - singular or chiasma
- plural
 nucleolus starts to disappear
20
e) Diakinesis
 chromosomes continue to condense
 nucleolus disappears
 nuclear membrane disappears
 chiasma move to the ends of the
bivalents
Force of separation causes the tangled
regions to move to the ends of the
bivalent.
classic ring bivalent
21
Metaphase I
 bivalents align on the metaphase plate.
Anaphase I
 homologous chromosomes separate and
move towards opposite sides of the cell.
Telophase I
 homologous chromosomes reach their
respective poles.
Interkinesis
 the homologous chromosomes relax. This
stage may or may not occur.
22
Equational division - similar to mitosis
1. Prophase II
2. Metaphase II
3. Anaphase II - separation and migration of
sister chromatids
4. Telophase II
The result of meiosis is the production of four
cells with half (N) the number of
chromosomes as the parental cell (2N)
In the female only one of the four cells
survives to be an egg cell (animals - ootid,
plants - megaspore)
23
Post-meiotic mitotic divisions - additional
cycles of mitosis that occur in the gametic cell
after meiosis.
In animals once meiosis has occurred
additional cell divisions are not necessary to
produce mature gametes.
In plants additional mitotic divisions are
necessary to produce mature gametes with
the proper number of nuclei.
 males: require two additional mitotic
divisions to produce 2 sperm nuclei and one
generative nucleus. The generative nucleus
directs the growth of the pollen tube. The
two sperm nuclei are for the double
fertilization of the egg and endosperm
nuclei in the female.
 females: require three additional mitotic
divisions to produce one egg nucleus, two
polar nuclei, 2 synergid nuclei and 3 antipodal nuclei.
24
Eukaryotic Recombination
3 ways for recombination to occur:
1. Sorting of non-homologous chromosomes
2. Crossing-over between non-sister
chromatids
3. Transposable elements
25
1. Sorting of non-homologous chromosomes
 The orientation of paternal and maternal
chromosomes at metaphase I of meiosis is
random.
 This results in the potential for different
combinations of maternal and paternal
chromosomes migrating to the centrioles.
 This results in the gametes having a unique
assortment of genes from the mother and
father
 This ‘mixing’ of genes by shuffling of nonhomologous chromosomes is the basis of
Mendel’s law of independent assortment.
26
2. Crossing-over between non-sister
chromatids - or recombination
 Prophase I: with synapsis of homologous
chromosomes there is the potential for
exchange of DNA between non-sister
chromatids.
 For a difference to be observed the nonsister chromatids must vary slightly for the
DNA sequence of a gene or genes.
 Crossing-over can occur between any of the
non-sister chromatids.
 Crossing-over can occur at more than one
site or between more than two non-sister
chromatids.
27
- The Holliday model is the best for
explaining how recombination occurs
between non-sister chromatids.
There are other models that have been
proposed that address differences between
what has been observed and what would be
expected with the Holliday model. One such
model is the Meselson-Radding model.
28
Transposable Elements
 First observed in maize
1938 - M. Rhodes
1950’s B. McClintock
 Observed that genes for certain mutations
appeared to ‘move’.
 In fact the changes that were observed were
due to pieces of DNA that could move and
insert into specific DNA sequences.
 Insertion and disassociation caused altering
of the DNA sequence of a gene resulting in
the changing of expression of that gene.
 Transposable elements have now been
identified in other eukaryotic systems
(yeast, drosophila) and in prokaryotes
(called transposons).
29
Characteristics of transposable elements
A. Has regions of direct or inverted repeat
DNA sequences.
B. Region that may carry specific genes,
possibly for movement of the element.
Movement of transposable elements
30
1) insertion
a) not necessary to have base
complementarity with the gene where the
transposable element will insert. The site of
insertion may depend on a recognition
sequence for the enzyme responsible for
insertion.
Insertion is initiated by a staggered cut in
the target gene (endonuclease activity).
b) After insertion the uneven ends are filled
in resulting in direct repeat sequences
flanking the transposable element.
Migration of transposable elements
31
There are 3 different ways for transposable
elements to move:
1) conservative transposition
2) replicative transposition
3) retro-transposition
32
Uses of transposable elements
 can be used to cause mutations
problem - may be unstable
 autonomous element can move so
would be unstable.
 non-autonomous element would not
be capable of independent
movement so would be stable unless
another autonomous element was
present in the genome.
 could be used to insert genes
problem - would not know where the
element would insert.
33
Chromosome variation
Chromosome number among species
Species
2N
N
human
dog
cat
mouse
fruit fly
46
78
38
40
8
23
39
19
20
4
barley
corn
tomato
wheat
potato
14
20
24
42
48
7
10
12
21
24
34
Chromosome number within specific tissues
salivary glands in drosophila fruit fly have
large polytene chromosomes that are the
result of multiple chromosome divisions
without cell division. Process is called
endomitosis.
The reason for the extra chromosome copies
in the cell is to produce high levels of salivary
proteins.
Polytene chromosomes made it possible to see
area of euchromatin and heterochromatin
that corresponded to specific genes.
Differential expression of genes could also be
observed with these ‘large’ chromosomes.
35
Chromosomes with specific functions
Sex chromosomes
There are two types of chromosomes in a cell
when considering sex determination:
1. sex chromosomes - carry genes that
determine the sex of an individual.
2. autosomes - chromosomes that are not
involved in sex determination.
Example - humans
2N - 46
autosomes - 44
sex chromosomes - 2
36
In humans have X and Y chromosomes
2X = female = homogametic sex
XY = male = heterogametic sex
Because of this the female’s gametes will be
100% X while the male’s gametes are 50% X
and 50% Y. So the male is the one who
determines the sex of the progeny.
in some species the male is homogametic and
the female is heterogametic (turkeys,
chickens)
37
What happens to the extra X chromosome in
females?
Good example of facultative heterochromatin
in most cells one X is extremely condensed.
This can be seen at interphase and is called a
Barr body
Females will have one Barr body
Males will not have a Barr body
In cats can tell that either X chromosome can
condense. The way to do this is to have
variation in a gene on the X chromosome.
38
Example - Coat color in tortoiseshell or calico
cats.
1 X carries a gene for red coat color
1 X carries a gene for non-red color
The color of a section of fur will depend on
which X chromosome condensed during
development. It also indicates that once an X
is designated as a Barr body it remains a Barr
body in that cell line.
The result of random condensing of the X
chromosomes is a female cat with a
patchwork (red and non-red) coat.
Why would you be surprised to see a male
tortoiseshell or calico cat?
39
Chromosome modifications
Euploidy - Changes in the number of sets of
chromosomes.
Aneuploidy - changes in the number of
chromosomes in a set.
Changes within a chromosome
a) deletion
b) duplication
c) inversion
d) translocation
40
Euploidy - change in the number of sets of
chromosomes.
If a species has more than two sets of
chromosomes the organism is said to be a
polyploid.
2 sets
3 sets
4 sets
6 sets
diploid
triploid
tetraploid
hexaploid
To describe the number of chromosome sets
in a species you use X to equal or represent
one chromosome set.
Humans - diploid
2N = 46 chromosomes
N = 23 chromosomes
2X = 46 chromosomes
X = 23 chromosomes - number of
chromosomes in one set
41
Wheat - hexaploid
2N = 42 chromosomes
N = 21 chromosomes
6X = 42 chromosomes
X = 7 chromosomes
42
If all the sets are homologous, i.e. originating
from the same species, then the species is
called an autopolyploid.
Examples of autopolyploids - potatoes,
strawberries
Advantages of an autopolyploid:
- plant parts and fruits are larger
- plant may be more vigorous
Disadvantages of an autopolyploid:
- problems in meiosis with chromosome
pairing and even distribution of
chromosomes.
- low or no seed set
Why is poor or no seed set not a problem with
potatoes and strawberries?
43
Another way to have no seed set is to have an
uneven number of chromosome sets in a
polyploid.
Example: banana, an autotriploid
Possible origin:
1. multiple fertilization (not likely)
2. fertilization of an unreduced (2N) gamete
3. hybridization between a tetraploid and a
diploid.
4X
x
2X

3X
44
Problems in gamete formation and seed set
are due to uneven chromosome pairing and
division in the reduction division of meiosis.
(i.e. how do you divided a trivalent equally?)
Advantages of autotriploids
plants - larger fruit
no seeds
oysters - no egg or sperm production
45
If the chromosome sets originated from more
than one related species then the organism is
said to be an allopolyploid.
The chromosomes that are from related
species but carry the same genes in relatively
the same linear order but do not pair during
meiosis are said to be homeologous
chromosomes.
Advantages of allopolyploids
 plants and seeds are larger
 more genetically diverse so able to
adapt to more environments
 fertile because only bivalents are
formed in meiosis because the
homeologous chromosomes will not
pair.
Examples: wheat
canola (rapeseed)
46
Example: wheat
An allohexaploid with chromosome sets
donated by 3 related species. All the donor
species (AA, BB, DD) were diploids with a 2N
number of 14.
AA
X
BB

AB
 double the number of chromosomes
through endomitosis
AABB now have allotetraploid (4X)

AABB
X DD

ABD
 double the number of
chromosomes by
endomitosis
AABBDD allohexaploid (6X)
2N = 42
N = 21
47
Chromosomes 1A, 1B, and 1D would be
called homeologous chromosomes because
they carry the genes for the same traits.
Why are mules sterile?
Example of an allodiploid
Horse (HH)
X
Donkey (DD)

Mule (HD)
In meiosis would you have proper
chromosome pairing and distribution?
Chromosome changes
48
 Changes in the number of chromosomes in
a chromosome set will result in aneuploidy.
 With aneuploidy the addition or deletion of
a chromosome may have a major or minor
effect.
 Effect depends on the genes that are on the
missing or additional chromosome and the
tolerance of the species to aneuploidy.
Mammals have a low tolerance while plants
have a high tolerance, especially polyploids.
 Aneuploidy occurs due to non-disjunction
of either the homologous chromosomes or
the sister chromatids during meiosis.
Non-disjunction during first or second
division of meiosis
49
first division
n+1
n+1
n+1
n-1
n-1
2n
n-1
second division
n+1
n
n-1
n
n
2n
n
Example of the addition of a chromosome
50
humans - 2N = 46
If one chromosome is added the condition is
called trisomic with 2N+1 = 47
If a pair of homologous chromosomes are
added the condition is called tetrasomic with
2N+2 = 48
If two non-homologous chromosomes are
added the condition is called double trisomic
with 2N+1+1 = 48
examples in humans
 trisomic for chromosome 21 - Down’s
syndrome
 trisomic for a sex chromosome Klinefelter’s syndrome (XXY)
Other examples of trisomy in plants include :
51
 barley
 corn
The presence of an extra chromosome can be
useful in determining the chromosome
location of a gene because the presence of the
extra chromosome disrupts normal
Mendelian segregation.
Trisomics can also occur that are only partial
chromosomes (acro-trisomic or telo-trisomic).
The presence of an extra chromosome
(trisomy) is observed more in diploid species
than the deletion of a chromosome
(monosomy) because diploids do not have the
necessary gene redundancy to tolerate the
loss of any genetic information.
Example of the deletion of a chromosome
52
wheat - 2N = 42
If one chromosome is missing the plant is said
to be monosomic with 2N-1 = 41.
If a homologous pair of chromosomes is
missing the plant is said to be nullisomic with
2N-2 = 40.
If two non-homologous chromosomes are
missing the plant is double monosomic with
2N-1-1 = 40.
Example in humans
Monosomic for a sex chromosome - Turner’s
syndrome (X instead of XX)
This form of aneuploidy in polyploid plants is
useful for genetic analysis.
Chromosome changes
53
 changes within a chromosome
a) deletions
b) additions
c) inversions
d) translocations
54
Deletions
 Loss of a section of a chromosome. This
loss could be from an end or from within
the chromosome.
 Deletions can be caused by unequal
crossing-over, by radiation or by movement
of transposible elements.
 Can observe deletions in meiosis during
prophase I if the individual is heterozygous
for the deletion.
55
Duplications
 Increase in the number of copies of a gene
or genes on a chromosome.
 Can be the result of uneven crossing-over.
Example: Bar eye in drosophila. A physical
change in the eye shape relates to
the addition of a band or bands
seen on polytene chromosomes.
Polytene
chromosome
Eye
shape
phenotype
56
Inversions
 When the gene order in a section of a
chromosome is reversed.
 Occurs when the chromosome is looped
and breakage and reunion occurs but
reunion is not to the original ends.
57
 Two types of inversions can occur:
a) paracentric inversion where the
centromere is not involved in the
inverted region.
b) pericentric inversion where the
centromere is involved in the inverted
region.
 Problems can occur in meiosis if the
individual is heterozygous (one normal one
inverted chromosome) for the inversion. To
get proper pairing of the chromosomes a
loop structure must form.
58
 The other requirement for a problem to
occur is that a cross-over must occur within
the inversion.
If a cross-over occurs in the inverted region
then duplications and deletions can occur
resulting in non-viable gametes. The
presence of the non-viable gametes makes the
individual partially sterile.
The type of duplication and deletion that may
occur depends on whether it is a paracentric
or pericentric inversion.
Result of being heterozygous for a
paracentric inversion.
Anaphase I - dicentric bridge and acentric
chromosome.
End of Meiosis - two functional gametes and
two non-functional gametes
with deletions and possibly
duplications.
59
Results of being heterozygous for a
pericentric inversion.
Anaphase I - no dicentric bridge, no acentric
chromosome.
End of meiosis - two functional gametes and
two non-functional gametes
with complementary
duplications and deletions.
60
Translocations
 The transfer of a section of one
chromosome to another chromosome.
If an exchange occurs between two nonhomologous chromosomes you now have
reciprocal translocations.
61
Only time that reciprocal translocations can
be a problem is when an organism is
heterozygous for the translocations.
The individual will appear normal since there
is no genetic information missing or
duplicated.
The problem occurs in meiosis when the
homologous regions of the chromosomes
attempt to pair in prophase I. Depending on
how chromosomes separate at anaphase I, the
resulting gametes could carry deletions and
duplications of regions of the chromosomes
involved in the translocations resulting in
non-viable gametes.
62
When the chromosomes pair a cross-shaped
structure can form.
When they separate they can either separate
in a adjacent formation or an alternate
formation.
Adjacent - open ring structure leads to nonviable gametes due to duplications
and deletions.
63
Alternate - figure eight or twisted circle
structure that leads to viable
gametes because all the
translocated chromosomes go to
one pole and all the normal
chromosomes go to the opposite
pole.
64
Why do duplications and deletions result in
non-viable gametes or if viable, result in a
modified organism?, dose effect.
Most systems depend on not only the presence
of specific genes but also the proper level of
expression of the gene. The absence or
duplication of genes leads to abnormally low
or high levels of expression of a gene product.
Example in humans - translocation of
chromosome 21 resulting in Down’s
syndrome.
65
But, mate this individual with a person with
normal chromosomes:
66
Speciation and Chromosome Modifications
Inversions and Translocations can lead to
speciation in two ways:
1. Creation of blocks of genes that are
inherited as a unit due to suppression of
recombination.
- in a heterozygote for an inversion
recombination will not be observed
for genes within the inversion.
1. Increase in sterility of heterozygotes as
the number of inversions and
translocations increase.
Example:
Individual A x Individual B
(3 inversions)
(no inversions)
F1 (heterozygous for 3
inversions)
Level of fertility: .5 x .5 x .5 = .125 or 12.5%
67