Download UNIT 1: INTRODUCTION TO BIOLOGY

SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
Chapter 8: Reproduction, Meiosis & Genetic variation
- Part II R
Reepprroodduuccttiioonn aanndd tthhee rroollee ooff ggeerrm
m cceellllss

Reproduction is one of the hallmark characteristics of all forms of life on planet
Earth

All living organisms reproduce; that means during reproduction they hand over (=
inherit) their individual genetic make-up and information over to a next generation
which assures the continuation of a species
 Reproduction (in biological terms) is a hallmark feature of living beings and means
the formation of a new organism from a pre-existing one
 for most organisms the new individual starts with a fertilized egg in
a process called fertilization and a series of subsequent cell
divisions
Sex and sexual reproduction from a biological perspective
As humans we may have asked ourselves the same simple question at some point of
our life. Why are there two separate genders established amongst humans and not
more … or just one? How come that not every creature on this planet does a living as a
hermaphrodite, comprising two genders in one body? Why is there two sexes
established in most higher evolved members of the living world and how come that
these two sexes of a given species somehow desire to get involved with the other
gender of the same species to perform acts for the purpose of sexual reproduction?
Well, we easily agree that sex and sexual reproduction is responsible for most what is
flamboyant, colorful and beautiful in nature, i.e. the colorful showy flowers of plants, the
melodies of bird songs, poetry and arts, it can’t be the sole purpose of it.
Biologists agree today, that all these practices, performances and show-offs associated
with the phenomenon of sexual reproduction are just the means to an end which is
sexual reproduction as an effective means to create an increased survival chance of a
species in a changing world due to increased genetic variability.
In order to survive as a species in an ever changing world, the species has to change in
morphology and behavior over time to be able to meet the new environmental
challenges. Since molecular biology tells us today that DNA, genes and genetic
programs dictate the outer appearance and morphology of living organisms, species
can only change if they change their genetic make-up. No genetic variation…no
evolutionary change of life on this planet! That is exactly where sexual reproduction and
the molecular process called meiosis kicks in. As you will see in the following sections
below, sexual reproduction and the connected biological process meiosis assure the
genetic material becomes shuffled and shuffled again during each generational cycle to
purposely increase the genetic variability of that very species.

Three forms of reproduction are established in the living world.
1
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
11.. S
Seexxuuaall rreepprroodduuccttiioonn
This is the predominant form of reproduction in m
muullttii--cceelllluullaarr eeuukkaarryyootteess and
aanniim
a
l
s
.
mals.
- it means the fusion of an egg cell and one sperm to form a so-called zygote,
which starts the life cycle of a new individual (see Figure of life cycle of humans
below)
- the haploid set of genetic material from each parent is fused to create a new
individual with a so-called diploid gene-set
22.. A
Asseexxuuaall rreepprroodduuccttiioonn
This type of reproduction is mostly found in bbaacctteerriiaa and aam
mooeebbaa
- e.g. bacteria reproduce in process called ‘binary fission’ (see Image below)
-- the production of new offspring by a single parent organism
 DNA of the offspring is inherited from one parent or cell
and is identical with the parental DNA
B
Biinnaarryy ffiissssiioonn ooff bbaacctteerriiaa
Rod-Shaped Bacterium, hemorrhagic E. coli, strain 0157:H7 (division) (SEM x22,810)
33.. P
Paarrtthheennooggeenneessiiss
-
This rare form of reproduction is performed by cceerrttaaiinn aam
mpphhiibbiiaannss aanndd rreeppttiilleess
the females of these species are able (dependent on environmental factors) to
switch between sexual and asexual reproduction
a very prominent example for this kind of reproduction is the whiptail lizard which
lives in the Southwestern part of the U.S.
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SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
Alternation of haploid (1n) and diploid (2n)
stages in the human life cycle
Gonads
Ovaries
Testis
Zygote
( 2n )
Gametes
( 1n )
Oocyte
(egg cell)
Sperm
23 chromosomes
( = 1n )
23 chromosomes
( = 1n )
3
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
TThhee rroollee ooff ggeerrm
m cceellllss iinn rreepprroodduuccttiioonn

While mitotic cell division (see Chapter 8) can be preformed by literally all somatic
cells in a multi-cellular organism, formation of reproductive cells, or so-called sex
cells (or  gametes), is only performed by a unique class of germ cells in sexually
reproducing organisms, the so-called gamete mother cells or germ cells

The germ cells perform a unique form of cell division, called meiosis, which has
major, crucial differences to mitotic cell division

One important difference to mitosis is the interesting fact that meiosis does not lead
to daughter cells with the same amount of chromosomes as the parental cell;
daughter cells resulting from meiosis have only half the chromosome number as the
parent cell
“ Meiosis leads to a 50% reduction of the number of chromosomes in
the resulting daughter cells …”

As you remember form the previous chapter, the total cellular DNA of a species (=
genome) is split into a certain number of chromosomes

The number of chromosomes is unique for each species; different species have
different number of chromosomes

Each cell of our human body, or so-called somatic cell, has 46 chromosomes in its
nucleus;
 the exception are the so-called gametes (= sperm and egg
cells) which have only 23 chromosomes

It is these gametes (or sex cells) in biological organisms which are produced by
earlier mentioned so-called germ cells. In humans and in higher organisms the
chromosomes in each somatic cell occur in pairs or sets of two; each cell which has
a double set of chromosomes is called diploid (= 2n)
 each set of chromosomes was received from the biological parents during
fertilization

Simple organisms or the gametes (eggs or sperm) of higher organisms have only
one set of chromosomes; they are haploid (= 1n)

Since most species, including humans, have two sets of chromosomes; they are
also called diploid (= 2n) organisms
 in diploid organisms one set of chromosomes comes from the egg
cell of the mother, the other set of chromosomes comes from the
sperm of the father

The pairs of identically shaped chromosomes within the somatic cells of diploid
organisms are also called the homologous chromosomes
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SAN DIEGO MESA COLLEGE
General Biology (Bio107)

SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
The characteristics of homologous chromosomes are:
1. each pair of homologous chromosomes contains its genes at the same
place or so-called locus on the DNA double helix
2. a gene on the identical locus on a homologous chromosome codes for the
same inherited characteristic, e.g. eye color, blood group
3. but the genes on the corresponding loci on the chromosomes are slighly
different, i.e. have a variation its DNA nucleotide sequence; biologists say the
gene exists in two different alleles
 Of the 23 pairs of homologous chromosomes in humans, 22 pairs are chromosomes
called autosomes
 autosomes are found in cells of males and females
 The other pair of chromosomes is called the sex or gender chromosomes
 the genes found on it determine a persons or species gender
 all females have a pair of X-chromosomes
 while males in contrary have one X and one so-called
Y-chromosome, which differs in size and shape
Gender chromosomes of other biological organisms & Gender determination
While the XX genotype in humans leads to the expression of female body features and
the XY combination determines maleness in humans, gender determination in other
organisms can be completely different. Indeed, biologists found a series of interesting
variations to this theme in nature.
For example, lemmings have three sex chromosomes, called W, X and Y. In these truly
interesting mammals, the XY genotype leads to male animals, while any of the
alternative chromosomal combinations XX, WX or WY, leads to femaleness. The also
possible YY genotype is not compatible with life and these sons die.
The Australian billed, egg-laying mammal Platypus has been shown to have 5 pairs of
sex chromosomes which determine the gender in this species; the XXXXXXXXXX
genotype is found in the female gender of this species, while males have five XY pairs
(= XYXYXYXYXY) in their cells!
In birds, the story is upside down. The XY or only X genotype leads to female birds,
while maleness is genetically dictated by the XX genotype.
Most peculiarly, some animals don’t have sex chromosomes at all.
 2 sets of chromosomes is a hallmark of cell nuclei of higher animals and humans

The complete set of chromosomes of an organism is called the diploid number
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SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
TThhee ddoouubbllee cchhrroom
moossoom
mee sseett ooff aa hhuum
maann ssoom
maattiicc cceellll
1 – 22: autosomes


gender or sex
chromosomes
The diploid number is species-dependent; it varies from species to species
 e.g. the diploid number for humans is 46
 the ant Myrmecia pilosula has a diploid number of 2; the
females have only a single pair of chromosomes; the
males of this group of ants develop from unfertilized
(haploid) eggs and, hence, each of their cells, has only
one single chromosome!!
 the extremely polyploid fern Ophioglossum reticulatum
has a diploid number of 630 (!!), which means that there
are 1260 chromosomes per cell
 All cells of the human body – with the exception of the gametes (or sex cells) - are
diploid
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SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
 The exception are the so-called sex cells or gametes
 the germ cells of ffeem
maallee organisms are called oocytes (or egg cells)
 the m
maallee gametes are called ssppeerrm
m cells
 Gametes have only have one single set of chromosomes in their nucleus; we say
these cells are haploid
 The human life cycle starts with the fusion of two haploid cells (= the female oocyte
with the male sperm cell) in a process called fertilization
 After fertilization the diploid fertilized egg or also called zygote starts to divide by
mitosis to form the diploid embryo
 Early during embryogenesis, haploid gametes are formed by specialized cells, the
so-called germ line cells, by a unique form of cell division called meiosis
M
ME
EIIO
OS
SIIS
S…
… ccrreeaatteess ggeenneettiicc vvaarriieettyy
Meiosis is a special type of cell division which creates:
1. haploid germ cells (eggs and sperm) from a diploid parent cell for
sexual reproduction
2. genetic variety due to tetrad formation and crossing over
 O
Onnee single diploid parent cell is divided to produce ffoouurr haploid daughter cells (=
gametes)
 these haploid germ cells are created in an very early stage of the
female human life cycle
 they remain slumbering in the G2/M-phase of meiosis I for many years
(= “meiotic arrest”)
 they grow out or also called differentiate into the functional
(= mature) oocytes after onset of puberty; this differentiation process is
triggered by the release of certain hormones e.g. GTRH, FSH
 this final maturation of the germ cells or differentiation occurs in
the so-called reproductive organs or gonads of m
maalleess (= testes)
and ffeem
maalleess (= ovaries)
 Meiosis involves a reduction of the genetic material from a double (= diploid, 2n)
chromosomal set to a single (= haploid, 1n) set
 Meiosis comprises two successive nuclear divisions with only one round of DNA
replication!
7
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
S
Sttaaggeess ooff m
meeiioossiiss
P
Prriim
moorrddiiaall ggeerrm
m cceellll
oorr ggaam
meettee m
mootthheerr cceellll

Gametes
((eegggg cceellllss or ssppeerrm
m))

diploid
haploid
 The two major steps of meiosis in more detail:
M
Meeiioossiiss II (= homologous chromosomes separate)
IInntteerrpphhaassee II
1. Chromosomes duplicate to from two identical sister chromatids (which are not
visible in the light microscope!)
2. Centrosomes duplicate
P
Prroopphhaassee II
1. Occupies 90% of the time required for cell division
2. Chromosomes become visible (‘coiling and folding’ of DNA occurs = DNA
condensation)
3. Pairing of homologous chromosomes = synapsis occurs, which leads to the
formation of the so-called tetrads
4. Tetrad formation facilitates the exchange of DNA segments of the neighboring
sister chromatids of homologous chromosomes in a process called
recombination by crossing over
8
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
Crossing over of homologous chromosomes during meiosis

During crossing-over chromatids break and are reattached to a different
homologous chromosome

Crossing over leads to a rearrangement (= shuffling) of the genetic material and
increases the genetic variability (see sections below!)
5. Nucleoli disappear
6. Centrosomes (= the structures where the later microtubules
attach with) move away from the center core to the poles
7. Chromosomes move away and the spindle apparatus forms
8. Nuclear envelope breaks into fragments
9. Tetrads are moved to the cell center guided by microtubules
9
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
M
Meettaapphhaassee II aanndd A
Annaapphhaassee II
1. Chromosomal tetrads are aligned on the metaphase or equatorial plate
- Tetrad alignment happens randomly at this step (Random alignment!)
- That means the maternal and paternal chromosomes of the tetrads will randomly
face different cell poles (see Graphic below)
Random alignment of homologous chromosomes in Meiosis I
General:
Haploid number
n=3
Ncomb = 2n
DNA-replication
For this example:
Prophase I
3
Ncomb = 2 = 8
or
or
or
or
or
or
or
or
Metaphase I
Alignments
2. Microtubules are attached to the kinetochores on the centromeres.
3. Tetrads (or homologous chromosomes) split and chromosomes of each tetrad
move toward opposite poles of the cell.
4. Migration of each homologous chromosome toward the two poles with the sister
chromatids attached to their centromeres
10
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
TTeelloopphhaassee II &
&C
Cyyttookkiinneessiiss
1. Chromosomes arrive at poles of the cell and each cell pole has the haploid
chromosomal set consisting of two sister chromatids
2. Chromosomes uncoil and the nuclear envelope re-appears
3. Cell prepares for the second meiotic division without
chromosome duplication (= no DNA replication occurs!)
M
Meeiioossiiss IIII (= sister chromatids separate)

The key events and processes are essentially the same as in mitosis, with one
important difference: no DNA replication occurs before the onset of meiosis II!
P
Prroopphhaassee IIII
1. Chromosomes condense again
2. Nuclear envelope breaks apart again
3. The spindle apparatus forms again and the 2 sister
chromatids move to the middle of the cell
11
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
M
Meettaapphhaassee IIII aanndd A
Annaapphhaassee IIII
1. Chromosomes are aligned in the cellular center area (= metaphase plate), with the
kinetochores of each sister chromatid pointing towards the opposite cell poles
2. The sister chromatids of each chromosome separate during anaphase II.
3. The (now) individual daughter chromosomes move towards opposite poles along
the spindle apparatus
TTeelloopphhaassee IIII &
& ccyyttookkiinneessiiss
1. Cell nucleus forms at each pole of the cell and cytokinesis occurs at the same time
2. 4 daughter cells are formed each with a haploid number of
chromosomes (= the former sister chromatid)
12
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
Meiosis: Overview of the two meiotic phases and the major cellular
events
Activation
by FSH
Diploid
gamete mother cell
DNA-replication
Prophase I
Metaphase I
Anaphase I
Tetrad formation &
Crossing over
Random alignment
of homologous chromosomes
Homologous chromosomes
segregate
Prophase II
Anaphase II
Sister chromatids
separate
Haploid
gametes
(sperm or oocytes)
13
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
S
Suum
mm
maarryy:: TThhee hhaallllm
maarrkk ffeeaattuurreess ooff m
meeiioossiiss
1. Two consecutive cell divisions but only one duplication of the
chromosomes
2. Four haploid daughter (= germ) cells result
3. Increase of genetic variability
H
Hoow
w ddooeess m
meeiioossiiss iinnccrreeaassee tthhee ggeenneettiicc vvaarriiaabbiilliittyy iinn ooffffsspprriinngg??
 the gene shuffling during meiosis is the raw material for the evolutionary process
of natural selection
 the increase in genetic variability during meiosis is achieved by three events
1. Independent orientation
 of the homologous chromosomes during Metaphase I
 the orientation of homologous chromosomes to form a
tetrad is a pure by-chance incident
 all homologous chromosomal pairs orient independently
at Metaphase I
 for any species the total number Nt of possible combinations of chromosomes
that are distributed into gametes is:
Nt = 2nnn
nn = haploid chromosome number
 for hhuum
maannss with a haploid chromosome number of 2233,
the number of possible chromosomal combinations in a gamete is:
2222333 or 8 million (!!)
 that means every gamete (oocyte or sperm) of humans
contains one of about 8 million possible combinations of
maternal and fraternal chromosomes!!
14
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
2. Random fertilization
-
-
fertilization = the biological process during which a haploid (1n) gamete (=
ooooccyyttee) from one female individual unites (= fuses) with a haploid (1n) gamete (=
ssppeerrm
m) from another male individual to form a diploid (2n) zygote
this process is random = it is not predictable which gamete will finally fuse with
which gamete
but the number of possibilities of chromosomal combinations in the new diploid
can be calculated
it is the product of the number of chromosomal combinations of each individual
e.g. for hhuum
maannss, a m
maann and a w
woom
maann can produce a diploid zygote with any of:
88 m
miilllliioonnss X 88 m
miilllliioonnss = 64 trillion (!!)
different chromosomal combinations !!
3. Crossing over
Electron microscopic picture of a crossing over event
 crossing over is the increase of genetic variability on the gene level
 the exchange of corresponding DNA segments between two
homologous chromosomes during Prophase I of meiosis
 the end of the corresponding sister chromatids of the tetrads occasionally cross
over during synapsis
 with crossing over, new kinds of gametes occur after
meiosis which harbor a new set of haploid chromosomes,
the so-called recombinant chromosomes
 one single crossing over event can affect many genes
and multiple crossing over events can in a cell
 considering that there are more than 1000 genes located in one side arm of a
sister chromatid, any crossing over event affects and rearranges many genes,
which are ‘bulk’-translocated
15
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
Nobody is perfect! Meiosis neither
Non-disjunctions during meiosis, chromosomal aberrations &
abnormalities

Since meiosis consists of many intricate steps, involving many enzymes and protein
components, it is a highly vulnerable cellular process which is prone to errors and
can be affected by many factors

Mistakes or errors which affect the symmetric separation of chromosomes
during meiosis I or II lead to alterations of chromosome numbers or so-called
chromosomal aberrations
 Possible accidents or malfunctions during meiosis which account for alterations
of chromosome numbers are:
1. Non-disjunction in meiosis I
 members of the homologous chromosome pairs fail to separate during
Metaphase I of meiosis (see Figure below)
 as a consequence n+1 or n-1 gametes form at the end of meiosis
Aberrant chromosome numbers in gametes due to a non-disjunction event in
meiosis I
Disturbing
Factor
Interpha s e
Propha s e I
M e ta pha s e I
Prop ha s e II
Graphic©E.Schmid/2001
M e ta pha s e II
n+1 or n-1
Gametes
16
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
2. Non-disjunction in meiosis II
 sister chromatids fail to separate equally during Metaphase II of meiosis
(see Figure below)
 as a consequence, either gametes with a normal set of chromosomes are
the result (n) or gametes with n+1 or n-1 numbers of chromosomes appear
Normal or aberrant chromosome numbers in gametes after a
non-disjunction event in meiosis II
Disturbing
Factor
Interpha s e
Propha s e I
M e ta pha s e I
Prop ha s e II
Graphic©E.Schmid/SWC2001
M e ta pha s e II
n, n+1 or n-1
Gametes
 Fertilization involving gametes with wrong (= aberrant) chromosome numbers or
chromosome patterns results in offspring with chromosomal abnormalities, =
additional/extra or missing chromosomes in the resulting zygote and embryo
 Errors of, or interference with meiosis can also lead to alterations in chromosome
structures (e.g. deletions or translocations) in the gametes; e.g. mistakes
happening during Prophase I of meiosis due to unequal crossing over events can
(if they remain undetected by the cell’s surveillance and repair system) lead to
alterations in chromosome structures in the gametes

Finally, more subtle changes in chromosome structures and shapes can also be
caused by a series of other factors, such as strong irradiation (e.g. X-rays,
radioactivity) and environmental factors (e.g. toxins, viruses).
17
SAN DIEGO MESA COLLEGE
General Biology (Bio107)

SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
Known events which lead to abnormal chromosomal structures are so-called:
1. Deletions
- a chromosomal piece breaks off and gets lost
2. Duplications
- a section of a chromosome duplicates and re-inserts (= dup) at the same
chromosome; often referred to as “partial trisomy”
3. Insertions
 chromosomal pieces/fragments insert into other
chromosomal sites
4. Inversions
 chromosomal piece breaks off a chromosomal site
and re-inserts in opposite direction at same site
18
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
5. Translocations:
- a chromosomal fragment excises and moves over to a non-homologous
chromosome where it inserts
- translocation in somatic cells can contribute to the development of cancer
- e.g. in the cancerous white blood cells of patients suffering from chronic
myelogenic leukemia (CML); in CML a translocation occurred which carried
a part of chromosome 22 over to chromosome 9 and vice versa; as a
result a so-called fusion protein, called Bcr-Abl is formed which leads to a
dysregulation of cellular processes
- a translocation between chromosomes 8 and 14 is found in most cases of a
form of blood cancer called Burkitt's lymphoma
Translocation of a segment of the human
chromosome 5 onto chromosome 18
Chr. #:
5
18
5
5,18
19
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
5. Chromosome rings:
- chromosomal ends of the short p and long q arm break off and get lost
- the remaining chromosome ends fuse and form typical chromosomal ring
structures
- often observed in cells after exposure to strong X-ray irradiation or radioactivity
which is often accompanied with DNA double strand breaks (DSBs)

Fertilization involving gametes with wrong (= aberrant) chromosome numbers,
aberrant chromosome patterns or structures results in offspring with
chromosomal abnormalities which in many cases are the cause for health
disorders and clinical symptoms

The most common chromosomal aberrations observed in sexually reproducing
organisms are monosomies (= cells with a missing chromosome) and trisomies (=
cells with an extra chromosome)

Severe chromosomal aberrations or defects in an individual or embyo can be
early detected and analyzed by preparing a so-called karyotype.

Environmental factors can not only have an effect on the number and shape of
chromosomes, but on the process of meiosis in general. In recent years more and
more scientific studies strongly indicate that bisphenol A (BPA), a chemical
commonly found in plastics and plastic products, may reduce sperm count and
fertility in man.
“According to a study conducted by doctors from Kaiser Permanente and
published in the journal Fertility and Sterility in 2010, men with higher
levels of the chemical BPA in their urine and semen were 2-4 times more
likely to have lower sperm count and to have problems with their sperm
fertility.”
20
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
Karyotype analysis

Genetic disorders caused by chromosomal aberrations or defects can be detected
with by performing a karyotype analysis

Many genetic disorders can be even detected by karyotyping long before birth by
fetal testing with the help of different clinical procedures and techniques

The most commonly applied fetal testing techniques in hospitals and cytogenetic
clinics are:
1. Amniocentesis
- this procedure is performed on pregnant women between the 14th and 16th
week of pregnancy
- a sample (2mL) of the amniotic fluid is consent-collected in a hospital and
biochemical tests as well as karyotyping are performed with the sample
-- the complication rate leading to an unwanted abortion of the embryo is about
1-2%
2. Chorionic villus sampling (CVS)
- performed between the 8th and 10th week of pregnancy
- a small amount of embryonic tissue, the so-called chorionic villi, are taken
from the placenta for testing
- this procedure is faster due to the rapid growth of the embryonic cells of the
villi

Both medical procedures are usually reserved for situations in which the possibility
of genetic disorders is significantly increased, such as in:
1. a 35 year old or older women with their first pregnancy
2. couples with a proven family history for a certain hereditary disorder

A karyotype analysis is usually performed with isolated white blood cells (Adults) or
sampled embryonic cells from the amniotic fluid or chorionic villi of the placenta;
karyotyping is a lab-performed, multi-step procedure which involves following steps
(see Figure below):
1. the sampled and washed cells are incubated with a potent mitogen, e.g. PHA
or concanavalin A (ConA) to induce mitosis in the cultivated cells
2. the cells are blocked in Metaphase of mitosis with the help of added
molecules = spindle blockers, such as colchicines, which interfere with proper
microtubule formation
3. the cells are fixed, the condensed Metaphase chromosomes are stained with
a certain dye (= Giemsa) and observed under a light microscope
21
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
The different steps of the karyotyping procedure
Amniocentesis
or
Venipuncture
(peripheral blood)
KCl
(hypo-osmotic shock)
Embryonic cells or
Lymphocytes
Colchicine
(= Mitosis blocker)
PH A
ConA
(= Mitosis
stimulus)
Centrifuge
Cell culture
(ca. 72h)
(ca. 2h)
(ca.
20 min)
Medium
Centrifuge
Pipette
transfer
Heating
& Drying
Graphics©E.Schmid/200
Microscope
glass slide
Centrifuge
Fixation
Giemsa dye
Methyl alcohol/
Acetic Acid Mix (3:1)
Stained & fixed mitotic cells
Chromosome staining
Microscopy
(250x)
22
SAN DIEGO MESA COLLEGE
General Biology (Bio107)
SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
4. a photo is taken of the observed chromosome smear (see Figure below)
Human metaphase chromosomes before establishing a karyogram
5. the observed metaphase chromosomes of the documented chromosome
smear are arranged according to size, banding pattern and centromere
location, in a typical karyogram (see Image below) and the karyogram
carefully analyzed for chromosome abnormalities, such as trisomies,
deletions or translocations

A typical karyotype analysis involves comparing chromosomes for their:
1.
2.
3.
4.
number (e.g. Trisomies)
length (e.g. deletions, translocations)
placement of centromeres
location and sizes of chromosomal G-bands (deletions, insertions,
inversions)
 the Giemsa dye stains regions of chromosomes that are rich in the base
pairs Adenine (A) and Thymine (T) where it produces the typical dark
bands, the so-called G-bands

The biomedicinal process leading to a karyotype is called karyotyping

During karyotyping, the homologous chromosomes are arranged in a so-called
karyogram and compared
“Every year over 400,000 karyotype analyses are performed in the U.S.
and Canada in the genetic labs of hospitals or clinics.”
23
SAN DIEGO MESA COLLEGE
General Biology (Bio107)

SCHOOL OF NATURAL SCIENCES
Instructor: Elmar Schmid, Ph.D.
The graphic below shows a typical karyogram of a human female after successful
karyotyping
- the homologous chromosomes are identified, cut out and arranged according to
size and chromosome number
- chromosome #1 is the largest human chromosome, while the small chromosome
#22 is shown second last in this typical karyogram below
Karyogram of a human female
1
2
8
9
16
17
3
10
18
Autosomes
11
19
4
5
6
7
12
13
14
15
20
21
22
XX
Sex chromosomes
E.Schmid/2001
24
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (Bio107)
Instructor: Elmar Schmid, Ph.D.
Examples of genetic disorders in humans due to changes in chromosome
numbers or chromosome structures

In the following section we will look at prominent disorders in humans which are
caused by chromosomal aberrations and which can be diagnosed with the help
of the karyotyping method
1. Trisomy 21 (= Down syndrome)
- characterized by cells which show an extra copy of the chromosome 21
in a karyogram
- Down Syndrome is - together with Trisomy 13 - the most common
chromosome number abnormality in humans
- affects about 1 out of every 700 children born in the US
- genetic basis of the rare human disorder called Down syndrome
- affected people suffer from heart defects, susceptibility to respiratory
infections, leukemia and usually have a shorter life-span
- many affected individuals exhibit varying degrees of mental retardation
- incidence of Down syndrome in the offspring of genetically normal parents
increases markedly with the age of the mother
- the chromosome alterations of Trisomy 21 are suspected to occur after
fertilization
2. Trisomy 13 (Pateu’s syndrome)
- Affected human individuals are characterized by cells which show an
extra copy of the chromosome 13 in a karyogram (see red circle in
Figure below)
- This chromosomal aberration is observed in about 1 out of every 5000
life births
- Trisomy 13 babies are frequently stillborn or die as newborns
- Physical abnormalities include: severe mental retardation, growth
retardation, mis-development of the brain/spinal cord, cleft lip and palate,
cyclopia (one eye) often with protruding facial proboscis (see Figure
below)
25
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (Bio107)
Instructor: Elmar Schmid, Ph.D.
Karyotype: Trisomy 13
Examples of clinical features of Trisomy 13:
26
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (Bio107)
Instructor: Elmar Schmid, Ph.D.
3. Trisomy 18 (= Edwards syndrome)
- characterized by cells which show an extra copy of the chromosome 18
in a karyogram
- 2nd most common autosomal aberration; incidence: 1/8000 live births
- severe facial (cleft lip, microcephalus) and skeletal malformations
- many organ malfunctions
- 90% of children with Trisomy 18 die by 12 months of age

In most cases of severe chromosomal aberrations or abnormal chromosomal
numbers, e.g. Triploidies, however, the affected human embryo is spontaneously
aborted long before birth (= miscarriage)
 all Trisomies, except Trisomie 21, produce infants who will usually die during
the first few months of life
 around half of all early spontaneous abortions are due to Trisomies
4. Fragile X-syndrome (Martin-Bell syndrome)
- The most common form of inherited mental retardation
- Affected individuals have cognitive/behavioral/neuropsychological
problems  mild-to-moderate autistic-like behavior
- occurs in 1/1500 male and 1/2500 female life births
- clinical phenotype results from Fragile X-chromosomal ends (Xq27.3)
- A short, extra chromosomal fragment is observable at one of the two long
sister chromatid arms of the X-chromosome
- This “chromosomal appendix” is caused by excessively long DNA
inserts, so-called (CGG) n repeats in the coding sequence of the FMR-1
gene which is located at the X-chromosomal end
 normal:
 pre-mutated
 fully mutated
-



5-55 CGG repeats
65-230 CGG repeats
> 230 CGG repeats
the karyotype is characterized by a thin strand of genetic material which
extends beyond the long arm at the highly conserved Xq27.3bof the Xchromosome  makes the X chromosome fragile looking
5. Ataxia telangietactica (AT) or Louis-Bar syndrome
- autosomal recessive inherited disorder
- characterized by chromosomal defects, most importanlty translocations
affecting the chromsomes # 7 and # 14
- juvenile development of excessive blood vessels (= telangiectasias) in the
conjuctivae of the eyes
- immune defiency leads to heavy, recurrent infections of the lungs and the
bronchial system
- many individuals suffering from AT have a high tendency (predisposition)
to develop tumors (lymphomas, leukemias)
- AT patients are extremely sensitive to X-rays and strong irradiation
27
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (Bio107)
Instructor: Elmar Schmid, Ph.D.
6. Chromosome 5p deletion syndrome or (“cat cry – cri du chat
syndrome”)
- cri du chat syndrome is caused by a deletion of the short (= p) arm of
the human chromosome 5
- affects 1/20,000 to 1/50,000 human life births
- infants with cri du chat syndrome commonly have a distinctive cat-like cry
- show severe mental retardation
- low birth weight, microcephaly, webbed fingers or toes
- cause is not known  spontaneous loss of a piece of chromosome 5
during development of an egg or sperm
7. Chromosome 13q deletion
- relatively common finding in human blood cancers, such as:
- B-cell chronic lymphocytic leukemia (CLL), non-Hodgkin's lymphoma
(NHL) and multiple myeloma (MM)
Image of chromosome #13 deletion
8. Chromosome 4p deletion (= Wolf-Hirschhorn syndrome)
- human disorder first described by Wolf (Freiburg, Germany) and
Hirschhorn (New York, U.S.A.)
- caused by mostly “de novo” deletion of a tiny chromosomal fragment from
the short arm of chromosome # 4
- affected children show pronounced physical and psychological
impairments with retarded development
- body shows several deformations affecting the face, but also scalp and
heart defects may be observed
9. 14/21 Translocation carrier
- most common translocation leading to Down syndrome
- caused by an exchange = translocation between chromosome #14 and
#21
- almost the complete chromosome #21 is attached to the short (p) arm of
chromosome #14
- affected individuals are phenotypic normal but bear high risk of having a
Down syndrome child
28
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (Bio107)
Instructor: Elmar Schmid, Ph.D.
10. 22q-“Philadelphia” chromosome translocations
- chromosomal anomalies which lead to certain forms of blood cancers,
e.g. acute lymphatic leukemia (ALL)
- it is caused by translocation of a fragment of the long arm of
chromosome 22 over to chromosome 9
- the t(9;22) translocation event brings two genes called c-ABL (chr. #9q34)
and BCR (chr. # 22q11) together and forms a new hybrid gene
- the expressed hybrid gene leads to a 190 or 210 kDa so-called fusion
protein Bcr-Abl, which plays a role in cancer development
Formation of the “Philadelphia chromosome” by translocation

non-disjunction in meiosis does not only affect autosomes , such as
chromosome 21, but can also lead to abnormal numbers of sex chromosomes
 this has usually less dramatic effects on the genetic balance and on the
phenotype of the carrier than unusual numbers of autosomes
 this probably due to the lesser number of genes on sex chromosomes and the
natural inactivation of the second X-chromosome in an XX (= female)
individual

the most common sex chromosome abnormalities are:
1. Extra Y chromosome (XYY)
- observed in 1 out of 2500 live births
- affected males have larger body stature and borderline intelligence
- new studies give hints to mild to severe social behavioral disturbances
with trend to accumulated criminal records
29
SAN DIEGO MESA COLLEGE
SCHOOL OF NATURAL SCIENCES
General Biology (Bio107)
Instructor: Elmar Schmid, Ph.D.
2. XXY = Klinefelter syndrome (XXXY or XXYY)
- cells of human males show an extra X chromosome
- this chromosomal aberration is observed in 1 out of 2000 live births
- affected males have abnormally small testes and the affected individual is
sterile
- often accompanied by breast enlargement
Karyotype of a human individual with Klinefelter syndrome
X X Y
3. XXX = Multi-X female (“super-female”)
- carriers have limited fertility
- around one woman in 1000 has three or more X-chromosomes
- most 47,XXX women are ‘normal’
- the 48,XXXX woman are usually mildly retarded, and 49,XXXXX produces
severe disability
4. XO = Turner syndrome
- affected individuals are females which show underdeveloped ovaries, poor
breast development and a so-called web of skin between neck and
shoulders
- carriers are sterile
30