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STUDY GUIDE FOR MODULE NO. ___
CYTOLOGICAL BASES OF HEREDITY
MODULE OVERVIEW
Figure 1: Chromosomes: EM image by Andrew Syred/
Science Source : https://rb.gy/cmnrtc
Figure 2: Chromosomes: 3D image
Credit: iStock: https://rb.gy/mdk7l6
This module reviews the structure of the simplest unit of a living organism- the cell, including its parts and
functions. It focuses on the nucleus where the chromosomes are located. The physical features of the
chromosomes are also discussed here.
Boveri and Sutton's chromosome theory of inheritance states that genes are found at specific locations on
chromosomes, and that the behavior of chromosomes during meiosis can explain Mendel’s laws of inheritance.
MODULE LEARNING OBJECTIVES
1. Recall the concept of Cell and cell theories
2. Compare and contrast the events happening in every stages of mitosis and meiosis and
. Analyze the importance of meiosis in the inheritance of characteristics from parents to offspring
3. Enumerate types and describe morphology of chromosomes
LEARNING CONTENTS (The Cell)
All living things are made up of a living material, the protoplasm, which, in most cases, has a protective and
absorptive membrane boundary between itself and its environment. The protoplasm and the boundary are
organized to form a unit, the cell, which had some important similarities in all organisms. Unicellular and
multicellular organisms have cell structure which usually consists of two distinct areas: the cytoplasm, the major
portion of the protoplasmic substance contained in the cell membrane, and the nucleus, a dark-staining body
within the cytoplasm.
There are two kinds of organisms according to the presence or absence of certain structures in the cell.
The prokaryotes, consisting of bacteria and blue-green algae, have no nuclear membrane that separates the
contents of the cytoplasm from the nucleus. The eukaryotes on the other hand, which consists of the majority of
living species and multicellular organisms, have nucleus membrane that separates the genetic material from the
cytoplasm. Viruses are neither of the two groups. They are organisms that must utilize the cellular activity of their
host and which does not have the cell membrane nor the nuclear membrane that are present in eukaryotes. The
only membrane present in viruses is the vital envelope enclosing primarily the viral genetic material.
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Within the cytoplasm is a number of organelles with active cell function. Their size and presence vary
between different organisms and between tissues. The most important chemical constituent of living organisms
are proteins, and various aspects of their function, structure, synthesis are important to consider. The nucleus, the
main focus for identifying the genetic material, is the primary director of cellular activity and inheritance. Under the
light microscope, staining by various chemical dyes the nucleus consist of dark network, called chromatin, which,
during the process of cellular division, becomes organized as distinct bodies called chromosomes.
NATURE OF CHROMOSOME
Chromosomes vary in size, shape, nature of banding, and the number among different species. It may be
as short as ¼ micron (1 micron = 1 micrometer = .001 millimeter) in fungi and birds, or as long as 30 microns in
Trillium plants. Most metaphase chromosomes range from 3.5 microns in Drosophila, 5 microns in man, and 8 to
10 microns in corn. The fibers that constitute the chromosome range in thickness from 100 angstroms (1
angstrom = .0001 micron =. 0000001 millimeter ) to about 500 angstroms depending upon the treatment to which
they have been subjected. Most often the main fibrous element observed is about 250 angstroms in diameter.
Chromosomes undergoing mitosis are usually rod-like bodies, each with a constriction at the centromere,
called the primary constriction. Further constrictions can be found in some chromosomes which results from
pinching off a small chromosomal section called a satellite. They are often associated with regions where the
nucleolus is formed or attached and are thus called nuclear organizers. Along the length of a chromosome are
knoblike regions, chromo meres, which show distinct sizes and occupy specific positions, giving non homologous
chromosomes their distinct morphological appearance.
Fig. 2.1 A generalized mitotic chromosome with structures often describe under the light microscope. Each thin
chromosome strand (chromonema) is believed to undergo extensive coiling in certain areas to produce the
densely visible chromosome. (After Strickberger)
Chromosome may be classified on the basis of the location of their centromere as:
(1)
(2)
(3)
(4)
Metacentric, centromere central arms of equal or essentially equal length,
Sub metacentric, centromere sub median, giving one longer and one shorter arm,
Acrocentric, centromere very near one end and arms very unequal in length,
Telocentric, centromere terminal with only one arm.
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Fig. 2.2 Digrammatic representation of three types of chromosome based on centromere position: (a)
metacentric; (b) acrocentric; (c) telocentric. (After Strickberger )
Fig. 2.3 Diagrammatic comparison of normal human male karyotype as observed with Q-, G-, and Rbanding techniques. The centromere is represented as observed in Q-banding only. (After Bums)
In interphase and early prophase, chromosomes are in the form of chromatin can be observed:
heterochromatin, tightly coiled or condensed and euchromatin, no condensed. The first is often in close
contact with the nucleolar organizer of a specific chromosome. During nuclear division heterochromatin may
occur in the region of the centromeres, at the ends of the arms or in other intercalary positions. They are known to
contain genetically active regions which include repetitive DNA, nucleolar organizers, and genes for some of the
RNA molecules.
Chromosome Banding. Positive identification of chromosomes was a breakthrough in1968 and 1969. It was
found that many dyes that have an affinity for DNA fluoresce under ultraviolet light. Chromosomes, after suitable
treatment with dyes, show bright and dark zones, or bands, which are specific in location and extent for that
chromosome. These bands are best seen in metaphasic chromosomes. Dyes such as quinacrine mustard and
giemsa stain produced Q-bands and G-bands respectively on the chromosomes. Cells are also treated with alkali
which leads to dense, bright staining in the centrometric region. This produces the C-banding which is specific for
heterochromatin.
Chromosome number. There are two kinds of cells, somatic and germ cells that are present in sexually
reproducing organisms. Somatic or body cells contain diploid number of chromosomes in contrast to matured
germ cells which are haploid, containing only half the number of chromosomes are somatic cells. Somatic
chromosomes of diploid organisms are found in pairs (homologous pair), the members of each pair being alike in
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size, in position of spindle attachment, and in bearing genes relating to the same hereditary characters. One
member of a pair of homologous chromosomes is paternal, the other is maternal.
The number of chromosomes found in various species of animals and plants varies. The lowest diploid
number in the cell of any organism I s2, which is found in certain round worms; the largest number, 300 or more,
is found in some protozoa. Man has 46.
Table 2.1
Chromosome Numbers of Different Species of Animals and Plants
Animals
Diploid number.
Man
46
Rhesus monkey
42
Cattle
60
Dog
78
Horse
64
Cat
38
Donkey
62
House mouse
40
Rat
42
Golden hamster
44
Guinea pig
64
Rabbit
44
Pigeon
80
Chicken
78
Alligator
32
Toad
22
Frog
26
Carp
104
starfish
36
silkworm
56
rod ant
48
house fly
12
mosquito
6
cockroach
23 male 24 female
grasshopper
24
honeybee
32
flatworm
16
nematode
1 male 12 female
Plants
yeast
green algae
barley
rice
spider wort
wheat
corn
snapdragon
squash
upland cotton
tomato
tobacco
evening primrose
kidney bean
white oak
pine
garden pea
potato
white clover
broad bean
fruit fly
slime mold
mold
pink bread mold
penicillin mold
freshwater hydra
Diploid number
18
20
16
14
24
24
20
16
40
52
24
48
14
22
24
24
14
48
32
12
8
haploid number 7
7
8
4
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Both male and female in most diploid organisms possess the full diploid number of chromosomes. In
some organisms as bees, the sexes differ in the number of sets of chromosomes, the females being diploid and
the males haploid.
LEARNING ACTIVITY 1
Exercise 2
http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping.html
Write your answers in your notebook
E-Book:
Chomatin Regulations available at:
https://www.nature.com/scitable/ebooks/chromatin-in-eukaryotic-regulation-16549786/134281460/
Make a Summary of Concepts from this reading material. Write your answers in your notebook
Video Clip:
https://youtu.be/TJfPbtXmngs. (Genetics - Chromosome Structure and Types)
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LEARNING CONTENTS (title of the subsection)
CELL DIVISION
Cell division is an important activity of all kinds of cells. Unicellular organisms divide in order to reproduce.
In multicellular organisms, cell division results to growth and development. Adult form is attained by increase in
the number of cells, size, and differentiation from zygote to maturity.
There are two integrated activities in the division of nucleated cells, nuclear division or karyokinesis and
cytoplasmic division or cytokinesis. Plants and animal cells exhibit two types of nuclear division, mitosis but only
reproductive cells undergo meiosis.
A. MITOSIS
The process of mitosis is divided for convenience into four stages or phases: prophase, metaphase,
anaphase, and telophase. Each stage merges into the next without sharp lines of transition. The sequence of
events transpiring from the close of one nuclear division to the beginning of the next one is the interphase stage.
Genetically, the DNA content of the nucleus is the important constituent that is duplicated between divisions.
Thus, when the cell begins mitosis, it has a double set of chromosomes.
Interphase. Interphase or intermitotic period consists of processes associated with growth and
preparation for mitosis. The duration of interphase (3 hours to 174 hours) is many times longer compared to the
period of entire mitosis, which may range from about ten minutes to a few hours. In humans, interphase occupies
some 18 to 24 hours. It is divided for convenience into the following stages.
G1. The first growth or gap stage of interphase is the stage in which nucleus and cytoplasm are enlarging
toward mature size. Active synthesis of ribonucleic acid (RNA) and protein takes place during this stage.
S. the synthesis stage is the stage whereby replication of DNA and synthesis of histones occur.
G2. The second growth or gap period is the period in which new DNA is rapidly complexed with
chromosomal proteins, and in which synthesis of RNA and proteins continues.
Fig.2.4 The eukaryotic cell cycle
Prophase. In prophase, the first stage of mitosis, the preparations for cell division new chromosomes are
now synthesized in those cases where they have not been formed in the interphase. Coiling of chromosomes and
their condensation make them visible as threadlike structures. Each of the mitotic prophase chromosomes
appears longitudinally split into two duplicates called chromatids. The two sister chromatids of each chromosome
are held together by strands in a specialized region, the centromere. Further movement of each half to opposite
sides of the nucleus, and the beginning of the breakdown of the nuclear membrane.
As prophase progresses, the mitotic apparatus, consisting of the spindle and associated structures, begins to
form. They are slender microtubules that often very nearly from one “end” of the cell to the other. They are
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classified as (1) continuous fibers, continuous from pole to pole of the mitotic apparatus, (2) chromosomal fibers,
extend from one pole to each chromosome, and (3) interzonal fibers, run between the centromeres of the
separating daughter later on in anaphase.
The close of prophase is marked by the movement of the longitudinally double chromosomes to the midplane or
equator, a period called prometaphase or metakinesis.
Metaphase. This stage is characterized by the alignment of the chromosomes at the equatorial plane.
The chromosomes are shortest and thickest during this stage. The sister chromatids are still held together by
connecting chromatin fibers at the centromere region.
Anaphase. During this stage, the sister chromatids separate and move as daughter chromosomes to the
spindle poles. Whereas in metaphase each chromosome is made up of two chromatids; in anaphase, each
daughter chromosome is no longer made up of chromatids. Anaphase accomplishes the quantitatively equal
distribution of chromosomal material to two developing daughter nuclei.
Fig.2.5 Various stages of mitosis in a somatic cell.
Telophase. The beginning of telophase is marked by the arrival of the daughter chromosomes at the
spindle poles; the end is marked by the reorganization of two new nuclei and their entry to the interphase stage.
Generally, telophase events are the reversed of prophase. New nuclei membranes are constructed, mitotic
apparatus gradually disappears nuclei are performed and chromosome resume their long, slender, extended form
as their coils relax.
Although initiated in anaphase, cytokinesis division of the cytoplasm occurs during telophase this
process is accomplished by formation of a cell plate in plant cells and by a cleavage furrow in animal cells.
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Significance of Mitosis. Mitosis accomplishes equal distribution of chromosomes to the resulting daughter cells.
Since genes are located in the chromosomes each daughter nucleus produce in the mitosis would be equal to the
parent nucleus in quality and in quantity of its genes.
B. MEIOSIS
The exact replication and splitting of each chromosome into two identical parts and their subsequent
separation into two cells would not ordinarily lead to any change in chromosome number between the parent and
daughter cells. The mitotic division of four chromosomes for example, could only be expected to give rise to
daughter cells with the similar number. In organism whose cells are always formed by asexual means, the
number of chromosomes should therefore remain constant between generations. In sexually reproducing
organism, however, where zygote is formed by fertilization between male and female gametes, the embryonic
cells would have doubled the chromosomes of each parent if no reduction in number occurred during sex cell
formation.
Sexually reproducing organisms have evolved a mechanism that enabled them to regularly reduce the
number of chromosomes in each gamete to half the usual number. If, for example, four were the regular diploid
chromosome number (2n) in somatic cells then the reduced or haploid number in the sex cells would be two.
Meiosis comes from a Greek word meaning “to diminish” is simply the process by which the chromosomes are
separated during the formation of the sex cells and their number reduced from the diploid to the haploid condition.
Fertilization then marks the event in which two haploid nuclei join to reform a diploid cell. Most organisms are
composed of diploid except for their gametes.
2n
meiosis
n
2n
meiosis
n
fertilization
2n
Some organisms are haploid for most of their life cycle but then, through fertilization of two haploid sex
cells; produce a diploid zygote that undergoes meiosis to form again a haploid stage. The number, as well as the
size and shape of the chromosomes of a species, is usually constant and is called its karyotype.
Meiosis involves two nuclear divisions, termed meiosis I first meiotic division) and meiosis II (second
meiotic division). The phases are given the same names as in mitosis although there are important differences
especially meiosis I. the first division represents a reduction in which members of homologous chromosomes are
separated into daughter cells without duplication.
Meiosis I
In multicellular organisms, meiosis occurs only in relatively few, specialized cells in reproductive organs.
Meiosis begins in these cells with the replication of the chromosomes. As in mitosis, the sister chromatids of each
chromosome are attached to one another at the centromere. From this point, meiosis I differs tremendously from
mitosis.
Prophase I. The first meiotic prophase often persists for a very long time, and may be measured in
weeks or months or even longer. In the human female it persists in each primary oocyte from a fetal age of 12 to
16 weeks until ovulation, generally once each 28 days, after sexual maturity. Some human oocyte, therefore,
persist in arrested prophase I until the end of the reproductive cycle, at menopause. This may be some 45 years
more or less.
The first meiotic prophase is subdivided into the following five stages: leptotene, zygotene, pachytene,
diplotene, and diakinesis (fig. 2.6). In leptotene stage, the chromosomes appear as long slender threads with
many beadlike structures (chromosomes) along their length. Studies show that replication has occurred in the
synthesis stage of the preceding interphase. During the zygotene stage homologous chromosomes appear to
attract each other and enter into a very close zipper like pairing, termed synapsis. This pairing is highly exact and
specific, taking place between all homologous chromosomes. A structure called the synaptonemal complex can
be observed between synapsed chromosomes through electron microscopy. It appears as a ribbon like group of
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three longitudinal components organized in two dense lateral elements and a thin central elements composed
primarily of proteins. The synaptonemal complex may function to pull chromosomes together helping them to pair
more precisely and efficiently.
Figure 2.6 stages of meiosis
The synaptonemal complex is completed at the next stage of the prophase I (pachytene), during which
the synapsed homologous now are clearly seen to be composed of two chromatids each. This group of four
chromatids is known as a bivalent or tetrad, and a series of exchanges of genetic material can occur only or has
already occurred between non-sister homologous chromatids (fig. 2.7). Such exchanges can be detected by
special means and signify the genetic mechanism of crossing over, or recombination. The point at which
exchange of material occurs between non-sister chromatids is evidenced by more or less X-shaped
configurations, the chiasmata. The longer the chromosome pair, the greater the likelihood of more than one
chiasmata, although one chiasmata appears to interfere with the formation of another in a closely adjacent region
of the chromosomes of the same side of the centromere.
In diplotene stage separation of homologous (except at points where chiasmata occur) is initiated.
Chromosomes continue to contract and the nucleolus begins to disappear.
Finally, in the last stage of prophase I, diakinesis, chromosomes reach maximum contraction. The
bivalents usually migrate close to the nuclear membrane and become evenly distributed. The nucleolus either
disappears or detaches from its associated chromosome. During the latter part of this stage, or the early part of
metaphase, the nuclear membrane dissolves and the bivalents attach themselves by their centromeres to the
rapidly forming spindle.
Metaphase I. In metaphase I the chromosomes reach their most condensed state and appear relatively
smooth in outline. The chiasmata that had first appeared during diplotene have now moved toward the ends of
each chromosomes, leaving only the single terminal attachment between the formerly paired arms of homologous
chromosomes. These remaining chiasmata prevent the separation of the homologous chromosomes which now
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lie on each side of the equatorial plate of the spindle stretched by their respective centromeres toward opposite
poles.
https://figures.boundless-cdn.com/18900/large/figure-11-01-04.jpeg
Figure 2.7 Pairing between homologous metacentric chromosomes, and subsequent exchange of chromosome
material in each arm leading to the formation of a bivalent with two chiasmata.
Anaphase I. Although previously duplicated along its entire length, each chromosome still maintains only
a single functional centromere for both of its sister chromatids. The separation, or disjunction of one homologous
chromosome from one another in anaphase toward opposite poles therefore results in this single centromere
(which may be structurally split) dragging both chromatids (dyad) along with it. The chiasmata slip off the ends of
the chromosomes as they are pulled apart, and the poleward chromatids are now bound together at only one
point, the centromere. Since two chromatids compose each dyad, their appearance depends on the position of
the centromere: a double V if the chromosome is either a metacentric or acrocentric, and a single V if telocentric.
Telophase I. These stages vary considerably between organisms. In most cases the dyads reach one of
the spindle poles, a nuclear membrane is forms around them, and the chromosome s pass into a short interphase
before the second meiotic division begins. In the plant Trillium the anaphase group of dyads enter immediately
into the second meiotic division, skipping telophase and interphase. Usually the sequence of events is so rapid
that the interphase chromosomes are not physically extended as in mitosis, nor is there sufficient time to form a
single large nucleolus. Cytokinesis may occur during this stage or may be postponed until simultaneous formation
of four daughter cells at the end of the second meiotic division.
Meiosis II
The chromosomes enter the prophase of the second meiotic division (prophase II) as dyads or two sister
chromatids connected together in their centromere region. Metaphase II is marked by completion of spindle
apparatus and migration of the chromosomes to the equator of the cell. As soon as the connected centromeres
divide, each chromatid (monad) separates from its sister and moves to the opposite pole in anaphase II.
Telophase II and cytokinesis follow rapidly, giving rise to four haploid cells from each initial diploid cell that
entered meiosis.
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In a cell which consists of a diploid number of two chromosomes, or one pair of homologous, the meiotic
events leading to reduction in chromosome number can be summarized as follows:
Meiosis I:
Metaphase I; one bivalent or tetrad (two chromosomes of four chromatids on the metaphase spindle)
Anaphase I; one dyad (one chromosome or two chromatids) pass to each pole.
Meiosis II:
Metaphase II; one dyad on the spindle in each daughter cell
Anaphase II; one monad (one chromatid) pass to each pole.
Significance of Meiosis
The basic significance of meiosis is the formation of four monoploid (haploid) nuclei from a single diploid
nucleus in two successive divisions, thereby preventing the doubling of chromosome number that result from
fertilization. The first meiotic division reduces the chromosome number from diploid to haploid, whereas the
second is similar to mitosis in distributing equal numbers of chromosomes to the resulting daughter cells. The
cellular products of meiosis directly become gametes and/or polar bodies in higher animals, but in vascular plants
the meiotic products are meiospores which give rise to reduced gamete- bearing plants.
Genetic variability is generated by meiosis in two ways: (1) random assortment of paternal and maternal
chromosomes and (2) crossing- over. This occurs during prophase of the first meiotic division when homologous
chromosomes pair, synapse and cross- over resulting to exchange in genetic materials. Each gamete therefore
has a unique combination of genes which is different from any other gamete.
Fig.2.8 Mitosis & meiosis compared
http://4.bp.blogspot.com/ /mitosis+vs+meiosis.jpg
Gametogenesis in Mammals
In mammals, the somatic cells are diploid, while the mature gametes are haploid. Meiosis immediately
precedes the formation of gametes. The diploid condition is restored in fertilization. The following steps are
involved. In the male: (a) Development of diploid primary spermatocytes, (b) Meiosis. The two haploid products of
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the first meiotic division are called secondary spermatocytes. The four cells resulting from the second meiotic
division are called spermatids, and (c) Maturation of spermatids into sperm. In the female: (a) Development of
diploid primary oocytes, (b) Meiosis.
The first division produces two unequal cells, a smaller first polar body and a larger secondary oocyte.
The second division produces two second polar bodies from the first polar body and, from the secondary oocyte,
a third second polar body and a lager ootid, and (c) Maturation of one egg from the ootid; degeneration of the
three second polar bodies.
http://cdn.yourarticlelibrary.com/wp-content/uploads/2014/03/clip_image00248.jpg
Fig. 2.9 Gametogenesis in higher animals. (After Strickberger)
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LEARNING ACTIVITY 2
Name:
Subj. & Sec.
Date
Prof.
Exercise 2
Cytological Bases of Heredity
1. Differentiate the sexual and asexual reproduction.
2. Is this statement true: Asexual reproductive modes occur only among prokaryotes;
eukaryotes rely exclusively on sexual reproduction.
3. If you were to examine the human chromosomes under an electron microscope,
would it be possible to identify homologous chromosomes? How?
4. Explain why you would expect genetic differences between cells to arise from meiosis
and not from mitosis.
5. In tomatoes the somatic chromosome number is 24. How many of each of the following
will be present in one somatic cell at the stage listed: (a) centromeres at prophase, (b)
chromatids at prophase, (c) chromatids at anaphase, (d) chromosomes at prophase (e)
chromosomes at anaphase.
6. A student examining a number of onion root tips counted 1000 cells in some phase of
mitosis. He noted 693 cells in prophase, 104 in metaphase, 36 in anaphase, and 67 in
telophase. From these data, give the longest and the shortest stage of mitosis.
7. In the fruit fly Drosophila, the diploid chromosome number is 8. How many of each of
the following are present in each cell at the stage of mitosis or meiosis indicated?
(a) centromeres at anaphase
(b)centromeres at anaphase I
(c) chromatids at metaphase I
(d) chromatids at anaphase (e) chromosomes at anaphase
(f) chromosomes at metaphase
(g) chromosomes at telophase I
(h) chromosomes at telophase II
(i) tetrads at prophase I
(j) dyads at anaphase
(k) dyads at anaphase I
(l) monads at anaphase II
8. Match stage of meiosis with the correct events.
____ 1. Each chromosome is separated from its homologue
and moves to the opposite pole
_____2. Two parcels of chromosomes move to the equator of
Two spindles
a. Prophase I
b. Metaphase I
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_____3. Sisters chromatids of each chromosome are separated
And move to opposite poles
_____4. Homologues pair with each other and crossing over occurs
_____5. Pair of homologous chromosomes align randomly at the
spindle equator.
c. Anaphase I
d. Metaphase II
e. Anaphase II
9. If a plant with four homologous pairs of chromosomes AA, BB, CC, and DD, is selffertilized, which of the following chromosome combinations would you expect to find
in its offspring: AB? CD? ABCD? AABB? CCDD? AABBCC? AABBCCDD?
AAAABBBBCCCCDDDD?
10. A diploid male organism has two homologous telocentric pairs of chromosomes, A and
B from the maternal parent and A’ and B’ from the paternal parent. (a) Draw the
anaphase of mitosis in this organism. (b) Draw one possible arrangement of these
chromosomes at anaphase I (c) On the basis of your drawing in b draw the appearance
of these chromosomes at Anaphase II. (d) these same chromosomes were involved
in meiosis in a female, would the kinds of egg nuclei produced be different from the
sperm nuclei?
11. The cat, Felis domestica, has a diploid number of 38 chromosomes in its somatic cells,
consisting of 19 homologous pairs (that is, 19 maternal and 19 paternal chromosomes).
A student stated that only one fourth of the gametes produced by meiosis in this animal
will have all of its chromosomes from either maternal or paternal origin. Explain whether
you think the student is wright or wrong.
12. How many human eggs will be formed from (a) 10 primary oocytes? (b) 10 secondary
oocytes? (c) 10 ootids?
13. How many human sperm will be formed from (a) 30 primary spermatocytes? (b) 30
secondary spermatocytes? (c) 30 ootids.
14. Among mammals, differentiate mitosis and meiosis in terms of:
a. number of cells formed in one cycle
b. number of chromosomes (diploid or haploid) in each daughter cell
c. the kinds of cells (somatic and sex cells) that undergo the cycle
d. importance of the cycle
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FM-AA-CIA-15 Rev. 0 10-July-2020
Study Guide in (Course Code and Course Title)
Module No.__
SUMMARY
Cells also contain the body's hereditary material and can make copies of themselves. Cells have many
parts, each with a different function. Some of these parts, called organelles, are specialized structures that
perform certain tasks within the cell.
Chromosomes vary in size, shape and number among species. It may be as short as ¼ micron, or as
long as 3.5 microns.
Under the light microscope, staining by various chemical dyes, the nucleus consist of a dark network
called chromatin in which during the process of cell division- becomes organized as distinct bodies called
CHROMOSOMES.
Chromosomes undergoing mitosis are usually rod-like bodies, each with a constriction at the
centromere. They are linear end to end arrangement of genes and DNA (some proteins and RNA).
Chromosomes are classified
Cell division is the production of two new and identical cells from one single cell. In both prokaryotes
and eukaryotes, DNA replication must occur before cell division. In prokaryotes, cell division occurs by binary
fission
Mitosis produces two diploid (2n) somatic cells that are genetically identical to each other and the
original parent cell, whereas meiosis produces four haploid (n) gametes that are genetically unique from each
other and the original parent (germ) cell. Gametogenesis is the process in which the diploid cells change into
mature gametes.
REFERENCES
Text Book:
Ramirez Dolores A., 2014, Genetics Seventh Edition
Kluggs William S. and Cummings Michael R., 2002. Essential of Genetics 4th ed. Prentice Hall. NJ.
E-Book:
https://rb.gy/zxs6cw
E-Sources:
http://www.biology.arizona.edu/human_bio/activities/karyotyping/karyotyping.html
Video clips:
https://youtu.be/QVCjdNxJreE
https://youtu.be/TJfPbtXmngs
PANGASINAN STATE UNIVERSITY
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