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Chapter
3. Reproduction and Chromosome Transmission
Study general features of
chromosomes.
Cellular division.
Sexual Reproduction.
Chromosomal theory of
inheritance.
General Chromosomal
Characteristics.
Chromosomes are the
structures within living cells that contain the genetic material.
Chromosomes
contain an extremely long molecule of DNA and proteins that give it an
organized structure.
Before we begin to study
transmission genetics we need to review the differences between prokaryotes
(prenucleus) and eukaryotes (true nucleus). Prokaryotes usually have a single circular chromosome in a nuclear area.
Eukaryotes have a membrane
bound nucleus that contains sets of linear chromosomes.
Eukaryotic chromosomes are
examined with karyotypes.
Usually accomplished by
observing chromosomes in actively dividing cells.
Chromosomes are visualized
in lymphocytes a somatic cell (not a gamete = germ cell).
Eukaryotic chromosomes are
inherited in sets.
Many eukaryotes are diploids
(each chromosome is member of a pair).
23 pairs in humans 39 pairs
in dogs (78).
The pair of chromosomes in
a diploid organism is referred to as a homologous pair.
Within each pair one
chromosome is a homologue of the other and means they are very similar. »
Similar in size and
genetic material. »
However they are not
identical: due to alleles. »
The physical location of a
gene is called its locus (plural = loci)
Cellular division.
One purpose of cell
division is asexual reproduction.
Often single celled
organisms (mother cell) divide producing two identical daughter cells
from.
Ex: bacteria, amoeba and Saccharomyces
cerevisiae. »
S.c. also has sexual repro
as well.
Another purpose is
building multicellular organisms.
Cell division requires the
duplication, organization and sorting of chromosomes. Can be simple (bacteria) or complex (eukaryotes).
Binary fission is a
bacterial cell dividing into two daughter cells.
Binary fission may occur
every 20-30 min. (E. coli).
Bacteria first replicate
(copy) their circular chromosome.
A septum then forms
dividing the two cells into daughter cells. This is an asexual form of reproduction because there is no contribution from two different gametes.
Transmission of
chromosomes in eukaryotic cells requires a sorting process known as mitosis.
Eukaryotic cell division is
a process aimed at producing two daughter cells that have the same number
and types of chromosomes as the original.
This a more complicated
process than binary fission. »
Stages:
G for growth, S for synthesis (of genetic material) and M for mitosis. »
There are two G phases G1 and G2.
Most of the cells lifetime
is spent in G1, S and G2. »
During G1 a
cell prepares to divide once it reaches a certain point it has reached the
restriction point and must continue on to cell division.
During S the chromosomes
are replicated and the two copies of the chromosome are called sister
chromatids. »
The sister chromatids are
attached at the centromere (kinetochore attachment site). »
Cells at this point (G2) have 46 pairs of sister
chromatids or 92 chromatids.
During G2 the
cell accumulates the necessary materials for nuclear and cellular
division. »
Progression to M phase
then occurs.
During M phase mitosis
occurs. »
Separates and distributes
a full compliment of chromosomes to each daughter cell.
Walter Flemming (1870s)
had the most careful study of mitosis and coined the term (mitos =
thread). »
Showed that both daughter cells got the identical group of threads
(chromosomes) comparable to the number from the parent. After the cell progresses through interphase it enters mitosis:
Ex: diploid cell with four
chromosomes (2 per set).
At prophase the chromosomes
have already replicated to produce sister chromatids (Ex: 8).
Distribution of
chromosomes during mitosis is brought about by the spindle apparatus. »
Spindle apparatus is
organized from two centrosomes. »
These centrosomes direct the formation of specialized proteins
called microtubules (MT).
The centrosomes will
migrate two either side of the cell one within each of the future daughter
cells. »
Microtubules are used to
guide half of the chromosomes to each of the poles. »
Condensation is necessary for to organize the chromosomes during
division.
After condensation the
compact chromosomes (sister chromatids) line up at the metaphase line in
the middle of the cell.
This occurs in late
prophase and the beginning of metaphase.
During metaphase the
formation of the spindle apparatus is complete.
Each pair of sister
chromatids is attached to both poles by the kinetochore microtubule. »
The kinetochore
microtubule emanates from the kinetochore which is part of the centrosome.
The pairs of sister chromatids are organized into a single row at
the center of the cell during metaphase.
Anaphase is the next step
of the sorting process.
The connection holding the
sister chromatids together is broken. »
Each chromosome is only
linked to one pole at this point.
The chromosomes move
towards the pole at which they are attached due to shortening of the
kinetochore microtubules.
Polar microtubules also tend to move the two poles further apart.
The chromosomes reach their
poles and de-condense during telophase.
The nuclear membrane now
reforms to produce two separate nuclei that each contains four
chromosomes.
Cytokinesis is the
separation of the two nuclei into daughter cells. »
Occurs in animal cells due
to cleavage furrow formation and constriction. »
Occurs in plant cells by the formation of a cell plate.
Barring mutation both
daughter cells should be identical genetically to each other and the
parental cell.
The development of
multicellularity relies on the repeated process of mitosis.
For a diploid organism most
somatic cells are genetically identical.
Germ cells (gametes) are
distinctly different.
Sexual Reproduction:
Sexual reproduction occurs
most often through parents producing gametes which fuse and produce
offspring through fertilization.
Some eukaryotic species
(like fungi) are isogamous meaning the gametes produced are
similar.
Sperm are usually small
and motile while the egg cell (ovum) is usually large and immobile.
Gametes are 1n or haploid.
In many species the haploid
gametes arise from diploid germ cells. Meiosis is a special sorting event that ensures that a gamete receives only one copy (1n) of every chromosome.
Meiosis produces gametes
that are haploid.
Edouard van Beneden (1883)
observed that gamete formation produced cells with only half of the number
of chromosomes.
Meiosis like mitosis occurs
after the cell has progressed through G1, S and G2
of the cell cycle (interphase).
However, there are then two successive cell divisions rather
than one. Ex: a cell with four chromosomes (2n).
The chromosomes are
replicated in S phase before meiosis occurs to produce pairs of sister
chromatids.
This is followed by two
subsequent cell divisions called meiosis I and meiosis II.
Meiosis I and II both
contain prophase, metaphase, anaphase and telophase. Prophase I is divided into periods known as:
Leptotene- chromosomes begin to condense, you can begin to notice
sister chromatids.
Zygotene- involves a
recognition process known as synapsis. »
The homologous chromosomes
recognize each other and then align themselves along their entire lengths. »
Synapsing of the
chromosomes is facilitated by a group of proteins called the synaptonemal
complex. »
These two pairs of sister chromatids are called bivalents.
Pachytene- after the
bivalents form crossing over may occur. »
This involves physical
exchange of chromosomal material between the bivalents. »
The site of crossing over
is the chiasma (chiasmata)
The diplotene stage- the
synaptonemal complex begins to disappear. »
Becomes easier to
visualize the bivalents (tetrad).
The synaptonemal complex completely disappears in the last stage,
diakenesis.
Prophase
I is also the stage at which the spindle apparatus and the
chromosomes are attached to the kinetochore microtubules.
Metaphase I the tetrads
line up along the metaphase plate.
Double row instead of a
single row (mitosis).
Random alignment of
homologues, many ways the homologues could randomly align themselves.
Kinetochore microtubules
attached to a pair of sister chromatids. »
One pair to one pole and
the other pair to the other pole. »
Different than in mitosis (attached to both poles).
Each pair of sister
chromatids migrate to a pole.
Telophase I decondensation
occurs, nuclear membrane reforms and the separate nuclei contain still
joined together sister chromatids (2 Sister chromatids in our example). Followed by cytokinesis then meiosis II.
Meiosis II identical to
what occurs during mitosis, except the starting point.
Meiosis II begins with the
two cells having sister chromatids (four sister chromatids (two pair) in
our example).
Other wise it is analogous
to mitosis.
What is produced:
Four haploid (1n) daughter cells.
Our example produces: »
Mitosis- two diploid
daughter cells with 4 chromosomes each. »
Meiosis- four haploid daughter cells with 2 chromosomes each.
The result with alleles is
also different between the two. How?
Mitosis the chromosomes
are? »
Genetically identical.
Meiosis the chromosomes
are? »
Not genetically identical due to only carrying one homologous
chromosome.
In mammals spermatogenesis
produces four haploid sperm and oogenesis produces a single haploid egg.
Spermatogenesis occurs in
the testes (gland).
Spermatogonial cells
divide by mitosis to form two cells one will remain a spermatogonial cell
while the other forms a spermatocyte.
The spermatocyte goes
through meiosis I and II.
Produces four haploid
spermatids. »
Mature into sperm cells
(still haploid). »
The head contains the genetic material and acrosome (organelle with
digestive enzymes).
Oogenesis occurs within
special cells of the ovary called oogonia.
Early in the development
of the ovary the oogonia initiate meiosis to produce primary oocytes. »
~ 1 million primary
oocytes per ovary before birth in a human female.
Primary oocytes are
arrested (dormant phase) at prophase I of meiosis until sexual maturity. »
One oocyte per month is
then matured.
Maturation only produces
one ovum instead of four.
Progresses through meiosis I to form a secondary oocyte and a polar
body.
The secondary oocyte then
continues through meiosis II and is also released from the ovary during
this process (ovulation).
If the sperm cell
penetrates the oocyte it completes meiosis II. »
Produces the haploid egg
and a second polar body. »
Sperm and egg nuclei unite to form diploid nucleus.
Plants alternate between
haploid and diploid generations.
Unlike animals the plant
life cycle alternates between
haploid (gametophyte) and diploid (sporophyte).
In complex plants the
organism we think of as the plant is the sporophyte.
The gametophyte are
microscopic haploid spores produced by meiosis within the sporophyte.
In an angiosperm plant
(true seed producing) meiosis occurs in the anthers and the ovaries.
In the anther diploid microsporocytes undergo meiosis to produce 4
haploid microspores. »
Microspores undergo
mitosis to form a two cell system (a haploid tube cell and generative
cell) this matures into the pollen grain. »
Generative cell undergoes
mitosis to form two sperm cells usually only occurs if the pollen grain
germinates. »
This is then considered a
mature male gametophyte.
Female gametogenesis
occurs within the ovules inside the ovaries.
Four megaspores are produced from a cell called a
megasporocyte by meiosis.
Three of the four
megaspores degenerate, while remaining haploid megaspore undergoes three
mitotic divisions accompanied by asymmetric cytokinesis. »
Produces seven cells
called the embryo sac.
At fertilization one sperm
nuclei enters the central cell that has two polar nuclei creating a
triploid cell (3n).
This becomes the endosperm
(food-storage tissue) after a mitotic division.
The other sperm nucleus
enters the egg cell the nuclei fuse to produce a diploid plant embryo.
After fertilization ovule
becomes the seed and ovary is the surrounding fruit.
Chromosome Theory of
Inheritance:
How is chromosomal
transmission related to inheritance of an individuals characteristics.
Really began to take off in
the late 19th century.
Mendel, Weismann and
Nageli.
Hertwig, Strasburger and
Flemming. »
Where the genetic material
was located. 1902-3 Theodore Boveri and Walter Sutton propose the chromosomal theory of inheritance from their microscopic studies of meiosis.
According to the
chromosomal theory of inheritance the inheritance patterns of traits can
be explained by the transmission patterns of chromosomes during
gametogenesis.
Sutton and Boveri realized
that Mendels segregation and independent assortment ideas mirrored
chromosomes during meiosis.
Tenets of the chromosomal
theory of Inheritance. 1)
2) Chromosomes are
replicated and passed along generation to generation and cell to cell
Each type of chromosome
retains its individuality during cell division and gamete formation.
3) Nuclei of most
eukaryotic cells contain chromosomes that are found in homologous pairs.
Chromosomes segregate into
separate gametes.
Offspring get one from one
parent and one from the other parent. 4) During gamete formation, different types of chromosomes segregate independently of each other.
5) Each parent contributes
one member of a set of chromosomes.
These individual
chromosomes are functionally equivalent.
Each set carries a full
complement of genetic determinants. We can see a relationship between Mendels laws of segregation and independent assortment and the chromosomal theory of inheritance.
Gender differences
correlate with the presence of the sex chromosomes.
1901 C.E. McClung (fruit
fly geneticist) suggested that male and female genders were due to the
inheritance of particular chromosomes.
We now know that there are
a pair of chromosomes, called the sex chromosomes.
In mammals we have the X-Y
gender system.
Male X-Y and female X-X.
The male is the
heterogametic sex and the female the homogametic sex. »
Why?
Other gender systems
include:
X-O operates in many
insects, the male only has one X (XO) where the female has two(XX). »
In some insects like
Drosophila they males are actually XY but they are still considered XO.
Why?
Z-W is used in birds and
some fish the male is ZZ and female is ZW. »
Z-W is used to differentiate it from mammals.
There is also a
haploid-diploid system like that found in bees. »
Male bees (drones) come
from an unfertilized haploid egg, while females the workers and queens
come from fertilized eggs.
Exp. Chromosomal
Inheritance.
Relationship between a
genetic trait and the inheritance of a sex chromosome in Drosophila
melanogaster.
Thomas Morgan studied
embryology and development.
Studied the affect of the
dark on the flies.
Saw no noticeable changes
even if the flies were treated with mutagen.
One day they recovered a
white-eyed male fly rather than the normal red-eye.
Must have arisen from a
new mutation, because it was a true-breeding strain.
Morgan and his graduate
student went on to study the inheritance of the white-eyed trait by making
crosses and quantitatively analyzing the data.
Did a test cross- a cross
between a recessive individual and an individual with an unknown phenotype.
Ex: w+
designates the wild-type allele (red-eyed) and w the recessive allele
(white-eye).
His results indicated that
the inheritance of the trait paralleled the inheritance of the X
chromosome.
Location of the w
alleles on the X chromosome is denoted Xw+ and Xw.
Interpreting the
Data:
Modern interpretation:
F1 all red-eyed
therefore?
The F2 is consistent with the idea that eye color is
located on the X-chromosome.
Ratio of 3470 : 784 is ~ 3
: 1.
Punnett square predicts no
female white eyed flies. »
Did this hold up?
Data indicates that the
eye color alleles are located on the X-chromosome these are now called X-linked
genes or X-linked alleles.
The test cross data are
also consistent with this form of inheritance.
Predicts a 1:1:1:1 ratio.
Observed data was 1.5 :
1.5 : 1 : 1. »
What could be an
explanation for the lower numbers of white-eyed flies? »
First geneticist to
receive the Nobel prize 1933.
These Punnett squares do
not yield the same results.
Genes on human sex
chromosomes can be transmitted in an X-linked, Y-linked or a
pseudoautosomal pattern.
Hemizygous
is used to describe the single copy of an X-linked gene in the male.
Sex linkage refers to a
gene residing on one sex chromosome and not the other.
There are also Y-linked
genes like SRY. »
Necessary for male development.
X- and Y- chromosomes also
have small regions of homology.
Carry the same genes in
these locations.
Promotes necessary
interactions during meiosis I.
MIC2
is a gene necessary for antibody production and is found in this region. »
Inherited in a pattern
similar to a gene on an autosome even though it is on the sex-chromosomes. »
What is the male
transmittance pattern of MIC2 and how does it differ from a sex-linked
gene?
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