Transcript Meiosis

4.2: Meiosis and Sexual
Life Cycles
The process of creating sex cells
i.e.: Eggs (females)
Sperm (males)
Cell Reproduction
Asexual (vegetative) reproduction
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A form of duplication using only mitosis.
Example, a new plant grows out of the root or a
shoot from an existing plant.
Produces only genetically identical offspring
since all divisions are by mitosis.
Offspring called clones meaning that each is an exact
copy of the original organism
This method of reproduction is rapid and effective
allowing the spread of an organism
Since the offspring are identical, there is no mechanism
for introducing diversity.
Sexual reproduction
Formation of new individual by a combination of two sex
cells (gametes).
Fertilization- combination of genetic information from two
separate cells
Gametes for fertilization usually come from separate
parents
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Female- produces an egg
Male produces sperm
Both gametes are haploid, with a single set of
chromosomes
The new individual is called a zygote, with two sets of
chromosomes.
Meiosis is a process to convert a diploid cell to a haploid
gamete, and cause a change in the genetic information to
increase diversity/variation in the offspring.
4.2.1: State that meiosis is a reduction division of a
diploid nucleus to form haploid nuclei. [Obj. 1]
The goal of meiosis is to make gametes.
Gametes are sperm cells and egg cells
Chromosomes
Chromosomes
In humans, each somatic cell has 46 chromosomes.
Made up of 23 pairs of homologous chromosomes
chromosomes that make up a pair – that have the
same length, centromere position and pattern of
coded genes.
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The two chromosomes of each pair carry genes controlling
the same inherited characters. (i.e. if a gene for eye colour
is situated at a particular spot (locus) on a certain
chromosome, then the homologue of that chromosome will
also have a gene specifying eye colour at the equivalent
locus.
Except for the two sex chromosomes – the chromosomes that
determine an individual’s sex (all other chromosomes are called
autosomes.
How Do Scientists Read
Chromosomes?
4.2.2 Homologous
chromosomes
Homologous chromosomes: two
chromosomes that are the same size and
show the same banding pattern.
4.2.5 State that, in karyotyping,
chromosomes are arranged in pairs
according to their size and structure.(1).
Make a Karyotype
Karyotype: a
visual
representation
of the
organization of
the
chromosomes in
the cell of an
organism.
Ploidy: Number of sets of
chromosomes in a cell
Haploid (n)-- one set chromosomes. In humans
n=23
Diploid (2n)-- two sets chromosomes. In humans
2n=2x23=46
Most plant and animal adults are diploid (2n)
Eggs and sperm are haploid (n)
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Each gamete has a single set of the 22 autosomes plus
a single sex chromosome, either X or Y. These are the
only cells in the body not produced by mitosis.
The formation of a gamete is considered a reduction
division because the number of chromosomes is
reduced by half.
Chromosomes in a Diploid Cell
Summary of chromosome
characteristics
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Diploid set for humans; 2n = 46 (2 sets of 23)
Autosomes; homologous chromosomes, one
from each parent (humans = 22 sets of 2)
Sex chromosomes (humans have 1 set of 2)
Female-sex chromosomes are homologous (XX)
Male-sex chromosomes are non-homologous (XY)
What is Meiosis?
Figure: Sexual
Life cycle
Meiosis involves two
successive nuclear
Gametes
divisions that produce four
n
n
haploid cells. The first
n
division (meiosis I) is the
FERTILIZATION
MEIOSIS
reduction division; the
second division (meiosis
II) separates the
chromatids.
ZYGOTE
Occurs in the ovaries or
2n
2n
2n
2n
testes.
Fertilization restores the Diploid
diploid condition.
multicellular
organism
Advantages of Meiosis
Mitotic cell division produces new cells
genetically identical to the parent cell.
VS
Meiosis increases genetic variation in the
population. – exchange of information
can occur during meiosis I while
mutations can also occur
Unique Features of Meiosis
Comparison of Meiosis and Mitosis
4.2.3: Outline
the process of
Meiosis
Interphase is an important stage preceding meiosis.
Without this stage meiosis would not occur.
During this stage, each individual chromatid replicates,
similar to mitosis..
At this stage, the chromosomes are long and stringy and
are not visible.
**Remember: All somatic cells are diploid in number
(2n), therefore for each chromatid there also exists its
homologue, which also replicates during interphase.
Prophase I
Prophase I is one of the most important stages
of meiosis.
The chromotid threads begin to twist and
condense, creating chromosomal structures
which are visible to the microscope.
In a process called synapsis, homologous
chromosomes, each made up of two sister
chromatids, come together as pairs.
After the homologous chromosomes pair, the
structure is referred to as a tetrad (four
chromatids). The point at which two non-sister
chromatids intertwine is known as a chiasmata
(sing. = chiasma).
Sometimes a process known as crossing over
occurs at this point. This is where two non-sister
chromatids exchange genetic material. This
exchange does not become evident, however,
until the two homologous pairs separate.
(10.1.1: Describe the behaviour of the
chromosomes in the phases of
meiosis. [Obj. 2])
10.1.2: Outline the process of the formation of
chiasmata during crossing over
Meanwhile, centrosomes move away from each other and
spindle fibres form between them.
Animations
The nuclear membrane disappears.
Metaphase I
At metaphase, each chromosome
has reached its maximum density.
The homologous pairs and their
sister chromatids also prepare for
separation.
They interact with spindle fibers
which form from either side of the
nuclear envelope of the cell.
There is a centriole at opposite
ends of the cell, which is referred
to as poles.
During metaphase, the
chromosomes are lined by the
spindle fibers at what is known as
the metaphase plate.
Anaphase I
Spindle fibres pull apart
the tetrad, separating
each homologous
chromosome. Sister
chromatids still remain
attached to each other
and move as a single
unit toward the same
pole.
It is by random chance
that a certain
chromosome is pulled to a
certain pole.
Telophase I varies from species to
species.
Sometimes Telophase I is skipped and
meiosis starts its second division
immediately.
In general, however, two nuclear
envelopes begin to surround the
separate chromosomes and
cytokinesis (splitting of the cytoplasm
into two separate entities) will
sometimes occur.
Each pole now has a haploid
chromosome set, but each
chromosome still has two sister
chromatids.
Then a phase called interkinesis will
follow, which essentially is a resting
period from Telophase I to Prophase II.
This differs from mitosis because DNA
replication does not occur again.
Telophase I
Prophase II
During Prophase II, each
dyad (1/2 a tetrad) is
composed of a pair of
sister chromatids and
they are connected by a
centromere.
The centrioles (replicated
during Telophase I) which
produce the spindle fibers
also start to move toward
the poles of the cell.
Metaphase II
Metaphase II is similar to Metaphase I
in that the dyads are lined up at a
metaphase plate by the spindle fibers.
Anaphase II
The centromeres of sister chromatids
finally separate, and the sister
chromatids of each pair, now individual
chromosomes, move toward opposite
poles of the cell.
Each sister chromatid ends up on one
side of the cell.
At the end of Telophase II,
the nuclear envelopes forms
around each set of DNA at
opposite poles of the cell
and the cytoplasm divides
once again (cytokinesis).
As a result, four haploid
daughter cells have formed
from one diploid cell.
The chromosomal content of
a haploid cell in one-half the
chromosomal content of a
diploid cell (n as opposed to
2n)
Animations
Telophase
II
Cell Photograph of Telophase II
the chromosomes are ½ the number in a somatic cell
Stages of Meiosis
•End result of meiosis  gametogenesis = production
of gametes
Spermatogenesis = process of male gamete
production, one diploid cell gives rise to 4 sperm cells
Oogenesis = process of female gamete production, one
diploid cell gives rise to 1 viable egg cell and three
polar bodies, occurs in the ovaries once a month
starting at puberty.
Meiosis
Cdck- cell division activity
10.1.3: Explain how meiosis results in an
effectively infinite genetic variety in gametes
through crossing over in prophase I and
random orientation in metaphase I. [Obj. 3]
10.1.4: State Mendel’s law of independent
assortment. [Obj. 1]
Genetic Variation
Independent Assortment of
Alleles
Independent Assortment and Gamete
Diversity
Random Orientation of Chromosomes
During Meiosis
1. Independent Assortment of Chromosomes:
Arrangements of chromosomes are sorted
out/moved to opposite poles by chance into
gametes; maternal and paternal (Metaphase I)
2. Crossing Over: Combining DNA inherited from
two parents into a single chromosome.
(Prophase I)
3. Random Fertilization: An egg cell has 1/8 million
possible chromosomes combinations, is fertilized
by a single sperm cell, which represents 1/8
million different possibilities. (Potentially
producing a zygote with any of 64 trillion diploid
combinations).
10.1.5: Explain the relationship between the
law of independent assortment and meiosis
Independent assortment occurs during
metaphase I of meiosis, when
homologous chromosomes line up along
the equatorial plane.
As chromosomes sort randomly, they
create opportunities for new
recombinants during fertilization.
4.2.4: Explain that non-disjunction can lead to changes in
chromosomes number
Mutations can be inherited if an error occurs in the formation
of one gamete and that gamete fertilizes another to form a
zygote  all the cells in the zygote will carry the mutation.
Nondisjunction: members of a pair of homologous
chromosomes do not move apart properly during meiosis I,
or in which sister chromatids fail to separate during meiosis
II.
 One gamete receives 2 of the same type of chromosome
and another gamete receives no copy
 Anueploidy = abnormal chromosome number
 Trisomy = if the chromosome is present in triplicate in the
fertilized egg (2n+1 chromosomes) e.g. double in egg
cell, normal in sperm cell; e.g.:Trisomy 21 = Down
Syndrome
 Monosomic = if a chromosome is missing (2n-1)
 Polyploidy = when organisms have more than 2
complete chromosome sets (triploidy – 3n, tetraploidy –
4n) occurs if the cell fails to divide after replicating its
chromosomes OR nondisjunction of all its chromosomes.
Down Syndrome
Karyotype
Using Karyotypes
to Predict Genetic
Disorders
3 of
chromosome
21
Other Errors
Syndrome Defect
Extra
Petau
chromosome 13
Problems
Causes numerous physical and mental
abnormalities, owing mostly to heart
defects
Lifespan is measured in days
Edward
Trisomy 18 extra
chrom 18
Turner
Females are
missing X
chromosome X
Usually naturally aborted or stillborn
The most common characteristics of
Turner syndrome include short stature and
lack of ovarian development. A number of
other physical features, such as webbed
neck, arms that turn out slightly at the
elbow, and a low hairline in the back of the
head are sometimes seen in Turner
syndrome patients. Individuals with Turner
syndrome are also prone to
cardiovascular problems, kidney and
thyroid problems, skeletal disorders such
as scoliosis (curvature of the spine) or
dislocated hips, and hearing and ear
disturbances.
More errors:
Syndrome
Defect
Problem
Triplo-X
Females
have an
extra X
chromosom
e: XXX
The symptoms of this syndrome vary widely. The
one consistent feature is tall stature. Some females
exhibit no or very few symptoms, while others have
more severe features of developmental delay and/or
Males have
an extra X
chromosom
e: XXY
XXY males usually have difficulty with expressive
language the ability to put thoughts, ideas, and
emotions into words. In contrast, their faculty for
receptive language-understanding what is said-is
Males have
an extra Y:
XYY males are fertile, they have testes of normal
size, and they have a normal sexual libido and
potency. In spite of a somewhat decreased sperm
quality with many so-called immature sperm cells,
the fertility seems to be normal. In the same way as
for triple-X women, who as a rule do not get children
with an extra X chromosome, males with XYY most
probably only very rarely get sons with two Y
Klinefelter
Jacobs
XYY
behavioral abnormalities.
close to normal.
chromosomes.
4.2.6: State how karyotyping is
performed
Karyotyping is performed using cells
collected by chorionic vilus sampling or
amniocentesis, for pre-natal diagnosis of
chromosome abnormalities.
http://www.massasoitbio.net/courses/136/136_courseassets/cummings_animat
ions/karyotype.html
4.2.7: Analyse a human karyotype to
determine gender and whether nondisjunction has occurred.
http://learn.genetics.utah.edu/content/begin/traits/predictdisorder/