the cell cycle - semantic scholar
TRANSCRIPT
The Cell Cycle
Meiosis
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Core Concepts
• Meiosis is a special kind of division that produces four (4)
haploid, non-identical cells.
• Errors may occur during cell division, producing cells with
abnormal chromosome number.
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Keywords
• anaphase
• centromere
• chromatin
• chromosome
• crossing-over
• cytokinesis
• diploid
• G1
• G2
• genome • haploid • homologue • interphase • kinetochore • meiosis • metaphase • mitosis
• non-disjunction • prophase • sister chromatid
• spindle • synthesis • telophase • tetrad
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The Cell Cycle
Meiosis
Reductional Division
for Sexual Reproduction
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Meiosis in
Sexual Life Cycles
• Homologous
chromosomes
– pair that has the same
length, centromere position,
staining pattern
– contain similar sets of
information
• Humans
– 22 pairs of autosomes
– 1 pair of sex
chromosomes
https://upload.wikimedia.org/wikipedia/commons/3/35/Sky_spectral_karyotype.png
http://legacy.hopkinsville.kctcs.edu/instructors/Jason-Arnold/VLI/m2celldivision/f9-01_homologous_chromo_c.jpg
Checkpoint 1 Complete the table.
Organism
Horsetail
Carp
Chicken
Pineapple
Haploid number,
N (a) 52 39 (d)
Diploid number,
2N 216 (b) (c) 50
https://en.wikipedia.org/wiki/List_of_organisms_by_chromosome_count
Checkpoint 2 For this cell,
a) How many chromosomes are present?
b) How many sister chromatids are present?
c) How many homologous pairs are present?
Checkpoint 3
Using the two colors of clay in front of you,
a) Designate one color to stand for paternal chromosomes and another for maternal chromosomes
b) Create three sets of homologous chromosomes to symbolize a cell in which 2N=6
c) Model how the cell would undergo mitosis
Meiosis involves 2
stages of nuclear
division
• Interphase
–G1, S, G2
• Meiosis
–Meiosis I
–Meiosis II
Meiosis I is reductional cell division
Meiosis I
Separating
homologous
chromosomes
Crossing-over during Prophase I Exchange of segments between homologous pairs
Homologues pair up tetrad
Synapsis “crossing-over” that occurs at a chiasma
Does not normally happen to sex chromosomes
increases genetic variation
Checkpoint 4
Using the clay, model what occurs during Prophase I of meiosis for a cell with 2N=6
a) Create at least one tetrad
b) Model at least one event of synapsis or crossing-over
c) Point out the chiasma
Meiosis I Separating
homologous
chromosomes
Checkpoint 5
Model what occurs during Metaphase I of meiosis:
a) RANDOMLY arrange the paired homologous chromosomes in two lines.
b) How many different arrangements are possible?
Independent assortment during Metaphase I Random separation of homologous chromosomes
Checkpoint 6 A cell with 46 chromosomes undergoes Metaphase I.
How many possible unique combinations can arise in the gametes given that homologous chromosomes randomly separate?
Meiosis I Separating
homologous
chromosomes
Checkpoint 7
a) Model what occurs during Anaphase I of meiosis.
b) How is the result different from the result at Anaphase of Mitosis?
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Meiosis I Separating
homologous
chromosomes
Checkpoint 8
Model the processes of Telophase I and Cytokinesis using your clay chromosomes.
How are the results of meiosis I different from the results of mitosis?
Checkpoint 9
For each daughter cell at the end of Meiosis I,
a) How many chromosomes are present?
b) How many sister chromatids are present?
c) How many homologous pairs are present?
Interphase between Meiosis I and Meiosis II
• In some species, chromosomes decondense and nuclear envelopes form.
•No chromosomes are duplicated after Meiosis I!
Meiosis II is equational cell division
Checkpoint 10
a) Model the entire process of Meiosis II using your clay chromosomes.
b) How is this similar to Mitosis?
c) How is it different?
Checkpoint 11
For each of the the four daughter cells formed at the end of meiosis II,
a) How many chromosomes are present?
b) How many sister chromatids are present?
c) How many homologous pairs are present?
Checkpoint 12 Mitosis Meiosis
When does DNA replication occur?
Interphase before mitosis Interphase before Meiosis I
Number of divisions One, divided into PMAT stages
Two, each including PMAT
Synapsis of homologous chromosomes
Does not occur Prophase I; along with crossing-over
Number of daughter cells and genetic composition
Two, each diploid (2N), genetically identical to parent cell
Four, each haploid (N), half as many chromosomes as parent cell, genetically unique
Role in the animal body
Adult from zygote, produces cells for growth, repair; asexual reproduction
Produces gametes and introduces genetic variability among gametes
(a)
(b)
(c)
(d)
(e)
How unique are you?
• Random fertilization
– 1/2,000,000 egg cells x
1/4,000,000,000 sperm cells
• Independent assortment
– 1/223 unique chromosome
combinations
• Crossing-over
– occurs an average of 2-3 times
per chromosome pair
Checkpoint 13
The following events contribute to
the great genetic diversity found in
sexually-reproducing individuals.
At which meiotic stage do they
take place?
Random fertilization
a) Independent assortment
b) Crossing-over
Human gametogenesis Production of gametes
Mitosis (1 diploid cell 2 diploid cells)
Meiosis I (1 diploid cell 2 haploid cells)
Meiosis II (2 haploid cells 4 haploid cells)
Maturation and differentiation (haploid cells undergo extensive changes and organization)
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Primordial germ cell in embryo (2N)
Spermatogonium (2N) Oogonium (2N)
Primary spermatocyte (2N)
Primary oocyte (2N)
Secondary spermatocytes (N)
Secondary oocyte (N)
Spermatids (N) Ootid (N)
Spermatozoa (N) Ovum (N)
Spermatogenesis Oogenesis
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Spermatogenesis
• Equal cytokinesis
• Continuous
• 100-650 million sperm cells
produced
Oogenesis
• Unequal cytokinesis
• Time table
• 400 oocytes ovulated
between puberty and
menopause 2 million 1 million
400,000 Suspended in prophase I before birth
One oocyte / month completes meiosis I
400 ovulated from
puberty to
menopause Halted at metaphase II until
fertilization
Only present after
fertilization
Checkpoint 14
How many sets of chromosomes are present in each of
the following cell types?
a) an oogonium
b) a 1o spermatocyte
c) a spermatid
d) a cell during anaphase I, from either sex
e) a cell during anaphase II, from either sex
f) a 2o oocyte
g) a polar body derived from a 1o oocyte
Checkpoint 15
A woman who is about 4 weeks pregnant
suddenly begins to bleed and pass some tissue
through her vagina. After a physician examines the
material, he explains to her that a sperm fertilized
a polar body instead of an ovum, and an embryo
could not develop.
Why do you think a polar body cannot support
the development of an embryo, whereas an ovum,
which is genetically identical to it, can?
Errors in cell division Chromosomal aberrations
Nondisjunction
• Failure to separate
normally
– pairs of homologous
chromosomes during
meiosis I
– sister chromatids fail
to separate in meiosis
II and mitosis
• Gametes contain two
copies or no copies of
a particular
chromosome
• What happens when
these gametes are
fertilized?
Aneuploidy
– offspring have an abnormal number
of a particular chromosome
– fertilization of gametes in which
nondisjunction occurred
• Trisomic zygote
• three copies
of a
particular
chromosome
(e.g.
Trisomy-21,
Trisomy-X)
• Monosomic
zygote
• one copy of
a particular
chromosome
(e.g.
Monosomy
X)
http://www.wyckoffps.org/cms/lib7/NJ01000588/Centricity/Domain/120/trisomy21.jpg,
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• Incidence of Down syndrome
1 in every 770 births.
• Among women over the age
of 35 years, incidence of
delivering a child with Down
syndrome increases.
Can sometimes occur in the early stages of
embryonic development
2N 2N 2N
2N 2N+1 2N-1
Checkpoint 16 Which of the following explains
how trisomy-21 occurs?
a. Nondisjunction of
chromosomes during meiosis
I of gamete formation
b. Nondisjunction of
chromosomes during meiosis
II of gamete formation
c. Nondisjunction of
chromosomes during mitosis
of embryonic cells
d. All of the above.
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Karyotype Name Common features Images
47, 13+ Patau Syndrome
http://www.lucinafoundation.org/assets/trisomy13.jpg
47, 18+ Edward Syndrome
http://syndromepictures.com/wp-content/uploads/2011/09/Edward-syndrome.jpg
47, 21+ Down Syndrome
• Mental impairment, stunted growth, umbilical hernia, increased skin on the back of neck
• Low muscle tone, narrow roof of mouth
• Flat head, large protruding tongue, flattened nose, slanted eyes
Patau Syndrome
47, 13+
Edward Syndrome
47, 18+
Down Syndrome
47, 21+
Metafemale Syndrome
47, XXX
Klinefelter Syndrome
47, XXY
Super male/ Jacobs
Syndrome
47,XYY Turner
Syndrome
45, XO
Polyploidy
– Extra sets of chromosomes (3N, 4N, 5N, 6N, 8N, 10N, 12N)
– Caused by nondisjunction of
ALL chromosomes
• Rare, usually fatal in animals
• Common in plants (30-80%)
– Polyploids often thrive
better and grow taller
– Solution to hybrid sterility
– May be preferred because of
sterility
Checkpoint 19
Cell karyotype Condition
a) 45, XO i. Aneuploid female with monosomy of sex chromosomes
b) 47, XX, 21+ ii. Aneuploid female with trisomy of autosomal chromosomes
c) 46, XY iii. Aneuploid female with trisomy of sex chromosomes
d) 47, XXX iv. Aneuploid male with trisomy of sex chromosomes
e) 47, XYY v. Normal male
f) 69, XXX vi. Polyploid female
Match the karyotypes with their description. All of these are
based on human cells.
Chromosomes
• contain units of heredity (genes)
• composed of chromatin (DNA + protein)
• organisms contain a specific number of chromosomes
Checkpoint 1
Arrange the following terms from most simple and loose to most complex and compact:
A. Beads on a string chromatin
B. Chromosome
C. Double helix
D. Nucleosome
Chromosomal aberrations changes in the chromosomes (mutations)
Alterations in the chromosome structure
• loss of part of a chromosome Deletion
• segment of a chromosome is repeated Duplication
• part of a chromosome is oriented in the reverse of its usual direction
Inversion
• part of a chromosome breaks off and attaches to another, non-homologous chromosome
Reciprocal translocation
A B C D E F G H A B C E F G H
A B C D E F G H A B C B C D E F G H
A B C D E F G H A D C B E F G H
A B C D E F G H
M N O P Q R A B P Q R
M N O C D E F G H
Checkpoint 2 Identify the type of alteration that has occurred.
A
B
C
D
Cri-du-chat
46, 5p-
Philadelphia chromosome
46, t(9;22)
Down Syndrome
Carrier
46, t(14;21)