Transcript
Practical 3
Meiosis
Gametogenesis
Chromosomal sex determination
• Drosophila type
• Male – XY
• Female - XX
Sexual reproduction
2n
2nmeiosis
meiosis
n
n
Gametes (haploid)
sperm
oocyte
fertilization 2n
n + n = 2n zygote
♂
♀Life cycle:
haploid phase (gametes)
diploid phase (somatic cells)
Recommended website
• http://genetics.gsk.com/chromosomes.htm#chromo
Written test
• 8 minutes
• Don't forget to put down your name, your
group and the test version.
• In multiple choice questions more than 1
statement could be correct.
• Don't write anything on the question sheet!
Meiotic division
1st meiotic division
2nd meiotic division
2n
n n
n n n n
First meiotic division
• Heterotypic or reduction• Prophase
– Leptotene– Zygotene– Pachytene– Diplotene– Diakinesis
• Metaphase• Anaphase• Telophase
Leptotene• Beginning of prophase• Chromosomes can't be
distinguished. • Condensation of
chromosomes begins.
Zygotene• Attachment of homologous
chromosomes – the chromosomes
begin to synapse.
• Assemblage of chromosomes is
enabled by synaptonemal complex.
Pachytene• Bivalents or tetrades are present.
– 1 bivalent = 2 chromosomes
– 1 tetrade = 4 chromatids
– bivalent = tetrade
• Crossing-over between homologous
chromosomes is possible.
Crossing-over
• Reciprocal exchange of chromatid segments
between chromosomal homologues
Importance of the crossing-over• New combinations of alleles of genes that are
located on the same chromosome. • New allele combinations lead to higher variability
of the offspring (important in evolution). • This increases the potential for genetic variability
between members of the same species.
Chromosomes X and Y• are attached in
pseudoautosomal regions (PAR) during the 1st meiotic prophase
X
Y
SRY – sex determining region – is located on chromosome Y in the neighborhood of PAR.X
Y
Task A:
• An infertile man with azoospermia had been examined in cytogenetic lab. He has no other striking phenotypic features.
• The cytogenetic analysis revealed abnormal finding because his cells contained two X chromosomes while chromosome Y is missing.
• Explain this surprising result.
Crossing-over in PAR
• Nonreciprocal crossing-over can result in translocation of the SRY gene from the chromosome Y to chromosome X
Task A – result
male
female
Normal combination of gonosomes X;Y translocation
„XX-male“
„XY-female“XX
XY
Hermaphroditism – possible consequence of sex chromosome
abnormalities
Prophase of the 1st meiotic division
next periods
Pachytene
Diplotene• Condensation of chromosomes
continues
• Homologous chromosomes are divided
• Disassembling of synaptonemal complex
• At the end of diplotene homologues are connected only in crossover sites. These sites are called chiasmata (singular: chiasma).
Diakinesis• Final period of the 1st meiotic
prophase. • Chiasmata are transferred to
terminal chromosomal regions.
• Under specific staining chromosomes could be well distinguished under the microscope.
• Nuclear membrane disappears.
Terminal phases of meiotic division
prophase
metaphase + anaphase
2nd meiotic division
(homeotypic)
1st meiotic division
(heterotypic)
Separation of double-chromatid
homologous chromosomes
Metaphase I
Metaphase I
Anaphase I X Anaphase II
Gametogenesis
• Spermatogenesis – sperm development
• Oogenesis – oocyte development
n n
1st meiotic division
2nd meiotic division
n n
primary spermatocyte
secondary spermatocyte
sperm
n n
2nSpermatogenesis
n n
1st meiotic division
2nd meiotic division
n n
primary oocyte
secondary oocyte
mature oocyte
nn
2n
polar bodies
polar body
Oogenesis
A schedule of oocyte development
Prophase I
fetus childhood – dictyotene mature ovary
fertilization
Spermatogenesis X Oogenesis
Task B: Risk of chromosomal nondisjunction defects
• Compare possible risk of chromosomal
abnormalities for two couples that visited
the genetic counselling clinic:
– Couple 1: female (21 years), male (38 years)
– Couple 2: female (36 years), male (32 years)
• Which family has higher risk of numerical
chromosomal aberrations?
Task B – result
• Oogenesis lasts much longer
than spermatogenesis – risk of
chromosomal nondisjunction
defects is age-related.
• The second couple has higher
risk due to 36 years old female.
• In pregnant females older than
35 years prenatal examination
of the fetus is recommended.
35 years … risk 1/365
Age-related risk of the Down syndrome child
Mother´s age
Practical tasks
Task 1: Observation of meiosis in the grasshopper testis
• The slide was prepared from the testis of the grasshopper. After removing from the individual the tissue was fixed with methanol + glacial acetic acid (3 : 1) and stained with the Giemsa-Romanowski solution. After dehydration the tissue was mounted in the Canada balsam.
• Find the spermatocytes on the slide using 10x objective lens.
• Change the objective magnification into 40 or 45x and observe different phases of meiosis. For their determination use adjacent photos.
Task 2: Disjunction of the sex chromosomes during meiosis
• Redraw the scheme of disjunction of the sex chromosomes during meiosis of the male to your materials and fill it.
• In the first scheme (A) distinguish the chromosomes according to their shape and number of chromatids.
• In the second scheme (B) designate the chromosomes using letters X or Y no matter of the number of chromatids.
XYA B
Task 3: Changes of nuclear DNA concentration during the cell cycle
• Contrast the two types of cell division (mitosis X meiosis) by completing the following graphs below.
• Redraw the picture to your materials and fill curves demonstrating changes in the DNA content per one cell:– during the cell cycle followed with mitosis– during the cell cycle followed with meiosis.
• (Value „C“ on y-axis represents DNA content per one cell in G1 phase.)
G1 G2S M G1
C
2C
G1 G2S M1 M2 G1
Cell cycle + mitosis Cell cycle + meiosis
Nuclear DNA content
Task 3: Changes of nuclear DNA concentration during the cell cycle
Results
Task 3: Observation of meiosis in the grasshopper testis
Task 4: Disjunction of the sex chromosomes during meiosis
XY
X Y
XX YY
Scheme A Scheme B
Task 5: Changes of nuclear DNA concentration during the cell cycle
G1 G2S M G1
C
2C
G1 G2S M1 M2 G1
Cell cycle + mitosis Cell cycle + meiosisNuclear DNA
Assisted reproduction
• A group of methods that allows reproduction of
infertile couple that can't be therapeuted by
common medical treatment (e.g. using drugs of by
psychological intervention)
• The assisted reproduction could be also used for
prevention of genetic defects in families with
normal fertility but with increased risk of genetically
conditioned disorder for the offspring.
Assisted reproduction methods
• Intrauterine artificial insemination
– application of sperm to the uterus of the
female.
• In vitro fertilization and embryo transfer
– fertilization is performed outside the organism
of the mother
Artificial insemination
• homologous – the
female is fertilized with
sperm of her husband;
• sperm of anonymous
donor are applied – the
donor and the couple
remain unknown to
each other
In vitro fertilization
• Application of gonadotropins (= fertility drugs) induce maturation of numerous oocytes.
• The oocytes are removed from ovaries by biopsy. In a special dish they are mixed with sperms of the husband.
• After short term cultivation in medium with nutrients 3 embryos are transferred to the female's uterus.
ICSI (intracytoplasmatic sperm injection)
ICSI protocol
Preimplantation diagnostics• Genetic or other examinations of oocytes or
embryo before transfer to the mother's • We use
– Polar body– 1 – 2 cells from 8-cell embryo– More cells from blastocyst
Presentation
Ethical issues of assisted
reproduction
Next seminar
• In vitro cell cultivation
• Recommended websites:
– http://www.research.umbc.edu/~jwolf/method5.htm
– http://homepages.gac.edu/~cellab/chpts/chpt12/intro12.html
• Be careful when handling cell cultures!
• Test: previous topics (only seminars)
See you next week!
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