Sexual reproduction Life cycle: haploid phase (gametes) diploid phase (somatic cells) 2n 2n meiosis meiosis n n Gametes (haploid) sperm oocyte fertilization 2n n + n = 2n zygote ♂ ♀
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Meiotic division 2n 1 st meiotic division 2 nd meiotic division n n n n n n
First meiotic division
Heterotypic or reduction
Beginning of prophase
Chromosomes can't be distinguished.
Condensation of chromosomes begins.
Attachment of homologous chromosomes – the chromosomes begin to synapse.
Assemblage of chromosomes is enabled by synaptonemal complex .
Bivalents or tetrades are present.
1 bivalent = 2 chromosomes
1 tetrade = 4 chromatids
bivalent = tetrade
Crossing-over between homologous chromosomes is possible.
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 1 st meiotic prophase
X Y SRY – sex determining region – is located on chromosome Y in the neighborhood of PAR. X Y
Task A :
A n infertile man with azoospermia had been examined i n 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 XY „ XX-male“ „ XY-female“
Hermaphroditism – possible consequence of sex chromosome abnormalities
Prophase of the 1 st meiotic division next periods
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).
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 2 nd meiotic division (homeotypic) 1 st meiotic division (heterotypic) Separation of double-chromatid homologous chromosomes
Metaphase I Metaphase I
Anaphase I X Anaphase II
Spermatogenesis – sperm development
Oogenesis – oocyte development
Spermatogenesis n n 1st meiotic division 2nd meiotic division n n primary spermatocyte secondary spermatocyte sperm n n 2n
Oogenesis n n 1st meiotic division 2nd meiotic division n n primary oocyte secondary oocyte mature oocyte n n 2n polar bodies polar body
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
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.
A B XY
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.)
Task 3 : Changes of nuclear DNA concentration during the cell cycle G 1 G 2 S M G 1 C 2C G 1 G 2 S M 1 M 2 G 1 Cell cycle + mitosis Cell cycle + meiosis Nuclear DNA content
Task 3: Observation of meiosis in the grasshopper testis
Task 4: Disjunction of the sex chromosomes during meiosis Scheme A Scheme B XY X Y X X Y Y
Task 5: Changes of nuclear DNA concentration during the cell cycle G 1 G 2 S M G 1 C 2C G 1 G 2 S M 1 M 2 G 1 Cell cycle + mitosis Cell cycle + meiosis Nuclear DNA
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
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)
Genetic or other examinations of oocytes or embryo before transfer to the mother's
1 – 2 cells from 8-cell embryo
More cells from blastocyst
Presentation Ethical issues of assisted reproduction