3LP. Transmission of genetic information from generation to generation Flashcards
Significance and source of genetic variability. Phases and events and of first prophase (leptotene, zygotene, pachytene, diplotene, diakinesis; synapsis, crossing over, chiazma).
Genetic variability
- Homologous recombination
- Independent assortment (homologous chromosomes in meiosis I: 2^23 variations)
Significance of meiosis
1) Genetic variability
2) Reduction of chromosome number (halved)
Stages of meiosis: meiosis I. and II
Meiosis I (homologous chromosomes separated in 2 cells)
- Prophase (leptotene, sygotene etc.) - crossing over
- Metaphase (chiasma visible)
- Anaphase
- Telophase
Meiosis II (sister chromatids separated in 4 cells)
- Same phases as above, but not the special prophases
Homologous chromosomes definition
Chromosomes that contain DNA that codes for the same genes - but different alleles
- one maternal and one paternal
Phases of first prophase
- Leptotene
- Zygotene
- Pachytene
- Diplotene
- Diakinesis
Leptotene phase
Prophase I
- Chromosome condensation
- Telomeres bind to nuclear envelope
Zygotene phase
Prophase I
- Synapsis of homologous chromosomes
- Bivalent forming ->Synaptonemal complex forming (Tetrad: 2 paternal sister chromatids + 2 maternal sister chromatids + between: Central element, transverse filament, lateral elements)
- X & Y synapsis: PAR1 (pseudoautosomal region)
Pachytene phase
Prophase I
- Crossing over = homologous recombination
- Recombination nodule in central region of synapsis
Diplotene phase
Prophase I
- Chiazma (pl: chiasmata) = site of crossing over, now visible (!) due to separation of homologous chromosomes
Diakinesis
Prophase I
- Further chromosome condensation
- Nuclear envelope fragmenting
- Bivalent ready for metaphase
Prophase I: synapsis
Begins in zygotene phase, complete in pachytene phase
Prophase I: crossing over
Aka homologous recombination
- Occurs in pachytene phase
Prophase I: Chiazma
Site of crossing over
- Visible from diplotene phase until end of metaphase
Random alignment and assortment of homologous chromosomes in meiosis I anaphase
Generates variability
2^23 different possibilities
Role of cohesin in separation in chromosome number reduction
Cohesin: one out of two protein complexes influencing DNA functions (the other being condensin)
- Composed of different SMC proteins (structure maintenance of chromosomes)
- Have ATPase activity and regulatory functions
- Associate in a ring-like structure
- Hold the 2 DNA molecules (sister chromatids) together
- *Much of it detaches during prophase and at end of metaphase it’s only close to the centromere (pericentromeric cohesin) - cleaved in early anaphase => separation of sister chromatids allowed
- **Also regulates kinetochor attachment according to wiki
- *** Cornelia de Lange
Meiosis II
Separation of sister chromatids -> 4 haploid cells
Atypical meiotic process: non-disjunction
Meiotic nondisjunction cause aneuploid genome mutations
- May happen both in oogenesis and spermatogenesis
- More frequent in oogenesis
- Sex chromosome nondisjunction is more frequent in spermatogenesis
Change of DNA amount and chromosome number during meiotic process
See notes
Comparison of spermatogenesis and oogenesis
Oogenesis:
- Primary oocyte arrested in prophase I (present from birth)
- Completion of meiosis I => enter meiosis II (1 polar body formed)
- Secondary oocyte arrested in metaphase II (only completed if fertilized (2nd polar body)
Spermatogenesis
- Normal meiotic division
- Not present from birth, produced continously
- Spermatids (haploid) formed after meiosis II => differentiate to sperm cells
Regulation of special meiotic events of oogenesis
1) Retinoic acid
- Metabolized (degraded) by CYP26B1 - higher amount in males => no retinoic acid => no STRA8 meiotic transcription factor (which is why males have delayed meiosis)
2) MPF
3) cAMP high levels inactivate MPF (decreased in puberty) - finishing diplotene arrest stage
4) CFS - inactivate APC via MAPK (in fertilization Ca levels increase and APC is reactivated) - finishing metaphase II arrest
Sex specific differences in meiosis I prophase (synapsis, synaptonemal complex, chiasmata)
Synapsis start
- M: at end of chr
- F: inside chr
Synaptonemal complex
- M: compact, shorter
- F: less compact, longer
Sites and number of chiasmata
- M: at end of chromosomes, less
- F: inside chromosomes, more
Role of kinetochores in chromosome number reduction
Kinetochores: protein structure found at the centromere of a chromatid to which microtubules attach during cell division
- Three-layered plate of protein - contains both dynein and kinesin-type motor proteins
- Associates to centromeres of chr. in prophase and prometaphase
- Help holding sister chromatids together ad plays a role in chromosome editing (wiki)
- Role: bind kinetochore microtubules(ca. 30-40 / sister chromatid)
Role of synapsis in chromosome number reduction
Synapsis/syndesis: the coming together and pairing of homologous chromosomes during prophase I
- Helps make sure the chromosomes are in the right place at right time - each daughter cell gets the correct number of chromosomes
- Possible chromosomal cross-over
- When homologous chromosomes synapse, their ends are first attached to nuclear envelope => migrate until matching ends have been paired
