3. Meiosis Flashcards
How many nuclear divisions in meiosis?
2
Describe the nuclear cycle in Saccharomyces cerevisiae?
- Haploid throughout most of the life cycle
- Two mating types, a and α
- One a and one α combine to give a transient diploid stage
- Then meiosis occurs
- Giving four haploid products
What are the 5 stages of prophase in meiosis 1? (5)
1) Leptotene - replicated chromosomes start to contract
(liverpool)
2) Zygotene - chromosomes line up in homologous pairs = synapsis
Held together by a protein complex called the synaptonemal complex
(zoomed)
3) Pachytene - crossing over (genetic exchange) between non-sister chromatids
(painfully)
4) Diplotene - chromosomes separate a bit but sites of crossing over are still visible
(doing)
5) Diakinesis -Chromosomes contract even further
(double)
How does meiosis generate variation? (2)
- Independent assortment (of chromosomes)
- Crossing over
Describe independent assortment?
Orientation of bivalents in meiosis 1 is random and leads to different combinations
Describe crossing over?
- At a molecular level, crossovers = sites of genetic exchange between homologous sequences on non-sister chromatids
- Double-strand breaks are generated by a specialised enzyme
- A process called homologous recombination occurs – relies on chromatids being lined up correctly so that identical (or nearly-identical) sequences can be accessed
What is the site of crossing over called?
Singular = chiasma Plural = chiasmata
What is a bivalent?
Arrangement of of 4 chromatids (2 chromosomes)
What is a tetrad?
Arrangement of of 4 chromatids (2 chromosomes)
What is a synaptonemal complex?
Protein structure that forms between homologous chromosomes (two pairs of sister chromatids) during meiosis and is thought to mediate synapsis and recombination during meiosis I in eukaryotes
What is a dyad?
2 sister chromatids with a single centromere
2 dyads is a bivalent
