16. Sexual life cycles and meiosis Flashcards
Explain asexual reproduction
- clones are produced - genetically identical
- examples: bacteria (unicellular) - binary fission, insects (multicellular) - in parthenogenesis in Aphids
What are the advantages of asexual reproduction?
- quick reproduction
- good for non changing environment
What are the disadvantages of asexual reproduction?
- no variation → less possibility to adapt to the changing environment
Explain sexual reproduction
- genetic info from mother and father
- gametes - reproductive cells: oocyte and sperm cells
- variation in offspring
What mechanism generates variation in sexual reproduction?
Behaviour of chromosomes during the sexual life cycle:
- in somatic cells: 23 pairs of homologous chromosomes (diploid 2n); in gametes: 23 chromosomes (haploid n) ← independent assortment and crossing over
How does the number of chromosomes change through the sexual lifecycle?
In meiosis decrease form diploid to haploid - for two gametes to fuse and form a collection of normal 23 pairs of chromosomes
Explain the behaviour of chromosomes in meiosis, mitosis vs meiosis
Meiosis:
1) Chromosomes duplicate (two homologous duplicated chromosomes)
2) Homologous chromosomes go into different cells (different genetic info)
3) Sister chromatids in duplicated chromosomes go into different cells - gametes (independent assortment)
From a diploid cell - 4 different haploid gametes produced
How are homologous chromosomes paired?
In pairing of homologous chromosomes:
1) Align homologous chromosomes
2) Meiotic cells initiate DNA breaks → find and hold homologous + initiate homologous recombination (DNA breaks by nuclease Spo11 in humans)
3) DNA repair mechanism after the break → matching info on homolog chrom - crossing over of homologous chromosomes → chiasmata formed - exchange of genes → genetic variation
How do homologous chromosomes are paired?
In pairing of homologous chromosomes:
1) align homolog chrom
2) DNA breaks used to find and hold (DNA breaks induced by nuclease Spo11 in humans)
3) DNA repair mechanism after the break - matching info on homolog chrom - crossing over of homologous chromosomes -> chiasmata formed - exchange of genes -> recombination
Explain what is synaptonemal complex
Protein complex which helps to keep chromosomes together (acts like glue)
Describe the types of chromosomes produced in meiosis in different gametes
4 genetically different gametes:
1) unchanged from the mother (red)
2) unchanged from the father (blue)
3) from the mother with father’s inserts (red-blue-red)
4) from the father with mother’s inserts (blue-red-blue)
Crossing over picture
Explain meiosis with all phases
Explain non-disjunction in meiosis
Explain the cause of Down syndrom and what is the correlation for it
What is the location of meiosis?
Gametes are created in the testes / ovaries - from germline cells GMCs (cells that create gametes) - gametogenesis (spermatogenesis and oogenesis)
What are the mechanisms of gametogenesis?
Contact between hub cells and the germ cells is crucial (in Drosophila)
How does gametogenesis occur in C. elegans
Hermaphrotide - both male and female gametes produced
- most body is formed from germline
- meiosis completed after fertilization
Explain what is meiotic arrest
- Oocytes (2n) are held in a meiotic arrest - not finished second meiosis process - completed after fertilisation to produce halpoid gamete (n) and a haploid polar body II (smaller cell not equivalent to the egg cell)
- Sperm cells complete two cycles of meiosis → at fertilisation haploid (n)
- Pronuclei will fuse after second meiosis is completed in oocyte → egg nucleus (2n)
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Why is the germline considered to be immortal?
- Because germline → gamete → embryo which has germiline → adult germline → gametes…
- Somatic cells die but germ cells are staying alive through the generations, so mutations in an organism can be passed to offspirng only if they occur in germ cells
What mechanisms generate additional variation in offspring?
Why is sexual reproduction effective in evolution?
Due to genetic variation: meiosis process instead of mitosis
Summary of sexual reproduction and meiosis
What are the sources of variation in meiosis?
