Meiosis Flashcards
Meiosis
Maintains chromosome numbers in cells of a species that undergoes sexual reproduction
Involves two rounds of cell division, results in 4 gametes
Meiosis 1
Centromeres and kinetochores of sister chromatids stay together, first phase of meiosis
Prophase I
Longest phase, 5 substages (leptotene, zygotene, pachytene, diplotene, diakinesis)
Prophase I: Leptotene (thin threads)
DNA already duplicated, chromosomes condense, centrosomes begin to move to each pole and spindle fibers
Prophase I: Zygotene (paired threads)
Condensed chromosomes find homologous partner, “zipper” together using proteins
- Tetrads/bivalents formed
Tetrad synaptonemal complex
A protein structure that connects homologous chromosomes together
Prophase I: Pachytene (thick threads)
Crossing over (genetic exchange between nonsister chromatids of a homologous pair)
Prophase I: Diplotene (two threads)
Synaptonemal complex disappears, crossover points appear as chiasmata, tetrads become visible as they begin to move apart
Prophase I: Diakinesis (moving through)
Nuclear membrane breaks down, and the spindle begins to attach to the kinetochore
Metaphase I
Tetrads line up on metaphase plate, each chromosome of the pair attaches to fibers from opposite poles
Anaphase I
Chromosomes no longer attached via chiasmata
- Separate and move to opposite poles
Telophase I
Sister chromatids still attached, nuclear membranes reform, cytokinesis
Prophase II
Chromosomes condense, centrosomes move to poles, sister chromatids attach to spindle fibers
Metaphase II
Chromosomes align at the metaphase plate (like Mitosis Metaphase)
Anaphase II
Sister chromatids separate, move to opposite poles
Recombination nodules
Exchange DNA at random points on chromosomes, found in synaptonemal complex, mixes maternal and paternal homologous chromosomes
Telophase II
Nuclear envelopes re-form, cell divides (cytokinesis), chromosomes begin to expand (for cell processes, transcription)
Oogenesis in humans
Meiosis only produces one egg, others are polar bodies and disintegrate
What phases in meiosis do human eggs freeze at?
Cells are stuck at diplotene until puberty, one at puberty, finishes meiosis I, cells arrest at Metaphase II until fertilization, at fertilization, completes meiosis II, sperm and egg fuse, divides by mitosis
Spermatogenesis
Continuous, meiosis produces 4 sperm
How do we know chromosomal theory of inheritance is true?
Hereditary information is on genes, on chromosomes
- Sex is associated with the inheritance of particular chromosomes
Egg and sperm contribute equally to the genetics of offspring
- Chromosome number is conserved in each cell
Criss Cross Inheritance
Female inherits trait from father, males inherit trait from mother
Hemizygous
Only having one copy of a gene/chromosome segment, when there is usually two
- Males only have one X chromosome
