Meiosis Flashcards
What is meiosis?
Division process by which gametes are made
What is result of meiosis?
Begin with 4n cell (diploid cell with double the normal amount of DNA) and end with 4 non-identical haploid cells, each with a single set of chromosomes
How many times does cell division occur?
Twice
Meiosis I –> 2 diploid cells
Meiosis II –> 4 haploid cells
What occurs before prophase I?
Interphase - cell copies all its chromosomes to prepare for division
What occurs during prophase I?
- Nucelar envelope disappears
- Chromosomes condense
- Spindle fibres appear
What occurs during prometaphase I?
- Spindle fibres attach to chromosomes at centromeres
- Chromsomes continue to condense
What occurs during metaphase I?
- Homologous chromosomes align at metaphase plate
- Crossing over occurs
- Each chromosome attaches to microtubules from just one pole of the spindle (2 homologues of a pair bind to microtubules from opposite poles)
What are homologous chromosomes?
Maternal and paternal versions of same chromosome
How do homologues chromosomes line up at equator? What is purpose of this?
Random assortment - line up randomly on either side of equator to ensure genetic diversity
What is crossing over?
Homologous chromosomes exchange parts of themselves so that one chromosome contains both maternal and paternal DNA - genetic diversity
What is the point at which crossing over occurs called on the chromosome?
Chiasmata
What occurs during anaphase I?
- Homologues pulled apart and move to opposite ends of cell as spindle fibres retract
- Sister chromatids of each chromosome remain attached
What occurs during telophase I in some organisms?
Nuclear membrane reforms, chromosomes decondense, spindle fibres disappear (this is sometimes skipped as cell will go straight into meiosis II)
Cytokinesis I
What does cytokinesis I form?
2 haploid daughter cells (2n)
Meiosis II - prophase II and prometaphase II?
Identical to meiosis I
Chromsomes condense and nuclear envelope breaks down IF NEEDED
What are cells like at beginning of meiosis II?
Haploid - have just one chromosome from each homologues pair but their chromosomes consist of 2 sister chromatids
What is purpose of meiosis II?
Sister chromatids separate, making haploid cells with non-duplicated chromosomes
What occurs during metaphase II? How is it different to metaphase I?
Chromosomes line up individually along metaphase plate, in contrast to metaphase I where chromosomes lined up in homologous pairs
What is purpose of anaphase II?
Sister chromatids separate and are pulled towards opposite poles of cell
What is purpose of telophase/cytokinesis II?
Nuclear membranes form around each set of chromosomes
Cytokinesis II splits chromsome sets into 4 non-identical haploid cells
What does each chromosome have in final product?
Just one chromatid (sperm/egg cells)
