6.3b - Meiosis Flashcards
What occurs during ‘Prophase 1’?
Spindle fibres form from centrioles
Nucleolus disappears, and nuclear membrane disintegrates
Homologous chromosomes pair up – now called bivalents (also referred to as a tetrad)
Crossing over (known as recombination) occurs: The exchange of genes between bivalents as chromatids entangle.
What occurs during ‘Metaphase 1’?
Bivalents line up across the equator of the cell, as opposed to in mitosis where each chromosome lines up single file.
- Leads to independent assortment, since the chromosomes are lined up in pairs, either the maternal or paternal chromosome of the bivalent will be in the 2 cells of meiosis 1, this leads to genetic variation as the orientation of the bivalents is random. One chromosome will face one pole of the cell, as will the other face the other pole. So, the combination of chromosomes in the 2 cells of meiosis 1 are completely random.
The chromosomes, with the 2 chromatids line up to be separated in mitosis, ensuring the 2 daughter cells still have the same number of chromosomes. However, in meiosis, since the bivalents line up, not the individual chromosomes, the 2 new cells, will have half the number of chromosomes, thus meaning it goes from 46 to 23 chromosomes, therefore meiosis 1 is called reduction division – the number of chromosomes is reduced in meiosis 1.
What occurs during ‘Anaphase 1’?
The homologous chromosomes are pulled away to the opposite poles of the cells as opposed to the individual chromatids, as in mitosis.
The chromosomes undergo crossing over (recombination). At the points where the chromosomes in the bivalents become entangled, sections of the sister chromatids break off their original chromosomes and re-join the new ones. The point at which the chromosomes break off and re-join is called chiasma (plural chiasmata).
The products of this recombination (crossing over) are called recombinant chromosomes.
During recombination, alleles of the same gene will switch over causing genetic variation.
What occurs during ‘Telophase 1’?
Same as in mitosis
Chromosomes reassemble and uncoil at each pole whilst the nuclear membrane reforms
Cell undergoes cytokinesis, reduction of chromosome number from 46 (diploid) to 23 (haploid) is complete.
What occurs during ‘Prophase 2’?
no crossing over occurs as there are no homologous chromosomes in pairs (bivalents), instead the spindle reforms, nucleolus disappears, and the nuclear membrane begins to disintegrate.
What occurs during ‘Metaphase 2’?
the bivalents don’t line up across the metaphase plate equator, instead the individual chromosomes do. However independent assortment still occurs due to crossing over, this makes the chromosomes unidentical.
What occurs during ‘Anaphase 2’?
In anaphase 2 the individual chromatids are pulled apart, this results in there eventually being 23 chromosomes and 23 chromatids in the 4 new daughter gamete cells.
What occurs during ‘Telophase 2’?
- Chromatids assemble at poles - Chromosomes uncoil and form chromatins again - Nuclear envelope reforms and nucleolus becomes visible - Cytokinesis - division of cells to 4 duaghter cells - Cells are haploid due to reduction division - All genetically different due to crossing over and independent assortment.
