Posted in P1 SBI3U Biology

SBI3U Meiosis

How Meiosis Works

From Interphase to Gametes

G1:  In this part of interphase, the primary gametocyte is in its gonad waiting for the go ahead signal.  It is a diploid cell which means that it has two sets of chromosomes (2n).  The n represents the number of chromosomes in a set for each organism.  In humans, n=23.  We get one set from each parent.  so we have 46 in all of our cells (somatic cells) except those going through or produced by meiosis (gametes).  At this point, both sets of chromosomes are in the form of single copy chromatin. When the go ahead signal is received the cell enters S phase of interphase.

S: S stands for synthesis.  Actually, ALL the DNA is replicated in this phase.  The chromosomes go from single copy chromatin to double copy chromatin.  Imagine a bowl of X-shaped spaghetti noodles.

G2:  In this part of interphase, the cell goes through the final preparations before entering meiosis.  So, at this point we have gone from having 2 sets of 3 billion base-pairs of DNA, to 2 sets of 6 billion base-pairs – a total of 12 billion base-pairs DNA in total.

Prophase I:  In prophase I, the nuclear membrane degrades, the nucleolus disappears and the double copy chromatin becomes supercoiled and we have 46 dyads.  Homologous pairs find each other and pair up, like a couple at a dance.  This process is called synapsis and results in the formation of 23 tetrads.  Tetra is the Greek prefix for four.  The chromatids opposite of each other go through unique cross-overs where sections are swapped which results in a tetrad made of four unique chromatids.  These cross-over events in our 23 tetrads are unique every time this happens.

Metaphase I:  The tetrads are aligned along the equator and connected to the centrioles at either pole.  They are assorted independently of each other and since each dyad of a tetrad is unique, their alignment is a 50/50 chance of a dyad lined up north or south.  For example, in tetrad 1, chromosome 1 from dad could be facing north and so chromosome 1 from mom would be facing south.  This has no bearing or impact on what happens with tetrad 2.  In tetrad 2, chromosome 2 from mom could be facing north and chromosome 2 from dad would be facing south.  This 50/50 chance alignment applies to every tetrad and those alignments will differ between every primary gametocyte that we have that will go through meiosis.  At this point the cell is still diploid.

Anaphase I:  The centrioles begin contracting the spindle fibres which pull the tetrads apart and the dyads are reeled into the north or south pole depending on the centriole they are attached to.  So in anaphase I, our chromsomes are recombined, mirror image (but not identical) dyads that are reeled into each pole.  The cell is still diploid at this point.

Telophase I:  The n number of dyads pile up at the poles and the membrane begins to pinch off at the equator.  Since it hasn’t fully split yet, this cell is still diploid.

Cytokinesis I:  The cell fully splits into two cells.

Prophase II:  These two cells are now haploid and contain a mirror image (but not identical) set of chromosomes in the form of dyads.  The centrioles duplicate again and spindle fibres emerge from them as they move to each pole of both cells.

Metaphase II:  In these two haploid cells, the dyads are aligned on the equator and attached to each centriole by spindle fibres connected to the centromere.  It is important to note, that since the chromatids are not identical to each other, their orientation (north or south) will vary from one to the next.   This variation is also seen between any cells going through meiosis in the gonads.  Thanks Matt for flipping coins here too!

Anaphase II:  The centrioles begin to contract the spindle fibres which split the dyads in these haploid cells into chromatids which are then reeled into each pole.

Telophase II:  The chromatids in each of the two haploid cells contain n chromatids at their poles and the membrane begins to pinch off.

Cytokinesis II:  The membrane in each cell pinches off resulting in four cells each containing one set of unique chromatids.

After cytokinesis, the cells will go through either differentiation into sperm cells if this is happening in males or they will become an ova and polar bodies if meiosis is happening in a female.  They await possible intercourse which would result in a zygote with two sets of chromatids, one set from each parent.  The zygote grows into offspring through mitosis and the cell cycle, and their gonads go through meiosis and the cycle is repeated generation after generation.  NOTE:  for your zygote in your project, just draw the chromatids that have been put there.

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