Posted in P6 SBI3U Biology

SBI3U Gametogenesis and Conception Assignment – NOW DUE THURSDAY APRIL 9

Instructions – Complete the spermatogenesis, oogenesis and conception handouts using the following instructions.

The analysis questions are to be submitted on the VLE under the Assessment –> Assignments section by the beginning of class on March 24.

Gametogenesis & Conception Data

  1. Use two different colours for the chromosomes from dad’s parents and complete the spermatogenesis diagram. For example, dark blue for the chromosomes from dad’s dad, light blue for the chromosomes from dad’s mom.
  2. Use two new colours for the chromosomes from mom’s parents.  For example, red for the chromosomes from mom’s dad and pink for the chromosomes from mom’s mom.
  3. Make sure each chromatid has a least one cross-over.
  4. Make sure that the cross-overs for spermatogenesis are different than oogenesis.
  5. Randomly choose one of the four sperm that were generated to create the zygote on the conception sheet.  Draw the chromosomes as they would be right after the sperm and egg meet.
  6. Below every cell in all of your diagrams identify it as diploid or haploid.
  7. Within every cell in all of your diagrams, identify the form the chromosomes are in.


  1. Whose DNA did the zygote get most of from:
    a) mom (i.e. mom’s mom or mom’s dad)? b) dad (i.e. dad’s mom or dad’s dad)?  Justify your answer based on (means refer to) the chromosomes within the zygote.
  2. Based on your drawings and knowledge from class, what is the overall purpose of meiosis I? (i.e. refer to your drawings and knowledge in your answer)
  3. Based on your drawings and knowledge from class, what is the overall purpose of meiosis II? (i.e. refer to your drawings and knowledge in your answer).
  4. In terms of chromosome distribution (independent assortment), we tossed coins to figure out where chromosomes would go. How many total coin tosses would there be in meiosis of a human cell?  Explain how you arrived at this number.
  5. Explain how it is impossible (other than for identical twins) for one zygote to be genetically identical to another.  Use the data from your charts to explain this.  (No citation needed).
  6. i) Where and when does meiosis happen in a) human males?  b) human females?
    ii) Why is the number of gametes that come from a single primary oocyte different than the number of gametes that come from a single primary spermatocyte?


  1. Other than disease protection, why does meiosis and sexual reproduction give a species an advantage over asexual reproduction?   Give an example to back up your answer.
  2. Using text and diagrams, explain how non-disjunction can happen and identify and describe a condition that non-disjunction can lead to.
    Use your textbook for this one – but you will have to cite it along with your other valid sources.
  3. What does the Catholic Church teach about the value of the zygote?  How does this teaching relate to the use of human embryonic stem cells?  What is a source of stem cells that could be used that would be acceptable to our religion?  (Note:  you will need to refer to the Catechism of the Catholic Church or the YOUCAT and other valid sources to answer these questions.)

Cite your sources using the same method we did for the Cheek Cell Investigation.

If you do not cite your sources, or you copy off of someone else’s wording, this constitutes plagiarism.  A mark of zero will be assigned as a result.

Wikis and discussion groups like Yahoo answers are NOT valid sources of information.  It is vital that you cite valid sources.

Posted in P6 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.