Meiosis

Somatic vs sex cells

Before we begin to talk about Meiosis, we have to remember where this process and where mitosis are occurring in your body.  Mitosis is occurring pretty much everywhere.  There are a few cell types that are stalled out in interphaseThe phase of the cell cycle in which the cell grows, performs its functions, and replicates DNA., like neuronsThe functional cells of the nervous system that transmit signals., but almost all other cellsThe basic structural and functional units of life. in your body do mitosis.  These cells are called somatic cellsAny body cell that is not a reproductive cell..  In contrast, what are generally called sex cells, are contained in ovariesThe female gonads that produce eggs and hormones. and testes, can be passed on to offspring, and do meiosis to create the gametes involved in sexual reproductionThe process of producing offspring..  It’s important to realize that you can’t pass on mutations in somatic cells.  Only cells that do meiosis contribute to the next generation.

terminology

I love this life cycle picture.  I feel like it has a lot of vocab you need to know for meiosis, and that it gives a nice visual that helps students to remember some of the important things about meiosis.  Let’s start with the key down on the bottom right of this picture.  Let’s get in our minds the background colors of beige and blue.  Beige is going to represent cells that are haploid or have only 23 chromosomes in them.  Blue is going to represent a term called diploidA cell with two sets of chromosomes, one from each parent (2n). and will represent cells that have 46 chromosomes in them.  Let’s start with a diploid structure….you.  Your somatic cells are diploid, with 46 chromosomes.  The cells that perform meiosis are contained in ovaries and testis.  Stem cells complete meiosis to produce what are called gametes or egg cells and sperm cells.  These gametes are haploid with only 23 chromosomes because meiosis reduces the number of chromosomes by half.  That’s an official definition of meiosis.  Don’t forget it!  Now that we have our haploid gametes, they will come together in a process known as fertilization.  The resulting cell is called a zygote and is diploid because 23 plus 23 equals 46.  This zygote then divides via mitosis, making more and more copies, until you get to the structure you are today.  And since the zygotes divides via mitosis, making clones, all your cells have the same 46 chromosomes.

Your Chromosomes

Sometimes, it’s helpful to take a look at a karyotypeA visual representation of an organism’s chromosomes arranged in pairs., which is the picture on the left here.  This is a picture of all 46 chromosomes laid out by size.  Remember that chromosomes come in pairs, one from Mom and one from Dad.  This picture clears up some of the visual aspects of your chromosomes and how they interact in inheritance.  Don’t forget it.  You have 46 chromosomes, but that’s like saying that you own 46 socks.  Now, if you are my husband, that’s no big deal because you buy your socks in bulk like once a year and they are actually all the same.  But, I have 46 socks that are in pairs.  Let’s pretend that my socks come in pairs with a left and a right specifically designated.  Your chromosomes comes in pairs, one from Mom and one from Dad.   Now let’s apply that to the karyotype on the right.  Which is from Mom and which is from Dad?  Can’t tell in from just the picture.   But the factA statement based on direct observation that is repeatedly confirmed. is that you have 23 pairs of chromosomes.  And you can sense how you got 23 of them from sperm and 23 of them from egg.  Think about how you might make gametes, giving away only 23 of these, one from each pair.

Chromosomes through meiosis

I also like this picture of meiosis.  I like it because you can read it from left to right, but I also like that it starts with a pair of homologous chromosomes in their unduplicated state.  This is usually a point of confusion for many students.  I have one big thing to say for you to remember: to know how many chromosomes are in your cell, count the centromeres.  Check out how the first circle on the left has two centromeres.  The next circle in sequence also have two centromeres, however, the chromosomes are in their duplicated state with sister chromatids.  Meiosis I separates the homologous chromosomes, or the matching pair from Mom and Dad.  Meiosis II separates sister chromatids (much like mitosis).  We know this is meiosis, because meiosis reduces the number of chromosomes by half.  The beginning cells has two chromosomes.  The ending cells have 1 chromosome.

Metaphase in Meiosis I and II

This is a great visual of the difference between meiosis I and II.  Meiosis I is separating the homologous chromosomes or the mom-dad pairs.  The pictures on the right are separating the sister chromatids by splitting the centromeres.

I’ve added in the next two slides just to provide the processes of meiosis I and II.  There are so many videos online.  I suggest watching the process of meiosis as a video.  It is much more meaningful to see it in motion.

Meiosis I separates homologous chromosomes

Both Meiosis I and Meiosis II proceed with the same 4 phases each and the same details like the nuclear envelopeA double membrane that surrounds the nucleus and separates it from the cytoplasm. disappearing and reappearing and the formation of the spindle.  So, if these details are fuzzy for you, I would strongly suggest going back to the mitosis mini-lecture instead of trying to digest them here.  We learned about these details with our learning about mitosis and questions on the quiz will ask about these details against the background of mitosis.  What will I ask in the questions about meiosis?  I’ll ask about the splitting of the chromosomes, focusing on the metaphaseA stage of mitosis where chromosomes align at the metaphase plate. states.  This is the key to understanding meiosis.

Let’s focus here on the third from the left cell in metaphase I of meiosis I.  Note two things that are different than a metaphase mitosis.  One: the chromosomes aren’t all one color.  The tips of them seem to have exchanged with their partners.  Two: the chromosomes are aligned along the metaphase plateThe imaginary plane in the center of the cell where chromosomes line up during metaphase. by pairs whereas in mitosis each chromosome lay along the metaphase plate.  This reflects the statement at the top of this slide: meiosis I separates homologous chromosomes.  These are the pairs of chromosomes your have (think about the karyotype).

Meiosis ii separates sister chromatids (like mitosis)

Now that we’ve separated the homologous chromosomes, let’s move on to meiosis II.  You only have to follow one level of this diagram, top or bottom, to get the understanding of meiosis II.  Let’s again focus in on the metaphase II pictures, third from the left.  These look like mitosis, where we are breaking apart those sister chromatids.  But, these cells have three chromosomes, which is half that in the original cells.  Yes, this is the reality of meiosis I.  Here in meiosis II  we are just returning the chromosomes to their original non-duplicated state.  Meiosis II ends with cells with half the number of chromosomes.    We also end with 4 cells as a result of the two rounds of division.

Independent assortment

Independent assortment is something that happens in meiosis I.  It is a source of genetic diversity that can create over 8 million variations in a human gamete A haploid reproductive cell (sperm or egg)..  This figure is based on our 2 sets of 23 chromosomes.  This picture here gives you a great visual of independent assortment.  Check out the cells at the top of the picture, which has 2 pairs of chromosomes in metaphase of meiosis I.  Note how there are two possibilities for this cell as to how the homologous chromosomes line up on the metaphase plate.  Now extend this to think about how our 23 pairs chromosomes can split up.  The key to understanding independent assortment is that one pair of homologous chromosomes lines up without regard to how the others line up.  And this makes 2 to the twenty-third possibilities or 8 million different kinds of gametes that you can create.

Crossing over

But we don’t stop at 8 million combinations.  It’s actually closer to infinite due to a process called crossing over.  This is a process that also happens in meiosis I.  When the homologous chromosomes pair up in prophaseThe first stage of mitosis where chromatin condenses into visible chromosomes., they exchange a bit of DNA with each other.  So, your Mom chromosome and Dad chromosome switch some DNA.  Take a look at this picture here.  We call chromosomes in their duplicated state tetrads.  So, tetrads are made up of sister chromatids for one chromosome.  The tetrad have a leg (or arm) that touches the tetrad of their homologous pair.  This touching site is called a chiasma.  It’s at this site that the exchange takes place.  Look at the resulting 4 chromosomes that will now be in the gametes.  If you think about it, these chromosomes are completely new to the human gene pool.  The combination of genes on these chromosomes have never before been seen.

Genetic recombination

Crossing over is sometimes called genetic recombination.  There can be more than one chiasma during crossing over, creating combinations like the one shown in this picture here. 

Crossing over also breaks linked genes, which are genes on the same chromosome and are commonly inherited together.  So, as a very general example, we know of certain pigmentation combinations that are common.  We have the light-skinned, red haired, green eye Irish combo and we have the olive skinned, dark hair, and dark eyes Mediterranean combo.  There are many examples of the offspring of these two phenotypes, one being the often present, auburn haired, dark eyed, light-skinned variant present in Ireland.