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Mitosis and Meiosis - the Easy Way
Transcript of Mitosis and Meiosis - the Easy Way
a big waste of time and energy... Okay, so body cells have to divide, cool story. How exactly is that accomplished? Excellent question, young grasshopper! But first, some vocabulary! Chromosome - one long DNA molecule that has certain functions (the human body has 23 unique chromosomes!) This is one chromosome - 1 piece of long DNA Disclaimer: chromosomes are usually
unbound, long messy strands of DNA.
This chromosome is bound for better visualization :) Chromatids - after 1 chromosome has been copied, it forms the familiar "X" shape.
1 of the legs is a chromatid, and both chromatids (called sister chromatids) come together to form what we usually call a "chromosome". 1 chromatid = 1 chromosome Sister Chromatids = 1 replicated chromosome Chromatin - DNA is packaged with proteins that allow it to stack on top of itself to become one neat leg
DNA is wound around protein spools - called nucleosomes - and the combination of DNA and the proteins is called CHROMATIN Chromosomes in this shape are the combination of DNA and proteins, which allow it to form into this leg shape Almost finished! Then the replication cycle! Centromere - the center of the "X" shape, where the sister chromatids meet up and... Kinetochore - 2 of these are in the centromere - 1 to each sister chromatid - and these are where proteins attach to pull the sisters apart Body cells are constantly dying to make way for new, fresh cells, so... Centromere, and Kinetochores (hidden
in the centromere) We are done with vocabulary! Now its time for the cell cycle and mitosis! The Cell Cycle Before a cell replicates and goes through mitosis, it must go through the cell cycle! G1 S G2 Mitosis Interphase Mitotic Phase Interphase - the phase when the cell grows and gets ready to replicate Mitotic Phase - the phase when the cell
undergoes Mitosis G1 - this is when the mother cell grows
up to be a big lady cell S - this is when the cell duplicates its
chromosomal DNA, but all of it is still in loose strands G2 - this is when the mother cell begins to
duplicate the necessary organelles for mitosis Now, the Cell is ready for Mitosis!! This is the first step of Mitosis:
the duplicated DNA will now wind up into its X shapes Prophase Here, two big things happen: Prometaphase 1. The nuclear envelope (membrane)
2. The spindle apparatus attaches
spindle fibers to the kinetochores Also, spindle apparatuses move to different parts of the cell. These will connect to the chromosomes and pull them apart later Spindle Apparatus At this point, the sister chromatids line up along the equator of the cell, with the spindle apparatuses at opposite ends of the cell Metaphase In this phase, the spindle fibers "pull" the sister chromatids apart towards the apparatuses Anaphase At this point, the cell begins to pinch off and form two daughter cells. Also, the nuclear envelope begins forming again Telophase When the cell membrane begins to divide,
it creates a "cleavage furrow." This is the
area where the membrane pinches off to
form two daughter cells. Cleavage Furrow Cytokinesis is not really a part of cell mitosis. It is when the two daughter cells finally split apart from each other. It also marks the point when Interphase and G1 begin. Cytokinesis Disclaimer: again, like before, the DNA
would not be in this shape right now. All the chromosomes would be in one big mess again. The chromosomes are shaped like this for clarification purposes. And that's it! That is the process of Mitosis - cell replication and division! But lucky for you, this is not the end...
There is another process - Meiosis - that you must still cover, young grasshopper! Meiosis - Sexual Replication This time, 1=4 Okay, so we went over Mitosis, but what is Meiosis? Meiosis is like Mitosis - cell division But...meiosis is the process of replication for sexual cells, i.e. sperm or eggs. The big differences between the two: in mitosis, a diploid cell becomes haploid there are two main parts to meiosis: meiosis 1 and meiosis 2. In mitosis, there is just mitosis chromosomes split apart differently in meiosis 1 than in meiosis 2 Woah woah, hold the phone...what is "diploid" and "haploid"? What are the two parts of meiosis? Slow down! Another fine question young grasshopper!
Before I start you on meiosis, let's go over some more vocabulary! Diploid - when a cell has two chromosome sets
*Recall that humans have 23 unique chromosomes. And then there is Haploid.
Haploid - when there is only 1 set of chromosomes.
This time, haploid cells really do only have 23 chromosomes (the individual strand of DNA). And now, the two parts of meiosis: 1. Meiosis 1 - this is what is unique to meiosis. This is where the two sets of each homologous pairs line up NEXT to each other, instead of in a straight line (picture coming up soon) Homologous pair - this is another term for sister chromatids. When the sisters join at the centromere, they form a homologous pair because each strand is identical to the other. The "X" is a homologous pair. Homologous Chromosome - this is a term for two sets of homologous pairs. The two sets of pairs are identical to each other, and that's important so that the daughter cells of meiosis can make sure to get all the chromosomes they need.
Imagine homologous chromosomes as "X X". (this is not denoting a girl, this is denoting two equal pairs of chromosomes) 2. Meiosis 2 - this is just like mitosis, except with haploid cells Big thing to remember! In meiosis, diploid cells become haploid! I know that was a lot of information that seems "out there." Trust me, when you see the picture of the meiosis cycle, it will all make sense! So, what are we waiting for!? Let's go to the Cycle!! :) Body cells are diploid, which means that there are really 46 chromosomes total. You have 1 chromatid from Mom and 1 from Dad for each unique chromosome. However, there are still only 23 unique chromosomes. Meiosis 1 - Prophase 1/Prometaphase 1 In this, Prophase 1 and Prometaphase 1 are combined together. Make special note to these things: 1. The red pairs are from Mom. The blue pairs are from Dad. (arbitrarily defined) 2. The Homologous Chromosomes (big from Mom and Dad, and small from Mom and Dad) have been duplicated, therefore there are two HOMOLOGOUS PAIRS for each type of homologous chromosome (big and small) 3. The nuclear envelope is in the process of being dissolved. The spindle apparatuses have moved. Homologous Chromosome Sister Chromatids = Homolgous Pair Cross Over Event Make note of the weird crossing between Homologous Chromosomes: This is called a "cross over event" where sections of 1 chromatid from Mom will cross over to 1 chromatid from Dad. The result? The switching of Mom's genes with Dad's. Side note: Cross over events are completely random, and variation occurs in how much will cross over. This is all random probability.
Cross over is good for genetic variation, keeps things from being the same over and over again! Meiosis 1 - Metaphase 1 In this part, pay attention to these things: 1. Instead of the sisters lining up single file, now the homologous chromosomes line up NEXT TO EACH OTHER. 2. The cross over events are still crossing over. They don't "break" until Anaphase 1. 3. The spindle apparatuses attach just like in mitosis The Homologous Chromosomes are lined up NEXT to each other. Spindle Apparatuses move just like in Mitosis Meiosis 1 - Anaphase 1 In addition to random cross over events, there is a thing called "independent variation." In a nut shell, it is the idea that the homologous pairs could like up in an way they want, independent of the other chromosomes. This too adds to genetic variation and diversity. Independent Variation starts here, and continues until the daughter cells are formed. Now, unlike Mitosis, instead of individual chromatids pulling apart, whole HOMOLOGOUS PAIRS pull away from their HOMOLOGOUS CHROMOSOME partner. This is where the cross over event "sticks" and the new crossed over genes stay on the new chromosome. Homologous Chromosomes pull apart Meiosis 1 - Telophase/CytoKinesis At this point, the, two new daughter cells break away from each other Unlike Mitosis, new nuclear envelopes do not really develop. Also, the daughter cells are now HAPLOID Cleavage Furrow Haploid Cell
Now the cell has half the original
amount of Chromosomes Independent Variation is responsible for why each cell received the homologous pairs they did. Meiosis 2 - Prophase/Prometaphase II At this point, it is just like mitosis would be Daughter cells Meiosis 2 - Metaphase II Again, just like Mitosis, the homologous pairs line up in single file. Now, independent variation is responsible for "deciding" which sisters go where, i.e. where the cross over chromatid goes. Spindle Apparatus
pulling the sisters apart, just like in mitosis Meiosis 2 - Anaphase II Now, this Anaphase occurs exactly like Anaphase does in mitosis Sisters are being pulled apart, and each side is getting 1 type of chromosome (big and small) Meiosis 2 - Telophase/Cytokinesis II This is now the final part of Meiosis The two daughter cells now divide one more time, forming two more daughter cells for the primary cell. The nuclear envelope now begins to form around the new genetic material (23 chromosomes in human sex cell - these are haploid, remember!) We now have four unique, distinct cells, each with a certain variation of genetic material, which adds wonderful genetic diversity and means a surprise to come when baby arrives! Whose features will it have? Mom's nose? Dad's eyes? It is all a surprise now! 2 haploid daughter cells for each 1 haploid daughter cell New Nuclear Envelope forming Now grasshopper, you are done! You went through the vigorous training sessions... You pressed on, carrying yourself through the wonders of Mitosis and Meiosis... You didn't think you would make it, but you did... Let's take a look at where you came from, and the journey you traveled... Bravo. You have done great work grasshopper. Now go, share your knowledge with the world! All images used respectfully from:
http://futurity.org/wp-content/uploads/2010/04/chromosome.jpg Prezi created by: Dominique Ramirez
For the Honors section of the Life102 Lab.
Lab Instructor: Jonathan Harris FIN