f you could send me an answer it would settle an argument.
Thank you and keep up the good work. We need
more sites that teach through entertainment.
I am afraid you have it backwards. Mature red blood
cells lack a nucleus, and therefore, do not have any DNA. Almost all
other cells do have a nucleus and DNA.
I have really enjoyed your site. As a 40 yr. old parent, helping her young student, you sure have made reviewing science easier than I would have thought. But I have a dilemma... How does one explain "Mitosis" in a language easily understood to a 4th grader? Any suggestions? Thanks for a great site!
Thanks for your kind words about our website. We truly believe that science is fun and really pretty easy. And, your child is quite lucky to have a parent who wants to help them get the concept of mitosis, of all things !
So, let's try this:
MITOSIS ISN'T ABOUT YOUR FEET !!!!!!
Its about how one cell makes two cells that are identical to each other. But, first, let's back up to simply define 'mitosis' (my-toe'-sis) versus 'meiosis' (my-o'-sis). Mitosis is the way that one cell creates two exact copies of itself, both with two of each chromosome = mi-two-sis. Meiosis is the way that one cell creates two halves of itself, both with one of each chromosome = mei-one-sis.
WHAT ARE 'PAIRS' OF CHROMOSOMES?
Before we get to mitosis, we really need to talk about the chromosomes in plant and animal cells. These types of organisms are created from a mother and a father. Each cell of the plant or animal contains pairs of chromosomes, one from the mother and one from the father. So, there are two chromosomes #1 and two chromosomes #2 and two chromosomes #3, etc etc. Now each chromosome #1 has the same genes in the same order along their length. But, the chromosome #1 from Mom probably has a different DNA sequence for many of those genes than does chromosome #1 from Dad. That is called genetic diversity and it is a very healthy thing for a species.
For clarity, lets look at each chromosome as a phone book for small town. Each phone book #1 has a single phone number for each house in Camby, Indiana. Even though each house has both a cell phone number and a land line number, only one number can be printed. Mom's phone book #1 has the same houses listed in the same order as Dad's phone book #1, but the phone numbers can be different. You can see how there are lots and lots and lots of versions of phone book #1 possible. Then, phone book #2 has a single phone number for each house in Corpus Christi, Texas. Again, each house has two phones, but only one number can be printed in any phone book #2. Phone book #1 may look like phone book #2, but they hold very different information.
When a cell divides by mitosis, two identical daughter cells are formed - each cell with a pair of each chromosome. In our phone book analogy - mitosis would make an exact copy of Mom's phone book #1 and Mom's phone book #2 and and exact copy of Dad's phone book #1 and and Dad's phone book #2 and make sure each daughter cell had a pair of each phone book - one Mom's phone book #1, one Dad's phone book #1 and one Mom's phone book #2, one Dad's phone book #2. In this way, the daughter cells are EXACTLY the same. Hope that was clear.
BACK TO MITOSIS......
So, here you have a cell going about its busy life doing what it does. When, its time to divide to make two cells that can go about their busy lives doing what they do: It's MITOSIS-TIME !!!!.
The cell must pull one copy of every chromosome into each daughter cells - no more, no less. Now, to do THAT takes a LOT of organization.
Well, the biggest problem is that DNA in one cell can be 5 meters long !! Its all unraveled inside the nucleus so that it is accessible for making messenger RNA and such. While it is unraveled, like a sweater unraveled on the floor, the cell makes an exact copy of each strand of DNA (chromosome). But, if it were to try to pull these two copies into two different cells, well, that would be like trying to separate spaghetti in a bowl without tangling it all up. Not easy.
So, the way things have evolved to work well is that each strand of DNA is 'knit' into a bigger package that is thick enough to be seen with a light microscope. This 'knitting' process is done by mixing specific proteins with the DNA. These proteins serve to wind the DNA and wind the DNA and wind the DNA into a more and more condensed shape. Take a piece of string about two feet long. Holding both ends, begin twirling one end. Keep at it and you'll find the string takes on a more condensed shape. This is much like the condensation of the chromosomes. Each strand turns into a chromosome this way.
Each chromosome has an area that seems a bit pinched - and that is called the 'centromere' because its somewhere near the center of the chromosome. This centromere holds the two DNA copies together. This part of Mitosis, where the DNA is duplicated and condensed with protein into chromosomes, is called 'prophase'. (pro- actually means initial or first)
A BRIEF MESSAGE ABOUT SCIENTISTS AND THEIR MICROSCOPES
Let's step back again to explain the progress of Science regarding cell division. Until the invention of the microscope, we didn't, and couldn't, know about cells. Once cells were discovered, scientists used their microscopes to watch what happened as cells divided. So they could talk to one another, and so they could investigate cell division in greater and greater detail, they separated cell division into phases by how things LOOKED. Remember, these scientists didn't know what chromosomes were or what DNA was. All they knew was what they saw under their microscopes. But, they knew it should be investigated and that more should be learned about this strange activity they were seeing. So, they named the stages of what they saw. The phase before Mitosis begins they called interphase -- under a microscope you can't see any DNA as it is unraveled. Prophase is where you see the chromosomes condense into recognizable shapes. After that is metaphase (meta- means middle), anaphase, telophase, and finally cytokinesis before returning to interphase.
Once they had named prophase and metaphase, the scientists decided they needed to key on a phase in between and they called that one prometaphase (see how clever scientists are?).
BACK TO MITOSIS........
Now, lets go back to what's happening in the cell. We covered what happens to the DNA during prophase. Also visible in the light microscope are little bodies called centrioles. During prophase these begin to migrate to opposite sides of the cell and fibers begin reaching out from the chromosomes (centromeres, specifically) to centrioles. These fibers will be the tracks the chromosomes follow to find their way to the two new daughter cells.
During prometaphase the nucleus is dissolving. When you think about it, this makes lots of sense. How can you pull one of each copy of a chromosome into a daughter cell if all the DNA stayed inside the nucleus???? Couldn't happen. So, the nuclear envelope dissolves.
Now the stage is set to pull the two copies of each chromosome apart. This phase is called metaphase. In metaphase the chromosomes move to the center of the cell, now that they have been released from the nucleus. This is an organizing step. This is all very much like a ballet, don't you think?
In anaphase we see the two copies of each chromosome being pulled to opposite ends of the cell, following the fibers that are stretched between the chromosome's centromere and the centrioles.
In telophase the chromosomes unwind again.
In cytokinesis the middle of the cell pinches down until there are two new cells. Also the nuclear membranes re-form to contain the chromosomes.
The two daughter cells move into interphase where they go about their busy lives doing what they do.
THE END - OR THE BEGINNING OF A CYCLE !!!
WHEW!!!!! Again, I hope all of that was clear. I am sending a link that has an animation of mitosis that is pretty cool. http://www.biology.arizona.edu/cell_bio/tutorials/cell_cycle/cells3.html
Please let me know where any confusion arises and I'll try to pin that area down better.
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