Growth and reproduction:
Organisms grow and reproduce. This is the last of the anatomy / physiology units for the year! It is a short unit, but it has some memorization in it.
We will learn the process by which multicellular creatures grow and simpler creatures reproduce asexually (without a partner). This process is called "mitosis" and has six steps.
Then we will learn about the many diverse methods organisms have evolved to reproduce asexually. You will even try your hand at doing one of them.
Then we will learn about how organisms go about making diverse offspring (sexual reproduction). This process is called "meiosis" and has 11 steps.
We will learn about the sexual parts of a flower and dissect a flower (if you can find one around your house).
Lastly, we will look in a little more depth at the special time of embryonic development.
So let's begin with mitosis:
The process begins with one cell and ends with two identical cells each a little smaller than the first. In you, this is how you grow. In a bacterium, this is how it reproduces.
There are 6 steps:
Interphase - this is sometimes called the resting phase of mitosis and most of the time they are resting. Most of your cells are in interphase most of the time since they are not actively reproducing themselves. (the name means between mitosis episodes). When a cell is large and has space next to it, it will trigger the disolving of the nucleus and duplication of the DNA (the material that codes for everything in you).
Prophase - this phase is when the DNA binds around proteins called "histones" so that they do not get tangled and torn when being pulled apart. You can actually begin seeing the DNA with just a microscope at this point. Each of your chromosomes becomes visible like a little "X" floating in the cell. You have 46 chomosomes.
Metaphase - this phase is when the centrioles (remember them from the cell organelles?) drift to oposite sides of the cell and begin pulling on the chromosomes. The DNA all lines up down the middle of the cell. That is how I remember this step--Meta = middle.
Anaphase - in this step, the two copies of DNA get pulled apart and go to the opposite sides of the cell.
Telophase - sometimes called the last step, this is when the DNA begins unbinding and new nuclei form around each set. But since this phase does not involve the cells pinching in two, it is not technically the last phase of mitosis.
Cytokinesis - the cell membrane pinches in two completing the process of making two identical cells from the one parent cell.
Students years ago dubbed this process IPMAT (forgetting about the cytokinesis), but I find that mnemonic memorable.
Here are your mnemonics:
1. Interphase- resting (interrupting=silent, not doing something)
2. Prophase- proteins (p=p) the DNA binds around those proteins called “histones”)
3. metaphase - middle (like you said on your website) DNA lines up in the middle
4. Anaphase-apart- the two copies get pulled APART
5. Telophase- (telo=end) begins unbinding, new set
6. Cytokines- cell membrane (c=c) pinches into two
Here are some images showing this:
Many organisms from bacteria to sea stars to plants can reproduce asexually. The biggest advantage is that you do not need a partner. One bacterium could populate the planet. The biggest disadvantage is that all the offspring are genetically identical to the parent. They are clones.
Some of the adaptations creatures have evolved to reproduce asexually are:
This looks just like mitosis. The one cell grows large and then splits into two similar cells.
Bacteria often reproduce this way.
This is similar to fission, but with one of the cells staying large and the daughter being significantly smaller.
Some more complex organisms bud:
This is when some cells that still can grow into any part of the whole organism are nurtured and allowed to grow. With plants, you can often cut a piece of stem or root and plant it after keeping it damp for a few days:
I want you to try to grow a cutting. Your parents may have a house plant you could ask to use (Jade plants work very well). If not, go outside and collect a stem (willow trees are terrific for this). Let me know in your journal how this went and keep me posted about your cutting plant.
This is when a plant shoots a stem out to the side to root elsewhere (think of strawberries). Then the little stem breaks and the plant has now reproduced:
Some plants have root bulbs that essentially bud. Breaking them off and planting them helps the plant reproduce asexually:
Some plants have buried stems that are full of those cells that can be anything. These are called tubers (think potatoes) and can be planted:
Some creatures can simply regenerate if cut in two:
These are both alive and came from one. They will eventually grow five legs each and be clones.
As you can see, there are lots of cool examples of asexual reproduction. The big benefit is that you can do it alone. But all offspring are identical to the parent.
Sexual reproduction means that the DNA gets cut in half and two halves get combined to make a new, different offspring.
The process is similar to mitosis and has similar phases. The big advantage of sexual reproduction is genetic diversity which strengthens the species. The big disadvantage is that you need to find a mate and exchange DNA.
The process is called meiosis, and it is essentially IPMAT followed by PMAT. In other words, the DNA doubles once at the beginning, and then the cell undergoes two cell splittings. The end result is four cells each with half the DNA that the parent cell had.
This process occurs in cells called gametes in the gonads.
You might wonder why have the first interphase at all. Why not just do PMAT? If you skip the DNA duplication phase, then you would still end up with half the DNA, but only in two cells rather than four. That is a good question. I could simply say that that is not the way organisms evolved, but I think it has to do with the benefits of that first phase. In males gametes, they need to produce lots of these haploid cells (haploid means having half the DNA as the whole organism). Whether it is pollen in a tree or sperm in a human, the male makes millions of these haploid cells so that they have a greater chance of success. Therefore 4 cells is better than 2. In the female, all of the cell stuff in each of the splits goes to only one side making one cell with half the DNA that is huge (an egg is one cell and only has half the DNA of the parent) and four tiny "polar bodies" that are not used for reproduction. Since the egg needs to have lots of nutrients and mitochondrial DNA and transcription factors, it is important to have four times the nutrients, hence splitting twice instead of only once.
Once the cells that have half the DNA are made, they just need to be brought together and a novel life will be brought into existence--sexual reproduction!
A good example of sexual reproduction is to look at the sexual parts of a plant--the flower:
The pistil is the female part in that it makes a few eggs and stays put. The Stamen is the male part in that it makes millions of pollen cells and sends them out to other flowers.
Find a flower and (after getting permission from your parents), dissect it. Pull it apart and look for the parts. You may need a sharp blade and a magnifying glass. Many flowers are not this straight-forward.
Whether a plant's seed, a chicken egg, or a human embryo, what does the developing organism need before it is "born?" Put this in your journal.
While not on the test, a closer look at human development is important:
Once the sperm and egg meet, we call that conception and the resulting single cell a zygote. That zygote drifts down the fallopian tubes and implants in the uterus. If it implants elsewhere, it is called an ectopic pregnancy and usually does not develop easily. There it will divide and part of it will develop into a placenta and send a message to the mother not to menstrate. The embryo (as it is called now that it has grown) begins to differentiate. This means that cells get cued to turn some of their DNA on and other DNA off. Once a cell differentiates, it can no longer develop into any type of cell. The cells are called stem cells before this differentiation and have obvious health benefits. After 8 weeks, the developing organism is called a fetus. Around 24 weeks, the human could (with help) survive. Around 40 weeks (give or take two weeks), the fetus sends a message to the mother that it is ready to be born and initiates the birth process. If the cell splits while still full of stem cells, two identical babies can develop. These are called identical twins. Fraternal twins is when two different eggs are fertilized by two different sperm and are no more identical than any two siblings who share a birthday.
If everything on the page makes sense, you are ready for the unit test! Can you answer what the advantages and disadvantages of asexual reproduction are? What about sexual reproduction? Can you put the stages of mitosis in order and tell me what is happening in each stage? Can you tell me why gametes go through meiosis instead of mitosis? Can you brainstorm a list of things that all developing embryos need (hint, it should be pretty similar to the units we've covered so far). Can you recognize the parts of a flower?