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A look into cell division

ImageIt’s that time in the general biology semester where we transfer our attention to cell division.  Having already discussed a number of basic principles like the laws of thermodynamics and a touch of chemistry, and cellular functions such as the flow of energy and the flow of information, it’s now time to look at how cells reproduce themselves.

In this chapter we should be recalling all the parts of the cell and accounting for how they get sorted into the developing ‘daughter cells’, and also recall the role of information, in the form of DNA, and how this is apportioned into the daughter. Of course we will spend most of our time focusing on the distribution of DNA, but we should always keep in mind what we know of other structures and organelles.

I previously wrote an essay describing cell division in humans that marries this information with the subject of the next unit, genetics and inheritance. You can find that text here. Therein, I briefly address one of the oddities of eukaryotic cells, the mitochondria. Mitochondria are odd because they live in our cells as strange symbiotes that share their energy with us in exchange for protection and a supply of nutrients. The theory describing this relationship was proposed by Lynn Margulis, and is widely accepted today. A description of her theory can be found here.

Because Mitochondria (and chloroplasts) are pseudo-autonomous cells, they must replicate themselves. A cartoon and some micrographs that illustrate this process have been borrowed from Nature Reviews.

ImageThe process involves an interaction with the Endoplasmic Reticulum, that guides an assembly of molecules that constrict around the Mitochondria eventually effecting its division into to smaller organelles. What this image does not include is the replication and separation of the mitochondria’s own circular DNA, a process that necessarily precedes the actual division of the organelle.

Altogether, there’s a lot to keep in mind when examining cell division. Why is this cell dividing? How are the instructions for life (DNA) being distributed between daughter cells? What does the daughter cell need in order to survive on its own? How do these parts / organelles handle their own division between the cells? And what would happen if any of this went wrong along the way?

 
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Posted by on October 7, 2013 in Education, Uncategorized

 

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Making Better Babies – A pragmatic solution to a problem and the ethical firestorm it provokes

Let’s say you are a young person thinking about having a baby, but you know there is a genetic disease in your family that worries you. It’s a reasonable concern that people have recognized for some time. There have always been diseases to concern parents, but technology is changing how we think about and face these concerns.

Image

A Downs Syndrome Karyotype

Once upon a time these was little more to do but cross your fingers and hope for a good result. Then chromosomal testing (Karyotyping) of a developing fetus became possible allowing parents to know what is happening inside the womb. Getting this information was not without risk, and even with the results in hand, would-be parents are faced with terrible choices. The way we dealt with this test in our family was to not have it done. We really wanted a baby and thought that  that having test results would not change our behavior. So we chose not to be put in a position of having to make that choice. It was what worked for us, but we also didn’t have specific concerns other than the fact that we were getting older.

Following the advent of chromosomal tests, it became possible to test the DNA of a fetus for specific, known problems. For example, If caner runs in the family, you could check to see if the baby’s p53 gene was normal. Having one or more bad copy of this gene dramatically raises the probability of developing cancer relatively early in life. Today, this is something we can know for certain.

But there are several sources of DNA in us. We typically think of the vast amount of DNA carried in the form of linear chromosomes that are packaged inside the nucleus of our cells. This is definitely the lion’s share of the DNA passed from one generation to the next, but there is another source as well: The Mitochondria. You may have learned about these organelles (little organs) as the ‘Powerhouse of the Cell’ for its role in generating much of the energy (ATP) your cells need to do their jobs. These organelles have a strange history in us. It is thought that many eons ago those things that are now mitochondria inside our cells were once free living organisms (possibly parasites, possibly a bigger cell’s dinner). However it happened these microbes were taken inside of our cells, but not digested as food or harmful enough to kill the host either.

Powerhouse

Why am I talking about this? Because those organelles still carry remnants of their former selves. They still have their own protein-making machinery and even their own DNA. This DNA isn’t large, but it does carry genes coding for vital proteins. And this is how we get back to our original story, because sometimes these mitochondrial genes are no good. If these genes aren’t right, they can’t make healthy, functional proteins. If they can’t make good proteins, then the host cell and the while organism can die.

Interestingly, all the mitochondria in every cell of your body came from your mother. This is one place where dad makes no contribution. Even though sperm have mitochondria, they don’t get incorporated into the new zygote, only those from the egg will remain.

Enter The Future of Fertility Medicine

Recent developments have shown that it is possible to replace the unhealthy mitochondria with healthy versions from a donor cell to make good eggs that can be fertilized and result in a healthy child. This was the subject of an excellent review in Nature and also discussed on the Nature Podcast this week. So how many parents is that? One mom, one dad and one mitochondria donor (I guess this could conceivably come from dad, but I just don’t know). This procedure has been done successfully with non-human primates, but so far not with humans.

So, pursuing a simple line of work aimed at helping parents make healthy babies is suddenly possible and suddenly a great ethical question. Have you ever seen Gattaca? If not, go out and watch it. I was sure this film was going to be miserable and be a poor representation of science, but I was totally wrong. They ask the same questions in that film that we are beginning to face in real life:

This isn’t the first time people have thought about this

When does Medicine become tampering with life? And does it matter? Don’t we want healthier, more able bodied people? Is it wrong to replace bad genes? What constitutes ‘bad’?

Personally, I don’t believe that there are universally right and wrong answers to these questions. Even if we decide that there are some less desirable consequences for mankind, that doesn’t mean that we wouldn’t do it. Much of plastic surgery isn’t really necessary and some might call it a perversion of medicine, but that doesn’t stop tons of people from getting it.

 
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Posted by on October 24, 2012 in Uncategorized

 

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