Posts in Biology
In the gender series we have examined gender from the standpoint of who gets to define you from a gender standpoint, why social boundaries trump biology for gender identification, and why the same cannot be said for race (thus the invalidity of the term “transracial”). In this fourth and final (for now) entry in the series, I will address some difficult and emotionally-charged issues around when transgender individuals should be integrated into current gender-based social categories and, critically, when they should not.
I will begin with the story of Fallon Fox, a transgender woman (born biologically male) who at 30 years old had gender reassignment surgery. A few years later, she joined the ranks of professional female mixed martial arts (MMA) fighters. This created significant controversy. She has had a fairly successful MMA fighting record and has left opponents injured in ways that are uncommon in female MMA matches. Should a person who was born male and up until 30 years of age experienced all of the hormonal strength and body-structural benefits of being male be allowed to participate in a potentially dangerous competitive female sport later in life?
My answer might surprise you. (more…)
Morality as an end result of self-interest causes people fits. For some, there is little more reason needed to reject the idea other than it negates the need for gods to have handed down our moral code. For others, however, they look at examples of super-morality – Gandhi being the most frequently expressed example of this – where a person’s self-sacrificing behavior seems to go beyond what you can explain via self-interest. In this post, I’m going to address that issue, and unless other great questions come up as a result, will close the loop on this part of the morality series.
For starters, let’s review some examples of moral behavior that results from self-interest. In my last post, I contrasted the behavior of human mothers to some marsupial mothers. Why is it that a human mother would go farther to defend her offspring than a marsupial? Can you explain that *just* via evolutionary biology? Well, yes you can. Life history theory posits that organisms develop behaviors that will result in the largest number of highest quality offspring, and furthermore, the higher the biological cost of reproduction, the more investment that is made in each one. Let’s examine our story in this light.
As a kid, I would rush to watch Saturday Morning Cartoons. My sisters and I would watch our shows, munch pop tarts, and try to stay quiet so that our parents would sleep a bit longer, giving us more time watching whatever adventure was being displayed in color or Technicolor. I didn’t really know how any of the technology worked – I just knew if I pushed a button, the TV turned on. Push another and the channel changed. My knowledge of the arcane workings of television has not progressed much over the years. I know how to make it do what it does and that’s good enough for me.
Most of us have a similar relationship with DNA. We know DNA holds genetic information for organisms, that our genes are encoded in DNA, and that’s about it. We know it looks like a double helix, might remember base pairs from High School science, but we would be hard pressed to explain how the information in DNA controls things like eye color, nose size, or height.
My wife and I have been on a recent miniature health kick. We are looking to replace common household snacks with healthy alternatives. Many alternatives exist, the trick is finding the ones actually good for you. We have cut out many of our typical snack items and replaced them with a nut mix. This mix is typically composed of items such as peanuts, cashews, yogurt covered cranberries, and various other odds and ends.
Recently, we have been including a hint of that taste of nature’s sweetness known as Reece’s Pieces. With every batch of our nut mix, we stir in a bag of the sugary concoction.
I have developed the nasty habit of pulling items from our nut mix bowl. My particular habits tend toward the cashews and Reece’s Pieces – I can’t get enough of them. Whenever I think I can get away with it, I sneak away supplies.
I recently noticed a trend with the Reece’s Pieces – these bits of chocolaty goodness come ready to attract the eye with three standard colors: yellow, orange, and brown. Humans have evolved to notice things that stand out from the environment, and the yellow and orange Reece’s tend to stand out when surrounded by an ocean of brown nuts. Safely hidden are the brown Reece’s Pieces.
During my once and former days as a pastor, I knew many couples with long marriages, many of whom had lost their spouses to illness and age. While just about all of these couples were admirable, one particular couple stood out.
After a long break for Christmas and a serious distraction known as Minecraft, it is time to get back to our sadly neglected series on DNA. Because it has been a while, let’s start with a quick refresher.
A DNA molecule is composed of two long strands (polymers) of nucleotides. A nucleotide is a nucleobase (nitrogenous base) joined to a five-carbon sugar (pentose) which is bound to a phosphate group. In DNA, the sugar molecule is deoxyribose. The nucleobase for DNA is one of the four nitrogen bases adenine, thymine, guanine, or cytosine. In RNA, uracil is found instead of thymine.
The nucleotides making up a DNA strand are joined to each other via the phosphate group which acts as a kind of glue to bond the five-carbon sugars to each other. The two strands of a DNA double helix are attached at the nitrogenous base: adenine bonding with thymine and guanine with cytosine in groupings known as the DNA’s base pairs.
Before we get to DNA replication, let’s look at the process that triggers replication. Cells are typically not static organisms but have a lifecycle all of their own which includes methods of cellular replication. Most of the cells in the body (cells that make up the body – including blood and organs – are called ) will replicate, forming new copies of themselves. Nerve cells are one exception – in general, nerve cells never replicate, which is one reason why nerve damage cannot be repaired.
In nature, there are two types of cells: and . Prokaryotes are basic and do not have a well-defined nucleus. Prokaryotic replication is a fairly straightforward process called binary fission. During binary fission, the cell literally splits itself in half with each half forming a new baby cell.
All multicellular life is composed of eukaryotic cells, cells with a well-defined nucleus and, typically, other complex machinery to regulate the life of the cell. Most eukaryotic cells, including most of our body cells, reproduce in a process called . We have already seen one exception: nerve cells, which don’t undergo mitosis because they simply do not replicate. Another exception is the body’s reproductive cells, which undergo .
Meiosis is part of the cycle of sexual reproduction and bears many similarities to mitosis but varies in ways significant for genetic differentiation.
are cells within the body of an organism. In general, somatic cells are distinguished from reproductive cells.
Prokaryotes are single-celled organisms without a well defined cellular nucleus.
Eukaryotes are single-celled organisms with a well-defined nucleus. All multicellular life is eukaryotic. Most eukaryotes also have additional internal elements not found in prokaryotes, such as mitochondria.
Eukaryotes reproduce in one of two ways: mitosis or meiosis.
Mitosis involves the division of a cell into two new cells, each genetically identical to each other and to the original cell. The overall process is divided into two stages: interphase and mitotic phase (M phase).
Interphase takes place between each M phase. Interphase itself can be divided into three stages: G1, S phase, and G2. G1 and G2 are essentially growing phases. Some cells never divide and remain perpetually in G1, carrying out normal metabolic activity. In cells that do undergo division, G1 and G2 are periods of growth for the cell, preparation for division. S phase is a bit more active. During this phase, the cell's chromosomes replicate their DNA molecules. Each chromosome will end up with two copies of its DNA molecule, attached to each other as sister chromatids.
M phase is the period of actual duplication and it takes place in five overlapping steps: prophase, metaphase, anaphase, telophase, and cytokinesis. During these phases, the chromosomes are shrunk down into tight spindles, the membrane of the cell's nucleus breaks down, chromosomes are pulled to the middle of the cell via mitotic spindles, the sister chromatids are separated via breakdown of the cohesin protein, mitotic spindles retract the chromosomes to each side of the cell then breaks down, a new nuclear membrane is formed around each new group of cellular components, and the cell is split in half via microfilaments.
The end product is two genetically identical cells.
Meiosis shares a lot of the methodology of mitosis but includes additional stages and leads to a different end result. In mitosis, somatic cells divide to form two genetically identical daughter cells. These cells are all diploid, each containing a copy of the cell's homologous chromosome pairs. In meiosis, the end result is four haploid cells. Whereas a diploid cell contains a homologous pair of each chromosome, the haploid cells contain only a single copy of the cell's chromosomes.
In meiosis, the four stages of mitosis take place twice, with slightly different results. The two cycles of meiosis are called Meiosis I and Meiosis II. During prophases of Meiosis I, the homologous chromosomes line up in a process called synapsis, pulling the sister chromatids of each homologous chromosome close together forming a structure called a tetrad. While the chromatids are arranged in a tetrad, it is possible for recombination to take place: close alleles can switch from one homologous chromosome to the other, resulting in unexpected genetic traits.
In anaphase, a cell's sister chromatids are not separated and pulled apart. Instead, the chromosome pairs are separated and pulled apart with each chromosome's sister chromatids remaining intact. The homologous pairs are separated essentially at random; maternal and paternal chromosomes will be mixed together, leading to genetic diversity.
Cell division takes place, resulting in two haploid cells that each contain both sister chromatids of the original chromosome (note: sister chromatids are replicated DNA molecules from one chromosome; they are not DNA molecules from each homologous chromosome, thus the cell is haploid). In meiosis II, the process is very similar to mitosis: during anaphase the sister chromatids separate and are pulled to opposite sides of the cell. At the end of meiosis, four haploid cells have been produced, two with copies of one set of homologous chromosomes, and two with copies of the other set of homologous chromosomes. Due to recombination, these copies will not be exact as some genetic differentiation would have taken place during prophase I.
Each haploid cell can mature into an organism's reproductive cells - gametes, in animals - and can fuse with the opposite-type gamete during fertilization (though in some organisms, the gametes types are not distinct; no male/female).
(Note: all images are from Wikipedia.)
Another filler of sorts on DNA. I anticipate my post on DNA replication should go up Monday. In the meantime:
So far I have focused exclusively on the shape of DNA we are most familiar with: the elongated right-handed double helix. This is the most common structure for DNA but is only one of the three major forms. They are: A-DNA, B-DNA, and Z-DNA. B-DNA is the familiar form.
In my previous post I took a close-up look at the structure of DNA. A few pieces were left out, partly to keep things short, partly from gaps in my own knowledge. I’m not yet ready with my next post, so this is a filler of sorts.
In art, negative space is the space around an object. While ordinarily attention goes to a particular object, for artists, negative space can be just as important. In DNA, negative space is the space between the strands of the double helix.
(Note: all images are from Wikipedia.)
“Dad, what are cars made of?”
“Well you see daughter, cars are built from many parts. There are car doors and chairs and tires and steering wheels and windshield wipers and so on.”
“But what about the part that makes the car go?”
“Oh, that’s the engine. Every car has an engine.”
“But what are engines made of?”
“Lots of parts, from the engine itself to other parts that help the engine do its thing! Gears and axels and belts and hoses and pipes and batteries and all sorts of other things.”
“And each one of those other things have to be there for the engine to work right?”
“Well, dad, tell me all about the engine and all about the parts it has to have to work right.”
A car is a complex machine. Even the most basic cars require many moving parts in order to transport you from your couch to the drive-through and back again. No less complex are the many DNA molecules found in every human cell. Let’s begin our look at DNA with a top-down view.