Boy, do I ever wish I could read the rest of this article. It sounds fascinating. Look out McGraw-Hill, those General Bio textbooks are in for a re-write.
Sorry Malcolm. It couldn’t be missed.
Tag Archives: chromosome
Groundbreaking work out of Penn State demonstrates cows are multi-Y chromosome-carrying PRIMATES!
Who said what about the molecular basis of inheritance? First steps towards the double helix
A workable Theory
Gregor Mendel did the first documented, accurate analysis of how one generation inherits traits from their parents’. However, being so far ahead of his time, and possibly because he may have been considered an outsider to the scientific community, the value of his work was not grasped and it was quickly forgotten. Mendel had stated a couple of theories in his paper making theoretical assumptions of how ‘factors’ responsible for inheritance are segregated during the formation of gametes and then rejoined in the zygote. Together, the two factors that every individual had for each trait determined the expression of that trait.
Colorful Bodies
From Zellsubstanz, Kern und Zelltheilung, 1882
A couple decades later Walther Flemming observed, under his microscope, some curious movements of things called ‘chromosomes’ (or colorful bodies). Unfortunately, he did so without knowledge of Mendel’s work and thus didn’t recognize the greater meaning of his discovery. At this time Germ Plasm Theory (not to be confused with Germ Theory) was being debated as a means of inheritance. This theory suggested that special germ cells carried the hereditary material from one generation to the next and that these cells were uninfluenced by the other cells of the body. The plasm itself took the place of Mendel’s idea of factors, but functioned as a more inclusive measure of material, more akin to what we call the genome today. I say ‘plasm’ is replacing ‘factors’, but it should be noted that no one intentionally renamed Mendel’s factors. The real problem was that no one knew about his work at all.
Edouard Van Beneden had been investigating similar chromosomal movements during meiosis that produced the gametes, or germ cells. However, he too was having a difficult time constructing a cogent theory concerning how these movements led to inheritance.
August Weismann
August Weismann was in the best position at the time to put the pieces together as he was already turning the available information over in his mind to construct a big picture. He recognized that the nucleus was the likely source of the ‘plasm’ and even considered the chromosomes as a possible candidate for this material. Further, he was concerned with how it was that germ cells could join in a way that did not double the heritable material with every generation.
Connecting the Dots
It was not until Sutton and Boveri independently unearthed Mendel’s work and recognized the similarity of chromosome movement during gametogenesis (making sperm and egg cells) to his descriptions of ‘factors’ that The Chromosomal Theory of Inheritance was first seriously proposed.
Altogether, these observations led many to believe that it was, indeed, the chromosomes that carried the genetic material. Still, this did not pinpoint what molecule was responsible or how it accomplished this for chromosomes are comprised of long strings of DNA and a number of proteins. Given the simplicity of DNA, many refuted this as potential genetic material as it simply could not carry the required information. Perhaps it served as a scaffold for the more important protein molecules? This was consistent with the chromosomal theory and had the benefit of allowing for complicated proteins to bear the information required to build new cells and organisms.
Griffith with his pal Bobby
Fredrick Griffith was one of the earliest workers to begin the march to identify the specific molecules that functioned as heritable material. While working to develop a vaccine against Streptococcus pneumoniae, he discovered their curious capacity for communicating information laterally, from one organism to another.
Griffith’s Experiments: Glimpsing the Answer
Two forms of the bacteria were known: One a virulent smooth form that secreted a protective carbohydrate capsule, and Secondly, a rough strain that lacks this protective coating and is non-virulent. Predictably, mice he infected with the smooth strain died as a result of bacterial overgrowth. Mice infected with the rough strain overcame the infection and lived. Also, as one would predict, heat-killed bacteria of both strains had no ill effect on mice. But what Griffith did next was much more interesting…
He took heat killed smooth bacteria and combined it with the non-virulent rough form and found that the resulting cocktail of bacteria not only killed mice, but these mice were also found to be overgrown with the smooth form of the bacteria.
What are the possible explanations?
- The S strain can come back to life in the presence of R strain bacteria.
- Some information was taken up by the live R strain bacteria that allowed it to take on a trait from the dead S strain bacteria.
Griffith chose the simpler second option and suggested that some transforming factor was responsible. This factor was released by the dead S strain and carried heritable information.
Unfortunately, Griffith wasn’t around long enough to see the story play out as he perished during the German air raids of London during WWII. But one last experiment done by another group illustrated that he was one the right path. This experiment repeated Griffith’s work carefully, but instead of combining S strain and R strain bacteria, the S strain bacteria were spun down into a pellet using a centrifuge and only the liquid supernatant was moved in with the R strain bacteria. As Griffith might have suggested, this liquid carried his transforming factor and was sufficient to cause a change in the R strain to make it produce and secrete the carbohydrate capsule.
This last experiment and some others were done by the laboratory of Avery, MacLeod and McCarty at the Rockefeller Institute in New York and will have to be discussed in a future post.
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.
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.
Aside
Cell Division is an essential part of life. However, this means different things to different cells. For single-celled organisms cell division is reproduction – one mother cell divides into two daughter cells. If these single-celled organisms are prokaryotic, then this division happens by the relatively simple process of binary fission. If the organism is eukaryotic, division occurs by a more structured process of mitosis.
Regardless of the differences between these organisms, cell division consists of three core components:
- Duplication of DNA
- Segregation of DNA (into areas that will become the new cells)
- Cytokinesis
In Prokaryotes, this describes the cellular events perfectly. The circular DNA (arguably a chromosome) is duplicated, the two copies of the DNA goes to opposite sides of the ce
ll and cytokinesis divides the cell in half along the midline.
An illustration of a number of bacterial cells undergoing binary fission:
The entire process is outlined below:
In eukaryotes, the same basic process occurs, but because of the differences in how DNA is organized and localized (in a nucleus), differences emerge.
The basic form of eukaryotic cell division is called mitosis. In single celled organisms this process functions as an asexual form of replication, while in multicellular eukaryotes, this form of division serves to add to the cell number (i.e. growth / healing).
Mitosis is actually the division of the nucleus and DNA, this process is accompanied by the division of the cell itself, cytokinesis.
There are four phases of mitosis:
- Prophase
- Metaphase
- Anaphase
- Telophase
- Cytokinesis overlaps somewhat with Telophase, but continues on afterwards to complete cell division.
Each stage of Mitosis is defined by a number of events describing the organization of the DNA, the state of the nuclear membrane and the development of mitotic spindles that maneuver the chromosomes within the cell. I won’t describe these here as this can be found in any number of other sources.
What I will discuss is the pattern of chromosome number, DNA copy number and the chromosome number.
The chromosome number is referred to as ‘n’.
The DNA copy number is referred to as ‘c’.
Humans have an n number of 23. What this number tells you is how many pairs of homologous chromosomes an organism has.* This immediately begs the question, ‘what’s a homologous chromosome?”
Homologous chromosomes are those that have the same genes – that is, they code for the same traits even if they do not have exactly the same versions of these genes on each chromosome. For each of the 23 different kinds of chromosomes humans have, each person gets one copy from their mom and one from their dad. So these 23 pairs of chromosomes are 23 pairs of homologous chromosomes.
This 23 number we keep talking about is the n number in humans. Each species n number may be different, but all the members of that species have the same n number. Further, the n number does not change at any time during the cell’s life
Because we have one chromosome from each of our parents in every one of the cells of our body we are diploid organisms (from the Latin, di – meaning and two ploid – well, ‘ploid’ doesn’t really come from Latin)
All the cells in our body are diploid – except the gametes (sex cells). These cells are formed by a special kind of cell division called meiosis. This type of division is very similar to mitosis, except that it consists of two separate rounds of division and the resulting sex cells have only one of each chromosome type and are therefore called haploid (think ‘half’)
Cells of the body are 2n = diploid.
Sex cells are n = haploid.
This leaves the c number…
The easiest way to determine the c number is to simply count the number of chromatids for each type of chromosome. Consider the cells below. On the left is a cell in G1 phase of the cell cycle. Then the cell goes through S phase where DNA replication occurs and looks like the cell on the right.
Note that the n number does not change, but the c number doubles when the number of chromatids doubles.
* It’s important to note here that not all organisms have similar patterns of chromosome arrangements as we humans do. In this essay I am referring to humans and organisms that handle their DNA as we do.
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