Chromosomes

04/10/2010 § 1 Comment

The next unit of the semester has to deal with genetics, DNA, and cellular reproduction. This unit’s essay question is simple: Why sex? (Or Why is there sex?) In class, we were faced with many arguments and reasons that showed that asexual reproduction (not involving sex) works well and there’s absolutely nothing wrong with reproducing the asexual way. So why sex with another cell, for example? According to class today, sex is actually something that humans invented on the journey to discovering reproduction. I’m pretty sure our ancestors didn’t know anything about the scientific reasons behind reproduction, they just knew, oh, if you did this, in a few months, there’d be a new human being in the house. But in that, a “new human being” or, in a broader sense, in a more generalized term, sex is somewhat more important than the asexual way to reproduce because it forms something new. The result of sex is a combination of genes, not just a copy (which is what asexual reproduction gives).

One example of asexual reproduction that we studied a little in class is the bacteria’s form of asexually reproducing itself — although the prokaryote is basically just copying itself. The DNA of a prokaryotic bacteria is a simple single loop of DNA. The structure of the DNA in a bacteria is simple so that copying the information is easy during reproduction. All in all, the entire process can be finished in about 10 to 15 minutes. This process, we learned, is called binary fission.

We know that prokaryotic bacterial cells reproduce asexually through the above process—binary fission—but eukaryotic cells can produce in two ways. A human, for example, can do both so that it is able to grow, develop and repair itself and also to reproduce and copy DNA to form offspring. We can reproduce through mitosis to grow, develop and repair. This process copies the DNA to form more cells and in reality, according to the science text book, p. 118, adults can produce up to 25 million new cells per second. That’s a lot of new cells. In contrast, we can also reproduce sexually through a male’s or female’s gametes. One way or another, the DNA in our cells is copied to produce more and more cells.

One very important idea we learned in class today was the structure and length of the genes, which form into chromosomes that stretch off as far as the length to the sun and back to earth five times. That’s an estimated 20 trillion meters of DNA in our bodies. We can fit all this DNA and genes into our body because the long strands of DNA coil again and again and again and again and again and again around proteins and around each other—very much so like a fishing line into a spool—and fits into all of our cells. And after the copying of chromosomes, the chromosome ends up with two identical chromatids, which then connect on a centromere. Chromosomes are the way our 20 trillion meters of DNA is arranged into our cells and into our bodies and help the cells reproduce properly.

Now, in chromosomes, there are 46 of them in a human body, but we consider chromosomes in their separate pairs, so to speak, there are 23 pairs. The two members of a pair are homologous and are matched by their shape, size and set of genes. While humans have 46 chromosomes, other animals or organisms can have much more or much less. Orangutans actually have 48 chromosomes and plants can have more than 200. The number of chromosomes in an organism differ depending on what the organism probably needs. (Since chromosomes hold DNA which hold information that tells ribosomes what proteins need to be made for the plant). A double set of homologous chromosomes is a diploid and has to do with sexual reproduction. When the egg and sperm fuses and after the egg fertilizes, a diploid zygote is formed, the baby or offspring of the two parents. This is meiosis, producing haploid gametes, and combining them to make the diploid zygote.

Also, as a review of something we learned in eighth grade, a particular chromosome determines what gender the offspring will be. In humans, the sex chromosomes are XX (for a female) and XY (for a male). If we look at the sex chromosomes (if we look at the karyotype pictures), then we’ll see that the male’s sex chromosome has a Y-shaped chromosomes next to a longer X-shaped chromosome. A female’s sex chromosomes are different, with two X-shaped chromosomes. This gender-determining trait chromosomes have is similar in almost all organisms, except not all organisms have X and Y chromosomes. Birds, moths and butterflies, for example, don’t have a Y-chromosome and the male gender can be determined if there is only one X-chromosome.

Finally, during the process of copying DNA, chromosomes can sometimes fail to separate appropriately during meiosis. This is called non-disjunction and can cause extra copies to be made in the offspring’s chromosomes. Down syndrome is an example of meiosis-gone-wrong because of the extra copy of chromosome 21. Other changes in a chromosome’s structure have to deal with deletion, which deals with parts of DNA that is lost—deleted—, duplication, which happens when DNA is copied, perhaps in the wrong place or the wrong amount, inversion, when certain parts of the DNA structures are switched and reversed, and translocation, when parts of the DNA are moved to places they don’t belong.

Amongst all the details of cellular reproduction, I basically understand everything. Throughout the next few weeks, I hope that I can understand a slightly deeper understanding of how sex can result into much more meaningful things other than copies of DNA. Maybe it includes the passing down of information to generations, or the change and evolution of the species/organism. I know that asexual reproduction doesn’t do that: form new things. But sex does.

§ One Response to Chromosomes

  • Dave Ferguson says:

    Kari,

    I’m continuing to enjoy your thoughtful and thorough posts. I like hearing how you put ideas together so well: excellent writing style.

    One minor error, “sex is actually something that humans invented” should be “sex is actually something that early eukaryotic cells invented”.

    Mr. F.

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