Thursday, January 27, 2011

Prime Numbers

This doesn't really qualify as biology, but it's still a branch of the natural sciences.

Here's a link to some fun prime numbers (only about 18 MB). Have fun!

Genetics: It Makes the World Go Round

Over the past week or so, I've become somewhat fascinated by genetics. Perhaps my background in mathematics is the cause, but I'm quite fascinated by the idea of simply building every idea out of another idea. 

To really understand genetics, we have to start out by going down to the molecular level. Every cell contains a nucleus, and within that nucleus, there's DNA. DNA is incredibly important to the body. It carries the information used to build an organism. But what makes it up?

DNA is composed of four different chemicals: guanine, adenine, thymine, and cytosine, which are typically abbreviated as their first letters. Within the strand itself, there are some rules these chemicals have to follow. A will always pair with T, and G will always pair with C. (Pair, by the way, means that they have been hydrogen bonded together. Picture a twisted ladder, and that's the shape of DNA. The rungs on the ladder can be thought of as hydrogen bonds between the two molecules. The sides of the ladder are actually made of sugar and phosphate bonded together.) 

Now, I read an excellent metaphor at the University of Utah website. It said that the letters of the strand can be thought of as letters of the alphabet. The letters come together to form words, and the words come together to form sentences. In the same way, different series of letters (for example, A T G T C A) can be thought of as coming together to form genes. 

Now, genes tell the cell to make certain proteins. Proteins, as we know, can give cells certain functions and abilities--for example, within a cell of the inner ear, they can allow the cell to work with other cells to hear sounds. Genes are composed of DNA, although there are many genes along a single strand of DNA. (There are approximately 25,000 genes within the human body!)

Of course, DNA isn't simply laying around the nucleus of the cell. It's packaged into units known as chromosomes. Chromosomes are simply big chunks of DNA with protein wrapped around it. Every human cell contains 23 pairs of chromosomes--46 in all. Each chromosome carries different DNA with different genes, which means that each one controls different traits. For example, the 23rd chromosome contains either an X and a Y chromosome or two X chromosomes. Whichever one of the pairs actually occurs defines the sex of the person.

This brings us into our next big topic, which is heredity. If you need a refresher, see my post on mitosis and meiosis before reading on. 

Because genes carry certain traits, and because each parent gives one set of 23 chromosomes to the child, a child will inherit certain traits from each parent. (Fans of the Harry Potter series will recognize that Harry inherited his father's hair and general appearance but his mother's eyes.) Because of this, each child has a different genotype (genetic makeup) and phenotype (physical appearance). When these children have children, they will pass on some genes from their mother and some genes from their father. This is how traits can pass through multiple generations.

Now, let's mix heredity and genes together. Genes are made up of what are called alleles. An allele can be either recessive or dominant. If it is dominant, its presence will be apparent in the child's phenotype regardless of whether another gene is present. If it is recessive, however, it will only be visible if it is paired with another recessive gene. Basically, dominant alleles are just that--dominant. They mask recessive alleles. 

Of course, it's possible for some interesting combinations to occur. If a person has two dominant alleles or two recessive alleles, they are known as homozygous. If they have a combination of  dominant and recessive alleles, they are heterozygous. Now, here is where inheritance becomes interesting. If two people, one who is homozygous dominant for a trait and another who is heterozygous for the same trait have a child, their child's phenotype will display the dominant allele. However, if they receive the recessive allele from their heterozygous parent, and they have a child with someone who also has a recessive allele for the same trait, then it is possible for their child to show the recessive trait! This is how traits can skip generations.

Well, I think that's all for now! Let me know if I mangled anything! 

Wednesday, January 26, 2011

Cells: They Reproduce

Well...I'm back. It's been a while.

DISCLAIMER: This material is confusing. I've done my best to explain it in a clear fashion, but there might be a few places where readers may get lost. My apologies! 

Recently, I've been studying the various ways in which cells divide and reproduce. There are two primary methods through which this is accomplished: mitosis and meiosis. On the surface, the differences are somewhat slight: one produces two cells with two pair of chromosomes; the other, four cells with only twenty-three chromosomes each. 

Now, I decided that I was going to stop starting paragraphs with, "Let's take a deeper look at each." With that said, I will instead finish this paragraph with: Let's take a deeper look at each.

To understand mitosis, one must comprehend the cell cycle. After one series of mitosis has ended, the daughter cells enter what is known as "G1," in which all that happens is growth of the cell. Then, during the next period of time, the chromosomes duplicate, causing this phase to be known as "synthesis." Another phase of growth, this time, "G2," occurs. The previous three phases are collectively known as "interphase."

Now, the mitotic cycle starts. This is where it gets really interesting. The chromosomes become visible under a microscope and the nucleolus dissolves. This period of time is known as "prophase." Now, the chromosomes appear in an X shape because their duplicates formed during the synthesis phase are joined in the middle, along what is known as the centromere. Next, the sister chromatids line up along the middle of a cell, known as the metaphase plate, as this time is called "metaphase." Then, the sister chromatids are pulled apart along the centromeres by fibers emitted from the centrioles (poles at both ends of the cell), and the chromosomes head to opposite ends of the cell in "anaphase." Finally, the cell's membrane splits the cell into two distinct cells. This is known as "telophase."

Meiosis has a few differences. First off, the goal of meiosis is to produce a cell with only twenty-three chromosomes so that it can share its chromosomes with another cell in order to produce a cell with unique genes that is then capable of developing into a baby of the species. Therefore, the cells undergo one more division than they do in mitosis.

Now, meiosis starts out just like mitosis does, with the chromosomes replicating and then condensing. However, the first difference comes in what is known as "Prophase I." Here, the condensed chromosomes pair up with their corresponding chromosomes (remember, each cell has a two sets of chromosomes). While they are paired up, enzymes cut sections of DNA from each chromosome and exchanges it with the other. This allows genes (more on those later!) to be transferred between the strands.

Then, the centrioles attach to the pairs of chromosomes--fibers from both centrioles to 23 chromosomes. The centrioles pull the chromosomes (as in metaphase), but instead of lining up along the metaphase plate, the chromosomes line up so that the pairs of chromosomes are divided by the plate. The pairs of chromosomes are now separated as one member of each pair is pulled to both sides of the cell. The sister chromatids, however, are still attached. The sister chromatids arrive at opposite ends of the cell, and nuclei form around them. Telophase I occurs and the cell divides into two cells, each with one set of 23 chromosomes that were duplicated during the synthesis phase.

So, quick recap: Originally, there were two pairs of 23 chromosomes. Each pair duplicated, creating four pairs. Then, the cell divided, creating two cells, each with two pairs of 23 chromosomes.

Now, the two cells basically perform mitosis again. The chromosomes condense into chromatids, line up along the metaphase pate, divide along the centromere, and a new membrane forms. Because each cell (which had two set of 23 chromosomes) has now divided into two, there are now four cells with one set of 23 chromosomes--the original goal of meiosis.

Well, that's that! See you again soon!