DNA: the molecular goddess

WOOPS! It's been a while (sick kids, getting my garden in, taxes, etc.) but not as long as I'd intended. I found I had posted this on my other blog, Over the Dither and Through the Words. Well, here it is now and I'm getting my next post prepared. Enjoy!

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In my last post on cell structure, we took a look at cell membranes which are made of a phospholipid bilayer. They protect the cell and cell organelles from the outside environment while letting a few players in and out to do their job. But what is it that membranes protect? Primarily, DNA (short for DeoxyriboNucleic Acid), a molecule that can make copies of itself. How DNA works was the next thing that blew my mind in biology, and pretty much cemented what I wanted to study.

DNA is the ultimate celebrity superstar of molecules. It’s the subject of many a science fiction story, volumes of textbooks and encyclopedias, various t-shirts, jewelry, and a scarf I want to knit someday. Everyone has heard of it, knows what it looks like, and knows that it carries our genes. If DNA is news to you, then you are probably fairly young or you’ve been living under a rock.

So how does this double helix encode our genes? You might think of a gene as that bit of information which gives us our eye color, makes us tall or short, shapes our nose, and gives us ADD (or not). While many of these things can be traced to a single gene, the more accurate definition of a gene is a sequence of DNA which gives instructions to put together a protein.

DNA has an alphabet of four letters, called nucleotides, nucleobases or bases for short. These nucleotides can fit together in two matching pairs called base pairs. The base pairs make up the rungs of the twisted ladder structure of DNA, with the sides being the twinned backbones of the macromolecule. The base pairs connect to each other across the rung, matching molecular shapes in the middle to make a light bond which can be zipped or unzipped. On one side of the ladder is the complementary template, and on the other is the actual code that creates a protein when transcribed.

These four nucleobases of DNA are Adenine (A), Guanine (G), Cytosine (C), and Thymine (T). A pairs up with T, and C pairs up with G. Ever watched the eugenics dystopian movie Gattaca? It's title refers to the bases of DNA.

Three of these bases together make a word, called a codon. This codon represents an amino acid. The twenty amino acids are the building blocks of proteins. There are also codons which say stop and start. This dictionary of codons which can be translated into amino acids, or stop and start is calledthe genetic code. How we got this genetic code is one of those mysteries that we still haven’t solved.

When making a protein, DNA is unzipped by certain enzymes, and a copy of it is made. This copy isn’t DNA, but RNA – a single strand instead of a double strand. Instead of Thymine, RNA uses a base called Uracil which still pairs with Adenine. Once the sequence of RNA is copied from the DNA (beginning at the start codons and ending at the stop codons), it is moved to a part of the cell where the protein can be built. This special sequence of RNA is called messenger RNA, or mRNA.

A DNA molecule being unzipped, transcribed, and it's mRNA template going on to a ribosome as a molecular blueprint for building a protein out of amino acids.

Another form of RNA is transfer RNA, or tRNA. tRNA is connected to an amino acid and has an anti-codon which has the three bases matching the codon for the amino acid. The mRNA goes to a ribosome, which is kind of a molecular reader. There can be up to 10 million ribosomes in a single cell. As the mRNA clicks through the ribosome, tRNAs connect to the proper mRNA codon, and then their amino acids are bound together. In this way, the long protein chain is created.

DNA doesn’t just code for proteins. In fact, only about 1.5% of the human genome represents protein sequences. It also has regulatory sequences, which control what proteins are made and how much and structural sequences for the chromosomes. And there can be a lot of repetitive DNA which doesn’t appear to do anything, though we can’t be positive about that right now. All of these can affect our genes and how they're expressed - in other words, what kind of inborn traits we have. But some of it does appear to be fossils of a type: broken copies of sequences we use or perhaps ones we no longer use. It’s this extra DNA which is another proof that species, including humans, have evolved over time. In the human genome, there are over 3 billion base pairs with approximately 23,000 protein coding genes. We are still exploring what each of those genes do.

So, cell membranes and DNA were the two sirens that pulled me into biology. Before I go any further into the other cool molecules, I think it's a good idea to get the lay of the land. In my next post on cell structure we'll take a brief look at a whole animal cell and get a simple description of each of the cell organelles.