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BIRDS AND BEYOND

How to make a chicken

By Claire Curry | Published Thursday, November 18, 2010

Which came first, the chicken or the egg?

This philosophical chestnut has stumped people for time immemorial. It has always seemed like a funny question to me. However, it makes me think of a different question, one that actually has an answer (although it was only discovered in the early and middle parts of the 20th century). How do we get from the egg to the chicken? In other words, how do chickens make more chickens?

The stuff of life is our genetic information. Today I’ll talk about DNA, which contains those genetic directions to make that chicken, hawk, or plant or anything living.

DNA stands for deoxyribonucleic acid. It is a long molecule made up of small units strung together in chains. Two chains of DNA twist around in what is called a double helix. Each unit in the chain is called a nucleotide. There are four types of nucleotides; each has different chemical properties. This allows them to pair with each other across the double helix, binding the two strands of DNA together like a coiling rope. These four nucleotides make up the whole genetic code.

Larger, more complicated molecular “machinery” attaches itself to the DNA and copies it into another chain of molecules. This transcription of the genetic code is made of ribonucleic acid, called RNA. More cellular machinery matches up chemicals called amino acids to every three nucleotides of the RNA translation.

Different combinations of three nucleotides (like a four-letter alphabet combining to make all combinations of three-letter words) code for different amino acids. These amino acids are linked together in chains called proteins. These protein chains fold up based on whether they are attracted to water. This is like oil and water – they don’t mix. So the chains fold up so that the “oily” parts stay on the inside of the folded protein.

These proteins are the hard workers of our chicken’s body. The muscle for its wings? Protein. The hemoglobin in its blood that carries oxygen to the brain? Protein.

Because there are more than 20 amino acids coded-for by just those four nucleotides in their combinations of three, and because proteins can be very long, you get essentially an infinite variety of proteins to make up the parts of the body, everything from the tiny “molecular machinery” to the muscles and the brain and the organs.

Different sections of DNA code for different proteins and these regions are called genes.

Every cell in our proverbial chicken’s body has a copy of this DNA. When these chickens get ready to lay those philosophical eggs, the DNA from each parent will each make up half of the egg’s DNA. This is where the double-stranded helix comes in. The two strands of DNA are un-wound with a protein rather like a zipper. This is why the chicken resulting from that egg will often end up being “in-between” its parents. It gets some DNA from both Mom and Dad.

Sometimes the offspring doesn’t look “in between,” though. Traits don’t always show up in the offspring (despite being visible in both parents) because of how the genes are read; sometimes one strand’s copy of the gene is dominant over the other.

So, why should we care about DNA? Because all organisms share the same basic machinery, this is what allows scientists to use mice, fruit flies and other critters to study diseases that humans get, too. For scientists studying populations of critters or plants, we can also use it to look at how the organisms are related to one another and even get clues as to how they are interacting. This is because of the genetic code, those four different nucleotides.

There are areas in the DNA that aren’t used any more, but were old genes or just appear to be random without currently known functions. These gradually change over time between generations because DNA has to be copied when cells are dividing to make new cells.

The cellular machinery involved is not perfectly accurate, so mutations in the DNA occur. These are inherited by the offspring, who may accumulate a few more mutations, and so on. Over many generations, there can be a lot of difference between their ancestors’ sequence of DNA. This lets us use the amount and type of differences to look at relatedness between organisms. I’ll talk more next month about how this works and what you can tell about a critter by looking at its DNA.

The next time you see any living thing, think about what amazing microscopic processes are going on. Even when we shed a skin cell, the little proteins have to go to work, reading out that DNA sequence. When our proverbial chicken lays its egg, it is combining two different strands of DNA that will be read by cellular machinery to eventually form a whole new chicken. Our world is full of beautiful intricacies great and small.


The next monthly field trips on the Lyndon B. Johnson National Grasslands will be Dec. 1 and Jan. 5. We will depart at 9 a.m. from the Forest Service District Office in Decatur. For more information, contact Mary Curry (see below) or the forest service at (940) 627-5475.

Claire and Mary Curry are nature enthusiasts based in Greenwood. If you would like to contact them, please e-mail them at larksparrow@eeclaire.com.

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