READ: DNA
READ: DNA
Heredity: Passing on Traits
Have you ever wondered why some people throw a baseball or softball with their left hand when most others use their right hand? Why do some people skateboard with their left foot forward while others skate with their right foot forward? Why are some tulips red and other tulips white? The answer to these questions is heredity. Heredity is the passing of traits and characteristics from parents to their offspring. Genetics is the study of how traits are passed from parents to their offspring.
What is DNA?
How do your cells know what to do? Just like builders have blueprints to tell them how to build a house, your cells also have instructions. Your cells' instructions are molecules of DNA. DNA is the material that stores our genetic information. When you build a house, you need a blueprint, a set of instructions that tells you how to build. The DNA is like the blueprint for living organisms. The genetic information is a set of instructions that tell your cells what to do.
DNA is an abbreviation for deoxyribonucleic acid. Nucleic acids are a type of molecule that store information. DNA may provide the instructions to make up all living things, but it is actually a very simple molecule. DNA is made of a very long chain of nucleotides. In fact, in you, the smallest DNA molecule has well over 20 million nucleotides. There are four different nucleotides: adenine (A), guanine (G), thymine (T), and cytosine (C).
The Genetic Code
The various sequences of the four nucleotide bases make up the genetic code of your cells. It may seem strange that there are only four letters in the “alphabet” of DNA. But since your chromosomes contain millions of nucleotides, there are many, many different combinations possible with those four letters.
DNA is made of two strands of nucleotides formed into a double helix, or a two-stranded spiral. The paired bases connect the two strands on the inside of the helix (Figure above). Adenine always pairs with Thymine, and Guanine always pairs with Cytosine in the DNA helix. Therefore, if one DNA strand reads ATGCCAGT, the other strand will be made up of the complementary bases: TACGGTCA.
What are Chromosomes?
Chromosomes are coiled structures made of DNA and proteins. Chromosomes are the form of the genetic material of a cell during cell division. When the cell is not dividing, DNA is not coiled into chromosomes. Instead, it exists as a grainy material called chromatin.
Human Chromosomes
Human cells normally have two sets of chromosomes, one set inherited from each parent. There are 23 chromosomes in each set, for a total of 46 chromosomes per cell. Each chromosome in one set is matched by a chromosome of the same type in the other set, so there are actually 23 pairs of chromosomes per cell. Each pair consists of chromosomes of the same size and shape that also contain the same genes.
Genes and Alleles
A gene is a segment of DNA which has the information to code for a protein. Genes are located at a particular place on a chromosome. For many genes, the protein product controls a particular trait.
A gene is a segment of DNA which codes for a protein. Proteins, in turn, determine traits. Changes in genes (mutations) cause changes in proteins, which in turn produce variation in traits.
For example, the gene for the enzyme tyrosinase controls the chemical reaction which makes melanin, a brown-black pigment which colors the skin and hair of many animals, including humans. The gene is the “recipe” for tyrosinase. Tyrosinase is the protein, and the trait is coloring of the skin or fur. The rabbit below has brown fur because its DNA contains a gene for tyrosinase.
Genes often have different forms – slightly different nucleotide sequences – known as alleles. In some rabbits, an alternative form of the gene for fur color makes non-functional tyrosinase. A change in the sequence of As, Ts, Cs, and Gs changes the sequence of amino acids in the protein and alters or destroys its activity. This allele causes lack of pigment – an albino rabbit like the ones below.
Summary
- DNA stores the genetic information of the cell in the sequence of its 4 bases: adenine, thymine, guanine, and cytosine.
- DNA is made of a long chain of nucleotides consisting of a 5-carbon sugar, a phosphate group, and nitrogen-containing base.
- Chromosomes are coiled structures made of DNA and proteins.
- Human cells normally have 46 chromosomes, made up of two sets of chromosomes, one set inherited from each parent.
- Genes are segments of DNA which determine traits by coding for proteins.
- Many genes have several different forms known as alleles.
- Alleles are responsible for the variation in traits.
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