Chromosomes Contain Our Genetic Code
All living organisms have one or more chromosomes that contain the code that directs the synthesis of proteins that are essential for its structure and function. In bacteria proteins can be structural and they can be enzymes that perform metabolic functions that can breakdown nutrients that provide energy and provide structural building blocks for growth and replication.
Each chromosome is, if fact, an enormous DNA molecule. Molecules are generally so small that they can't be seen even with a microscope, but chromosomes can be seen with a microscope under certain circumstance, particularly when a cell is about to divide. The illustration below shows the 46 chromosomes that contain the human genome.
There are 22 homologous pairs and two sex chromosomes (the X and Y chromosomes). One chromosome in each pair is inherited from one's mother and one from one's father. Each chromosome is a single molecule of DNA. The illustration below illustrates this by imagining that we have grabbed one end of a chromosome and pulled it out to reveal that it is an extremely long polymer consisting of a double helix. In fact, if we were to take a single human chromosome and stretch it out, it would be about 5 centimeters long (about 2 inches), and all 46 chromosomes would be about 2 meters long if they were stretched out and laid end to end. Our cells have all 46 chromosomes, but they are coiled around proteins and highly coiled into the form of the chromosomes that are seen to the right. The chromosomes of eukaryotes are contained within the membrane-bound nucleus.
But DNA provides the essential genetic code for all living organisms, including bacteria. The bacterium E. coli has a single circular chromosome (DNA molecule) which is also coiled, supercoiled, and packaged with proteins, but in prokaryotes the chromosome is located in the cytoplasm instead of being contained in a membrane- bound nucleus.
Structure of DNA
DNA is an abbreviation for deoxyribonucleic acid, which is an extremely long polymer made from units called nucleotides. The illustration below shows the structure of both DNA and RNA (ribonucleic acid.)
The backbone of each molecule is composed of alternating sugars (the pentagon with the "S") and phosphate groups (shown with "P), and each sugar is also covalently bonded to one of the following nucleotide bases:
- adenine (A),
- thymine (T),
- cystosine (C)
- guanine (G)
- uracil (U)
A nucleotide "unit" (outlined by the red box in the illustration] consists of a sugar molecule, a phosphate, and one of the five. Consequently, one can think of DNA as an extremely long double-stranded polymer of nucleotides. Note also that the two strands of DNA are held together by hydrogen bonds between complementary bases on the two strands. The figure below demonstrates this complementarity. In DNA the base thymine always bonds to adenine, while cytosine always bonds to guanine because of their complementary chemical structure and "fit". As a result of this complementary structure, if the base sequence of one strand is known, then the structure of the other strand can be deduced.
DNA and RNA differ in several ways:
- DNA is double stranded, while RNA is single stranded (although RNA forms loops by hydrogen-bonding to itself).
- DNA contains the sugar deoxyribose, while RNA has the sugar ribose.
- RNA contains the base uracil in place of thymine.