The structure of DNA and RNA (3.1.5.1)
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Nucleotides (or mononucleotides) are monomers that are linked by condensation reactions to form nucleic acids (or polynucleotides; DNA and RNA).
Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are information–carrying molecules found in nearly all living cells. They are polymers of nucleotides.
- DNA stores genetic information.
- RNA has several roles, including transferring genetic information from DNA to ribosomes (mRNA), combining with proteins to form ribosomes (rRNA), and being involved in protein synthesis (tRNA).
Early scientists doubted that DNA carried the genetic code because it seemed too simple. This changed after experimental observations:
- Avery showed that DNA from a dead virulent strain of bacteria could transform a live non-virulent strain into a virulent one. When the DNA was destroyed, transformation did not occur.
- Hershey and Chase used bacteriophages (viruses that infect bacteria). They labelled DNA with the radioactive isotope and protein with the isotope. They found that only labelled DNA entered bacteria and caused infection.
Nucleotides (mononucleotides) consist of three primary components joined together by condensation reactions:
- a pentose sugar, which is a 5-carbon sugar molecule
- a phosphate group, which is attached to the 5′ carbon of the pentose sugar
- a nitrogenous base (nitrogen-containing organic base), which is attached to the 1′ carbon of the pentose sugar.
By convention, the carbon numbers in the sugar are numbered 1′ to 5′.
DNA and RNA nucleotides differ in the type of pentose sugar they contain:
- DNA (deoxyribonucleic acid) contains the pentose sugar deoxyribose which lacks an oxygen atom on the 2′ carbon.
- RNA (ribonucleic acid) contains the pentose sugar ribose which has a hydroxyl attached to the 2′ carbon.
DNA and RNA nucleotides differ in one of their nitrogenous bases (nitrogen-containing organic bases):
DNA and RNA polynucleotides both contain adenine, cytosine, and guanine.
The fourth nitrogenous base DNA contains is thymine, but in RNA it is uracil.
DNA and RNA have similar structures, but also several important differences.
Adenine (A) pairs with thymine (T) through 2 hydrogen bonds.
Guanine (G) pairs with cytosine (C) through 3 hydrogen bonds.
Di- and polynucleotides are formed when condensation reactions link nucleotides by phosphodiester bonds.
Phosphodiester bonds occur between the phosphate group of one nucleotide and the hydroxyl group on the 3’ carbon of the pentose sugar of another nucleotide.
This linkage forms the sugar–phosphate backbone of DNA and RNA strands.
A DNA molecule is made up of 2 DNA polynucleotide chains.
The two strands run antiparallel to each other; 1 strand runs in the 5’ to 3’ direction, while the other runs in the 3’ to 5’ direction.
Hydrogen bonds between complementary base pairs hold the two chains together.
The two bonded DNA strands twist to form a double helix structure.
Unlike DNA, RNA molecules are relatively short, single–stranded polynucleotide chains.