Brent Cornell

Nucleic acids are the genetic material of a cell and determine how both individual cells and entire organisms will develop and function

Nucleic acids function as the basic unit of inheritance by encoding for the synthesis of proteins (which carry out the genetic instructions)

Nucleotides

Nucleic acids are composed of recurring monomeric units called nucleotides, each of which contain three principal components:

• A central 5-carbon pentose sugar (pentagon), which is attached to a phosphate group (circle) and a nitrogenous base (rectangle)
• The carbon atoms of the sugar are numbered (1–5): the base connects to the 1’–C (right), the phosphate connects to the 5’–C (left)
  
  

Nucleotides are connected together via condensation reactions to form long single-strand molecules (nucleic acid = polymer)

• The phosphate group attaches to the sugar of a different nucleotide at the 3’–C position via a covalent (phosphodiester) bond
• The resulting chain of sugars and phosphates form a backbone, with the nitrogenous bases facing outwards as a sequential code

Nitrogenous Bases

Each nucleotide contains one of four different nitrogenous bases – adenine, guanine, cytosine and thymine / uracil

• These bases can occur as either a double-ringed structure (purines) or as a single-ringed structure (pyrimidines)
• Thymine and uracil are almost chemically identical and perform the same function in different molecules (T = DNA ; U = RNA)

When the nucleotides are joined together in a chain, the order of these bases will form a genetic sequence that may encode for a particular characteristic of the cell or organism (via the synthesis of a specific protein)

• Hence, variations in the base sequence will result in different genetic sequences and give rise to the production of unique traits

Types of Nucleic Acids

There are two main types of nucleic acids that differ in both their function and certain key structural aspects:

• DNA (deoxyribonucleic acid) functions as the master copy of all genetic instructions and is stored in the nucleus of eukaryotic cells
• RNA (ribonucleic acid) is a transient copy that functions to transfer the genetic instructions from the nucleus to the rest of the cell

DNA and RNA possess certain structural differences that are reflective of their different roles within the cell

• RNA is a single-stranded molecule, whereas DNA is a double-stranded molecule (promotes conservation of the base sequence)
• RNA includes the base uracil, whereas DNA uses thymine instead (but the bases are almost identical and fulfil the same function)
• RNA and DNA are composed of different pentose sugars (RNA contains ribose, whereas DNA contains deoxyribose)

Structure of DNA

Two chains of DNA are held together by hydrogen bonding between complementary bases (i.e. purine and pyrimidine)

• Adenine and Thymine pair via two hydrogen bonds (A = T) 
• Guanine and Cytosine pair via three hydrogen bonds (G Ξ C)

In order for the bases to be facing each other and thus able to pair, the strands must be running in opposite directions 

• The two DNA strands are described as being antiparallel (one strand runs 5’ → 3’ and the other strand runs 3’ → 5’)

As the antiparallel chains lengthen, the atoms will organise themselves into the most stable energy configuration  

• This atomic arrangement results in the double-stranded DNA forming a double helix (roughly 10 – 15 bases per twist)

Main Types of RNA

RNA functions to transfer genetic instructions from the nucleus to the cytoplasm, where the information is decoded (by ribosomes)

There are three main types of RNA which cooperate to complete this goal:

• Messenger RNA (mRNA) – a transcript copy of a specific DNA sequence (which encodes for the synthesis of a polypeptide)
• Transfer RNA (tRNA) – carries the polypeptide subunits (amino acids) to the organelle responsible for synthesis (ribosome)
• Ribosomal RNA (rRNA) – a primary component of the ribosome and is responsible for its catalytic activity

In essence, the relationship between the three types of RNA can be described as follows:

• Genetic instruction (within mRNA) → Protein ingredients (brought by tRNA) → Manufacture of product (catalysed by rRNA)