DNA & RNA

DNA is the master molecule of heredity, and the section below opens up what it is and what it carries.

• DNA is a polymer of nucleotides — a long chain of repeating units called nucleotides.
• Two nucleotide chains coil around each other to form a double helix.
• The double helix carries the genetic instructions for the growth, development, functioning and reproduction of all known living organisms and many viruses.
• The double-helix diagram is labelled with a base pair and the helix of sugar–phosphates (the backbone).
• DNA is the master molecule of life; without DNA, life cannot exist.

Knowing what DNA does leads to the question of how its shape was worked out.

• DNA structure is a double-stranded helix held together across the middle.
• DNA's function is to store and transmit genetic information.
• The discovery drew on the X-ray photographs of Rosalind Franklin and Maurice Wilkins.
• Watson, Crick and Wilkins won the Nobel Prize in 1962 for this work.
• DNA as a substance had been isolated much earlier by Friedrich Miescher in 1869.

Having met the molecule and its discoverers, we ask where in the cell it actually lives.

• DNA is also found in the cell organelles Chloroplast and Mitochondria, which carry their own small DNA.
• The bulk of a eukaryotic cell's DNA (nuclear DNA) sits in the nucleus.
• In prokaryotes (bacteria) there is no true nucleus, so DNA floats freely in the cytoplasm in a region called the nucleoid.
• A human cell packages its DNA into 46 chromosomes, stretching to roughly 2 metres if fully uncoiled.

DNA is a polymer of nucleotides, so the next step is to open up a single nucleotide.

• The nucleotide diagram is labelled Phosphate Group, Deoxyribose (Sugar) and Nitrogen Base.
• Each nucleotide is composed of one of the 4 nitrogen-containing nucleobases.
• In DNA the sugar is deoxyribose, while in RNA it is ribose — this single difference gives the two acids their names.
• The sugar and phosphate groups link up to form the two sugar–phosphate backbones that run along the outside of the helix.
• Did you know: the order of bases in DNA holds the genetic code that makes each organism unique.

The real information in DNA lies in its four nitrogenous bases, split into two chemical families.

• Purines are the double-ring bases: Adenine (A) and Guanine (G).
• Pyrimidines are the single-ring bases: Cytosine (C) and Thymine (T).
• A purine always pairs with a pyrimidine, joined by hydrogen bonds.
• Adenine pairs with Thymine (A–T) by 2 hydrogen bonds. (The source's "A forms a double bond with T" is a mistake — these are hydrogen bonds, not double covalent bonds.)
• Guanine pairs with Cytosine (G–C) by 3 hydrogen bonds, the stronger pair.
• In RNA, Thymine is replaced by Uracil (U), so there A pairs with U instead.

With the chemistry settled, we describe the precise shape and size of the helix, usually given in Angstroms (1 Å = 10⁻¹⁰ m).

• The distance between base pairs is 3.4 Å.
• The total diameter of DNA is 20 Å.
• One half-turn (mid-point to tilt-point) of DNA = 10 Å.
• One complete turn (distance of 2 tilts) of DNA = 34 Å.
• The width (thickness) of a single DNA strand/thread is 2.8 Å.
• Since one full turn is 34 Å and bases stack 3.4 Å apart, exactly 10 base pairs fit per turn.
• The helix diagram is labelled base pair, one half-turn, one complete turn, thickness of one strand, one complete turn = 34 Å, and diameter = 20 Å.
• The two strands run in opposite directions (antiparallel), one 5'→3' and the other 3'→5'.
• Remember: DNA is very long and thin, made up of repeating units called nucleotides.

Because the two backbones are unevenly spaced as they wind around each other, the helix surface shows two kinds of channels.

• The grooves are important for the attachment of DNA-binding proteins involved in replication and transcription.
• The major groove occurs where the backbones are far apart.
• The minor groove occurs where the backbones are close together.
• The diagram is labelled major groove, minor groove, DNA-binding protein, backbones far apart (major groove) and backbones close together (minor groove).
• Remember: Major groove = wide and deep; Minor groove = narrow and shallow.

We have now seen the structure in full, so it helps to gather exactly what this molecule does for the cell.

• DNA contains the genetic instructions for the development and function of living things.
• All known cellular life and some viruses contain DNA.
• DNA is a biological macromolecule that carries hereditary information, made of many nucleotides.
• DNA is necessary for the production of proteins, the regulation, the metabolism and the reproduction of the cell.
• Remember: DNA is the master molecule of life; without DNA, life cannot exist.

DNA does not build proteins by itself; it works through a partner molecule, which brings us to RNA.

• RNA is a polymeric molecule essential in coding, decoding, regulation and expression of genes.
• Unlike DNA, RNA is single-stranded.
• An RNA nucleotide contains Ribose (5-carbon sugar), one of 4 nitrogenous bases (Adenine, Uracil, Guanine or Cytosine) and a phosphate group.
• RNA helps in gene expression and determines the sequence of amino acids in a protein.
• The RNA diagram is labelled nucleobases, single helix of sugar–phosphate, single-stranded, and ribose sugar + phosphate backbone.
• Remember: RNA is shorter and less stable than DNA; RNA contains ribose (DNA has deoxyribose); RNA uses Uracil (U) instead of Thymine (T).

RNA comes in three main kinds, each with its own job in protein synthesis, differing in size and abundance.

• tRNA (Transfer RNA) is 15–20% of total RNA; it is stable and is the physical link between mRNA and the amino acid sequence of proteins (diagram: amino acid, anticodon, transfer RNA).
• mRNA (Messenger RNA) is 5–10% of total RNA, formed from DNA and short-lived.
• mRNA conveys genetic information from DNA to the ribosome and specifies the amino acid sequence of the protein product of gene expression.
• RNA polymerase transcribes pre-mRNA (primary transcript) into processed, mature mRNA.
• Remember: rRNA forms the structural and functional core of ribosomes; tRNA acts as an adaptor molecule during translation; together they are key to protein synthesis.
• Remember: mRNA carries the genetic message from DNA to the ribosome; it is short-lived compared to rRNA and tRNA and is crucial to protein synthesis.

Bringing the types together shows what RNA achieves as a team.

• RNA's main function is to carry the amino-acid-sequence information from genes to the ribosomes, where proteins are assembled in the cytoplasm.
• DNA makes RNA, and RNA in turn makes protein (DNA → RNA → Protein).
• RNA starts the synthesis of peptide bonds, translating RNA information into protein information (RNA → peptide bonds → protein).
• Remember: RNA is the messenger between DNA and ribosomes; without RNA, protein synthesis cannot occur.

Now that both molecules are described in full, the cleanest way to fix them in memory is side by side — among the most frequently asked exam points.

• DNA is located in the nucleus and mitochondria; RNA in the cytoplasm, nucleus and ribosomes.
• DNA has 2-deoxyribose sugar; RNA has ribose sugar.
• DNA is double-stranded; RNA is single-stranded.
• DNA is self-replicating; RNA does not replicate and is synthesized from DNA.
• DNA is a long chain of nucleotides; RNA has a shorter chain.

• Remember: DNA = storehouse of genetic information; RNA = messenger and worker that helps in protein synthesis; both are nucleic acids but differ in structure and function.