DNA and RNA: Structure and Function

• Nucleic acids include DNA and RNA.

Functions of DNA & RNA

• DNA (deoxyribonucleic acid) carries the genetic instructions for development, functioning, and reproduction in all living organisms and some viruses.
• RNA (ribonucleic acid) carries genetic instructions in some viruses.
• 3 types of RNA convert DNA code into polypeptides (proteins).
• Some RNA molecules act as catalysts, like enzymes.
• RNA can have complex regulatory roles in cells that include regulating gene expression and modifying other RNA.

Molecular Structure

• A nucleotide monomer consists of a sugar, a nitrogenous base, and a phosphate group.
• Nitrogen-containing bases project from the sugar's carbon via a glycosidic bond.
• A phosphodiester bond links sugars to form the backbone of a DNA strand.
• Chromosome 1 in humans has 249 million base pairs, as DNA is a large polymer.

Nitrogenous Bases

• DNA has four bases: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G).
• RNA has four bases: Adenine (A), Uracil (U), Cytosine (C), and Guanine (G).
• Uracil (U) replaces Thymine (T) in RNA.
• Purines (Adenine and Guanine) have a two-ring structure.
• Pyrimidines (Cytosine, Thymine, and Uracil) have a one-ring structure.
• Adenine (A) hydrogen bonds with Thymine (T) in DNA.
• Guanine (G) hydrogen bonds with Cytosine (C) in DNA.

Sugars in DNA and RNA

• Sugars in nucleic acids are 5-carbon sugars.
• DNA contains deoxyribose.
• RNA contains ribose.
• A phosphate group bonds at the 5' carbon and to the 3' carbon of the next sugar.
• Bonds to nitrogenous bases occur at the 1' carbon.

DNA Structure

• DNA strands are antiparallel, running in opposite directions (5' to 3' and 3' to 5').
• A 5' sugar bonds to a phosphate group.
• A 3' hydroxyl group is on the sugar.
• DNA is most stable when double-stranded and twists into a 3-D helix.
• Hydrogen bonding between bases occurs due to antiparallel strands.
• Stability of the DNA molecule results from this hydrogen bonding.
• In eukaryotes, DNA wraps around histone proteins, forming chromatin.

Further Compacting

• Structure is compressed by supercoiling, forming chromosomes.
• Most prokaryotes lack histones but have supercoiled DNA held by special proteins.

RNA Structure

• RNA is typically single-stranded.
• It forms double-stranded structures important to its function.
• RNA can form secondary structures stabilized by hydrogen bonds between nitrogenous bases through folding.
• Unlike DNA, RNA contains a hydroxyl group on the 2'-carbon atom of its sugar component. This makes RNA more reactive than DNA.

Roles of Sugar and Phosphate

• Sugar (deoxyribose in DNA, ribose in RNA) and phosphate groups form the nucleic acid backbone via phosphodiester bonds.
• The backbone provides stability, directionality (5'→3' orientation), and solubility due to its hydrophilic, negatively charged surface.
• Sugars link to nitrogenous bases, while phosphate groups connect adjacent sugars.
• This enables the polymer chain to store and transmit genetic information.

Genes and Alleles

• A gene is a DNA segment coding for a protein or functional RNA.
• An allele is a specific variant of a gene.

Additional Nucleic Acids

• mRNA carries the genetic code from DNA to ribosomes.
• tRNA transfers amino acids during protein synthesis.
• rRNA is a structural component of ribosomes.

Nucleotides

• A nucleotide includes a sugar (ribose or deoxyribose), a phosphate group, and a nitrogenous base (A, T, C, G in DNA; A, U, C, G in RNA).
• It is the monomer unit of nucleic acids.

The Nucleus

• The nucleus stores all the genetic instructions for manufacturing proteins and regulates all cellular activities and is called the "control center" of the cell."
• Chromosomes are located in the nucleus of each cell.

Other Locations for DNA

• DNA can be found in the nucleus, mitochondria, and in plants, chloroplasts.
• DNA's structure was established by James Watson and Francis Crick with contributions from Rosalind Franklin and Maurice Wilkins.

DNA Replication

• Replication is “semiconservative” because half of the original strand is conserved in each new DNA molecule.

DNA Ladder

• Sides consist of alternating sugar and phosphate groups, forming the sugar-phosphate backbone.

RNA vs DNA

• RNA is single-stranded.
• RNA contains the sugar ribose instead of deoxyribose.
• RNA uses uracil (U) instead of thymine (T).

Why is DNA Called a Blueprint?

• DNA contains the instructions for building and maintaining all living organisms, guiding development, function, and reproduction.
• Different genes are turned on or off in each cell type, so each cell expresses the proteins needed for its specific function.

The Copy Process

• The process of copying DNA is called replication.
• DNA's shape is a double helix.
• RNA (specifically mRNA) carries genetic information from DNA in the nucleus to ribosomes in the cytoplasm, where proteins are made.
• Proteins are created at ribosomes.
• Ribosomes are either free in the cytoplasm or attached to the rough endoplasmic reticulum.

Hydrogen Bonds

• Individual hydrogen bonds are weak, but the cumulative effect of millions in a DNA double helix provides stability.
• Base stacking (hydrophobic interactions) further reinforces the structure.