Nucleic Acids and DNA Structure

Nucleic Acids

• Nucleic acids store and express genetic information.
• There are two types of nucleic acids: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

DNA Structure

• DNA’s structure was first described by James Watson and Francis Crick in 1953.
• DNA is a double helix, consisting of two complementary strands of deoxyribonucleotides (dAMP, dGMP, dCMP, dTMP) connected by phosphodiester bonds.
• The phosphodiester bonds are formed between the 3’-OH group of the deoxyribose sugar on one nucleotide and the 5’ phosphate group on the adjacent one.
• The phosphate and sugar form the DNA strand's backbone.
• The nitrogenous bases project inwards.
• The two complementary strands of DNA run antiparallel, meaning the 5’ end of one strand is base-paired with the 3’ end of the other.
• Hydrogen bonds hold together the two strands of DNA within the double helical structure.
• Adenine forms two hydrogen bonds with thymine, while guanine and cytosine are connected by three.
• This base pairing stabilizes the interior of the double-stranded DNA.

RNA Structure

• There are three main types of RNA involved in protein synthesis: rRNA, tRNA, and mRNA.
• These are unbranched polymeric molecules composed of nucleoside monophosphates joined by 3'→5'-phosphodiester bonds.
• They differ in size, function, and structural modifications.

Ribosomal RNA (rRNA)

• rRNA accounts for approximately 80% of total RNA in the cell.
• Ribosomes are responsible for protein synthesis and contain peptidyl transferase activity, which is catalyzed by ribozymes.

Transfer RNA (tRNA)

• tRNA is the smallest of the three RNAs.
• It carries amino acids during protein synthesis.

Messenger RNA (mRNA)

• mRNA carries genetic information from DNA to the cytosol for translation.

DNA Replication

• The process of producing two identical copies from one original DNA molecule is called DNA replication.
• Important enzymes involved in replication are DNA polymerases.

Transcription

• The synthesis of RNA from a DNA template is called transcription.
• Genes are transcribed by enzymes called RNA polymerases.
• Eukaryotic cells have several distinct RNA polymerases, each responsible for a major class of RNA:
  • RNA polymerase I: synthesizes most rRNA.
  • RNA polymerase II: synthesizes mainly mRNA.
  • RNA polymerase III: synthesizes mainly tRNA.

Post-Transcriptional Modification of RNA

• This includes:
  • Capping of mRNA
  • Addition of a poly(A) tail
  • Splicing

Translation

• The process of producing proteins occurs on ribosomes and is directed by mRNA.
• The genetic message encoded in DNA is first transcribed into mRNA, and the nucleotide sequence in the coding region of the mRNA is then translated into the amino acid sequence of the protein.
• Codons are the genetic words used in this process. Each codon consists of three successive nitrogenous bases.

Post-Translational Modifications

• These include:
  • Trimming
  • Covalent modifications