Molecular Biology: DNA and RNA Structure

Structure of Genetic Material

• DNA (deoxyribonucleic acid) consists of 2 long anti-parallel chains of nucleotides, forming a double-stranded structure.
• A nucleotide consists of a phosphate group, a pentose sugar (deoxyribose), and a nitrogenous base (purines: Adenine and Guanine, pyrimidine: Cytosine and Thymine).
• Complementary base pairing occurs between Adenine and Thymine, and Guanine and Cytosine.

RNA (Ribonucleic Acid)

• RNA is single-stranded and contains a pentose sugar (ribose) and Uracil instead of Thymine.
• There are 3 types of RNA: mRNA (messenger), rRNA (ribosomal), and tRNA (transfer).

DNA Replication

• DNA replication occurs in the cytoplasm (prokaryotes) and nucleus (eukaryotes).
• The process is semi-conservative, resulting in one old strand and one new strand.
• The process involves:
  • Unwinding the double helix using enzymes gyrase and helicase.
  • Proteins stabilizing the unwound DNA.
  • Leading strand extending continuously.
  • RNA primers being synthesized and removed by DNA polymerase.
  • Discontinuous fragments being sealed by DNA ligase.

Protein Synthesis

• The central dogma of molecular biology states that DNA → mRNA → protein.
• Transcription involves making a mRNA copy of DNA.
• Translation involves mRNA → protein.

Transcription

• Transcription occurs in the cytoplasm (prokaryotes) and nucleus (eukaryotes).
• The process involves:
  • RNA polymerase binding to the promoter site.
  • Unwinding the DNA and synthesizing complementary mRNA.
  • RNA polymerase continuing to move down the DNA until it reaches the terminator.
• mRNA processing in eukaryotes includes:
  • Removing introns (intervening, non-coding regions) and joining exons (expressed, coding regions).
  • Using small nuclear ribonucleoproteins (snRNPs).

Translation

• Translation occurs in the cytoplasm (both prokaryotes and eukaryotes).
• The process involves:
  • mRNA being sandwiched by ribosomal subunits.
  • tRNA carrying amino acids and binding to the mRNA.
  • Amino acids forming peptide bonds and becoming a strand.
  • The process repeating until the stop codon.

Genetic Code

• The genetic code is read in codons, which are groups of 3 nucleotides (A,C,G,U).
• Each codon codes for:
  • An amino acid (sense).
  • A stop signal (non-sense).
• The genetic code is degenerate, meaning multiple codes can code for one amino acid.
• There are 64 possible codons but only 20 amino acids.
• The start codon is AUG (methionine).