Nucleic Acids: DNA and RNA Structure

Nucleic Acids

• Nucleic acids are composed of nucleotides.
• Each nucleotide is composed of:
  • A phosphate group
  • A pentose sugar (deoxyribose in DNA or ribose in RNA)
  • Nitrogenous bases (purines and pyrimidines)
• Purines are larger and double-ringed (adenine and guanine)
• Pyrimidines are smaller and single-ringed (cytosine, thymine, and uracil)

DNA Structure

• DNA is a double helix, with two strands of nucleotides connected and twisted.
• Hydrogen bonds connect complementary bases (adenine and thymine, or guanine and cytosine).

The Search for the Genetic Material

• Frederick Griffith (1928) demonstrated transformation, changing the genotype and phenotype of a cell by assimilation of genetic material from an outside source.
• Oswald Avery, Maclyn McCarty, and Colin MacLeod (1944) repeated Griffith's experiment, showing that DNA is the transforming agent.

The Search for the Structure of DNA

• Erwin Chargaff (1947) discovered that the amount of adenine in a DNA molecule is equal to the amount of thymine, and the amount of guanine is equal to the amount of cytosine (Chargaff's Rule).
• Maurice Wilkins and Rosalind Franklin (1952) used X-ray crystallography to determine the shape of DNA, showing that it is a three-dimensional double helix.
• James Watson and Francis Crick (1953) determined the true structure of DNA, winning the Nobel Prize.

Biological Properties of DNA

• DNA carries information from generation to generation.
• DNA must be copied accurately.
• DNA sometimes mutates, and mutations are copied accurately.
• DNA must be translated and its information put into action.

DNA Replication

• Occurs during the S period of interphase.
• Takes place within the nucleus of a eukaryotic cell.
• Involves the production of new DNA from existing DNA templates.
• The original parent strands separate, and new complementary daughter strands are synthesized.
• Helicase untwists and unzips the DNA helix, breaking the hydrogen bonds.
• DNA polymerase builds new DNA nucleotides using a single strand DNA template.
• One strand (3' to 5') is replicated continuously, while the other strand is antiparallel (5' to 3') and replicated discontinuously.

DNA Repair

• Nuclease enzymes remove damaged DNA.
• DNA polymerase builds new DNA to repair the mistake.
• DNA ligase pieces together the repaired region to the existing correct region.

The Central Dogma

• The flow of genetic material in a eukaryotic cell: DNA → RNA → Protein.
• Transcription: the production of RNA from a single strand DNA template.
• Translation: the production of protein from a messenger RNA strand.

Transcription

• Helicase untwists and unzips the DNA helix, breaking the hydrogen bonds.
• RNA polymerase produces new RNA nucleotides from a single strand DNA template.
• RNA codons are built from DNA codons.
• Transcription occurs in the nucleus.

RNA

• A single strand of nucleotides.
• RNA base nucleotides are complementary to DNA base nucleotides.
• If a DNA single strand base sequence reads…., then RNA polymerase would build the complementary RNA sequence.

Types of RNA

• mRNA (messenger RNA): RNA that is ultimately translated and results in the production of protein.
• tRNA (transfer RNA): RNA that carries an amino acid to be used in the building of protein.
• rRNA (ribosomal RNA): a structural RNA that comprises the ribosome.

Translation

• RNA → Protein.
• mRNA leaves the nucleus and goes to a ribosome.
• Each mRNA codon codes for the production of an amino acid.
• tRNA molecules each carry an amino acid and an anticodon that is complimentary to the mRNA codon.
• Amino acids are linked by peptide bonds.

Ribosome Structure

• Consists of a large and a small subunit.
• mRNA enters the ribosome between the two subunits to be read.
• The A site accepts a new tRNA molecule, which delivers a new amino acid.
• The P site holds a tRNA molecule that has a growing chain of amino acids.
• The E site is the exit site that releases previously used tRNA molecules.

Genetic Code

• A dictionary that indicates which amino acids result from each mRNA codon.
• It is a triplet code, non-overlapping, universal, and degenerate code.
• Uses a table to determine the amino acid coded for by each mRNA codon.

Point Mutations

• A point mutation is a mutation involving a base substitution.
• If the altered codon does not change the amino acid, it has no effect.
• If the altered codon codes for a different amino acid, a different protein results (missense mutation).
• If the altered codon codes for STOP, no protein results (nonsense mutation).

Frame Shift Mutations

• Alter the complete base sequence that follows the mutation.
• A base addition shifts the reading frame to the right, while a base deletion shifts the reading frame to the left.
• Can lead to missense or nonsense mutations.
• More harmful than point mutations.