DNA Structure and Replication

Structure of DNA

• Nucleotides

  • Building blocks of nucleic acids

  • Composed of:

  • Nitrogenous base

  • Pentose sugar (ribose in RNA, deoxyribose in DNA)

  • Phosphate group

• Bonds

  • Hydrogen bonds between nitrogenous bases:

  • 2 between Adenine (A) and Thymine (T)

  • 3 between Guanine (G) and Cytosine (C)

  • Covalent bonds in the sugar-phosphate backbone

  • Phosphodiester bonds connect the 3' carbon of one nucleotide to the 5' carbon of the next

• Shape of DNA

  • Double helix structure: twisted ladder

  • Antiparallel strands (5' & 3' ends)

  • Consistent diameter of 2 nm due to pairing of purines (A, G) with pyrimidines (C, T)

  • Telomeres consist of repeated sequences that protect chromosome ends.

Differences in Base Types

• Purines

  • Double-ring structure (Adenine and Guanine)

• Pyrimidines

  • Single-ring structure (Cytosine and Thymine)

DNA Structure Discovery

• Erwin Chargaff (1949)

  • Found that A = T and G = C in all DNA sources

• Wilkins & Rosalind Franklin

  • Used X-ray crystallography to reveal the structure of DNA

• Watson & Crick (1953)

  • Built a model of DNA as a double helix

DNA Replication Process

• Key Enzymes

  • Helicase: Unwinds and unzips DNA at replication fork by breaking hydrogen bonds

  • Single-strand binding proteins (SSB): Stabilize unwound strands

  • DNA polymerase: Adds new nucleotides (5' to 3' direction)

  • Primase: Synthesizes short RNA primers to initiate replication

  • DNA ligase: Connects Okazaki fragments on lagging strand

• Steps of DNA Replication:

  1. Unwinding: Helicase unwinds the double helix

  2. Template Binding: Each strand serves as a template for synthesis. Complementary base pairing occurs (A with T, G with C)

  3. Leading and Lagging Strands:

  • Leading strand synthesized continuously

  • Lagging strand synthesized in fragments (Okazaki fragments)

  1. Bond Formation: DNA ligase joins fragments together

• Proofreading: DNA polymerase checks and corrects mismatched nucleotides.

Eukaryotic DNA Replication

• Similar to prokaryotes but takes longer (8-10 hours) due to larger and more numerous chromosomes

• Replication occurs at multiple origins to ensure efficiency

• Human genome consists of ~3 billion base pairs distributed across 23 chromosomes

Telomeres and Cellular Aging

• Telomeres shorten with each cell division, playing a role in cellular aging. This may lead to cellular senescence and eventually cell death after critical shortening.

DNA Packaging

• DNA in eukaryotic cells is packaged into chromosomes with proteins called histones, forming nucleosomes, allowing for efficient storage and regulation of gene expression.

Translation and Transcription

Transcription

• The process by which a segment of DNA is copied into RNA (specifically mRNA) by RNA polymerase. This occurs in the nucleus in eukaryotes.

• Steps of Transcription:

  1. Initiation: RNA polymerase binds to the promoter region of the gene.

  2. Elongation: RNA polymerase moves along the DNA template strand, synthesizing mRNA in the 5' to 3' direction.

  3. Termination: RNA polymerase encounters a termination signal, signalling the end of transcription.

Translation

• The process by which the sequence of nucleotide bases in mRNA is translated into a sequence of amino acids to form a protein. This occurs in the ribosome.

• Steps of Translation:

  1. Initiation: The small ribosomal subunit binds to the mRNA at the start codon (AUG).

  2. Elongation: tRNA molecules bring amino acids to the ribosome in accordance with the mRNA codon sequence.

  3. Termination: The ribosome reaches a stop codon, releasing the completed polypeptide chain.

  4. Post-Translation Modifications: After translation, the polypeptide chain may undergo various modifications, such as phosphorylation or glycosylation, which are crucial for its final functional form.