In-Depth Notes on DNA Structure and Function

DNA Structure

• DNA (Deoxyribonucleic Acid) is the genetic material found in all living organisms and some viruses.

• The DNA molecule is structured as a double helix, resembling a twisted ladder.

Key DNA Facts

• 8% of human DNA consists of ancient viruses.

• Human DNA is 95% identical to chimpanzee DNA and about 50% identical to that of bananas.

• Cabbages share approximately 40-50% of their DNA with humans.

• Each DNA molecule strand is about 1.8 meters long but is packed into a space of 0.09 micrometers.

• The DNA in every cell is damaged between 1,000 to 1,000,000 times daily, but the body has a repair mechanism to fix this damage to prevent cell death or cancer formation.

DNA Composition

• Nucleotide Structure: Composed of a phosphate group, a 5-carbon sugar (ribose in RNA and deoxyribose in DNA), and a nitrogenous base (adenine, guanine, cytosine, thymine in DNA, and uracil in RNA).

• Sugar-Phosphate Backbone: The backbone of DNA consists of alternating sugar and phosphate groups.

Base Pairing and Complementarity

• Bases pair through hydrogen bonds:

  • Adenine (A) pairs with Thymine (T)

  • Guanine (G) pairs with Cytosine (C)

• The pairing is complementary and is critical in DNA replication and transcription.

• Chargaff's Rules: The amount of adenine is equal to thymine, and guanine is equal to cytosine within a DNA molecule.

DNA Packing

• In eukaryotic cells, DNA is located within the nucleus and tightly packed around proteins called histones, forming structures known as nucleosomes and higher-order chromatin.

• Chromatin can condense to form chromosomes, especially during cell division.

Role of DNA in Cells

• Gene Expression: The process of DNA coding for RNA and subsequently for proteins follows the Central Dogma: DNA → RNA → Protein.

Transcription Process

1. Initiation:

   • RNA polymerase binds to the promoter region on DNA, signaling the start of transcription.

2. Elongation:

   • RNA polymerase synthesizes RNA from the template strand of DNA, reading the DNA code in triplets called codons (each codon codes for an amino acid).

3. Termination:

   • Transcription ends when RNA polymerase reaches a termination sequence, releasing the newly formed mRNA.

RNA Processing in Eukaryotes

• Initial RNA transcripts (pre-mRNA) undergo several modifications:

  • 5' Capping: Addition of a modified guanine nucleotide to the 5' end.

  • Polyadenylation: Addition of a tail of adenines to the 3' end.

  • Splicing: Removal of non-coding regions (introns) and joining of coding regions (exons) to form mature mRNA.

Translation Process

• Translate mRNA into a polypeptide chain (protein) through:

1. Initiation:

   • Small ribosomal subunit binds to the mRNA and the initiator tRNA (transfer RNA) attaches at the start codon (AUG).

2. Elongation:

   • Ribosome moves along the mRNA, adding new amino acids to the growing polypeptide chain as specified by each codon on the mRNA.

3. Termination:

   • Process stops when a stop codon (UAA, UAG, or UGA) is reached, releasing the newly synthesized polypeptide.

Operons in Prokaryotes

• Operon: A group of genes regulated together, can be turned ON or OFF based on environmental conditions (e.g., Lac and Trp operons).

• Lac Operon: Activated in the presence of lactose; the repressor detaches, allowing gene expression.

• Trp Operon: Active only when tryptophan is absent, thus regulating its own synthesis.

Bacterial Transformation Using Plasmids

• Plasmids are small circular DNA molecules used in genetic engineering to deliver genes (e.g., insulin gene).

• Important components of a plasmid include:

  • Origin of Replication: Ensures the plasmid is copied during cell division.

  • Selectable Marker: Provides resistance to antibiotics, allowing selection of transformed bacteria.

Experimentation with PGLO Plasmid

• The PGLO plasmid contains genes for green fluorescent protein (GFP) and antibiotic resistance.

• The transformation procedure involves adding plasmid DNA to bacterial cells, subjecting them to heat shock, and allowing growth on selective media to isolate successfully transformed colonies.