DNA: The Genetic Material

Historical Perspective and Requirements for Genetic Material

Early scientists debated whether DNA or proteins were the genetic material responsible for inheritance. Proteins, with 20 amino acids, were considered more variable than DNA, which has only four nucleotides. However, DNA fulfills three essential requirements for genetic material:

• Storage of genetic information: Must contain instructions for traits.

• Stability and accurate replication: Must be reliably copied and transmitted during cell division.

• Ability to mutate: Must undergo changes to provide genetic variability (e.g., alleles).

Example: The blue eye allele originated from a single mutation thousands of years ago.

Discovery of DNA Structure

Understanding DNA's structure was crucial to confirming its role as genetic material. Key contributors include:

• Rosalind Franklin: Used X-ray diffraction to reveal DNA's double helix structure.

• Maurice Wilkins: Shared Franklin's data with Watson and Crick.

• James Watson & Francis Crick: Modeled DNA as a double helix with anti-parallel strands and complementary base pairing.

DNA's structure consists of:

• Sugar-phosphate backbone: Forms the sides of the ladder.

• Hydrogen-bonded bases: Form the rungs.

• Anti-parallel strands: Strands run in opposite directions (5' to 3' and 3' to 5').

• Complementary base pairing: Purines (adenine, guanine) pair with pyrimidines (thymine, cytosine).

Additional info: Purines have a double-ring structure; pyrimidines have a single-ring structure.

DNA Replication

Semi-Conservative Replication

DNA replication is the process of copying DNA before cell division. It is semi-conservative, meaning each new DNA molecule contains one parental (old) strand and one daughter (new) strand.

• Parental strands serve as templates for new strands.

• Complementary base pairing ensures accuracy.

Equation:

Mechanism of Replication

• DNA helicase: Unwinds and unzips the DNA double helix.

• Single-stranded binding proteins (SSBs): Stabilize separated strands.

• DNA primase: Lays down RNA primers to signal where replication starts.

• DNA polymerase: Adds new nucleotides to the template strand.

• Leading strand: Synthesized continuously.

• Lagging strand: Synthesized in short fragments (Okazaki fragments), joined by DNA ligase.

Additional info: Okazaki fragments are short DNA segments on the lagging strand.

Replication in Prokaryotes vs. Eukaryotes

• Prokaryotes: Single circular DNA; replication starts at one origin and proceeds in both directions.

• Eukaryotes: Linear chromosomes; replication begins at multiple origins (replication bubbles).

Additional info: Replication bubbles expand until they meet, ensuring rapid and efficient DNA copying.

Proofreading and Error Correction

• DNA polymerase: Proofreads and corrects errors during replication.

• Errors occur about once every 10,000 bases but are usually corrected.

Example: DNA polymerase removes mismatched nucleotides and resumes synthesis.

Gene Expression: From DNA to Protein

Role of RNA in Gene Expression

Gene expression is the process by which genetic information in DNA is used to produce proteins, which determine traits. RNA is essential for this process:

• DNA: Contains the genetic blueprint (like a cookbook).

• RNA: Reads the DNA and helps synthesize proteins (like following a recipe).

• Protein: The final product, responsible for structure and function.

Additional info: Gene expression involves two main steps: transcription and translation.

Types of RNA

• Messenger RNA (mRNA): Carries genetic information from DNA to ribosomes.

• Transfer RNA (tRNA): Brings amino acids to the ribosome; acts like a "taxi" for amino acids.

• Ribosomal RNA (rRNA): Combines with proteins to form ribosomes, the site of protein synthesis.

Comparison of DNA and RNA

Additional info: Uracil (U) replaces thymine (T) in RNA.

The Genetic Code and Protein Synthesis

Codons and the Genetic Code

The genetic code is the set of rules by which information in mRNA is translated into proteins. Each codon is a sequence of three RNA nucleotides that specifies an amino acid.

• Four bases (U, A, C, G) combine to form 64 possible codons.

• Each codon codes for one amino acid (unambiguous).

• Multiple codons can code for the same amino acid (redundant).

• Start codon: AUG (codes for methionine; signals start of translation).

• Stop codons: UAA, UGA, UAG (signal end of translation).

Protein Synthesis: Translation

At the ribosome, mRNA is read three nucleotides at a time (codons), and tRNA brings the corresponding amino acids to build a polypeptide chain.

• Ribosome reads mRNA codons.

• tRNA matches codons with amino acids.

• Polypeptide chain forms and folds into a functional protein.

Example: Codon UCU codes for serine; tRNA brings serine to the ribosome.

Summary Table: Codon Properties

Key Terms and Definitions

• Allele: Variant form of a gene.

• Replication bubble: Region where DNA replication occurs simultaneously at multiple sites.

• Okazaki fragment: Short DNA segment synthesized on the lagging strand.

• Codon: Three-nucleotide sequence in mRNA specifying an amino acid.

• Gene expression: Process by which genetic information is used to produce proteins.