DNA Structure and Function

Introduction to DNA

Deoxyribonucleic acid (DNA) is the hereditary material in almost all living organisms. It contains the instructions needed for an organism to develop, survive, and reproduce. DNA's structure and function are central topics in general biology.

• Definition: DNA is a molecule composed of two polynucleotide chains that coil around each other to form a double helix.

• Function: DNA stores genetic information used in the growth, development, functioning, and reproduction of all known living organisms.

• Example: Human DNA contains genes that determine traits such as eye color, blood type, and susceptibility to certain diseases.

Historical Discoveries in DNA Structure

The discovery of DNA's structure was a major milestone in biology, leading to a deeper understanding of genetics and heredity.

• Watson and Crick (1953): Published the double helix model of DNA structure.

• Rosalind Franklin: Produced "Photo 51" using X-ray diffraction, providing crucial evidence for the double helix.

• Nobel Prize: Awarded to Watson, Crick, and Wilkins for their work on DNA structure.

• Additional info: Franklin's contributions were foundational but not formally recognized by the Nobel committee.

Chemical Structure of DNA

DNA is made up of nucleotides, each consisting of a phosphate group, a deoxyribose sugar, and a nitrogenous base.

• Nucleotides: The building blocks of DNA, each containing:

  • Phosphate group

  • Deoxyribose sugar

  • Nitrogenous base: Adenine (A), Guanine (G), Cytosine (C), Thymine (T)

• Base Pairing: A pairs with T, G pairs with C via hydrogen bonds.

• Double Helix: Two strands run antiparallel (5' to 3' and 3' to 5').

• Additional info: Purines (A, G) are larger, double-ringed bases; Pyrimidines (C, T) are smaller, single-ringed bases.

DNA Replication

Overview of DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. This ensures that each daughter cell receives an identical set of genetic instructions.

• Enzymes involved: DNA polymerase, helicase, ligase.

• Leading and Lagging Strands: DNA is synthesized continuously on the leading strand and discontinuously (Okazaki fragments) on the lagging strand.

• Directionality: DNA polymerase adds nucleotides in the 5' to 3' direction.

Equation:

Central Dogma of Molecular Biology

Flow of Genetic Information

The central dogma describes the flow of genetic information from DNA to RNA to protein. This process is fundamental to gene expression.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated into a protein by ribosomes, with the help of transfer RNA (tRNA).

• One-way flow: Information moves from DNA → RNA → Protein.

Equation:

The Genetic Code

Structure and Function of the Genetic Code

The genetic code is a set of rules by which information encoded in DNA is translated into proteins. It is universal among most organisms.

• Codons: Sequences of three nucleotides (triplets) in mRNA that specify amino acids.

• 20 Amino Acids: The code must account for 20 standard amino acids.

• 64 Possible Codons: With four nucleotides, there are possible codons.

• Redundancy: Multiple codons can code for the same amino acid.

• Start and Stop Codons: AUG codes for methionine (start), UAA, UAG, UGA are stop codons.

Genes and Genetic Variation

Definition and Role of Genes

Genes are functional units of heredity located at specific positions on DNA. They code for proteins that determine an organism's traits.

• Gene: A sequence of DNA that codes for a specific protein or functional RNA.

• Phenotype: Observable characteristics resulting from gene expression.

• Human genome: Contains about 3 billion base pairs; only a fraction codes for proteins.

Genetic Variation and Mutations

Genetic variation arises from changes in DNA sequences, which can occur naturally or due to environmental factors. Mutations are a primary source of genetic diversity.

• Mutation: Any change in the nucleotide sequence of DNA.

• Types of mutations:

  • Substitution: One nucleotide is replaced by another.

  • Insertion: Addition of one or more nucleotides.

  • Deletion: Removal of one or more nucleotides.

• Effects: Mutations can alter protein function, sometimes causing diseases (e.g., sickle-cell anemia).

Example: Sickle-Cell Anemia

Sickle-cell anemia is caused by a substitution mutation in the gene coding for hemoglobin. This single nucleotide change leads to abnormal hemoglobin structure and function.

• Normal DNA: Codes for glutamic acid.

• Mutant DNA: Codes for valine, resulting in sickle-shaped red blood cells.

Equation:

Summary

DNA is the master molecule of life, encoding genetic information that is transcribed and translated to produce proteins. The structure of DNA, the process of replication, the central dogma, the genetic code, and the mechanisms of genetic variation are foundational concepts in general biology.