🔬General Biology I Unit 14 – DNA Structure and Function

DNA Basics

• DNA (deoxyribonucleic acid) is the hereditary material in humans and almost all other organisms
• Carries genetic information for the development, functioning, growth and reproduction of all known organisms
• Consists of two long polynucleotide chains composed of four types of nucleotide subunits
  • Nucleotides contain a phosphate group, a sugar group and a nitrogen base
  • Sugar in DNA is 2-deoxyribose, which contains one less hydroxyl group than RNA
• DNA is located in the cell nucleus and stores the instructions for making all the proteins required by an organism
• DNA molecules are double-stranded helices, with the two strands running in opposite directions
• The double helix structure provides a mechanism for DNA replication and inheritance of genetic information

Nucleotide Structure

• Nucleotides are the building blocks of DNA and consist of three components
  • A phosphate group
  • A sugar molecule (deoxyribose in DNA)
  • A nitrogenous base
• Four types of nitrogenous bases in DNA
  • Adenine (A)
  • Thymine (T)
  • Guanine (G)
  • Cytosine (C)
• Phosphate group is attached to the 5' carbon of the sugar molecule
• Nitrogenous base is attached to the 1' carbon of the sugar molecule
• Nucleotides are linked together by phosphodiester bonds between the phosphate group of one nucleotide and the 3' hydroxyl group of the next nucleotide
• The order of nucleotides along the DNA strand encodes genetic information

Double Helix Model

• DNA exists as a double helix, with two complementary strands coiled around each other
• Proposed by James Watson and Francis Crick in 1953 based on X-ray crystallography data from Rosalind Franklin
• The two strands run in opposite directions (antiparallel)
  • One strand runs 5' to 3', while the other runs 3' to 5'
• The sugar-phosphate backbones are on the outside of the helix, and the nitrogenous bases are on the inside
• The nitrogenous bases of the two strands form hydrogen bonds with each other, stabilizing the double helix structure
• The double helix has a right-handed twist and makes a complete turn every 10 base pairs
• The width of the double helix is 2 nm, and the distance between two adjacent base pairs is 0.34 nm

Base Pairing Rules

• In DNA, nitrogenous bases pair with each other according to specific rules
  • Adenine (A) always pairs with Thymine (T)
  • Guanine (G) always pairs with Cytosine (C)
• Base pairing is mediated by hydrogen bonds between the bases
  • A-T base pairs form two hydrogen bonds
  • G-C base pairs form three hydrogen bonds
• The base pairing rules ensure that the two strands of DNA are complementary to each other
• During DNA replication, each strand serves as a template for the synthesis of a new complementary strand
• Base pairing is essential for the stability of the double helix and the accurate transmission of genetic information

DNA Replication

• DNA replication is the process by which a DNA molecule makes an identical copy of itself
• Occurs during the S phase of the cell cycle, before cell division
• DNA replication is semiconservative, meaning each newly synthesized DNA molecule contains one original strand and one newly synthesized strand
• Steps of DNA replication
  1. Initiation: DNA helicase unwinds the double helix at the origin of replication
  2. Elongation: DNA polymerase synthesizes new strands using each original strand as a template
     • Leading strand is synthesized continuously in the 5' to 3' direction
     • Lagging strand is synthesized discontinuously as Okazaki fragments, which are later joined by DNA ligase
  3. Termination: Replication continues until the entire DNA molecule is copied
• DNA replication is highly accurate due to proofreading mechanisms and error correction by DNA polymerases

Genetic Code and Transcription

• The genetic code is the set of rules that defines how the sequence of nucleotides in DNA is translated into the sequence of amino acids in proteins
• Genetic code is read in triplets called codons, with each codon specifying a particular amino acid or a stop signal
• Transcription is the process by which the genetic information in DNA is copied into a complementary RNA strand
  • Occurs in the nucleus and is catalyzed by the enzyme RNA polymerase
• Steps of transcription
  1. Initiation: RNA polymerase binds to the promoter region of the gene and separates the DNA strands
  2. Elongation: RNA polymerase synthesizes a complementary RNA strand using one of the DNA strands as a template
     • RNA contains uracil (U) instead of thymine (T)
  3. Termination: RNA polymerase reaches a termination sequence and releases the newly synthesized RNA strand
• The resulting RNA molecule, called messenger RNA (mRNA), carries the genetic information to the ribosomes for protein synthesis

DNA Mutations

• DNA mutations are changes in the nucleotide sequence of DNA
• Can be caused by errors during DNA replication, exposure to mutagens (UV light, chemicals), or viral infections
• Types of DNA mutations
  • Point mutations: Single nucleotide changes
    • Substitutions: One nucleotide is replaced by another
    • Insertions: One or more nucleotides are added
    • Deletions: One or more nucleotides are removed
  • Chromosomal mutations: Large-scale changes in the structure or number of chromosomes
    • Duplications: A segment of DNA is copied and inserted into the genome
    • Deletions: A segment of DNA is removed from the genome
    • Inversions: A segment of DNA is reversed in orientation
    • Translocations: A segment of DNA is moved to a different location in the genome
• Effects of DNA mutations
  • Silent mutations: Do not change the amino acid sequence of the protein
  • Missense mutations: Change one amino acid in the protein
  • Nonsense mutations: Introduce a premature stop codon, resulting in a truncated protein
  • Frameshift mutations: Alter the reading frame, often resulting in a nonfunctional protein
• DNA repair mechanisms can detect and correct many types of mutations, maintaining the integrity of the genetic information

DNA in Real Life

• DNA technology has numerous applications in various fields
• DNA fingerprinting: Used in forensic science to identify individuals based on their unique DNA profiles
  • Can be used to solve crimes, establish paternity, or identify missing persons
• Genetic engineering: Involves the manipulation of DNA to modify the characteristics of an organism
  • Used to create genetically modified organisms (GMOs) with desired traits (pest-resistant crops, insulin-producing bacteria)
• Gene therapy: Involves the introduction of functional genes into cells to replace or correct defective genes
  • Potential treatment for genetic disorders (cystic fibrosis, sickle cell anemia)
• Personalized medicine: Utilizes an individual's genetic information to tailor medical treatments and preventive strategies
  • Can help predict disease risk, optimize drug dosages, and minimize side effects
• Evolutionary studies: DNA analysis helps to understand the evolutionary relationships between species and track population migrations
  • Mitochondrial DNA and Y-chromosome DNA are often used in these studies due to their unique inheritance patterns
• DNA data storage: DNA has the potential to be used as a high-density, long-term storage medium for digital data
  • DNA can store vast amounts of information in a compact space and remain stable for centuries