Exhaustive Notes on DNA Structure, Function, Genomes, and Protein Synthesis

Course Overview & Learning Objectives

LO 2.1: Explain how the structure of DNA relates to its function.
LO 2.2: Predict the sequence of a complementary strand of DNA when given one strand.
LO 2.3: Explain the differences between chromosomes, genes, DNA, RNA, and proteins.

Introduction to DNA Function & Biological Identity

Definition & Function of DNA:
- DNA is the abbreviation for deoxyribonucleic acid.
- DNA serves as the long-term molecular storage medium for genetic instructions within living cells.
- It contains the information required to build proteins.
Analogies for DNA Function:
- Cookbook Analogy: DNA is comparable to a master cookbook. It contains individual recipes (genes) that provide instructions for making proteins.
- Baker Analogy: Ribosomes act like bakers. DNA does not directly synthesize proteins; instead, ribosomes read the genetic recipes stored in DNA to assemble proteins (analogous to baking a cake).
- Blueprint Analogy: A DNA strand functions as a biological blueprint for building and maintaining an organism.
Genetics vs. Environment & Life Experience:
- Although DNA provides the foundational instructions for constructing an organism, it does not exclusively define an individual's complete identity or character.
- An individual's physical traits, behavior, and outcome are shaped by a combination of genetics, environmental factors, decisions, and life experiences.
- Example of Identical Twin Divergence:
  - Identical twins possess identical DNA sequences, meaning their cells carry the exact same genetic instructions to build a human.
  - Environmental influences and individual choices can lead identical twins down vastly different life paths.
  - For example, identical twins Gary Hall and Larry Hall shared identical genetic material and participated in Civil War reenactments together. However, Gary Hall lived a conventional life, whereas Larry Hall became a serial killer. This demonstrates that identical DNA can produce distinct individuals.

Historical Discovery of DNA Structure

Key Molecular Biologists:
- In $1953$ , Francis Crick and James Watson (along with Maurice Wilkins) published the structural description of DNA as a double helix.
- Watson, Crick, and Wilkins were awarded the Nobel Prize in Physiology or Medicine in $1962$ for this discovery.
Contributions of Rosalind Franklin:
- Watson and Crick's model relied heavily on experimental work conducted by Rosalind Franklin.
- Franklin produced detailed X-ray diffraction patterns (notably Photo 51) that provided crucial measurements and revealed the helical structure of the DNA molecule.

$Rosalind Franklin and Photo 51 X-ray diffraction pattern$

Watson and Crick presenting their physical model of the DNA double helix

Molecular Structure of DNA

Nucleotides as Monomer Subunits:
- DNA is a biological macromolecule composed of repeating monomer units called nucleotides (named for their presence inside the cell nucleus).
- Each DNA nucleotide consists of three chemical parts:
  1. A phosphate group
  2. A deoxyribose sugar (a five-carbon sugar)
  3. A nitrogenous base

Chemical structure of a DNA nucleotide composed of a phosphate group, deoxyribose sugar, and adenine base

The Four Nitrogenous Bases:
- Four distinct nitrogenous bases exist in DNA, creating four different nucleotide monomers:
  - Adenine (A)
  - Thymine (T)
  - Guanine (G)
  - Cytosine (C)
- Each of the four bases possesses a distinct chemical structure and geometric shape.
Sugar-Phosphate Backbone:
- Single strands of DNA are constructed by linking the deoxyribose sugar of one nucleotide to the phosphate group of the adjacent nucleotide.
- These sugar-phosphate links are strong covalent bonds that resist breaking, providing a durable structural backbone for the strand.
Double-Stranded Architecture & Hydrogen Bonding:
- DNA consists of two nucleotide strands held together by hydrogen bonds between opposing nitrogenous bases.
- Hydrogen bonds are individually weak interactions compared to covalent backbone bonds, allowing the two strands to separate during biological processes.
- In a flattened ladder model of DNA, the structural sugar-phosphate backbones form the outer side rails, while paired nitrogenous bases form the inner rungs.
The Double Helix:
- When two complementary nucleotide strands pair, their chemical bonds cause the assembly to twist into a right-handed spiral known as a double helix.
- The spiraling geometry creates repeating structural features known as major grooves and minor grooves along the outer surface of the molecule.

Diagram of the DNA double helix illustrating nitrogenous base pairs, major and minor grooves, and sugar-phosphate backbone

Complementary Base Pairing & Sequence Prediction

Base Pairing Rules:
- Base pairing between opposing DNA strands is highly specific:
  - Adenine (A) always pairs with Thymine (T).
  - Cytosine (C) always pairs with Guanine (G).
Geometric Requirements of the Helix:
- The DNA double helix maintains a uniform diameter throughout its length.
- Uniform width is achieved because single-ring nitrogenous bases (pyrimidines: Thymine and Cytosine) always pair with double-ring nitrogenous bases (purines: Adenine and Guanine).
- Pairing two double-ring bases together would make the helix too wide, whereas pairing two single-ring bases together would make the helix too narrow.

Purine and pyrimidine nitrogenous base pairing maintaining fixed double helix width

Sequence Prediction Example:
- Given a single DNA strand sequence: $\text{GATTACA}$
- Applying complementary pairing rules (\text{A} \n\n\leftrightarrow \text{T}, $\text{C} \leftrightarrow \text{G}$ ):
  - $\text{G} \rightarrow \text{C}$
  - $\text{A} \rightarrow \text{T}$
  - $\text{T} \rightarrow \text{A}$
  - $\text{T} \rightarrow \text{A}$
  - $\text{A} \rightarrow \text{T}$
  - $\text{C} \rightarrow \text{G}$
  - $\text{A} \rightarrow \text{T}$
- The resulting sequence of the complementary strand is: $\text{CTAATGT}$

Genetic Variation, Genomes & Real-World Applications

Storage of Genetic Information:
- Because the sugar-phosphate backbone is structurally identical across all DNA molecules, genetic information is stored strictly in the precise linear sequence of nitrogenous bases ( $\text{A}$ , $\text{T}$ , $\text{C}$ , and $\text{G}$ ).
- The specific base sequence provides the instructions that differentiate individuals at the molecular level.
Human DNA Quantitative Facts:
- The human genome contains approximately $3 \times 10^9$ ( $3 \text{ billion}$ ) base pairs per cell.
- Unrelated humans share roughly $99.9\%$ of their DNA sequence, differing by approximately $0.1\%$ .
- A $0.1\%$ difference across $3 \times 10^9$ base pairs means two unrelated individuals differ by roughly $3 \times 10^6$ ( $3 \text{ million}$ ) base pairs.
Definition & Comparative Sizes of Genomes:
- A genome is defined as the complete suite of genetic material present in an organism or cell.
- Genome sizes vary widely across different organisms:

Virus / Organism	Genome Size in Base Pairs
T2 virus	$200,000$
E. coli bacterium	$4.6 \times 10^6$ ( $4.6 \text{ million}$ )
Yeast	$1.2 \times 10^7$ ( $12 \text{ million}$ )
Fruit fly	$1.8 \times 10^8$ ( $180 \text{ million}$ )
Chicken	$1.2 \times 10^9$ ( $1.2 \text{ billion}$ )
Human	$3.0 \times 10^9$ ( $3 \text{ billion}$ )
Pea	$5.0 \times 10^9$ ( $5 \text{ billion}$ )

Table 14.3 Sizes of Different Genomes in Base Pairs

Forensic Applications & Investigative Genetic Genealogy:
- DNA analysis allows law enforcement to identify suspects by comparing forensic samples with genealogical databases.
- Case Study: Joseph James DeAngelo (the Golden State Killer, responsible for $13$ murders, $50$ rapes, and over $100$ burglaries) was identified and arrested in $2018$ after investigators uploaded crime-scene DNA to the public genealogy website GEDmatch.
Exoneration of the Wrongfully Convicted:
- Forensic DNA testing is used to establish innocence and overturn wrongful convictions.
- According to the National Registry of Exonerations, $575$ wrongfully convicted individuals have been exonerated through post-conviction DNA testing since $1989$ .
- These exonerations include $196$ clients represented by the Innocence Project, including individuals such as Leonard Mack and Kirk Bloodsworth.
Evolutionary Relatedness & Sequence Alignment:
- All living organisms utilize DNA as their genetic storage material.
- Species that share a recent common evolutionary ancestor exhibit a higher percentage of identical nitrogenous base sequences.
- Distantly related species display greater sequence divergence.
- Comparative DNA sequence alignments between species (e.g., humans, chimpanzees, gorillas, and orangutans) help scientists trace evolutionary lineages.

Sequence alignment comparison between human, chimpanzee, gorilla, and orangutan genomes

Hierarchical Scale of Genetic Structures

Scale of Organization:
- Biological structures containing genetic material are organized hierarchically by scale: $\text{Nucleotides} < \text{DNA Strand} < \text{Gene} < \text{Chromosome} < \text{Genome}$
Structural Definitions:
- Nucleotide: The individual monomer consisting of a phosphate, sugar, and nitrogenous base.
- DNA: The double-stranded polymer chain formed by bonded nucleotides.
- Gene: A specific sequence segment along a DNA strand that contains the code for a single protein. An average gene spans thousands of nucleotides.
- Chromosome: A single, large DNA molecule tightly coiled and packaged around specialized proteins inside the nucleus.
- Genome: The complete collection of all chromosomes and genetic material contained within a cell.

Diagram illustrating hierarchical relationship between Cell, Nucleus, Chromosome, DNA, Gene, and Nucleotides

Information Transfer: DNA, RNA & Protein Synthesis

The Nuclear Compartmentalization Problem:
- In eukaryotic cells, DNA is located inside the nucleus.
- Because a complete DNA molecule is extremely large and bulky, it cannot cross the nuclear envelope to enter the cytoplasm.
- Protein synthesis occurs at ribosomes located outside the nucleus in the cytoplasm.
- To overcome this, cells generate short, mobile transcripts of individual genetic instructions that can exit the nucleus.
Transcription (DNA to RNA):
- Transcription is the biological process where a protein enzyme reads a DNA gene segment inside the nucleus to construct a complementary single-stranded RNA (ribonucleic acid) transcript.
- Differences Between DNA and RNA:

Structural Property	DNA	RNA
Strand Number	Double-stranded	Single-stranded
Length	Very long (entire chromosome)	Short (length of a single gene)
Nitrogenous Bases	Adenine, Thymine, Cytosine, Guanine	Adenine, Uracil, Cytosine, Guanine
Nuclear Mobility	Confined inside nucleus	Able to exit nucleus into cytoplasm

Base Substitution: RNA uses Uracil (U) in place of Thymine (T). During transcription, Adenine on DNA pairs with Uracil on RNA.

Comparison of double-stranded DNA structure and single-stranded RNA structure

Translation (RNA to Protein):
- Translation is the process where ribosomes in the cytoplasm decode the nucleotide sequence of an RNA molecule to synthesize a protein.
- Ribosomes match sets of nitrogenous bases on the RNA strand to specific amino acids (delivered by tRNA).
- The ribosome links these amino acids together via peptide bonds to form a functional protein chain.
The Central Flow of Molecular Information: $\text{DNA (Gene in Nucleus)} \xrightarrow{\text{Transcription}} \text{RNA (Mobile Copy)} \xrightarrow{\text{Translation}} \text{Protein (Functional Molecule)}$

Overview of transcription inside the nucleus and translation at ribosomes in the cytoplasm

Course Schedule, Readings & Assessment Deadlines

Required OpenStax Biology Textbook Readings:
- Current Material: OpenStax Sections $6.1$ & $9.1$
- Upcoming Material: OpenStax Sections $6.2$ & $6.3$
Unit 2 Key Deadlines & Exam Dates:
- Unit 2 Quiz: Opens $\text{09/30}$ , Closes $\text{10/02}$
- Unit 2 In-Class Activity: Scheduled for $\text{10/05}$
- Unit 2 Science in Action: Due $\text{10/07}$
- Unit 2 Video Questions: Due $\text{10/07}$
- Unit 2 Exam: Scheduled for $\text{10/12}$