BIOB11 Lecture 1
Historical Discoveries on the Nature of Genes
Key Figures - Groundbreaking scientists whose work collectively unveiled the secrets of heredity and the molecular structure of genes:
Gregor Mendel: Established the fundamental laws of inheritance through pea plant experiments.
Walther Flemming: First observed and described chromosomes during cell division.
Walter Sutton: Proposed the chromosome theory of inheritance, linking Mendel's units of heredity to chromosomes.
Thomas Hunt Morgan: Demonstrated that genes are located on specific chromosomes using Drosophila.
Sturtevant: Developed the first genetic linkage map based on recombination frequencies.
Avery, MacLeod & McCarty: Provided experimental evidence that DNA is the genetic material.
Watson & Crick: Proposed the double helix structure of DNA, elucidating its mechanism for replication and information storage.
Gene as a Unit of Inheritance
Gregor Mendel's Experiments
Mendel, an Augustinian friar, conducted meticulous crossbreeding experiments with garden pea plants (Pisum sativum) in the mid-19th century.
He analyzed seven distinct observable characteristics (e.g., seed shape, seed color, flower color, plant height) over several generations.
His methodical approach, using true-breeding lines and quantitative analysis, allowed him to identify patterns of inheritance.
Concept of Genes
Mendel proposed that discrete physical units, which he called "hereditary factors" (later termed genes by Wilhelm Johannsen), govern observable traits.
These genes are specific segments of DNA located on chromosomes, coding for particular proteins or functional RNA molecules.
Each diploid individual inherits two copies of each gene, one from each biological parent, which dictates their genetic makeup.
The sum of an organism's genes is its genome.
Alleles
Alleles are alternative forms or variants of a gene that occupy the same locus (position) on homologous chromosomes.
Alleles can be distinguished by their phenotypic effects:
Dominant Allele: An allele that expresses its associated phenotype even when only one copy is present in a heterozygous individual.
Recessive Allele: An allele that expresses its associated phenotype only when two identical copies are present (homozygous recessive), being masked by a dominant allele in heterozygotes.
Genotype Classification:
Homozygous: An individual possesses two identical alleles for a particular trait (e.g., purebred dominant AA or purebred recessive aa).
Heterozygous: An individual possesses two different alleles for a particular trait (e.g., hybrid Aa).
During meiosis, each reproductive cell (gamete – sperm or egg) receives only one allele for each trait due to the segregation of homologous chromosomes, ensuring that the offspring receives a complete set of two alleles (one from each parent) upon fertilization.
Dominant and Recessive Alleles
Definitions
Dominant Allele: The allele whose phenotypic effect is expressed in a heterozygous individual, masking the presence of a recessive allele. It determines the observable trait.
Recessive Allele: The allele whose phenotypic effect is hidden or unexpressed when a dominant allele is present. It only manifests phenotypically in a homozygous recessive state.
Law of Dominance
In a cross between two true-breeding individuals with different traits (e.g., pure tall vs. pure short), the F1 generation will uniformly express the dominant trait.
Example:
'A' represents the dominant allele for tall height (uppercase nomenclature, e.g., T for tall).
'a' represents the recessive allele for short height (lowercase nomenclature, e.g., t for short).
If a pure tall pea plant (TT) is crossed with a pure short pea plant (tt), all F1 offspring will be heterozygous tall (Tt) because the tall allele (T) is dominant.
Genotype and Phenotype Examples
Short Genotype: aa (homozygous recessive) | Phenotype: short (the recessive trait is expressed).
Tall Genotype: Aa (heterozygous) | Phenotype: tall (the dominant trait is expressed, masking the recessive).
Tall Genotype: AA (homozygous dominant) | Phenotype: tall (the dominant trait is expressed).
These simple examples illustrate Mendelian inheritance patterns for monogenic traits. More complex traits involve multiple genes or environmental factors.
Example Ratios: In a cross between two heterozygous tall plants (Aa \times Aa), the offspring would exhibit an approximate genotypic ratio of 1 AA : 2 Aa : 1 aa and a phenotypic ratio of 3 tall : 1 short (i.e., 75% tall plants and 25% short plants).
Mendel’s Laws of Inheritance
Law of Segregation
This law states that during the formation of gametes (sperm and egg cells) during meiosis, the two alleles for a heritable character (gene) separate (segregate) from each other such that each gamete receives only one allele for that character.
Consequently, an offspring inherits one allele from each parent for each gene.
Example: For a parent with genotype 'Rr' (heterozygous for seed shape, where 'R' is dominant for round seeds and 'r' is recessive for wrinkled seeds), this parent will produce two types of gametes in equal proportions: 50% carrying the 'R' allele and 50% carrying the 'r' allele.
Law of Independent Assortment
This law posits that the alleles for different genes segregate independently of one another during gamete formation.
In other words, the inheritance of one trait does not influence the inheritance of another trait, provided the genes for these traits are located on different chromosomes or are far apart on the same chromosome.
This independent segregation leads to all possible combinations of alleles in the gametes.
Example: In a dihybrid cross, the inheritance of seed color (such as yellow 'Y' vs green 'y') is independent of seed shape (round 'R' vs wrinkled 'r'). A parent with genotype RrYy can produce four types of gametes: RY, Ry, rY, and ry, each with approximately 25% frequency.
Mendelian Genetics Example (Dihybrid Cross)
Consider a cross between two true-breeding pea plants:
Parent 1: RRYY (homozygous for round and yellow seeds)
Parent 2: rryy (homozygous for wrinkled and green seeds)
Gametes generated:
Parent 1 can only produce RY gametes.
Parent 2 can only produce ry gametes.
First Filial Generation (F1): All offspring will have the genotype RrYy (heterozygous for both traits) and phenotype of round, yellow seeds, demonstrating the Law of Dominance for both traits.
If these F1 individuals were self-crossed (RrYy \times RrYy), the F2 generation would exhibit a phenotypic ratio of 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green, illustrating independent assortment.
Chromosomes as Carriers of Genetic Information
Historical Context of Chromosomes
In the 1880s, the scientific community began to establish that chromosomes, observable structures within the nucleus, act as the physical carriers of genetic information.
Walther Flemming, a German biologist, developed dyes that allowed him to observe and describe visible thread-like structures (which he named chromatin) within the nucleus of dividing cells.
He documented the process of mitosis, showing how these structures condense into distinct chromosomes and are meticulously distributed equally among daughter cells, suggesting their crucial role in heredity.
Concept of Homologous Chromosomes
Building upon Flemming's work, Walter Sutton articulated the chromosome theory of inheritance in 1902-1903, proposing that Mendel's hereditary factors (genes) are located on chromosomes.
He explained that each diploid cell contains paired chromosomes, known as homologous chromosomes, with one chromosome from each pair inherited from each parent.
These homologous pairs carry genes for the same traits at corresponding loci.
During meiosis I, homologous chromosomes associate closely, forming structures called tetrads (because they consist of four chromatids) or bivalents (because they consist of two homologous chromosomes).
These bivalents are held together by a protein-RNA complex called the synaptonemal complex, which facilitates crucial processes like crossing over.
Meiotic division I is a reductional division, separating homologous chromosome pairs and reducing the chromosome number by half. Meiosis II is an equational division, separating sister chromatids, similar to mitosis.
Meiosis vs Mitosis
Comparison Features
Number of cell divisions: Mitosis involves one nuclear division, resulting in two daughter cells. Meiosis involves two sequential nuclear divisions (Meiosis I and Meiosis II), resulting in four daughter cells.
Number of times DNA is replicated: In both mitosis and meiosis, DNA replication occurs only once, during the S phase preceding the first division.
Homologous chromosomes vs sister chromatids:
In mitosis, sister chromatids separate during anaphase.
In meiosis I, homologous chromosomes separate during anaphase I. In meiosis II, sister chromatids separate during anaphase II.
Ploidy of daughter cells: Mitosis produces diploid (2n) daughter cells from a diploid parent cell. Meiosis produces haploid (n) daughter cells from a diploid parent cell.
Diversity of offspring:
Mitosis produces two genetically identical daughter cells, crucial for growth, repair, and asexual reproduction.
Meiosis produces four genetically non-identical daughter cells, essential for sexual reproduction and genetic diversity due to crossing over and independent assortment.
Purpose: Meiosis is a reductive process that reduces the chromosome number by half and generates genetic variation, enabling sexual reproduction. Mitosis maintains the chromosome number and genetic identity, facilitating growth and repair.
Chromosomal Structure and Genetic Information
Interphase Chromosomes
During interphase (the non-dividing phase of the cell cycle), chromosomes are decondensed and not individually visible under a light microscope.
Each interphase chromosome consists of a single DNA molecule, which in eukaryotes is a long, linear double helix, tightly coiled and packaged with proteins (histones) into chromatin.
Prokaryotic chromosomes, in contrast, are typically circular DNA molecules lacking histones.
Genes located on the same chromosome are said to be linked and constitute a linkage group. These linked genes tend to be inherited together more frequently than genes on different chromosomes.
Crossing Over and Genetic Recombination
Crossing over (or chromosomal crossover) is a crucial genetic event that occurs during prophase I of meiosis.
It involves the physical exchange of genetic material (DNA segments) between non-sister chromatids of homologous chromosomes.
This process generates genetic recombination, creating new combinations of alleles on a chromatid that were not present on the original parental chromosomes. This reshuffling significantly contributes to genetic diversity within a species.
The specific points where crossing over occurs and homologous chromosomes remain connected are termed chiasma (plural: chiasmata).
The frequency of crossing over between two genes is directly proportional to the physical distance separating them on the chromosome, a principle used to construct genetic maps.
The Molecular Basis of Genes
Experiments to Determine the Nature of Genes (Griffith's Transformation)
In 1928, Frederick Griffith conducted experiments with Streptococcus pneumoniae, a bacterium that causes pneumonia.
He identified two strains: a virulent S strain (smooth, encapsulated, pathogenic) and a non-virulent R strain (rough, non-encapsulated, non-pathogenic).
His key observation was that heat-killed S bacteria, when mixed with live R bacteria, could "transform" the R bacteria into a virulent S strain, causing disease and death in mice.
He concluded that some factor, a "transforming principle," from the dead S cells was responsible for conferring virulence to the R cells, though the chemical nature of this principle remained unknown.
Avery, MacLeod, and McCarty's Contribution
In 1944, Oswald Avery, Colin MacLeod, and Maclyn McCarty embarked on identifying Griffith's transforming principle.
They systematically purified different macromolecules (DNA, RNA, proteins, lipids, carbohydrates) from heat-killed S bacteria.
They then tested each component for its ability to transform live R bacteria.
Crucially, they found that only the DNA fraction was capable of transforming R cells into S cells.
Furthermore, treatment with DNAse (an enzyme that degrades DNA) abolished the transforming activity, while proteases and RNases had no effect.
Their meticulous work provided strong, direct evidence that DNA, and not protein, is the genetic material responsible for heredity.
Hershey-Chase Experiment
In 1952, Alfred Hershey and Martha Chase conducted elegant experiments using bacteriophages (viruses that infect bacteria) to further confirm DNA as the genetic material.
They selectively labeled components of the phage:
DNA was labeled with radioactive phosphorus-32 (^{32}P) because phosphorus is abundant in DNA but not in protein.
Proteins were labeled with radioactive sulfur-35 (^{35}S) because sulfur is found in proteins but not in DNA.
Bacteriophages with labeled DNA or protein were allowed to infect E. coli bacteria.
After infection, the cultures were blended to shear off external phage particles and then centrifuged to separate the heavier bacterial cells from the lighter phage remnants.
Results showed that ^{32}P (DNA) entered the bacterial cells and was passed on to progeny phages, while most of the ^{35}S (protein) remained outside the cells.
This definitively demonstrated that DNA is the substance transferred from viral particles to bacteria during infection, carrying the genetic instructions for viral replication.
Structure and Composition of DNA
DNA Composition
Deoxyribonucleic Acid (DNA) is a complex macromolecule, a polymer composed of repeating monomer units called nucleotides.
Each nucleotide in DNA consists of three main components:
A deoxyribose sugar (a five-carbon sugar).
A phosphate group.
One of four nitrogenous bases:
Adenine (A) – a purine
Guanine (G) – a purine
Cytosine (C) – a pyrimidine
Thymine (T) – a pyrimidine
The sugar-phosphate backbone forms the structural framework of the DNA strands, with the nitrogenous bases projecting inward. These bases are connected to each other across the two strands by specific hydrogen bonds.
Nucleotide Structure
Nucleoside: Formed by the covalent attachment of a nitrogenous base to the 1' carbon of a deoxyribose sugar. Examples: deoxyadenosine, deoxyguanosine.
Nucleotide: A nucleoside additionally esterified with one or more phosphate groups, typically at the 5' carbon of the sugar. Examples: deoxyadenosine monophosphate (dAMP), deoxyguanosine triphosphate (dGTP).
Nucleotides within a DNA strand are linked together by phosphodiester bonds.
A phosphodiester bond forms between the phosphate group attached to the 5' carbon of one nucleotide sugar and the hydroxyl group on the 3' carbon of the adjacent nucleotide sugar. This creates the backbone with a distinct 5' to 3' polarity.
Base Pairing in DNA
The nitrogenous bases form specific pairs between the two DNA strands, mediated by hydrogen bonds, a principle known as complementary base pairing:
Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds. This strong pairing contributes to DNA stability.
Adenine (A) always pairs with Thymine (T) via two hydrogen bonds.
Chargaff's Rules (1950): Erwin Chargaff's analysis of DNA composition from various organisms revealed key ratios:
The amount of Adenine (A) always approximately equals the amount of Thymine (T): [A] = [T].
The amount of Guanine (G) always approximately equals the amount of Cytosine (C): [G] = [C].
Consequently, the total amount of purines ([A] + [G]) always equals the total amount of pyrimidines ([T] + [C]).
These rules were instrumental in Watson and Crick's deduction of the double helix structure and the specific base pairing, implying a balanced composition crucial for genetic specificity and replication accuracy.
Functional Requirements of DNA Structure
Key Functional Requirements
Storage of Genetic Information: DNA must be capable of reliably storing vast amounts of instructional information (genes) for all cellular processes, including protein synthesis and regulation. The linear sequence of nucleotides acts as a genetic code.
Replication and Inheritance: The genetic information must be accurately copied (replicated) before cell division to ensure faithful transmission to daughter cells and subsequent generations of offspring. The complementarity of base pairing provides a simple mechanism for template-based replication.
Gene Expression: The stored information must be accessible and able to direct the synthesis of proteins, which perform most cellular functions. This involves two main processes:
Transcription: The genetic information from a DNA segment is copied into an RNA molecule.
Translation: The RNA molecule then guides the synthesis of a protein.
Capacity for Variation (Mutation): While maintaining stability for accurate inheritance, DNA must also allow for occasional changes (mutations) to produce genetic variation, which is the raw material for evolution.
Watson-Crick Model Significance
The Watson-Crick double helix model (1953) elegantly explained how DNA fulfills these functional requirements.
It demonstrated that the specific linear sequence of bases (A, T, G, C) along the polynucleotide strands encodes genetic information.
Crucially, the complementary base pairing and antiparallel nature of the two strands suggested a straightforward mechanism for DNA replication: the two strands could separate, and each would serve as a template for the synthesis of a new complementary strand, ensuring the accurate transmission of genetic instructions.
The structure also implied how information could be accessed and transcribed into RNA without permanently altering the DNA molecule.
The Double Helix Model of DNA
Structure of the DNA Double Helix
Proposed in 1953 by James Watson and Francis Crick, building on X-ray diffraction data from Rosalind Franklin and Maurice Wilkins, and Chargaff's rules.
The model describes DNA as a right-handed double helix, composed of two long polynucleotide strands coiled around a central axis.
The two complementary strands run in opposite directions (are antiparallel): one strand runs 5' \to 3' and the other runs 3' \to 5'.
The sugar-phosphate backbones are on the outside of the helix, forming the structural framework, while the nitrogenous bases are stacked in the interior.
Each full turn of the helix spans approximately 3.4 nanometers (nm) and contains roughly 10 base pairs.
The double helix features major and minor grooves on its surface. These grooves are important for the binding of various proteins (e.g., transcription factors) that regulate gene expression and DNA replication.
Hybridization and DNA Functionality
Hybridization refers to the process where two complementary single strands of nucleic acid (DNA-DNA, DNA-RNA, or RNA-RNA) anneal to form a stable double helix or a double-stranded region.
This process is driven by the formation of hydrogen bonds between complementary bases.
Strand separation (denaturation) occurs when the hydrogen bonds between bases are broken, typically by elevated temperature or extreme pH, causing the double helix to unwind into two single strands (e.g., during replication or transcription).
Renaturation (reannealing) is the reverse process, where separated complementary strands recombine to form a double helix when favorable conditions are restored.
This denaturing and renaturing property is fundamental to DNA's functionality, allowing localized unwinding for access to genetic information and subsequent rejoining of strands.
Recap of Genetic Information Flow
Flow of Information (The Central Dogma)
The fundamental principle describing the flow of genetic information in biological systems is known as the Central Dogma of Molecular Biology.
The information transfer sequence is primarily: DNA (replication) \xrightarrow{\text{transcription}} RNA \xrightarrow{\text{translation}} Protein.
This illustrates DNA's role as the stable hereditary information blueprint, which is transcribed into a working copy (RNA), and then translated into the functional molecules (proteins) that carry out most cellular tasks.
Upcoming Topics in Next Lecture
Next Lecture Focus - The subsequent lecture will delve into:
The complex and varied structure of the genome across different organisms.
The physical processes of denaturation and renaturation of DNA in more detail, and their applications.
The intricate complexity of genome organization and packaging within cells, particularly the role of chromatin in eukaryotes.