Genetics and Molecular Biology

Molecular Genetics

  • Discovery of DNA Structure (1953)
      - Researchers: James Watson and Francis Crick.
      - Significance: Initiated the field of molecular genetics.
      - Gene Recognition: Defined as sequences of nucleic acids that could be isolated and characterized.
      - Mutations: Enhanced understanding of mutations and their implications in genetics.

Structure of DNA

  • Components of Chromosomes: Composed of two large molecule types: DNA and protein.
      - DNA Structure: Organized into a chain of nucleotides, which consist of:
        - Nitrogenous base
        - 5-carbon sugar (deoxyribose)
        - Phosphate group
      - Nucleotide Types: Four types based on unique nitrogenous bases:
        - Purines (two linked rings):
          - Adenine (A)
          - Guanine (G)
        - Pyrimidines (single ring):
          - Cytosine (C)
          - Thymine (T)

DNA Molecule

  • Structure: Nucleotides bond, forming a twisted ladder (helix).
      - Ladder Composition:
        - Sides formed by alternating sugar and phosphate groups.
        - Rungs formed by hydrogen bonds between paired bases:
          - Purines (A and G) pair with pyrimidines (C and T).
          - Pairings:
            - G-C
            - A-T

Functions of DNA

  • Storage of Genetic Information:
      - Genetic information resides in the sequence of nucleotides.
      - Gene Definition: Segment of DNA directing protein synthesis.
        - Proteins: Serve as structural or storage materials or function as enzymes influencing cellular activities.
      - Genome: Total DNA content in an organism's chromosomes.

Replication of Information

  • Process Timing: Occurs during the S phase of the cell cycle.
      - Mechanism: Strands of the double helix unzip, serving as templates for creating new strands.
      - Nucleotide Addition: Performed by DNA polymerase, following specific pairings:
        - G-C
        - A-T
      - Replication Type: Semi-conservative replication.

Expression of Information

  • Gene Expression Variability: Different subsets of genetic information are read based on cell type and environmental influences.
      - Two Main Processes:
        - Transcription: Creating a copy of the gene message from DNA using RNA.
          - RNA Characteristics: Contains ribose, is single-stranded, has uracil instead of thymine.
        - Translation: RNA is translated to produce proteins, occurring in the cytoplasm.

Transcription Process

  • Types of RNA Produced:
      - Messenger RNA (mRNA): Encodes proteins.
      - Transfer RNA (tRNA): Functions in translation machinery.
      - Ribosomal RNA (rRNA): Also part of the translation machinery.

  • RNA Synthesis:
      - Nucleotides are added by RNA polymerase, using complementary base pairing.
      - Only certain genome portions are transcribed; the rest is noncoding DNA.

  • Transcription Regulation:
      - Promoter Region: Signals the start for transcription enzymes.
      - Terminator Sequence: Signals the end of transcription, allowing enzymes to detach.

  • Outcome: Produces a single-stranded RNA transcript with nonprotein-coding DNA essential for controlling gene expression.

Translation Process

  • tRNA Role: Translates the coded mRNA.
      - Structure of tRNA:
        - One end binds to mRNA.
        - Other end binds to specific amino acids.
        - Each tRNA type has a unique anticodon loop.

  • Codon-Anticodon Interaction: Anticodon of tRNA corresponds to the mRNA codon.

  • Ribosome Function: Facilitates translation, acting as the site for protein assembly.

Genetic Code

  • mRNA Codon Structure: Composed of triplet sequences (codons).
      - Combinations: 64 possible codons for 20 amino acids.
      - Order Impact: The nucleotide order determines the resulting amino acid sequence.
      - Universality: The genetic code is consistent across various life forms (bacteria, plants, animals).

Central Dogma of Molecular Genetics

  • Information Flow: Defined by the sequence: DNA → RNA → Protein.

Mutations

  • Definition: Changes in a DNA sequence.

  • Mutagens: Agents that can cause alterations in DNA, including:
      - Ultraviolet light
      - Ionizing radiation
      - Certain chemicals

  • DNA Repair Mechanisms: Repair enzymes can often correct DNA damage.

  • Types of Mutations:
      - Somatic Mutation: Occurs in body cells; non-heritable.
      - Germ-Line Mutation: Arises in sex cell lineage; can be inherited and contribute to genetic variability.

Cytogenetics

  • Field Definition: Study of chromosome behavior and structure from a genetic perspective.

  • Chromosome Structure Changes:
      - Inversion: A chromosomal piece breaks off and reinserts in the opposite orientation, inherited in blocks, unaffected by meiosis.
      - Translocation: A chromosomal piece breaks off and attaches to a different chromosome. Both inversion and translocation can drive speciation.

Changes in Chromosome Number

  • Result of Non-Disjunction: Errors during pairing/separation can yield gametes with abnormal chromosome numbers.
      - Aneuploid: Organisms with an extra or missing chromosome.
      - Polyploid: Organisms with at least one complete extra set of chromosomes, often leading to larger size or higher yield in crops (e.g., cotton, wheat).

Mendelian Genetics

  • Pioneering Researcher: Gregor Mendel, Austrian monk, conducted foundational experiments in genetics around 1860, rediscovered later.

Mendel’s Experiments

  • Model Organism: Pea plants, selected for different trait forms (e.g., tall vs. short).

  • Generations:
      - Parental Generation (P): Tall and short individuals crossed, all offspring tall (F1 generation).
      - First Filial Generation (F1): All offspring tall; self-crossing leads to second filial generation (F2).
      - Second Filial Generation (F2): Produces a phenotypic ratio of 3 tall to 1 short.

Mendelian Laws

  • Law of Unit Characters: Characters controlled by pairs of alleles, located at specific loci on homologous chromosomes.

  • Law of Dominance: One allele (dominant) can mask the effect of another allele (recessive).
      - Terminology:
        - Phenotype: Observable physical attributes.
        - Genotype: Genetic makeup influencing phenotype.
        - Homozygous: Identical alleles.
        - Heterozygous: Different alleles.

Monohybrid Cross

  • Experiment Design: Cross true-breeding parents differing in one trait (producing F1 generation).

  • Results: An F2 generation yields a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio.

Dihybrid Cross

  • Design: Crosses parents differing in two traits, demonstrating the law of independent assortment.
      - Independent Segregation: Genes controlling multiple traits segregate independently.
      - Linked Genes: Genes on the same chromosome do not assort independently.
      - Genotypes: Punnett square used to determine gametes and phenotypic ratios resulting in a 9:3:3:1 ratio.

The Backcross

  • Definition: A cross between a hybrid and one of its parents, expecting a phenotypic ratio of 1:1 in offspring.

The Testcross

  • Procedure: Cross a dominant phenotype organism with a homozygous recessive to determine genotype (homozygous vs. heterozygous).

Incomplete Dominance

  • Description: In heterozygotes, the phenotype is intermediate between the two homozygotes, indicating a lack of dominance.

Interactions among Multiple Genes

  • Combination of Genes: More than one gene may control phenotypic traits and biochemical pathways (e.g., blue-eyed Mary plants, influenced by genes W and M).

Genetic Control of Phenotype

  • Dominant vs. Recessive Alleles: Dominant alleles code for functional proteins while recessive alleles may represent mutant forms incapable of catalysis.

Quantitative Traits

  • Type of Traits: Exhibit ranges of phenotypes, often influenced by multiple genes (quantitative trait loci - QTLs).
      - Environmental Impact: Identical genotypes may express different phenotypes under varying environmental conditions.

Extranuclear DNA

  • Definition: DNA found in mitochondria and chloroplasts.
      - Endosymbiont Hypothesis: Suggests mitochondria and chloroplasts originated from free-living bacteria, leading to an evolutionary symbiotic relationship with plant cells.
      - Inheritance: Maternal inheritance; sperm typically do not transmit organelle DNA.

Linkage and Mapping

  • Concepts:
      - Linked Genes: Genes located close together on chromosomes are more likely to be inherited together.
      - Crossing Over: More probable between distant genes, used to construct genetic maps.
      - Genetic Map Unit: One map unit corresponds to 1% crossing over between gene pairs.

The Hardy-Weinberg Law

  • Law Definition: States that allele proportions in a large, random mating population remain constant across generations in the absence of evolutionary influences.
      - Changing Factors:
        - Small population sizes can lead to random loss of alleles.
        - Selection pressures can contribute to deviations from Hardy-Weinberg proportions.