16.16 Chapter Summary

BioCast:

1. Haploid (n) and Diploid (2n)

• Haploid (n):
  • Definition: A cell containing one complete set of chromosomes.
  • Example: Gametes (sperm and egg cells).
  • Significance: Ensures chromosome number is maintained across generations.
• Diploid (2n):
  • Definition: A cell containing two complete sets of chromosomes, one from each parent.
  • Example: Somatic (body) cells.
  • Significance: Allows for genetic diversity and proper chromosome pairing during meiosis.

2. Homologous Pairs of Chromosomes

• Definition: Pairs of chromosomes that are similar in shape, size, and genetic content. One chromosome of each pair is inherited from each parent.
• Characteristics:
  • Contain the same genes at the same loci, but may have different alleles.
  • Pair up during meiosis I, specifically in prophase I.
• Example: Humans have 23 pairs of homologous chromosomes (46 in total).

3. Need for Reduction Division during Meiosis in Gamete Production

• Reduction Division (Meiosis I):
  • Purpose: Reduces the chromosome number by half from diploid (2n) to haploid (n).
  • Reason: Ensures that upon fertilization, the resulting zygote has the correct diploid number of chromosomes.
• Consequences of No Reduction:
  • Chromosome number would double each generation, leading to genetic imbalances.

4. Behaviour of Chromosomes in Plant and Animal Cells during Meiosis

• Key Structures Involved:
  • Nuclear Envelope: Breaks down during prophase I and prophase II; re-forms during telophase I and telophase II.
  • Spindle Fibers: Formed from centrosomes (animal cells) or spindle poles (plant cells) to segregate chromosomes.
  • Cell Surface Membrane: Undergoes cytokinesis to divide the cell into two daughter cells.
• Main Stages of Meiosis:
  1. Meiosis I:
     • Prophase I: Homologous chromosomes pair up (synapsis) and crossing over occurs.
     • Metaphase I: Homologous pairs align at the metaphase plate.
     • Anaphase I: Homologous chromosomes are pulled to opposite poles.
     • Telophase I: Nuclear envelopes may reform; cytokinesis divides the cell into two haploid cells.
  2. Meiosis II:
     • Prophase II: Spindle fibers form in each haploid cell.
     • Metaphase II: Chromosomes align individually at the metaphase plate.
     • Anaphase II: Sister chromatids are separated to opposite poles.
     • Telophase II: Nuclear envelopes reform; cytokinesis results in four genetically distinct haploid gametes.

5. Interpreting Photomicrographs and Diagrams of Cells in Different Stages of Meiosis

• Identifying Stages:
  • Prophase I: Chromosomes appear thickened and paired; crossing over visible as chiasmata.
  • Metaphase I: Homologous pairs align side by side at the metaphase plate.
  • Anaphase I: Homologous chromosomes move to opposite poles.
  • Telophase I: Two haploid cells may show reformed nuclear envelopes.
  • Prophase II: Similar to prophase but occurs in haploid cells.
  • Metaphase II: Chromosomes align individually at the metaphase plate.
  • Anaphase II: Sister chromatids separate.
  • Telophase II: Four distinct haploid cells are formed.
• Tips for Interpretation:
  • Look for Chromosome Pairing: Indicates prophase I.
  • Observe Chromosome Alignment: Metaphase stages differ between I and II based on pairing.
  • Note Separation Patterns: Homologous chromosomes separate in anaphase I; sister chromatids in anaphase II.

6. Crossing Over and Random Orientation (Independent Assortment)

• Crossing Over:
  • Definition: Exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I.
  • Effect: Creates new allele combinations, increasing genetic diversity.
  • Result: Each gamete has a unique set of genes different from both parents and siblings.
• Independent Assortment:
  • Definition: Random orientation of homologous chromosome pairs during metaphase I.
  • Effect: Determines the combination of maternal and paternal chromosomes that go into each gamete.
  • Result: Leads to numerous possible genetic combinations, enhancing variation.
• Genetic Outcome:
  • Genetic Variation: Both crossing over and independent assortment contribute to genetically unique gametes.

7. Random Fusion of Gametes at Fertilisation Produces Genetically Different Individuals

• Fertilisation Process:
  • Random Fusion: Any sperm can fuse with any egg, each carrying a unique combination of genetic information.
• Genetic Implications:
  • Unique Zygote: The combination of two unique gametes results in a zygote with a distinct genetic makeup.
  • Genetic Diversity: Ensures each individual is genetically different from their siblings (except identical twins) and parents.

Key Terms to Remember

• Haploid (n): Single set of chromosomes.
• Diploid (2n): Double set of chromosomes.
• Homologous Chromosomes: Pair of chromosomes with the same genes but possibly different alleles.
• Meiosis: The process of cell division that reduces chromosome number by half, resulting in gametes.
• Crossing Over: Exchange of genetic material between homologous chromosomes.
• Independent Assortment: Random distribution of maternal and paternal chromosomes into gametes.
• Gametes: Reproductive cells (sperm and egg) that are haploid.
• Zygote: The diploid cell formed by the fusion of gametes during fertilisation.

The Roles of Genes in Determining the Phenotype

8. Key Definitions

• Gene: A unit of heredity located on a chromosome that codes for a specific trait.
• Locus: The specific physical location of a gene on a chromosome.
• Allele: Different versions of a gene (e.g., A and a).
  • Dominant Allele: Expressed in the phenotype even if only one copy is present (e.g., A).
  • Recessive Allele: Expressed in the phenotype only when two copies are present (e.g., a).
  • Codominant Alleles: Both alleles are fully expressed in the phenotype (e.g., AB blood type).
• Phenotype: The observable characteristics of an organism (e.g., flower color).
• Genotype: The genetic makeup of an organism (e.g., AA, Aa, aa).
• Homozygous: Having two identical alleles for a trait (e.g., AA or aa).
• Heterozygous: Having two different alleles for a trait (e.g., Aa).
• Linkage: When genes are located close to each other on the same chromosome and tend to be inherited together.
• Test Cross: A cross between an individual with an unknown genotype and a homozygous recessive individual to determine the unknown genotype.
• F₁ Generation: The first generation of offspring from a cross.
• F₂ Generation: The second generation of offspring, resulting from crossing individuals from the F₁ generation.

9. Genetic Diagrams and Punnett Squares

Monohybrid Cross

• Definition: A cross between two organisms focusing on a single trait.
• Punnett Square:
  • Example: Crossing Aa × Aa for flower color.
  • Results:
    • Genotypes: AA, Aa, Aa, aa
    • Phenotypes: 3 dominant, 1 recessive

Dihybrid Cross

• Definition: A cross between two organisms focusing on two traits.
• Punnett Square:
  • Example: Crossing AaBb × AaBb for seed shape and color.
  • Results:
    • Genotypes: 9 A-B-, 3 A-bb, 3 aaB-, 1 aabb
    • Phenotypes: 9 dominant for both traits, 3 dominant for one and recessive for the other, etc.

Codominance

• Definition: Both alleles in a gene pair are fully expressed.
• Example: AB blood type (A and B alleles both expressed).

Multiple Alleles

• Definition: More than two alleles exist for a gene.
• Example: ABO blood group system (A, B, O alleles).

Sex Linkage

• Definition: Genes located on sex chromosomes (X or Y).
• Example: Hemophilia is linked to the X chromosome.

Autosomal Linkage and Epistasis

• Autosomal Linkage: Genes located on non-sex chromosomes; may be inherited together.
• Epistasis: Interaction between genes where one gene masks or modifies the expression of another.
  • Example: Coat color in Labrador Retrievers (B for black, b for brown; E allows color expression, e results in yellow regardless of B/b).

10. Test Crosses

• Purpose: To determine the genotype of an individual exhibiting the dominant phenotype.
• Procedure:
  • Cross the individual with a homozygous recessive individual.
  • Analyze the offspring’s phenotypes to infer the unknown genotype.
• Example:
  • If unknown genotype is AA × aa → All offspring are Aa (phenotypically dominant).
  • If unknown genotype is Aa × aa → Approximately 50% dominant, 50% recessive.

11. Chi-Squared Test

• Purpose: To determine if observed genetic data significantly deviates from expected ratios.
• Formula:
  • O: Observed frequency
  • E: Expected frequency

• Steps:
  1. Calculate expected frequencies based on genetic ratios.
  2. Use the formula to compute χ².
  3. Compare χ² to the critical value from χ² distribution tables to determine significance.

12. Gene-Protein-Phenotype Relationships

TYR Gene, Tyrosinase, and Albinism

• TYR Gene: Codes for the enzyme tyrosinase.
• Tyrosinase: Essential for melanin production.
• Albinism: Result of mutations in TYR leading to non-functional tyrosinase, resulting in lack of pigment.

HBB Gene, Haemoglobin, and Sickle Cell Anaemia

• HBB Gene: Codes for the β-globin subunit of haemoglobin.
• Haemoglobin: Protein in red blood cells carrying oxygen.
• Sickle Cell Anaemia: Mutation in HBB causes abnormal haemoglobin, leading to distorted (sickle-shaped) red blood cells.

F8 Gene, Factor VIII, and Haemophilia

• F8 Gene: Codes for clotting factor VIII.
• Factor VIII: Essential for blood clotting.
• Haemophilia: Mutation in F8 leads to deficient or dysfunctional factor VIII, causing prolonged bleeding.

HTT Gene, Huntingtin, and Huntington’s Disease

• HTT Gene: Codes for the huntingtin protein.
• Huntingtin: Involved in neuronal function.
• Huntington’s Disease: Mutation leads to faulty huntingtin protein, causing neurodegeneration and movement disorders.

13. Gibberellin and Stem Elongation

• Gibberellin: A plant hormone that promotes stem elongation.
• Alleles:
  • Dominant Allele (Le):
    • Codes for a functional enzyme in the gibberellin synthesis pathway.
    • Results in normal stem elongation.
  • Recessive Allele (le):
    • Codes for a non-functional enzyme.
    • Results in reduced or no stem elongation (dwarfism).
• Genotypic Effects:
  • LeLe or Lele: Tall stems (phenotypically dominant).
  • lele: Dwarf stems (phenotypically recessive).

Example Problems

Monohybrid Cross:

• Question: Cross Aa × Aa for flower color (A = purple, a = white). What are the expected phenotypic ratios in F₂?
  • Answer: 3 purple : 1 white.
• Dihybrid Cross with Codominance:
  • Question: If A and B are codominant alleles, what is the phenotype of AaBb individuals?
    • Answer: Both traits are expressed equally (e.g., red and white flowers resulting in pink flowers).

Test Cross:

• Question: An individual with purple flowers (genotype unknown) is crossed with a white-flowered homozygous recessive. Offspring show a 1:1 ratio of purple to white. What is the unknown genotype?
  • Answer: Aa (heterozygous).
• Chi-Squared Test:
  • Question: Observed F₂ ratios: 90 dominant, 30 recessive. Expected ratio: 3:1. Is the difference significant?
  • Calculation:
    • Total: 120
    • Expected: 90 dominant, 30 recessive.
    • χ² = [(90-90)²/90] + [(30-30)²/30] = 0 + 0 = 0 → Not significant.

14. Structural Genes vs. Regulatory Genes

Structural Genes

• Function: Encode proteins that perform cellular functions (e.g., enzymes, structural proteins).
• Example: Genes that code for hemoglobin or insulin.

Regulatory Genes

• Function: Control the expression of structural genes.
• Mechanism: Produce proteins (regulators) that increase or decrease the transcription of specific structural genes.
• Example: LacI gene in the lac operon that codes for the repressor protein.

Repressible Enzymes vs. Inducible Enzymes

Repressible Enzymes

• Function: Typically involved in anabolic (biosynthetic) pathways.
• Regulation: Enzyme production is inhibited (repressed) when the end product is abundant.
• Example: Tryptophan synthesis in bacteria; high tryptophan levels repress the enzymes needed for its synthesis.

Inducible Enzymes

• Function: Usually involved in catabolic (breakdown) pathways.
• Regulation: Enzyme production is activated (induced) in the presence of a substrate.
• Example: Lactose metabolism in bacteria; presence of lactose induces enzymes for its breakdown.

15. Genetic Control of Protein Production in Prokaryotes: The Lac Operon

Components of the Lac Operon

• Structural Genes:
  • lacZ: Codes for β-galactosidase.
  • lacY: Codes for lactose permease.
  • lacA: Codes for thiogalactoside transacetylase.
• Regulatory Elements:
  • Promoter: Binding site for RNA polymerase.
  • Operator: Binding site for the repressor protein.
  • Regulator Gene (lacI): Produces the repressor protein.

Mechanism

1. Absence of Lactose:
   • Repressor protein binds to the operator.
   • RNA polymerase cannot bind to the promoter.
   • Structural genes are not transcribed; enzymes are not produced.
2. Presence of Lactose:
   • Lactose binds to the repressor, causing a conformational change.
   • Repressor releases from the operator.
   • RNA polymerase binds to the promoter and transcribes structural genes.
   • Enzymes for lactose metabolism are produced.

16. Transcription Factors in Eukaryotes

Definition

• Transcription Factors: Proteins that bind to specific DNA sequences to regulate gene expression.

Functions

• Activation: Increase the rate of transcription by facilitating the binding of RNA polymerase to the promoter.
• Repression: Decrease the rate of transcription by blocking RNA polymerase access or by altering chromatin structure.

Mechanism

1. Binding: Transcription factors bind to enhancer or silencer regions of DNA.
2. Interaction: They interact with other proteins and the transcription machinery to modulate transcription rates.
3. Outcome: Regulation of gene expression ensures appropriate protein levels for cellular function.

17. Gibberellin Activation of Genes in Eukaryotes

Role of Gibberellin

• Gibberellin: A plant hormone that promotes growth and influences various developmental processes.

Mechanism of Action

1. Activation:
   • Gibberellin binds to its receptor in the cell.
2. Breakdown of DELLA Proteins:
   • DELLA proteins are repressors that inhibit transcription factors involved in growth.
   • Gibberellin signaling leads to the degradation of DELLA proteins.
3. Promotion of Transcription:
   • With DELLA repressors degraded, transcription factors are free to activate gene expression.
   • Genes involved in growth and development are transcribed, leading to protein synthesis that promotes these processes.

Summary

• Without Gibberellin: DELLA proteins inhibit growth-promoting genes.
• With Gibberellin: DELLA proteins are degraded, allowing transcription factors to activate growth-promoting genes.

Key Points to Remember

• Structural vs. Regulatory Genes: Structural genes make proteins; regulatory genes control when and how much structural genes are expressed.
• Repressible vs. Inducible Enzymes: Repressible enzymes are shut off when their product is plentiful; inducible enzymes are turned on in the presence of a substrate.
• Lac Operon: A model for understanding gene regulation in prokaryotes; controlled by the presence or absence of lactose.
• Transcription Factors: Essential for regulating gene expression in eukaryotes by modulating transcription rates.
• Gibberellin and DELLA Proteins: Illustrates hormone-mediated gene activation by removing repression on transcription factors.