Photosynthesis, Cell Division, and Mendelian Genetics: Study Guide

Photosynthesis

Overview and Equation

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, producing glucose and oxygen from carbon dioxide and water.

• Overall Equation:

$6\ \mathrm{CO}_2 + 6\ \mathrm{H}_2\mathrm{O} + \text{light energy} \rightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 + 6\ \mathrm{O}_2$

• Redox Reactions: Photosynthesis involves oxidation-reduction (redox) reactions, where water is oxidized and carbon dioxide is reduced.

• Importance: Redox reactions drive the transfer of electrons, essential for energy conversion in photosynthesis.

Light and Pigments

• Light: Light is a form of electromagnetic radiation. Photosynthetically active radiation (PAR) includes wavelengths from about 400–700 nm.

• Important Colors: Blue (around 450 nm) and red (around 680 nm) light are most effective for photosynthesis; green light is least absorbed.

• Pigments: Molecules such as chlorophyll a, chlorophyll b, and carotenoids absorb light energy. Chlorophyll appears green because it reflects green light.

Light-Dependent and Light-Independent Reactions

• Light-Dependent Reactions: Occur in the thylakoid membranes; use light energy to produce ATP, NADPH, and O2 (as a byproduct).

• Electron Transport Chain: Transfers electrons from water to NADP+, forming NADPH and generating a proton gradient used to synthesize ATP.

• Light-Independent Reactions (Calvin Cycle): Occur in the stroma; use ATP and NADPH to fix CO2 into glucose.

• Relationship: ATP and NADPH produced in the light-dependent reactions power the Calvin Cycle.

Rubisco and Photorespiration

• Rubisco: The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase catalyzes the first step of the Calvin Cycle, fixing CO2.

• Photorespiration: Occurs when Rubisco binds O2 instead of CO2, reducing photosynthetic efficiency.

• C4 and CAM Pathways: Adaptations in some plants to minimize photorespiration by spatial (C4) or temporal (CAM) separation of carbon fixation.

Cell Division

Mitosis

Mitosis is the process by which a eukaryotic cell divides to produce two genetically identical daughter cells.

• Replicated Chromosome Structure: Each chromosome consists of two sister chromatids joined at a centromere.

• Phases of Cell Cycle: G1 (growth), S (DNA synthesis), G2 (preparation), M (mitosis).

• Phases of Mitosis: Prophase, Metaphase, Anaphase, Telophase.

• Cytokinesis: Division of the cytoplasm; in animals, via cleavage furrow; in plants, via cell plate formation.

• Cell Cycle Regulation: Checkpoints (G1, G2, M) ensure proper division; failure can lead to cancer.

• Cancer: Uncontrolled cell division due to loss of cell cycle regulation.

Binary Fission

• Definition: A form of asexual reproduction in prokaryotes (e.g., bacteria) where the cell divides into two identical cells.

Meiosis

Meiosis is the process by which diploid cells divide to produce four genetically unique haploid gametes.

• Homologous Pair: Two chromosomes (one from each parent) with the same genes but possibly different alleles.

• Allele: Different versions of a gene.

• Phases of Meiosis: Meiosis I (homologous chromosomes separate), Meiosis II (sister chromatids separate).

• Genetic Variation: Created by crossing over (prophase I) and independent assortment (metaphase I).

• Errors in Meiosis: Nondisjunction leads to gametes with abnormal chromosome numbers (e.g., Down syndrome).

Genetics

Key Terms

• Gene: A segment of DNA that encodes a functional product.

• Genome: The complete set of genetic material in an organism.

• Heredity: Transmission of traits from parents to offspring.

• Character: Observable heritable feature (e.g., flower color).

• Trait: Variant of a character (e.g., purple or white flowers).

• True-breeding: Organisms that produce offspring identical to themselves when self-pollinated.

• Hybrid: Offspring of parents with different traits.

• Heterozygous: Having two different alleles for a gene.

• Homozygous: Having two identical alleles for a gene.

• Dominant/Recessive: Dominant alleles mask the effect of recessive alleles.

• P, F1, F2 Generations: Parental, first filial, and second filial generations in genetic crosses.

• Phenotype: Observable traits.

• Genotype: Genetic makeup.

Gregor Mendel and His Experiments

• Gregor Mendel: The "father of genetics"; used pea plants to study inheritance patterns.

• Method: Cross-pollinated true-breeding plants and analyzed offspring over several generations.

Mendel’s Laws of Inheritance

• Law of Segregation: Each individual has two alleles for each gene, which segregate during gamete formation.

• Law of Independent Assortment: Genes for different traits assort independently during gamete formation.

• Law of Dominance: In heterozygotes, one allele may mask the expression of another.

Genetic Crosses

• Monohybrid Cross: Cross involving one gene (e.g., Aa x Aa).

• Dihybrid Cross: Cross involving two genes (e.g., AaBb x AaBb).

• Test Cross: Crossing an individual with a dominant phenotype to a homozygous recessive to determine genotype.

• Punnett Squares: Diagrammatic tool to predict genotypic and phenotypic ratios.

Complex Patterns of Inheritance

• Multiple Alleles: More than two alleles exist for some genes (e.g., ABO blood types).

• Sex-Linked Traits: Genes located on sex chromosomes (e.g., color blindness).

• Genetic Linkage: Genes located close together on the same chromosome tend to be inherited together (exception to independent assortment).

• Incomplete Dominance: Heterozygote shows intermediate phenotype (e.g., pink flowers from red and white parents).

• Codominance: Both alleles are fully expressed (e.g., AB blood type).

• Lethal Alleles: Some allele combinations are fatal (e.g., homozygous dominant for certain genes).

Blood Types and Rh Incompatibility

• ABO Blood Types: Determined by three alleles (IA, IB, i); exhibit codominance and multiple alleles.

• Rh Factor: Presence (+) or absence (−) of Rh antigen; incompatibility can cause hemolytic disease of the newborn.

Chromosomal Theory and Structural Rearrangements

• Chromosomal Theory of Inheritance: Genes are located on chromosomes, which segregate and assort independently during meiosis.

• Structural Rearrangements: Changes in chromosome structure, such as translocations (exchange of segments between chromosomes) and inversions (reversal of a chromosome segment), can affect gene expression and lead to genetic disorders.

Type of Inheritance	Key Features	Example
Complete Dominance	Dominant allele masks recessive	Purple vs. white pea flowers
Incomplete Dominance	Heterozygote is intermediate	Red, pink, white snapdragons
Codominance	Both alleles expressed	AB blood type
Sex-linked	Gene on X or Y chromosome	Color blindness
Multiple Alleles	More than two alleles in population	ABO blood group

Additional info: This guide expands on the study list by providing definitions, examples, and context for each topic, ensuring a comprehensive review for exam preparation.