Photosynthesis, Cell Cycle, Meiosis, Mendelian Genetics, and Chromosomal Inheritance: Study Notes

Study Guide - Smart Notes

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Photosynthesis

Chloroplasts and Their Structure

Chloroplasts are the organelles where photosynthesis occurs in plants and algae. They contain internal membranes called thylakoids, which are stacked into grana, and are surrounded by a double membrane.

Thylakoid: Flattened sacs where the light reactions take place.
Grana: Stacks of thylakoids.
Stroma: Fluid-filled space surrounding the grana; site of the Calvin cycle.
Chlorophyll: Main pigment located in the thylakoid membranes.

Additional info: Chloroplasts also contain their own DNA and ribosomes, supporting the endosymbiotic theory.

Electromagnetic Spectrum and Pigments

The electromagnetic spectrum encompasses all wavelengths of light. Photosynthetic pigments absorb specific wavelengths to drive photosynthesis.

Chlorophyll a: Main pigment; absorbs violet-blue and red light, reflects green (hence plants appear green).
Accessory pigments: Chlorophyll b and carotenoids broaden the spectrum of absorbed light.
Absorption spectrum: Graph showing the wavelengths absorbed by pigments.
Action spectrum: Graph showing the effectiveness of different wavelengths in driving photosynthesis.

Example: The action spectrum of photosynthesis closely matches the absorption spectrum of chlorophylls.

Light Reactions

Light reactions convert solar energy to chemical energy (ATP and NADPH) and release O2 as a byproduct. These occur in the thylakoid membranes.

Linear electron flow: Involves both Photosystem II (PSII) and Photosystem I (PSI); produces ATP, NADPH, and O2.
Cyclic electron flow: Involves only PSI; produces ATP but not NADPH or O2.

Equation for the light reactions:

Calvin Cycle

The Calvin cycle uses ATP and NADPH from the light reactions to fix CO2 into organic molecules. It occurs in the stroma.

Enzyme rubisco: Catalyzes the first step; usual substrates are CO2 and RuBP (ribulose bisphosphate).
Inputs: ATP, NADPH, CO2.
Outputs: G3P (glyceraldehyde-3-phosphate), which can be used to form glucose and other carbohydrates.

Equation for the Calvin cycle:

Photorespiration

Photorespiration is a process where rubisco uses O2 instead of CO2, leading to the loss of fixed carbon and energy.

Substrates for rubisco: O2 and RuBP during photorespiration.
C3 plants: Use only the Calvin cycle; susceptible to photorespiration.
CAM plants: Fix CO2 at night to minimize photorespiration.

Example: Succulents like Crassula use CAM photosynthesis.

The Cell Cycle and Mitosis

Overview of the Cell Cycle

The cell cycle is the series of events that cells go through as they grow and divide. It consists of interphase and the mitotic phase.

Interphase: Period of cell growth and DNA replication; includes G1, S, and G2 phases.
Mitotic phase (M phase): Includes mitosis (nuclear division) and cytokinesis (cytoplasmic division).

Phases of Interphase

G1 phase: Cell grows and carries out normal functions.
S phase: DNA is replicated.
G2 phase: Cell prepares for mitosis.

Mitosis

Mitosis is the process by which a eukaryotic cell separates its duplicated chromosomes into two identical nuclei. It consists of five stages:

Prophase: Chromosomes condense, spindle forms.
Prometaphase: Nuclear envelope breaks down, spindle fibers attach to kinetochores.
Metaphase: Chromosomes align at the metaphase plate.
Anaphase: Sister chromatids separate and move to opposite poles.
Telophase: Nuclear envelopes reform, chromosomes decondense.

Cytokinesis: Division of the cytoplasm, forming two daughter cells.

Key Terms in Mitosis

Somatic cells: Body cells (not gametes); diploid.
Gametes: Reproductive cells (sperm and egg); haploid.
Sister chromatids: Identical copies of a chromosome, connected at the centromere.
Kinetochore: Protein structure on chromatids where spindle fibers attach.
Mitotic spindle: Structure made of microtubules that segregates chromosomes.
Centrosome: Microtubule-organizing center.
Aster: Radial array of microtubules around the centrosome.
Metaphase plate: Imaginary plane where chromosomes align during metaphase.
Cleavage (in animals): Process forming the cleavage furrow during cytokinesis.
Cell plate (in plants): Structure that forms during cytokinesis to divide plant cells.
Binary fission: Asexual reproduction in prokaryotes; simpler than mitosis.

Meiosis and Sexual Life Cycles

Asexual vs. Sexual Reproduction

Asexual reproduction: Offspring are genetically identical to the parent (clones).
Sexual reproduction: Offspring inherit a unique combination of genes from two parents.

Chromosome Terminology

Karyotype: Ordered display of an individual's chromosomes.
Homologous chromosomes: Chromosome pairs with the same genes but possibly different alleles.
Autosomes: Non-sex chromosomes.
Sex chromosomes: X and Y chromosomes; determine sex.
Diploid (2n): Two sets of chromosomes.
Haploid (n): One set of chromosomes.
Fertilization: Fusion of gametes to form a zygote.
Zygote: Fertilized egg; diploid.

Meiosis

Meiosis is a two-division process that reduces chromosome number by half, producing four haploid cells.

Meiosis I: Homologous chromosomes separate.
Meiosis II: Sister chromatids separate.
Synapsis: Pairing of homologous chromosomes during Prophase I.
Crossing over: Exchange of genetic material between homologs at chiasmata.

Origins of Genetic Variation

Independent assortment of chromosomes
Crossing over
Random fertilization

Mendel and the Gene Idea

Key Terminology

Character: Heritable feature (e.g., flower color).
Trait: Variant of a character (e.g., purple or white).
Allele: Alternative versions of a gene.
F1 hybrids: First filial generation offspring of the parental (P) generation.
P generation: Parental generation.
F1 generation: First generation of offspring.
F2 generation: Second generation of offspring.
Heterozygote: Individual with two different alleles for a gene.
Homozygote: Individual with two identical alleles for a gene.
Phenotype: Observable traits.
Genotype: Genetic makeup.
Dominant: Expressed allele in heterozygote.
Recessive: Masked allele in heterozygote.

Mendel's Laws

Law of segregation: Two alleles for a gene separate during gamete formation.
Law of independent assortment: Alleles of different genes assort independently during gamete formation.

Genetic Crosses and Punnett Squares

Monohybrid cross: Cross involving one gene.
Dihybrid cross: Cross involving two genes.
Punnett square: Diagram to predict genotype and phenotype ratios.

Multiplication Rule: Probability of two independent events = product of their probabilities.

Addition Rule: Probability of either of two mutually exclusive events = sum of their probabilities.

Extensions of Mendelian Genetics

Pleiotropy: One gene affects multiple traits.
Epistasis: One gene affects the expression of another gene.
Pedigree: Family tree showing inheritance patterns.
Codominance: Both alleles are expressed (e.g., AB blood type).
Incomplete dominance: Heterozygote phenotype is intermediate (e.g., pink snapdragons).
Recessively inherited disorders: Cystic fibrosis, sickle cell disease.
Dominantly inherited disorders: Huntington's disease, achondroplasia.

Chromosomal Basis of Inheritance

Linked Genes

Linked genes are located close together on the same chromosome and tend to be inherited together.

Autosomal vs. Sex-Linked Traits

Autosomal traits: Genes located on non-sex chromosomes.
Sex-linked traits: Genes located on sex chromosomes (often X-linked).

Example: Color blindness is an X-linked recessive trait.

Punnett Squares with X-Linked Genes

When predicting outcomes, remember that males (XY) have only one X chromosome, so recessive X-linked traits are more common in males.

Chromosomal Abnormalities

Nondisjunction: Failure of chromosomes to separate properly during meiosis, leading to aneuploidy (e.g., Down syndrome).
Polyploidy: More than two complete sets of chromosomes; common in plants.

Summary Table: Key Differences in Cell Division

Process	Purpose	Number of Divisions	Number of Daughter Cells	Genetic Identity
Mitosis	Growth, repair, asexual reproduction	1	2	Identical to parent
Meiosis	Sexual reproduction	2	4	Genetically unique

Additional info: Table summarizes the main differences between mitosis and meiosis for quick review.