Midterm 3 Study Guide: Photosynthesis, DNA Technology, Cell Division, and Genetics

Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy, primarily in the form of carbohydrates. In eukaryotes, this process occurs in the chloroplasts, which contain specialized membranes and pigments to capture light energy.

• Chloroplast Structure: Chloroplasts have three membranes, creating three distinct spaces. Most photosynthetic reactions occur in the thylakoid membrane (light-capturing reactions) and the stroma (Calvin cycle).

• Photosynthetic Pigments: Located in the thylakoid membrane, these molecules absorb specific wavelengths of light. The color of a pigment is due to the wavelengths it does not absorb.

• Major Pigment Classes:

  • Chlorophylls: Absorb purple/blue and red light; appear green.

  • Carotenoids: Absorb blue/green light; appear yellow, orange, or red. They also act as antioxidants, neutralizing free radicals formed by high-energy light.

Stages of Photosynthesis

• Light-Capturing Reactions: Occur in the thylakoid membrane. Light energy is stored in ATP and NADPH.

• Calvin Cycle: Occurs in the stroma. Uses ATP and NADPH to build carbohydrates from CO2.

• Interdependence: Both stages are linked by ATP/ADP and NADPH/NADP+. If one stage stops, the other cannot continue for long.

Mechanism of Light Absorption and Energy Transfer

• Photosynthetic pigments absorb photons, exciting their electrons.

• Energy is transferred between pigment molecules by resonance (not by electron transfer) until it reaches the reaction center.

• At the reaction center, an electron is transferred to an electron carrier, converting light energy to chemical energy.

Photosystems and Electron Transport

• Photosystem II: Uses an electron transport chain to pump H+ across the thylakoid membrane, creating an electrochemical gradient used by ATP synthase to produce ATP (photophosphorylation). Electrons are replaced by splitting water, releasing O2.

• Photosystem I: Receives electrons from Photosystem II, excites them with light, and transfers them to NADP+ to form NADPH.

The Calvin Cycle

• Uses ATP and NADPH to convert CO2 into G3P (a 3-carbon sugar).

• G3P can be used to build sucrose (when photosynthesis is slow) or starch (when rapid).

• The enzyme rubisco fixes CO2 to a 5-carbon sugar, forming a 6-carbon intermediate that splits into two 3-carbon molecules.

• Some G3P exits the cycle; most is used to regenerate the 5-carbon sugar.

Photorespiration and Adaptations

• Photorespiration: Rubisco can fix O2 instead of CO2, wasting energy and releasing CO2. This is more common when leaf pores are closed to prevent water loss.

• C4 Plants: Separate initial CO2 fixation and the Calvin cycle into different cells (e.g., corn, sugar cane).

• CAM Plants: Fix CO2 at night and run the Calvin cycle during the day (e.g., cacti).

• C3 Plants: Do not have these separations; most plants are C3.

DNA Technology

Organization of Genetic Material

• Genome: All genetic material in a cell; in eukaryotes, divided among multiple chromosomes.

• Chromosome: A continuous double-stranded DNA molecule with associated proteins; contains many genes at specific loci.

• Gene: A DNA segment coding for a protein.

• Allele: Different versions of a gene.

• Humans have 23 pairs of chromosomes (22 autosomes, 1 pair of sex chromosomes).

Basic Tools of DNA Technology

• Restriction Enzymes: Cut DNA at specific sequences; some create "sticky ends" for recombination.

• PCR (Polymerase Chain Reaction): Amplifies specific DNA sequences using temperature cycles, primers, and DNA polymerase.

• Transgenic Organisms: Organisms with recombinant DNA from different sources; used in agriculture, medicine, and research.

• Plasmids: Circular DNA in bacteria used to introduce new genes.

CRISPR/Cas9 Gene Editing

• Based on a bacterial defense mechanism against viruses.

• Uses RNA to guide the Cas9 enzyme to a specific DNA sequence, which is then cut.

• If no homologous DNA is provided, repair often results in insertions/deletions (gene knockout).

• If homologous DNA is provided, new genes can be inserted (gene knock-in).

• Applications include gene drives (e.g., making mosquitoes unable to carry malaria) and gene therapy for diseases.

Gene Therapy

• Involves introducing healthy alleles into cells to correct genetic diseases.

• Most commonly uses viral vectors; CRISPR-based methods are emerging.

Mitosis and Cancer

Cell Division in Eukaryotes

• Mitosis: Produces two genetically identical cells; used for growth, repair, and asexual reproduction.

• Meiosis: Produces gametes with half the normal DNA content; increases genetic diversity.

• Before division, cells undergo interphase (growth and DNA replication).

• Diploid (2n): Two sets of chromosomes (e.g., human somatic cells).

• Haploid (n): One set of chromosomes (e.g., gametes).

Phases of Mitosis

1. Prophase: Chromosomes condense; spindle apparatus forms.

2. Prometaphase: Nuclear envelope dissolves; spindle fibers attach to kinetochores.

3. Metaphase: Chromosomes align at the cell's equator.

4. Anaphase: Sister chromatids separate and move to opposite poles.

5. Telophase: Nuclear envelopes reform; chromosomes decondense.

6. Cytokinesis: Cytoplasm divides, forming two separate cells (cleavage furrow in animals, cell plate in plants).

Control of the Cell Cycle

• Checkpoints ensure proper DNA replication and chromosome alignment.

• MPF (Maturation Promoting Factor): A cyclin-Cdk complex that triggers mitosis.

• Cell cycle is regulated by internal and external signals.

Cancer Biology

• Cancer Cells: Exhibit uncontrolled division and loss of differentiation.

• Metastasis: Spread of cancer to other body parts.

• Angiogenesis: Formation of new blood vessels to supply tumors.

• Genetic Causes:

  • Proto-oncogenes: Normally promote cell growth; mutations create oncogenes that drive cancer.

  • Tumor Suppressor Genes: Inhibit cell growth; loss of function can lead to cancer (e.g., p53).

  • Mutator Genes: Involved in DNA repair; mutations increase mutation rates.

• Multiple mutations are usually required for cancer to develop.

• The immune system can help eliminate cancer cells; immunotherapies are a new treatment approach.

Meiosis and Chromosomal Inheritance

Chromosome Number and Variation

• Polyploidy: More than two sets of chromosomes (e.g., 4n, 8n).

• Aneuploidy: Abnormal number of chromosomes (e.g., trisomy = 2n+1, monosomy = 2n-1).

Phases of Meiosis

• Meiosis I: Homologous chromosomes separate; cells become haploid.

• Meiosis II: Sister chromatids separate; similar to mitosis.

Sources of Genetic Diversity

• Independent Assortment: Random distribution of maternal and paternal chromosomes during meiosis I.

• Random Fertilization: Any sperm can fertilize any egg, increasing combinations.

• Crossing Over: Exchange of genetic material between homologous chromosomes at chiasmata during prophase I.

Consequences of Chromosomal Abnormalities

• Most aneuploidies result in embryonic death; some can be carried to term but often cause health problems.

• Nondisjunction: Failure of chromosomes to separate properly during meiosis, leading to aneuploidy.

• Mosaicism: Nondisjunction during mitosis in the embryo leads to some cells with abnormal chromosome numbers.

• Sex chromosome aneuploidies are generally less severe than autosomal ones.

• X Inactivation: In individuals with more than one X chromosome, all but one X is inactivated in each cell, leading to a mosaic pattern.

• Other species have different sex determination systems (e.g., ZW in birds, environmental sex determination in some reptiles).

Genetics Introduction

Basic Genetic Concepts

• Alleles: Different versions of a gene; each individual inherits two alleles per gene (one from each parent).

• Homozygous: Two identical alleles for a gene.

• Heterozygous: Two different alleles for a gene.

• Genotype: The genetic makeup of an individual.

• Phenotype: The observable traits of an individual.

• Dominant and Recessive Alleles: Dominant alleles mask the effect of recessive alleles in heterozygotes.

Mendelian Genetics

• Gregor Mendel's experiments with monohybrid crosses led to the discovery of dominant and recessive traits.

• The chromosomal theory of inheritance links gene inheritance to chromosome behavior during meiosis.

• Punnett Squares: Used to predict offspring genotypes and phenotypes based on parental genotypes.

• Principle of Segregation: Each gamete receives only one allele of each gene due to separation of homologous chromosomes during meiosis.

• Each offspring's genotype is independent of their siblings.

• Testcross: Crossing an individual of unknown genotype with a homozygous recessive individual to determine the unknown genotype.

Skills for Exam

• Interpret figures related to photosynthesis and predict outcomes of experimental changes (e.g., enzyme inhibition).

• Construct and interpret Punnett squares for single-gene traits with complete dominance.