Key Takeaways
- The Science section has 60 questions (55 scored) in 60 minutes — about 1 minute per question
- Biology makes up approximately 20 items (~33%) of the Science section — the largest subsection
- Cells are the basic unit of life; prokaryotic cells lack a nucleus, eukaryotic cells have a membrane-bound nucleus
- Key organelles: nucleus (DNA), mitochondria (energy/ATP), ribosomes (protein synthesis), cell membrane (selective barrier)
- Mitosis produces 2 identical diploid cells (growth/repair); meiosis produces 4 unique haploid cells (gametes)
- DNA is a double helix with complementary base pairs: Adenine-Thymine, Guanine-Cytosine
- Genetics: dominant alleles are expressed when present; recessive alleles are expressed only when homozygous (aa)
- Punnett squares predict offspring genotype and phenotype ratios from parent allele combinations
- Ecology topics include food chains, ecosystems, producers/consumers/decomposers, and symbiotic relationships
Biology Fundamentals
The Science section of the NLN NEX contains 60 questions in 60 minutes (55 scored, 5 pretest). Biology accounts for approximately 20 items (~33%) — the largest science subsection.
Cell Biology
Cells are the basic structural and functional units of life. All living organisms are made of cells.
Prokaryotic vs. Eukaryotic Cells
| Feature | Prokaryotic | Eukaryotic |
|---|---|---|
| Nucleus | No membrane-bound nucleus | Membrane-bound nucleus |
| DNA | Circular, in nucleoid region | Linear chromosomes in nucleus |
| Organelles | Few; no membrane-bound organelles | Many membrane-bound organelles |
| Size | Smaller (1-10 µm) | Larger (10-100 µm) |
| Examples | Bacteria, archaea | Animals, plants, fungi, protists |
| Reproduction | Binary fission | Mitosis/meiosis |
Key Eukaryotic Organelles
| Organelle | Function | Analogy |
|---|---|---|
| Nucleus | Contains DNA; controls cell activities | "Brain" of the cell |
| Cell membrane | Selectively permeable barrier; controls what enters/exits | "Gatekeeper" |
| Mitochondria | Cellular respiration; produces ATP (energy) | "Powerhouse" |
| Ribosomes | Protein synthesis | "Protein factories" |
| Endoplasmic Reticulum (ER) | Rough ER: protein processing; Smooth ER: lipid synthesis | "Assembly line" |
| Golgi apparatus | Packages and ships proteins | "Shipping department" |
| Lysosomes | Digestion of waste and foreign material | "Recycling center" |
| Cell wall | Rigid outer layer (plants, fungi only) | "Protective wall" |
| Chloroplasts | Photosynthesis (plants only) | "Solar panels" |
| Vacuole | Storage (large central vacuole in plants) | "Storage tank" |
Cell Membrane and Transport
| Transport Type | Description | Energy Required? | Example |
|---|---|---|---|
| Passive diffusion | Molecules move from high to low concentration | No | O₂ and CO₂ across membranes |
| Osmosis | Water moves from low to high solute concentration | No | Water entering/leaving cells |
| Facilitated diffusion | Molecules move through channel/carrier proteins | No | Glucose transport into cells |
| Active transport | Molecules move from low to high concentration (against gradient) | Yes (ATP) | Sodium-potassium pump |
| Endocytosis | Cell engulfs large particles | Yes | White blood cells engulfing bacteria |
| Exocytosis | Cell expels large particles | Yes | Neurotransmitter release |
DNA and Protein Synthesis
DNA (Deoxyribonucleic Acid) stores genetic information in the nucleus.
- DNA is a double helix with two complementary strands
- Base pairing rules: Adenine (A) pairs with Thymine (T); Guanine (G) pairs with Cytosine (C)
- A gene is a segment of DNA that codes for a specific protein
- DNA → RNA → Protein (the Central Dogma of molecular biology)
Key processes:
| Process | What Happens | Location |
|---|---|---|
| DNA Replication | DNA copies itself before cell division | Nucleus |
| Transcription | DNA template is used to make mRNA | Nucleus |
| Translation | mRNA is read by ribosomes to build proteins | Ribosomes (cytoplasm) |
Mitosis vs. Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Production of gametes (sex cells) |
| Starting cell | Diploid (2n) | Diploid (2n) |
| End result | 2 identical diploid cells | 4 unique haploid cells |
| Number of divisions | 1 | 2 (meiosis I + meiosis II) |
| Genetic variation | No (identical copies) | Yes (crossing over + independent assortment) |
| Where it occurs | All body (somatic) cells | Reproductive (germ) cells only |
Phases of Mitosis (PMAT):
- Prophase: Chromatin condenses into chromosomes; nuclear membrane dissolves
- Metaphase: Chromosomes line up at the cell's equator (middle)
- Anaphase: Sister chromatids are pulled apart to opposite poles
- Telophase: Nuclear membranes reform; chromosomes decondense; cytokinesis divides the cell
Genetics Basics
| Term | Definition |
|---|---|
| Gene | A segment of DNA that codes for a trait |
| Allele | A variant form of a gene (e.g., brown eye allele vs. blue eye allele) |
| Dominant | Allele expressed when at least one copy is present (represented by capital letter, e.g., B) |
| Recessive | Allele expressed only when two copies are present (represented by lowercase, e.g., b) |
| Homozygous | Two identical alleles (BB or bb) |
| Heterozygous | Two different alleles (Bb) |
| Genotype | The genetic makeup (BB, Bb, or bb) |
| Phenotype | The observable trait (brown eyes, blue eyes) |
Punnett Square Example: Parent 1: Bb (heterozygous brown eyes) Parent 2: Bb (heterozygous brown eyes)
| B | b | |
|---|---|---|
| B | BB | Bb |
| b | Bb | bb |
- Genotype ratio: 1 BB : 2 Bb : 1 bb
- Phenotype ratio: 3 brown : 1 blue (75% brown, 25% blue)
Which organelle is responsible for producing ATP, the cell's main energy currency?
In DNA base pairing, adenine (A) pairs with:
What is the primary difference between mitosis and meiosis?
If both parents are heterozygous (Bb) for a trait, what percentage of offspring are expected to show the recessive phenotype?
The process by which water moves across a selectively permeable membrane from an area of low solute concentration to high solute concentration is called _____.
Type your answer below
Arrange the phases of mitosis in the correct order.
Arrange the items in the correct order
Which type of cell transport requires ATP (energy)?
Which of the following are found in plant cells but NOT in animal cells? (Select all that apply)
Select all that apply