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Biology study guides

Cells, genetics, ecology, and the systems of living things — answered with worked explanations and diagrams.

34 explained questions, each with the answer, a worked explanation, and something interactive to help it stick.

Which of the following correctly organizes these genetic terms in order from smallest to largest?From smallest to largest: nucleotide -> gene -> DNA molecule -> chromosome. A nucleotide is the single building block, a gene is a stretch of nucleotides, a DNA molecule contains many genes, and a chromosome is one long DNA molecule coiled with proteins.Read the guide
Which statement accurately describes the relationship between cytokinesis and mitosis?Cytokinesis follows mitosis: mitosis divides the nucleus (the genetic material) and cytokinesis divides the cytoplasm. Together they complete cell division, producing two genetically identical daughter cells. Cytokinesis usually begins in late mitosis and finishes after it.Read the guide
Which of the Following Does Not Occur During Mitosis?Synapsis and crossing over of homologous chromosomes do not occur during mitosis — those are unique to meiosis. DNA replication also does not occur during mitosis; it happens earlier, in the S phase of interphase. Mitosis itself only separates already-copied chromosomes into two identical nuclei.Read the guide
The Endosymbiotic Theory Helps to Explain the Origin of Which Structures?Mitochondria and chloroplasts. The endosymbiotic theory proposes these double-membrane organelles descended from free-living prokaryotes — aerobic bacteria and cyanobacteria — that were engulfed by an ancestral eukaryotic cell and became permanent internal symbionts rather than being digested.Read the guide
What is the role of a pioneer species in primary succession?Pioneer species create soil. Organisms like lichens and mosses colonize bare rock, secrete acids that weather it into particles, and add organic matter when they die — forming the first thin soil that allows grasses, shrubs, and trees to grow later.Read the guide
How Are Proteins and Nucleic Acids Related?Proteins and nucleic acids are related because nucleic acids (DNA and RNA) store and carry the genetic instructions that determine a protein's amino-acid sequence. Information flows DNA to RNA to protein, so DNA ultimately specifies which protein a cell builds.Read the guide
What is the most likely reason that horses and mountain goats have hooves?Because they share a common ancestor. Horses and mountain goats are both hoofed mammals (ungulates), and their hooves are homologous structures inherited from a shared ancestral foot, later adapted for their different terrains rather than evolving independently.Read the guide
Which three processes are methods of genetic recombination?In sexually reproducing eukaryotes, the three processes are crossing over, independent assortment, and fertilization. Together they shuffle and combine alleles to create genetically unique offspring. (In prokaryotes, the equivalent trio is transformation, transduction, and conjugation.)Read the guide
What is most likely to result immediately after a rainforest in Brazil is clear-cut?Secondary succession. Because clear-cutting leaves soil, roots, seeds, and some organisms intact, the community regrows through secondary succession rather than starting from bare rock, which would be primary succession.Read the guide
Which is the best definition of directional selection?Directional selection is a process by which one extreme of a trait is favored, shifting the population's trait distribution toward that extreme over successive generations. The average value of the trait moves in one direction as the favored phenotype becomes more common.Read the guide
Which structural characteristic is seen in RNA but not in DNA?RNA contains the nitrogenous base uracil (which replaces thymine) and the sugar ribose, which has a hydroxyl (–OH) group on its 2′ carbon. RNA is also typically single-stranded. DNA instead uses thymine, deoxyribose (no 2′ –OH), and is double-stranded.Read the guide
What is the end result of mitosis?The end result of mitosis is two genetically identical diploid daughter cells, each containing the same number of chromosomes as the original parent cell. Mitosis produces exact copies used for growth, repair, and asexual reproduction.Read the guide
Homologous pairs of chromosomes are lined up independently of other such pairs during _____.During metaphase I of meiosis. Homologous chromosome pairs (tetrads) line up at the metaphase plate, and each pair orients randomly and independently of every other pair. This independent assortment shuffles maternal and paternal chromosomes, creating genetic variation in the resulting gametes.Read the guide
Is It Advantageous for a Predator to Prey Exclusively on a Single Prey Species?False. Specializing on one prey species is generally risky: if that prey declines, the predator has no fallback and may starve. Dietary flexibility (generalist foraging) buffers against prey fluctuations, so a broad diet is usually more advantageous for survival.Read the guide
Which of These Is Not a Carcinogen?Fat is not a carcinogen. Among the usual options (fat, UV light, cigarette smoke, testosterone), fat does not directly damage DNA or trigger malignant change; it is linked to cancer only indirectly. The other three are established carcinogens or tumor promoters.Read the guide
Glucose + glucose → _____ by _____.Glucose + glucose → maltose by dehydration synthesis (also called condensation). Two glucose monosaccharides bond through an α-1,4 glycosidic linkage, and one water molecule is removed to form the disaccharide maltose.Read the guide
Which Processes Lead to Most Genetic Variation in Sexually Reproducing Organisms?Three processes generate most genetic variation in sexually reproducing organisms: crossing over (recombination) in prophase I, independent assortment of chromosomes in metaphase I, and random fertilization the random union of two genetically unique gametes.Read the guide
Which Statement Is Most Likely to Apply to a Cell That Has DNA Within Its Cytoplasm?The cell is a prokaryote, so the statement that applies is that it lacks a nucleus (and other membrane-bound organelles). Its single circular DNA floats free in the cytoplasm in a region called the nucleoid, unlike eukaryotes that enclose DNA in a nucleus.Read the guide
Which structure represents a component of the HRP cofactor?Heme is the answer. Horseradish peroxidase (HRP) uses a heme cofactor, an iron-containing porphyrin ring. The iron (Fe) at the center of that porphyrin is the redox-active site. Phosphate, ribose, and pyridoxal are not part of heme.Read the guide
Human eggs and sperm each contain 23 ______, which are composed of long strands of ______?Human eggs and sperm each contain 23 chromosomes, which are composed of long strands of DNA. Gametes are haploid (half the normal set), and each chromosome is a tightly coiled molecule of DNA wrapped around proteins.Read the guide
In Muscle Cells, Fermentation Produces _____?In muscle cells, fermentation produces lactic acid (lactate). When oxygen is scarce, cells reduce pyruvate to lactate, regenerating NAD+ so glycolysis can keep making a small amount of ATP anaerobically.Read the guide
Which Organelle Plays a Role in Intracellular Digestion?The lysosome. It is a membrane-bound sac filled with hydrolytic enzymes that break down proteins, lipids, carbohydrates, and nucleic acids, digesting food particles, invading microbes, and worn-out organelles inside the cell.Read the guide
Correctly Label the Following Parts of the Male Reproductive SystemThe main parts are the penis, scrotum, testis (testicle), epididymis, vas (ductus) deferens, seminal vesicle, prostate gland, ejaculatory duct, and urethra, with the bladder nearby. Testes make sperm and testosterone; ducts and glands transport and nourish the sperm.Read the guide
How Might Environmental Manipulation of a Crop Have Unexpected Consequences?An engineered or relocated crop may demand more resources — such as water — than the original, or become poorly adapted to local pests, diseases, or weather. These trade-offs can disrupt the local ecosystem, spread invasively, or reduce yields, producing unintended downstream harm.Read the guide
Select All the Significant Threats to WaterfowlThe significant threats to waterfowl are habitat loss (especially wetland drainage), pollution and environmental contaminants, and disease such as avian influenza and botulism. Regulated, well-managed hunting is NOT a significant threat because harvests are set sustainably.Read the guide
Short Segments of Newly Synthesized DNA Are Joined Into a Continuous Strand By What?Short segments of newly synthesized DNA are joined into a continuous strand by DNA ligase. On the lagging strand, DNA ligase seals the gaps between Okazaki fragments by forming a phosphodiester bond between the 3'-OH of one fragment and the 5'-phosphate of the next.Read the guide
Which of the Following Statements Best Describes the Stork?A stork is a large, long-legged, long-necked wading bird (family Ciconiidae) that feeds in wetlands and shallow water. It is NOT a true waterfowl like ducks, geese, or swans, and it lacks their webbed feet.Read the guide
Homologous chromosomes migrate to opposite poles during _____.Homologous chromosomes migrate to opposite poles during anaphase I of meiosis. The paired homologs separate and move apart while their sister chromatids remain joined; sister chromatids do not separate until anaphase II.Read the guide
Which of the Following Have a Spatulate Bill?Spoonbills and the Northern Shoveler (Spatula clypeata) have spatulate, spoon- or shovel-shaped bills. Among typical duck choices, the Northern Shoveler is the classic answer, using its broad flattened bill for tactile filter-feeding.Read the guide
Which of the Following Events Occurs During Transcription?During transcription, RNA polymerase synthesizes a complementary mRNA strand from a DNA template, reading 3'→5' and building RNA 5'→3' with uracil replacing thymine. In eukaryotes, the pre-mRNA is then processed and non-coding introns are removed by RNA splicing.Read the guide
Carbon Fixation Involves the Addition of Carbon Dioxide to What?Carbon fixation adds carbon dioxide to RuBP (ribulose-1,5-bisphosphate). In the Calvin cycle, the enzyme RuBisCO attaches CO2 to the five-carbon RuBP, forming an unstable six-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA).Read the guide
The Heart Is Actually One, Two, or Three Pumps?The heart is actually two pumps. The right side pumps deoxygenated blood to the lungs (the pulmonary circuit), while the left side pumps oxygenated blood to the rest of the body (the systemic circuit). Both sides beat simultaneously as one organ.Read the guide
How Do You Sort Nucleotide Building Blocks by Name and Classification?Sort the nitrogenous bases into two ring classes: purines (double-ring) are adenine and guanine; pyrimidines (single-ring) are cytosine, thymine (DNA only), and uracil (RNA only). Every nucleotide is a base plus a pentose sugar plus a phosphate.Read the guide
Which of the Following Compete for Space on Intertidal Rocks?Algae, barnacles, and mussels compete for space on intertidal rocks. All three are sessile organisms that must permanently attach to a hard surface to survive, so on the limited area of exposed rock they directly compete with one another for that attachment space.Read the guide