Ace the Free Biology Practice Exam (2025)

What is the basic unit of life?

Cell

Organelle

Tissue

Molecule

Cells are the basic functional and structural units of all living organisms, capable of performing all life processes. Multicellular organisms are composed of many cells, while some organisms are unicellular. Understanding the cell theory is fundamental in biology. source

Which molecule carries genetic information?

Protein

Lipid

RNA

DNA

DNA stores and transmits genetic information in cells, directing the synthesis of proteins and RNA. It is composed of nucleotide sequences that encode genes. Although RNA also carries information for some viruses, DNA is the primary genetic material in most organisms. source

Which process produces energy in mitochondria?

Cellular respiration

Chemosynthesis

Fermentation

Photosynthesis

Cellular respiration in mitochondria uses oxygen to convert glucose into ATP, the main energy currency of the cell. The process includes the Krebs cycle and electron transport chain. Photosynthesis occurs in chloroplasts, while fermentation happens in the cytoplasm without oxygen. source

What class of biomolecules are enzymes?

Nucleic acids

Proteins

Carbohydrates

Lipids

Enzymes are proteins that act as biological catalysts, speeding up chemical reactions without being consumed. Their three-dimensional structure determines their specificity for substrates. Other classes of biomolecules serve different roles such as storage, structure, or genetic information. source

Which organelle is known as the powerhouse of the cell?

Ribosome

Chloroplast

Nucleus

Mitochondrion

Mitochondria generate most of the cell's ATP through oxidative phosphorylation, earning them the nickname 'powerhouse of the cell.' They have their own DNA and double membrane. Chloroplasts perform photosynthesis in plants and algae. source

Which organelle is the site of photosynthesis in plant cells?

Mitochondrion

Chloroplast

Endoplasmic reticulum

Golgi apparatus

Chloroplasts contain chlorophyll, which captures light energy to convert carbon dioxide and water into glucose and oxygen. They have internal thylakoid membranes where light-dependent reactions occur. Mitochondria, in contrast, generate ATP by respiration. source

What is the monomer unit of proteins?

Amino acid

Monosaccharide

Nucleotide

Fatty acid

Amino acids are the building blocks of proteins, linked by peptide bonds in a specific sequence to form polypeptides. The sequence of amino acids determines the protein's structure and function. Other monomers include nucleotides for nucleic acids and monosaccharides for carbohydrates. source

During which phase of the cell cycle does DNA replication occur?

M phase

S phase

G1 phase

G2 phase

The S phase (synthesis phase) is when the cell replicates its entire DNA, ensuring each daughter cell receives a complete genome. G1 and G2 are gap phases where the cell grows and checks for errors. M phase is mitosis, where cell division occurs. source

What is the name of the semiliquid material inside the cell in which organelles are suspended?

Lysosome

Peroxisome

Cytoskeleton

Cytoplasm

Cytoplasm includes the cytosol and all organelles between the cell membrane and the nucleus. It is the site of many metabolic pathways. The cytoskeleton is a network of fibers, while lysosomes and peroxisomes are organelles with specific functions. source

What type of bond holds the two strands of the DNA double helix together?

Covalent bonds

Peptide bonds

Ionic bonds

Hydrogen bonds

Hydrogen bonds form between complementary nitrogenous bases (adenine-thymine and cytosine-guanine), stabilizing the DNA double helix. Covalent bonds connect the sugar-phosphate backbone. Peptide bonds link amino acids in proteins. source

Which element is the primary building block of organic molecules?

Carbon

Oxygen

Nitrogen

Hydrogen

Carbon atoms can form four covalent bonds, allowing for diverse and complex organic structures. It is the backbone of carbohydrates, lipids, proteins, and nucleic acids. Other elements like hydrogen and oxygen also play roles but carbon is central. source

What is the process that splits water into oxygen, protons, and electrons during photosynthesis called?

Photolysis

Carbon fixation

Electron transport

Calvin cycle

Photolysis occurs in the light-dependent reactions of photosynthesis, where water molecules are split to release oxygen, electrons, and protons. This process replenishes electrons in photosystem II. The Calvin cycle is a light-independent process. source

Which class of macromolecules includes starch and cellulose?

Carbohydrates

Proteins

Lipids

Nucleic acids

Carbohydrates are composed of sugar units and include energy-storage molecules like starch and structural molecules like cellulose. Lipids are fats and oils, proteins are amino acid polymers, and nucleic acids carry genetic information. source

Which blood cells are primarily responsible for fighting infections?

White blood cells

Erythrocytes

Red blood cells

Platelets

White blood cells (leukocytes) defend the body against pathogens through various mechanisms including phagocytosis and antibody production. Red blood cells transport oxygen, and platelets are involved in clotting. Erythrocytes is another term for red blood cells. source

In the genetic code, how many nucleotide bases make up a codon?

Three

Four

Two

Five

Codons are triplets of mRNA nucleotides that correspond to specific amino acids during protein synthesis. The three-base structure allows for 64 possible codons. Two-base codons would not provide enough combinations. source

In osmosis, water moves across a semipermeable membrane from a region of

Higher solute concentration to equal solute concentration

Lower solute concentration to higher solute concentration

Equal solute concentration to lower solute concentration

Higher solute concentration to lower solute concentration

Osmosis is the diffusion of water across a semipermeable membrane from lower to higher solute concentration to equalize concentrations. This movement drives many physiological processes. Water potential differences govern the direction. source

Which human organ system secretes hormones that regulate growth, metabolism, and reproduction?

Endocrine system

Respiratory system

Circulatory system

Nervous system

The endocrine system consists of glands that secrete hormones directly into the bloodstream, affecting distant organs. It works in concert with the nervous system for homeostasis. The circulatory system transports these hormones. source

Which structure in eukaryotic cells separates the nucleus from the cytoplasm?

Golgi apparatus

Nuclear envelope

Plasma membrane

Endoplasmic reticulum

The nuclear envelope is a double membrane that encloses the nucleus, controlling transport of molecules via nuclear pores. The plasma membrane surrounds the entire cell. Endoplasmic reticulum is connected to the nuclear envelope internally. source

Which condition is required for a population to be in Hardy-Weinberg equilibrium regarding allele frequencies?

No mutations

High mutation rate

Frequent migration

Strong natural selection

Hardy-Weinberg equilibrium assumes no mutation, random mating, no gene flow, infinite population size, and no selection. These conditions maintain constant allele frequencies across generations. Real populations often deviate from these ideals. source

Which enzyme unwinds the DNA double helix during replication?

DNA polymerase

Helicase

Primase

Ligase

Helicase breaks hydrogen bonds between DNA strands, unwinding the double helix to allow replication. DNA polymerase synthesizes the new strand; primase lays down RNA primers; ligase joins Okazaki fragments. source

What is the end product of glycolysis?

Citrate

Acetyl-CoA

Lactate

Pyruvate

Glycolysis converts one glucose molecule into two pyruvate molecules, producing ATP and NADH. In the presence of oxygen, pyruvate enters the mitochondrion for further oxidation. Under anaerobic conditions it may be converted to lactate. source

What part of a phospholipid molecule is hydrophobic?

Phosphate head

Fatty acid tails

Glycerol backbone

Polar head

Phospholipids have hydrophilic (water-attracting) phosphate heads and hydrophobic (water-repelling) fatty acid tails. This amphipathic nature drives membrane bilayer formation. The glycerol backbone connects the head to the tails. source

Which type of RNA transports amino acids to the ribosome during protein synthesis?

tRNA

mRNA

rRNA

snRNA

Transfer RNA (tRNA) carries specific amino acids and matches its anticodon to the mRNA codon in the ribosome. Messenger RNA (mRNA) provides the template, and ribosomal RNA (rRNA) forms part of the ribosome structure. source

What term describes organisms that use light as their energy source?

Heterotrophs

Autotrophs

Phototrophs

Chemotrophs

Phototrophs use light energy to synthesize organic molecules. Autotrophs fix carbon dioxide, and heterotrophs consume organic compounds. Chemotrophs derive energy from chemical compounds. source

During mitosis, in which phase do sister chromatids separate?

Telophase

Anaphase

Prophase

Metaphase

Anaphase is characterized by the separation of sister chromatids, which are pulled toward opposite spindle poles. Metaphase aligns the chromatids at the equatorial plate. Telophase follows where nuclear membranes reform. source

What is the pH of pure water at 25°C?

14

1

7

Pure water ionizes to equal concentrations of H+ and OH? ions at 25°C, resulting in a neutral pH of 7. pH values below 7 are acidic, and above 7 are basic. Temperature affects the exact value slightly. source

Which vitamin can the human body synthesize with UV exposure to the skin?

Vitamin D

Vitamin B12

Vitamin K

Vitamin C

Ultraviolet light converts 7-dehydrocholesterol in the skin to vitamin D3, which is then activated in the liver and kidneys. Vitamins C, B12, and K cannot be synthesized in this way. Vitamin D is crucial for calcium metabolism. source

Which cellular process involves engulfing large particles or cells?

Exocytosis

Endocytosis

Phagocytosis

Pinocytosis

Phagocytosis is a form of endocytosis where cells extend membranes to engulf large particles or microbes. Pinocytosis involves uptake of fluids. Exocytosis releases materials outside the cell. source

Which pigment molecule captures light energy in plant photosynthesis?

Hemoglobin

Chlorophyll

Melanin

Keratin

Chlorophyll absorbs light primarily in the blue and red wavelengths to drive photosynthesis in chloroplasts. Melanin protects against UV radiation in animals. Hemoglobin carries oxygen in blood. source

The sequence of DNA bases determines the sequence of

Nucleotides in RNA

Sugars in carbohydrates

Fatty acids in lipids

Amino acids in proteins

Triplets of DNA bases (codons) specify amino acids in proteins during translation. This genetic code links nucleic acid sequences to protein sequences. Other biomolecules are not directly coded by DNA. source

Which organelle is responsible for modifying, sorting, and packaging proteins for secretion?

Ribosome

Lysosome

Endoplasmic reticulum

Golgi apparatus

Proteins synthesized in the rough endoplasmic reticulum are transported to the Golgi apparatus, where they undergo further modifications such as glycosylation, are sorted, and packaged into vesicles for secretion or delivery. Ribosomes synthesize proteins, and lysosomes degrade cellular waste. source

What describes the primary structure of a protein?

Alpha helix and beta sheet folding

Linear sequence of amino acids

Three-dimensional shape

Arrangement of subunits

The primary structure refers to the specific order of amino acids linked by peptide bonds in a polypeptide. Secondary and tertiary structures describe folding patterns. Quaternary structure involves multiple subunits. source

In PCR, what is the role of primers?

To define the start and end points of DNA amplification

To join Okazaki fragments

To unwind the DNA helix

To synthesize new DNA strands

Primers are short DNA oligonucleotides that anneal to complementary sequences on the template DNA, providing a free 3?-OH for DNA polymerase to extend. They determine the region to be amplified. DNA polymerase synthesizes new strands, and helicase unwinds DNA. source

Which Mendelian principle describes the separation of allele pairs during gamete formation?

Law of Dominance

Law of Independent Assortment

Law of Segregation

Law of Uniformity

Mendel's Law of Segregation states that allele pairs separate during gamete formation, ensuring each gamete carries only one allele for each gene. Independent Assortment refers to different genes sorting independently. source

What term describes when one gene masks the expression of another gene at a different locus?

Epistasis

Incomplete dominance

Pleiotropy

Codominance

Epistasis occurs when the presence of specific alleles at one locus affects the expression of alleles at a different locus. Pleiotropy is when one gene influences multiple traits. source

Which stage of cellular respiration generates the most ATP molecules?

Glycolysis

Krebs cycle

Fermentation

Electron transport chain

The electron transport chain produces the majority of ATP via oxidative phosphorylation, using the proton gradient generated by the Krebs cycle and electron carriers. Glycolysis and the Krebs cycle generate far fewer ATP directly. Fermentation does not produce additional ATP beyond glycolysis. source

What is the primary function of LDL in the body?

Synthesize cholesterol

Remove cholesterol from the bloodstream

Transport cholesterol to peripheral tissues

Break down dietary fats

Low-density lipoprotein (LDL) carries cholesterol from the liver to peripheral tissues. High levels of LDL can lead to plaque formation in arteries. High-density lipoprotein (HDL) removes cholesterol from tissues. source

Which DNA repair mechanism removes thymine dimers caused by UV radiation?

Mismatch repair

Homologous recombination

Base excision repair

Nucleotide excision repair

Nucleotide excision repair recognizes and removes bulky DNA lesions like thymine dimers, replacing them with correct nucleotides. Base excision repair corrects small, non-helix-distorting lesions, and mismatch repair fixes replication errors. source

What type of receptor has intrinsic tyrosine kinase activity?

Ligand-gated ion channel

G-protein-coupled receptor

Enzyme-linked receptor

Nuclear receptor

Enzyme-linked receptors, such as receptor tyrosine kinases, dimerize and autophosphorylate tyrosine residues upon ligand binding, initiating intracellular signaling cascades. GPCRs use G-proteins, ion channels alter ion flow, and nuclear receptors regulate gene expression. source

Which process during meiosis enhances genetic diversity by exchanging DNA between homologous chromosomes?

Binary fission

Transcription

Crossing over

Translation

Crossing over occurs during prophase I of meiosis when homologous chromosomes exchange segments, creating recombinant chromosomes and increasing genetic diversity. Binary fission is prokaryotic cell division, and transcription and translation are molecular processes. source

What is the role of the sinoatrial node in the heart?

It acts as the heart's natural pacemaker

It oxygenates blood

It filters blood returning from the body

It prevents backflow of blood

The sinoatrial (SA) node generates electrical impulses that initiate each heartbeat and set the heart rate. It is located in the right atrium. Valves prevent backflow, and the lungs oxygenate blood. source

Which human blood type is considered the universal donor?

O negative

A positive

B negative

AB positive

O negative blood lacks A, B, and Rh antigens, minimizing the risk of an immune reaction in most recipients. AB positive is the universal recipient. Other types can cause agglutination if mismatched. source

What is the main buffering system in human blood?

Sulfate buffer system

Bicarbonate buffer system

Protein buffer system

Phosphate buffer system

The bicarbonate buffer system (HCO3?/CO2) maintains blood pH by converting strong acids and bases into weak equivalents, regulated by the lungs and kidneys. Phosphate and proteins contribute but play smaller roles. source

Which checkpoint ensures that all chromosomes are properly attached to the spindle before anaphase?

Metaphase checkpoint

G2/M checkpoint

Spindle checkpoint

G1/S checkpoint

The spindle checkpoint, also called the metaphase checkpoint, verifies that all kinetochores are attached to spindle microtubules before allowing anaphase to proceed. G1/S checks DNA damage, G2/M verifies DNA replication, and metaphase is synonymous with the spindle checkpoint. source

What term describes individuals with two identical alleles for a gene?

Heterozygous

Polyploid

Hemizygous

Homozygous

Homozygous individuals carry two identical alleles for a particular gene, whereas heterozygous individuals carry two different alleles. Hemizygous refers to a single allele in diploids, and polyploid organisms have multiple sets of chromosomes. source

What is gene conversion in genetics?

Crossing over during meiosis

Equal exchange of chromosome segments

Nonreciprocal transfer of genetic information between homologous sequences

Mutation of a single base pair

Gene conversion is a process where information is unidirectionally transferred from one DNA helix (donor) to another (acceptor), altering the sequence without reciprocal exchange. It often occurs during homologous recombination. It differs from standard crossing over. source

How does alternative splicing contribute to protein diversity?

By changing the start codon

By adding extra introns

By removing different combinations of exons from pre-mRNA

By modifying DNA methylation

Alternative splicing allows a single gene to produce multiple mRNA variants by including or excluding certain exons during processing. This expands the proteome without increasing gene number. Introns are removed, not added. source

What is the primary function of telomerase?

To shorten telomeres

To unwind DNA during replication

To repair mismatched bases

To extend telomeric DNA sequences at chromosome ends

Telomerase adds repetitive nucleotide sequences to the ends of chromosomes, counteracting shortening during DNA replication. It contains an RNA template and reverse transcriptase activity. It does not function in base repair or unwinding. source

Which kinetic model describes enzyme activity that exhibits cooperativity?

Michaelis-Menten equation

Arrhenius equation

Lineweaver-Burk plot

Hill equation

The Hill equation models cooperative binding, where substrate binding to one active site affects binding at other sites, resulting in a sigmoidal curve. Michaelis-Menten assumes independent sites. Lineweaver-Burk is a linear transformation of Michaelis-Menten. source

What is the founder effect in population genetics?

Gene flow between adjacent populations

Natural selection favoring colonizers

Reduced genetic diversity in a new population founded by a few individuals

Increased mutation rate in large populations

The founder effect occurs when a small group of individuals establishes a new population, carrying only a subset of the genetic variation. This can lead to reduced diversity and unique allele frequencies. It is not directly related to mutation rate or selection. source

Which mechanism mediates RNA interference (RNAi) to regulate gene expression?

tRNA charging

DNA methylation

siRNA-guided mRNA degradation

rRNA processing

Small interfering RNAs (siRNAs) guide the RNA-induced silencing complex (RISC) to complementary mRNA, leading to its cleavage and degradation, effectively silencing the gene. DNA methylation and rRNA processing are different regulatory mechanisms. source