Molecular Biology of the Cell · Part 1 of 2 · 10 MCQs per part
Part 1 of 2
From Cell Theory to Eukaryotic Origins
Every organism alive today — whether a soil bacterium or a blue whale — shares the same fundamental biochemistry, the same genetic code, and the same cellular machinery. Understanding why begins with comparing the two great domains of cellular life.
1.1 All Cells Share Common Features
The cell theory, established in the 19th century, holds that all living organisms are composed of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells. Despite the enormous variety of life on Earth, all cells share a set of fundamental features: they are enclosed by a plasma membrane, they carry genetic information in the form of DNA, they use ribosomes to synthesize proteins, and they use ATP as the universal energy currency.
Key term
Cell theory
The principle that all living things are made of cells, that the cell is the fundamental unit of life, and that all cells come from pre-existing cells.
All cells are also bounded by a lipid bilayer membrane, use the same twenty amino acids to build proteins, and transcribe DNA into RNA before translating that RNA into protein. This universality of molecular biology strongly supports the idea that all life on Earth descended from a common ancestor.
1.2 Prokaryotic Cells
Prokaryotes — the Bacteria and Archaea — are unicellular organisms that lack a membrane-enclosed nucleus. Their DNA typically exists as a single circular chromosome located in the nucleoid region of the cell. Prokaryotes also lack membrane-bound organelles, although they do possess ribosomes (70S type) and sometimes elaborate internal membrane systems.
Key term
Prokaryote
An organism whose cells lack a membrane-enclosed nucleus and other membrane-bound organelles; includes Bacteria and Archaea.
Bacteria are astonishingly diverse and metabolically versatile. They can be found in nearly every habitat on Earth, from hydrothermal vents to the human gut. Many play essential ecological roles in nutrient cycling, while others cause infectious diseases. Archaea, once thought to be exotic extremophiles, are now known to be ubiquitous in marine and soil environments.
1.3 Model Organisms in Molecular Biology
Much of what we know about fundamental cellular processes comes from studying model organisms — species chosen for their experimental advantages. The bacterium Escherichia coli (E. coli) grows rapidly, is genetically tractable, and has been the workhorse of molecular biology for decades. The budding yeast Saccharomyces cerevisiae is the premier unicellular eukaryotic model. Other key models include Drosophila melanogaster (fruit fly), Caenorhabditis elegans (nematode), Danio rerio (zebrafish), Mus musculus (mouse), and Arabidopsis thaliana (thale cress).
These organisms share key features: short generation times, small genomes, well-developed genetic tools, and large research communities. Because of the deep evolutionary conservation of cellular mechanisms, discoveries in model organisms frequently reveal universal principles applicable to human biology and disease.
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Pause & Recall
Why does the universality of the genetic code and ribosome structure support a common ancestor for all life?
If life had originated independently multiple times, different organisms might use different codons or different amino acid alphabets. The fact that all organisms use the same 64-codon table and the same 20 amino acids is far more likely if all descended from one ancestral cell that fixed this system.
1.4 Evolution: Unity and Diversity
Charles Darwin's theory of evolution by natural selection explains both the unity and the diversity of life. Unity arises because all organisms share common ancestors; diversity arises because populations accumulate genetic changes over time, and natural selection favors variants that reproduce more successfully in their environments.
At the molecular level, evolution is visible in DNA sequence comparisons. Genes that perform essential, conserved functions — such as those encoding ribosomal RNA or histones — change very slowly over evolutionary time because mutations in them are almost always harmful. By comparing gene sequences across species, biologists can reconstruct phylogenetic trees that depict evolutionary relationships.
Practice Questions — Part 1Score: 0 / 10
1. Which of the following is a feature shared by ALL cells?
All cells — prokaryotic and eukaryotic — possess ribosomes to translate mRNA into protein. Nuclei, chloroplasts, and flagella are absent in many cell types.
2. Prokaryotic cells differ from eukaryotic cells primarily in that prokaryotes:
The defining feature of prokaryotes is the absence of a membrane-enclosed nucleus. Their DNA resides in a nucleoid region but is not surrounded by a nuclear envelope.
3. The nucleoid region of a prokaryotic cell contains:
The nucleoid is the region in prokaryotes where the single circular chromosome is located. It is not enclosed by a membrane, and prokaryotic DNA is generally not wrapped around histones (though archaea use histone-like proteins).
4. Which of the following is the best example of a model organism used to study basic eukaryotic cell biology?
Saccharomyces cerevisiae (budding yeast) is the premier unicellular eukaryotic model organism. The others are prokaryotes (bacteria or archaea).
5. Which statement best explains why discoveries in model organisms often apply to humans?
Because all eukaryotes share a common ancestor, many core cellular processes (DNA replication, transcription, translation, cell division) are highly conserved. Insights from yeast or flies frequently reveal mechanisms operating in human cells.
6. According to the cell theory, which of the following is correct?
The cell theory states: (1) all organisms are made of cells, (2) the cell is the basic unit of life, and (3) all cells arise from pre-existing cells — refuting spontaneous generation.
7. Which molecule serves as the universal energy currency in all living cells?
ATP (adenosine triphosphate) is the universal energy currency used by all cells to power biochemical reactions. While NADH and GTP also carry energy, ATP is the primary currency.
8. Natural selection acts on:
Natural selection requires heritable variation. Variants that reproduce more successfully leave more offspring, and their heritable traits become more common over generations. This applies to all living organisms, unicellular or multicellular.
9. Which of the following gene families changes most slowly over evolutionary time, and why?
Ribosomal RNA is essential for protein synthesis in all organisms. Almost any mutation in its sequence disrupts ribosome function, so mutations are strongly selected against, making rRNA genes among the most highly conserved in all of life.
10. The two prokaryotic domains are:
Life is divided into three domains: Bacteria, Archaea, and Eukarya. Bacteria and Archaea are both prokaryotic (lacking a nucleus), while Eukarya includes all organisms with a true nucleus.
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Section B · Recall Questions · Part 1
Type your answer, then click Check to reveal the sample answer.
B1
State the three tenets of cell theory.
Sample answer: (1) All living organisms are composed of cells. (2) The cell is the basic structural and functional unit of life. (3) All cells arise from pre-existing cells.
B2
What structural feature most fundamentally distinguishes prokaryotic cells from eukaryotic cells?
Sample answer: Prokaryotes lack a membrane-enclosed nucleus; their DNA resides in the nucleoid region without a surrounding nuclear envelope.
B3
What molecule serves as the universal energy currency in cells, and what does this universality tell us about the history of life?
Sample answer: ATP is the universal energy currency. Its use by all cells suggests that this molecule was adopted by the last universal common ancestor (LUCA), supporting the single-origin hypothesis for life.
B4
Name two model organisms used in molecular biology and state one experimental advantage of each.
Sample answer: E. coli: short generation time (~20 min), easy genetic manipulation. S. cerevisiae (budding yeast): eukaryote with powerful genetics, easy to culture, shares many genes with humans.
B5
Why are ribosomal RNA genes used to reconstruct the evolutionary relationships (phylogeny) between organisms?
Sample answer: Ribosomal RNA genes are highly conserved across all life because mutations are almost always lethal. Their slow rate of change makes them informative molecular clocks for comparing even distantly related organisms.
B6
Name the three domains of life and indicate which are prokaryotic.
Sample answer: The three domains are Bacteria, Archaea, and Eukarya. Bacteria and Archaea are both prokaryotic (lacking a nucleus); Eukarya are eukaryotic.
B7
What is the plasma membrane and what is its basic molecular composition?
Sample answer: The plasma membrane is the lipid bilayer that encloses every cell, separating the interior from the external environment. It is composed primarily of phospholipids arranged in a bilayer, with embedded proteins.
B8
Briefly explain the mechanism of natural selection and its three requirements.
Sample answer: Natural selection requires: (1) variation among individuals, (2) that the variation is heritable (passed to offspring), and (3) that the variation affects reproductive success (fitness). Individuals with favorable variants produce more offspring, shifting the population over generations.
B9
Define "genome" as used in molecular biology.
Sample answer: The genome is the complete set of genetic information (DNA) of an organism, including all of its genes and non-coding sequences. In most organisms this is encoded in DNA; in some viruses it is RNA.
B10
What does "evolutionary conservation" of a gene mean, and what does it imply about the gene's function?
Sample answer: A gene is evolutionarily conserved if its sequence has changed little across divergent species. This implies the gene performs an essential function; mutations are strongly selected against because they disrupt a process critical for survival or reproduction.
Section C · Critical Thinking · Part 1
Develop analytical responses, then compare with the sample.
C1
Viruses are not considered cells and are not covered by cell theory. Does this mean they fall outside evolutionary biology? Explain your reasoning.
Sample answer: No. Viruses carry heritable genetic information (DNA or RNA), and their sequences evolve by mutation and natural selection. They are subject to evolutionary processes even though they are not cellular and cannot replicate independently. Phylogenetic analysis of viral genes is routinely used to track viral evolution.
C2
What are the limitations of using model organisms to understand human biology, and how might these limitations be addressed?
Sample answer: Model organisms may not replicate human-specific features (e.g., complex brain architecture, immune nuances). Drug targets found in yeast may behave differently in human tissues. These limitations can be addressed by using multiple models, human cell culture systems, organoids, and careful validation in human data before clinical translation.
C3
If two species share 95% DNA sequence identity in a conserved gene, what can you infer about their evolutionary relationship compared to two species that share only 60% identity in the same gene?
Sample answer: Higher sequence identity indicates more recent common ancestry and less evolutionary divergence. The 95%-identical pair diverged more recently, giving less time for mutations to accumulate. The 60%-identical pair diverged earlier, accumulating more sequence differences. This reasoning is the basis for using molecular sequences to build phylogenetic trees.
C4
How did Pasteur's swan-neck flask experiment support cell theory, and why was this experiment difficult to refute?
Sample answer: Pasteur showed that boiled broth remained sterile in an intact swan-neck flask (which allowed air but trapped microbes) but became cloudy when the neck was broken. This demonstrated that microbes come from the air — not from spontaneous generation — supporting the tenet that all cells arise from existing cells. The elegance of the control (same flask, air access, but different microbial entry) made it hard to explain away.
C5
Even though both Bacteria and Archaea are prokaryotes, they are placed in separate domains. What molecular evidence justifies this classification?
Sample answer: rRNA sequence analysis by Carl Woese revealed that Archaea are as distinct from Bacteria as either is from Eukarya. Additional differences include: archaea use ether-linked lipids in their membranes (bacteria use ester-linked), archaea use histone-like proteins, and their RNA polymerases more closely resemble eukaryotic enzymes than bacterial ones.
Section D · Interactive Questions · Part 1
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D1
What is the name of the region in a prokaryotic cell where the chromosome is located? (one word)
D2
What molecule is the universal energy currency of all cells? (abbreviation)
D3
Which model organism is the most commonly used prokaryote in molecular biology? (genus and species, e.g. "Homo sapiens")
D4
How many domains of life are recognized in the modern three-domain system? (number)
D5
Do prokaryotes have membrane-bound organelles? (yes or no)
Part 2 →
Having established the basic distinction between prokaryotes and eukaryotes and the principles of evolutionary unity, we now turn to eukaryotic cell structure, the origin of eukaryotes through endosymbiosis, and the enormous diversity of genomes across life.
Part 2 of 2
Eukaryotic Cell Structure, Endosymbiosis, and Genome Diversity
1.5 The Eukaryotic Cell
Eukaryotic cells are far more structurally complex than prokaryotes. They possess a nucleus — a membrane-enclosed compartment containing the cell's chromosomes — as well as numerous other membrane-bound organelles. The endoplasmic reticulum (ER) is a network of membranes involved in protein and lipid synthesis. The Golgi apparatus modifies, sorts, and packages proteins for secretion or delivery to other organelles. Lysosomes contain digestive enzymes for breaking down cellular debris and foreign material.
Key term
Nucleus
The membrane-enclosed organelle in eukaryotic cells that houses the cell's chromosomal DNA and is the site of transcription.
Mitochondria are the sites of aerobic respiration, generating most of the cell's ATP. Plant cells and algae also contain chloroplasts, which perform photosynthesis. Both mitochondria and chloroplasts have their own DNA and ribosomes — a key clue to their origin.
1.6 The Endosymbiotic Theory
The endosymbiotic theory, championed by Lynn Margulis, proposes that mitochondria and chloroplasts evolved from free-living prokaryotes that were engulfed by a host cell but not digested. Over evolutionary time, the engulfed prokaryotes became permanently integrated as organelles, transferring most of their genes to the host nucleus.
Key term
Endosymbiotic theory
The hypothesis that mitochondria and chloroplasts originated as free-living bacteria that were engulfed by ancestral eukaryotic cells and became permanent intracellular symbionts.
Evidence supporting endosymbiosis includes: mitochondria and chloroplasts are similar in size to bacteria; they replicate by binary fission; their ribosomes (70S) resemble bacterial ribosomes rather than eukaryotic cytoplasmic ribosomes (80S); and their genome organization resembles bacterial chromosomes. Phylogenetic analysis places mitochondria closest to alpha-proteobacteria and chloroplasts closest to cyanobacteria.
1.7 Genome Diversity and Size
Genome size varies enormously across life. The E. coli genome is approximately 4.6 million base pairs (Mb) encoding about 4,300 genes. The human genome is roughly 3,200 Mb (3.2 Gb) but encodes only ~20,000–25,000 protein-coding genes — far fewer than expected given the genome's size. This discrepancy reflects the large amount of non-coding DNA in the human genome, including regulatory sequences, introns, and repetitive elements.
The relationship between genome size and organismal complexity is not straightforward — this is called the C-value paradox. Some salamanders and lilies have genomes many times larger than the human genome, yet are far less complex by most measures. Much of the extra DNA in large genomes consists of transposable elements and other repetitive sequences.
1.8 Genotype and Phenotype
The genotype is the complete genetic constitution of an organism — the sequence of bases in its DNA. The phenotype is the physical and biochemical characteristics of an organism, resulting from the interaction of its genotype with the environment. The same genotype can produce different phenotypes in different environments (phenotypic plasticity), and conversely, the same phenotype can sometimes arise from different genotypes.
Practice Questions — Part 2Score: 0 / 10
1. Which organelle is the site of aerobic respiration and ATP production in eukaryotic cells?
Mitochondria are the sites of aerobic respiration (oxidative phosphorylation), producing the majority of the cell's ATP via the electron transport chain and ATP synthase.
2. Which of the following is NOT evidence supporting the endosymbiotic theory?
Mitochondria retain their own genome (though most genes have transferred to the nucleus over time). The fact that they still possess their own DNA is evidence FOR endosymbiosis — if all genes had moved to the nucleus, this evidence would be lost. Option D is incorrect as a piece of supporting evidence.
3. The prokaryotic ancestor of mitochondria is thought to be most closely related to:
Phylogenetic analysis places the mitochondrial ancestor within the alpha-proteobacteria (e.g., Rickettsia). Chloroplasts, by contrast, are most closely related to cyanobacteria.
4. The C-value paradox refers to the observation that:
The C-value paradox notes that genome size (C-value) does not correlate with complexity. Some salamanders and plants have genomes many times larger than humans, largely due to repetitive elements and transposable DNA rather than more protein-coding genes.
5. Which of the following organelles is found in plant cells but NOT in typical animal cells?
Chloroplasts are the organelles of photosynthesis; they are found in plant cells and algae but not in animal cells. All eukaryotic cells (plant and animal) have mitochondria, ER, and ribosomes.
6. The term "phenotype" refers to:
The phenotype encompasses all observable characteristics of an organism — morphology, physiology, behavior — that result from the interaction of its genotype with the environment. The set of all proteins is the proteome, not the phenotype.
7. The Golgi apparatus is primarily responsible for:
The Golgi apparatus receives proteins from the ER, modifies them (e.g., glycosylation), sorts them, and packages them into vesicles for secretion outside the cell or delivery to lysosomes and other organelles.
8. Approximately how many protein-coding genes does the human genome contain?
The human genome encodes approximately 20,000–25,000 protein-coding genes. ~4,300 is the gene count for E. coli. The ~3 billion figure refers to the total base pairs in the haploid human genome, not genes.
9. Lynn Margulis is primarily known for:
Lynn Margulis is celebrated for her vigorous and ultimately successful advocacy for the endosymbiotic theory, proposing that mitochondria and chloroplasts evolved from engulfed bacteria. Carl Woese is known for the three-domain system; Watson, Crick, Franklin, and Wilkins for DNA structure.
10. What does the term "genotype" refer to?
The genotype is the genetic makeup of an organism — the specific sequence of bases in its DNA. The phenotype is what we observe. The proteome is the set of proteins expressed. Chromosome number is the karyotype.
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Section B · Recall Questions · Part 2
Type your answer, then click Check to reveal the sample answer.
B1
Summarize the endosymbiotic theory of mitochondrial origin.
Sample answer: Mitochondria evolved from free-living alpha-proteobacteria that were engulfed by an ancestral eukaryotic host cell. Over time, most bacterial genes transferred to the host nucleus, and the bacterium became permanently integrated as the mitochondrion.
B2
List three lines of evidence supporting the endosymbiotic origin of mitochondria.
Sample answer: (1) Mitochondria contain their own circular DNA resembling bacterial chromosomes. (2) Mitochondrial ribosomes are 70S (bacterial type), not 80S. (3) Mitochondria replicate by binary fission independently of cell division.
B3
What type of bacterium is thought to be the ancestor of chloroplasts, and what evidence supports this?
Sample answer: Cyanobacteria are the ancestors of chloroplasts. Both perform oxygenic photosynthesis, have similar pigment systems and thylakoid membranes, and phylogenetic analysis of chloroplast rRNA genes places them within cyanobacteria.
B4
Explain the C-value paradox and what accounts for unexpectedly large genomes in some organisms.
Sample answer: The C-value paradox is the lack of correlation between genome size and organismal complexity. Large genomes in organisms like salamanders often result from accumulation of transposable elements and other repetitive sequences, not from having more protein-coding genes.
B5
What is the nucleus and what key processes occur there?
Sample answer: The nucleus is the membrane-enclosed organelle in eukaryotes that houses the chromosomal DNA. Key processes include DNA replication, transcription (synthesis of RNA from DNA), and RNA processing.
B6
Distinguish between genotype and phenotype, and give an example of how the same genotype can produce different phenotypes.
Sample answer: Genotype is the DNA sequence; phenotype is the observable characteristics. Example: genetically identical twins raised in different environments may differ in height or susceptibility to disease — phenotypic plasticity.
B7
What are the two types of endoplasmic reticulum and what is the primary function of each?
Sample answer: Rough ER has ribosomes on its surface and synthesizes membrane and secretory proteins. Smooth ER lacks ribosomes and is involved in lipid synthesis, detoxification, and calcium storage.
B8
What is the function of lysosomes in animal cells?
Sample answer: Lysosomes are membrane-bound organelles containing hydrolytic (digestive) enzymes. They degrade macromolecules from endocytosis, worn-out organelles (autophagy), and foreign particles, recycling the components for reuse.
B9
What are transposable elements and how do they contribute to genome size?
Sample answer: Transposable elements are DNA sequences that can move (transpose) to new locations within the genome. They often replicate as they transpose, increasing their copy number. In large eukaryotic genomes like humans, they make up more than 45% of the genome and are a major cause of genome size variation.
B10
How do eukaryotic cytoplasmic ribosomes differ from prokaryotic ribosomes in terms of sedimentation coefficient?
Sample answer: Eukaryotic cytoplasmic ribosomes are 80S (composed of 60S and 40S subunits), while prokaryotic ribosomes are 70S (50S and 30S subunits). Mitochondrial ribosomes in eukaryotes are also 70S, reflecting their bacterial ancestry.
Section C · Critical Thinking · Part 2
Develop analytical responses, then compare with the sample.
C1
If the endosymbiotic theory is correct, why do mitochondria still retain a genome rather than transferring all genes to the nucleus?
Sample answer: Several hypotheses exist: some mitochondrial proteins are too hydrophobic to be imported through the inner membrane after cytoplasmic synthesis; local gene expression allows faster response to local energy demands; and some sequences may have regulatory roles. The retained genes encode proteins central to the respiratory chain, where immediate local control may be advantageous.
C2
A student argues that humans must have the most genes of any organism because we are the most complex. How would you refute this argument using specific examples?
Sample answer: Gene number does not correlate with complexity. Humans have ~20,000–25,000 genes, but the water flea Daphnia pulex has ~31,000 genes. Rice (Oryza sativa) has more protein-coding genes than humans. Complexity in animals is achieved partly through alternative splicing, post-translational modification, and regulatory networks rather than simply having more genes.
C3
Explain how phenotypic plasticity challenges a simple genotype-determines-phenotype model.
Sample answer: Phenotypic plasticity — the ability of one genotype to produce different phenotypes in response to environmental conditions — shows that phenotype is not solely determined by DNA sequence. For example, identical twins share the same genotype but can develop different diseases due to different environmental exposures and epigenetic changes. This means understanding biology requires studying environment-gene interactions, not just DNA sequences alone.
C4
Recent genomic evidence suggests that eukaryotes evolved from within the Archaea (specifically Asgard archaea). How does this change the classical three-domain tree of life?
Sample answer: The classical three-domain model places Bacteria, Archaea, and Eukarya as separate lineages diverging from LUCA. If eukaryotes evolved from within Asgard archaea, then Eukarya would be a branch of Archaea, making the tree a two-domain model (Bacteria + Archaea, with Eukarya nested inside Archaea). This would mean eukaryotes are not an independent domain but an archaeal lineage that acquired bacterial genes (mitochondria) through endosymbiosis.
C5
Transposable elements were once dismissed as "junk DNA." What evidence suggests they can play beneficial roles in genome evolution?
Sample answer: Transposable elements can insert near genes and alter their regulation, creating new expression patterns. They can donate regulatory sequences (promoters, enhancers) to nearby genes. They contribute to genome rearrangements that can generate novel gene combinations. Some elements have been "domesticated" by host genomes to serve new functions — for example, the RAG1/RAG2 genes central to V(D)J recombination in the immune system appear to have derived from transposon sequences.
Section D · Interactive Questions · Part 2
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D1
What type of ribosome (70S or 80S) is found in the cytoplasm of eukaryotic cells?
D2
Which organelle carries out photosynthesis in plant cells? (one word)
D3
What group of bacteria is the accepted ancestor of mitochondria? (two words)
D4
The complete genetic information of an organism is called its what? (one word)
D5
Do mitochondria replicate by mitosis or binary fission? (two words)