Cell biology: 50 essentials from organelles to DNA

All 50 cards

1. 1
   Cell theory
   All living things are made of cells, the cell is the basic unit of life, and cells arise only from existing cells
2. 2
   Prokaryotic vs eukaryotic cells
   Prokaryotes (bacteria and archaea) have no nucleus or membrane-bound organelles; eukaryotes (animals, plants, fungi and protists) have both
3. 3
   Nucleus
   Holds the cell’s DNA and is where DNA is replicated and transcribed; enclosed by a double-membrane nuclear envelope with pores
4. 4
   Ribosome
   Builds proteins by translating mRNA; made of rRNA and protein, found free in the cytosol or bound to the rough ER

   Prokaryotic ribosomes are 70S; eukaryotic cytoplasmic ribosomes are 80S.
5. 5
   Rough endoplasmic reticulum
   Membranes studded with ribosomes that make and fold proteins bound for secretion, membranes or lysosomes
6. 6
   Smooth endoplasmic reticulum
   Makes lipids and steroids, detoxifies drugs and stores calcium ions; it has no ribosomes
7. 7
   Golgi apparatus
   Modifies, sorts and packages proteins and lipids from the ER into vesicles bound for their destinations
8. 8
   Lysosome
   A vesicle of digestive enzymes, active at acidic pH (about 4.5–5), that breaks down macromolecules, worn-out organelles and engulfed material
9. 9
   Mitochondrion
   The site of aerobic respiration (Krebs cycle and oxidative phosphorylation), where most of the cell’s ATP is made

   It has a double membrane with folded cristae, its own circular DNA and bacteria-like ribosomes.
10. 10
    Chloroplast
    The site of photosynthesis in plants and algae: light reactions in the thylakoid membranes, the Calvin cycle in the stroma

    Overall: $\mathrm{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2}$, powered by light.
11. 11
    Endosymbiotic theory
    Mitochondria and chloroplasts descend from free-living bacteria that were engulfed by an ancestral cell

    Evidence: their own circular DNA, bacteria-like ribosomes, double membranes, and division by binary fission.
12. 12
    The three kinds of cytoskeleton filament
    Microfilaments (actin, about 7 nm), intermediate filaments (about 10 nm) and microtubules (tubulin, about 25 nm)

    Microtubules form the mitotic spindle, cilia and flagella.
13. 13
    What are cell walls made of in plants, fungi and bacteria?
    Cellulose in plants, chitin in fungi, peptidoglycan in bacteria
14. 14
    Structures plant cells have that animal cells lack
    A cellulose cell wall, chloroplasts and a large central vacuole that keeps the cell turgid

    Animal cells have centrioles, which most plant cells lack.
15. 15
    Fluid mosaic model
    The membrane is a fluid phospholipid bilayer with proteins embedded in or attached to it, free to move sideways

    Phospholipid heads are hydrophilic and face the water; the fatty-acid tails are hydrophobic and face inward. Cholesterol moderates fluidity in animal cells.
16. 16
    Simple diffusion
    Net movement of particles from high to low concentration with no energy input; small nonpolar molecules like $\mathrm{O_2}$ and $\mathrm{CO_2}$ cross the bilayer directly
17. 17
    Facilitated diffusion
    Passive movement down a concentration gradient through channel or carrier proteins

    Examples: glucose through GLUT transporters, water through aquaporins, ions through ion channels.
18. 18
    Osmosis
    Diffusion of water across a selectively permeable membrane, toward the side with the higher solute concentration
19. 19
    An animal cell in a hypotonic vs a hypertonic solution
    Hypotonic: water flows in, and the cell swells and may burst (lysis). Hypertonic: water flows out, and the cell shrivels (crenation)

    Plant cells in a hypotonic solution become turgid instead of bursting; in a hypertonic one they plasmolyze.
20. 20
    Active transport
    Moving a substance against its concentration gradient using energy, usually from ATP, through a pump protein
21. 21
    Sodium–potassium pump
    Pumps $3\,\mathrm{Na^+}$ out of the cell and $2\,\mathrm{K^+}$ in for each ATP used

    It maintains the resting membrane potential of neurons and other cells.
22. 22
    Secondary active transport
    Uses the gradient built by a pump to drive another substance against its own gradient, through a symporter or antiporter

    Example: the $\mathrm{Na^+}$–glucose symporter (SGLT) in the small intestine.
23. 23
    Endocytosis vs exocytosis
    Endocytosis takes material in by wrapping it in membrane vesicles (phagocytosis, pinocytosis, receptor-mediated); exocytosis releases vesicle contents outside the cell
24. 24
    Phases of the cell cycle
    G1 (growth), S (DNA replication), G2 (preparation for division) and M (mitosis and cytokinesis)

    G1, S and G2 together are interphase. Cells that stop dividing, like most neurons, enter G0.
25. 25
    Stages of mitosis, in order
    Prophase, prometaphase, metaphase, anaphase, telophase

    Cytokinesis usually overlaps telophase. The classic mnemonic for the main four is PMAT.
26. 26
    Prophase
    Chromatin condenses into visible chromosomes, the spindle starts to form and the nucleolus disappears
27. 27
    Metaphase
    Chromosomes line up along the middle of the cell (the metaphase plate), attached to spindle fibers from both poles
28. 28
    Anaphase
    Sister chromatids separate and are pulled to opposite poles of the cell
29. 29
    Result of mitosis
    Two daughter cells genetically identical to the parent cell, with the same chromosome number
30. 30
    Result of meiosis
    Four genetically different haploid cells, such as sperm or egg cells in animals
31. 31
    Meiosis I vs meiosis II
    Meiosis I separates homologous chromosomes, halving the chromosome number; meiosis II separates sister chromatids, much like mitosis
32. 32
    Crossing over
    Exchange of DNA segments between homologous chromosomes during prophase I, producing new combinations of alleles
33. 33
    Independent assortment
    Each homologous pair lines up independently at metaphase I, so one person can make $2^{23} \approx 8.4$ million chromosome combinations in their gametes
34. 34
    Cytokinesis in animal vs plant cells
    Animal cells pinch in two with a cleavage furrow (a ring of actin and myosin); plant cells build a cell plate from Golgi vesicles
35. 35
    Cell cycle checkpoints
    G1/S, G2/M and the spindle checkpoint in metaphase; they pause the cycle if DNA is damaged or chromosomes aren’t properly attached

    The p53 protein halts damaged cells and can trigger apoptosis (programmed cell death); it is mutated in about half of human cancers.
36. 36
    Human diploid and haploid chromosome numbers
    $2n = 46$ in body cells, $n = 23$ in gametes
37. 37
    Structure of DNA
    A double helix of two antiparallel strands, with a sugar–phosphate backbone outside and paired bases inside, held together by hydrogen bonds
38. 38
    Base-pairing rules
    A pairs with T (two hydrogen bonds) and G with C (three); in RNA, A pairs with U
39. 39
    DNA vs RNA
    DNA has deoxyribose and thymine and is usually double-stranded; RNA has ribose and uracil and is usually single-stranded
40. 40
    Semiconservative replication
    Each new DNA molecule keeps one original strand and gains one newly built strand

    Shown by the Meselson–Stahl experiment in 1958.
41. 41
    In which direction does DNA polymerase build DNA?
    Only 5′ → 3′, adding nucleotides to the 3′ end, so the lagging strand is made in short Okazaki fragments joined by DNA ligase

    Helicase unwinds the helix first, and primase lays down short RNA primers.
42. 42
    Central dogma of molecular biology
    DNA → RNA → protein: DNA is transcribed into RNA, and messenger RNA is translated into protein
43. 43
    Transcription
    RNA polymerase reads the DNA template strand and builds a complementary RNA; in eukaryotes this happens in the nucleus
44. 44
    mRNA processing in eukaryotes
    A 5′ cap and a 3′ poly-A tail are added, and introns are spliced out so only exons remain
45. 45
    Codon
    Three mRNA bases that code for one amino acid or a stop signal

    There are 64 codons. AUG (methionine) is the start codon; UAA, UAG and UGA are stop codons.
46. 46
    Role of tRNA
    Carries a specific amino acid to the ribosome; its anticodon pairs with the matching codon on the mRNA
47. 47
    Silent, missense and nonsense mutations
    Silent: the same amino acid. Missense: a different amino acid. Nonsense: an early stop codon
48. 48
    Frameshift mutation
    An insertion or deletion of a number of bases not divisible by three, which shifts the reading frame and changes every codon after it
49. 49
    ATP
    Adenosine triphosphate, the cell’s energy currency: hydrolysis to ADP and phosphate releases energy for cellular work
50. 50
    Overall equation of aerobic respiration
    $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O}$

    It yields about 30–32 ATP per glucose molecule.