LET Elementary Biological Science — Cell Biology, Genetics and EvolutionRevision Notes

Exam Tips

• LET items often ask: 'Which scientist contributed the idea that all cells come from pre-existing cells?' Answer: Rudolf Virchow.
• A question asking to distinguish bacteria from plant cells tests prokaryote vs. eukaryote knowledge.
• If the question says 'lacks a membrane-bound nucleus,' the answer is prokaryotic/bacteria.
• Remember the mnemonic: 'PRO = no nucleus' (prokaryote has no true nucleus).

Key Points

• Cell theory has THREE classic statements: (1) All living things are composed of one or more cells. (2) The cell is the basic unit of structure and function in living organisms. (3) All cells come from pre-existing cells (contributed by Rudolf Virchow).
• The cell theory was built on the contributions of Robert Hooke (named 'cells' from cork, 1665), Anton van Leeuwenhoek (observed living microorganisms), Matthias Schleiden (plants are made of cells), Theodor Schwann (animals are made of cells), and Rudolf Virchow (cells come from pre-existing cells).
• PROKARYOTIC CELLS: Small, no true membrane-bound nucleus, DNA floats freely in the cytoplasm. Example: Bacteria. No membrane-bound organelles.
• EUKARYOTIC CELLS: Larger, possess a membrane-bound NUCLEUS and specialized organelles. Examples: plant cells, animal cells, fungi, protists.
• The prokaryote vs. eukaryote distinction is a VERY FREQUENT LET exam item — know it thoroughly.
• Key difference to memorize: Prokaryotes = NO true nucleus; Eukaryotes = TRUE nucleus with nuclear membrane.

Definitions

Section Title

Section 1: Cell Theory and Types of Cells

Common Mistakes

• Confusing who contributed which part of cell theory — remember Virchow specifically added 'cells come from pre-existing cells.'
• Saying bacteria have a nucleus — they do NOT. Bacteria are prokaryotic.
• Thinking prokaryotes have no DNA — they DO have DNA, it just lacks a nuclear membrane around it.
• Forgetting that viruses are NOT cells and are NOT covered by cell theory — they are not living under the strict definition.

Exam Tips

• Memorize the 'nickname' for each organelle: Nucleus = control center; Mitochondria = powerhouse; Ribosome = protein factory; Golgi = post office; Lysosome = recycling center; Chloroplast = solar panel.
• For osmosis questions, think: 'Water follows the salt' — water moves toward where there is MORE solute.
• Plant vs. animal cell comparisons frequently ask: 'Which structure is found in plant cells but NOT in animal cells?' Answer: Cell wall, chloroplast, large central vacuole.
• Passive vs. active transport: passive = no energy, downhill; active = needs ATP, uphill.

Key Points

• NUCLEUS: 'Control center' of the cell. Contains DNA. Directs all cell activities. Has a nuclear membrane (envelope).
• CELL MEMBRANE (Plasma Membrane): Controls what enters and exits the cell. Described as SELECTIVELY PERMEABLE. Present in ALL cells.
• CYTOPLASM: Jelly-like fluid that fills the cell and holds organelles. Many chemical reactions occur here.
• MITOCHONDRION: 'Powerhouse of the cell.' Site of CELLULAR RESPIRATION. Produces ATP (energy currency of the cell). Present in animal AND plant cells.
• RIBOSOME: Site of PROTEIN SYNTHESIS. Can be free in cytoplasm or attached to rough endoplasmic reticulum (ER). Present in ALL cells, including prokaryotes.
• ENDOPLASMIC RETICULUM (ER): ROUGH ER (studded with ribosomes) makes and transports proteins. SMOOTH ER makes lipids and detoxifies substances.
• GOLGI APPARATUS: 'Post office' or 'shipping department' of the cell. Packages and sends proteins to their destinations inside or outside the cell.
• LYSOSOME: Contains digestive enzymes. Breaks down waste materials, worn-out organelles, and foreign substances. Mostly in ANIMAL cells.
• CHLOROPLAST: Site of PHOTOSYNTHESIS. Contains CHLOROPHYLL (green pigment). Found in PLANT cells ONLY (and algae).
• CELL WALL: Rigid outer layer that gives support and shape. Found in PLANTS (made of CELLULOSE), FUNGI, and BACTERIA. NOT in animal cells.
• VACUOLE: Storage organelle. Plant cells have one LARGE CENTRAL VACUOLE that stores water and keeps the cell firm (turgid). Animal cells have small, temporary vacuoles.
• CENTRIOLES: Help in cell division (organizing the spindle fibers). Found in ANIMAL cells; generally absent in plant cells.
• PLANT vs. ANIMAL CELLS — HIGH YIELD COMPARISON: Plant cells HAVE cell wall, chloroplasts, large central vacuole; Animal cells HAVE centrioles, lysosomes, NO cell wall, NO chloroplasts.
• MOVEMENT ACROSS THE MEMBRANE: PASSIVE TRANSPORT (no energy needed) moves substances from HIGH to LOW concentration — includes simple diffusion and osmosis. ACTIVE TRANSPORT (requires ATP/energy) moves substances from LOW to HIGH concentration (against the gradient) — example: sodium-potassium pump.
• OSMOSIS: Diffusion of WATER across a selectively permeable membrane from an area of LOW solute concentration to HIGH solute concentration.
• ISOTONIC solution: Equal solute concentration inside and outside — cell stays the same size.
• HYPOTONIC solution: Lower solute concentration OUTSIDE than inside — water moves INTO the cell — cell SWELLS (may burst in animal cells; becomes TURGID in plant cells — this is why plants stay upright).
• HYPERTONIC solution: Higher solute concentration OUTSIDE than inside — water moves OUT of the cell — cell SHRINKS (PLASMOLYSIS in plant cells). This is why salting vegetables draws out their water.

Definitions

Section Title

Section 2: Cell Structure and Function

Common Mistakes

• Mixing up the Golgi apparatus and the endoplasmic reticulum — remember: ER makes/transports, Golgi PACKAGES and SHIPS.
• Saying chloroplasts are in animal cells — they are NOT. Only plant cells (and algae) have chloroplasts.
• Confusing osmosis direction — water always moves FROM the less concentrated solution (hypotonic) INTO the more concentrated solution (hypertonic).
• Saying active transport moves substances from high to low concentration — it does the OPPOSITE (low to high, against the gradient) and REQUIRES energy.
• Forgetting that ribosomes are in prokaryotic cells too — they are the ONLY organelle found in all cell types.

Exam Tips

• Use PMAT as your mnemonic for mitosis stages: Prophase, Metaphase, Anaphase, Telophase.
• The LET often asks: 'How many chromosomes will a human gamete have?' Answer: 23 (haploid).
• A key distinguishing question: 'Which type of cell division produces genetically varied cells?' Answer: MEIOSIS.
• Remember: Meiosis = Making Eggs and sperm = FOUR cells = HALF the chromosomes.
• If a question mentions 'crossing over' or 'genetic recombination,' it is referring to MEIOSIS (specifically Prophase I).

Key Points

• Cells divide for THREE reasons: growth, repair of tissues, and reproduction.
• MITOSIS produces TWO daughter cells that are GENETICALLY IDENTICAL to the parent cell. Daughter cells are DIPLOID (2n) — they have the full set of chromosomes. Used for GROWTH and REPAIR of body tissues.
• Stages of MITOSIS in order: PROPHASE → METAPHASE → ANAPHASE → TELOPHASE → CYTOKINESIS. Memory aid: PMAT (Plus Cytokinesis).
• PROPHASE: Chromosomes become visible and condense. Nuclear envelope breaks down.
• METAPHASE: Chromosomes line up at the cell's middle (metaphase plate). Spindle fibers attach.
• ANAPHASE: Sister chromatids are pulled APART to opposite poles of the cell.
• TELOPHASE: Nuclear envelopes reform around each set of chromosomes.
• CYTOKINESIS: Cytoplasm divides — in animal cells, a cleavage furrow forms; in plant cells, a cell plate forms.
• MEIOSIS produces FOUR daughter cells that are GENETICALLY VARIED and HAPLOID (n — half the chromosome number). Used to produce GAMETES (sperm and egg cells).
• Meiosis involves TWO rounds of division: MEIOSIS I and MEIOSIS II.
• CROSSING OVER occurs during PROPHASE I of meiosis — homologous chromosomes exchange segments of DNA. This is a MAJOR SOURCE OF GENETIC VARIATION.
• DIPLOID (2n) vs. HAPLOID (n): Human body cells are diploid with 46 chromosomes (23 pairs). After meiosis, gametes are haploid with 23 chromosomes. Fertilization restores the diploid number (23 + 23 = 46).
• KEY COMPARISON — MITOSIS vs. MEIOSIS: Mitosis = 1 division, 2 cells, diploid, identical; Meiosis = 2 divisions, 4 cells, haploid, varied.
• FERTILIZATION: The fusion of sperm (n) and egg (n) to form a ZYGOTE (2n). The zygote then develops by mitosis.

Definitions

Section Title

Section 3: Cell Division — Mitosis and Meiosis

Common Mistakes

• Saying mitosis produces haploid cells — WRONG. Mitosis produces DIPLOID cells. Only meiosis produces haploid cells.
• Forgetting that meiosis involves TWO divisions — students often treat it like mitosis with only one division.
• Mixing up the PMAT stages — Anaphase is when chromatids are PULLED APART (think 'A' for Apart).
• Saying 'meiosis is for growth' — WRONG. Meiosis is for producing GAMETES. MITOSIS is for growth and repair.
• Forgetting that a human body cell with 46 chromosomes will produce gametes with 23 chromosomes through meiosis, and fertilization restores 46.

Exam Tips

• Memorize base pairing: A=T, G=C (in DNA); A=U, G=C (in RNA).
• LET item type: 'A strand of DNA reads: 5'-AATGCC-3'. What would the complementary DNA strand be?' Answer: TTACGG.
• Central Dogma sequence: DNA → (transcription) → RNA → (translation) → Protein.
• RNA uses Uracil (U), not Thymine (T). If a sequence has U, it is RNA, not DNA.
• Watson and Crick described the double helix — Rosalind Franklin's X-ray data was crucial. Know these names for historical LET questions.

Key Points

• DNA (Deoxyribonucleic Acid) carries the HEREDITARY INSTRUCTIONS of all living organisms.
• The structure of DNA is a DOUBLE HELIX — like a twisted ladder. This structure was described by James Watson and Francis Crick in 1953, based critically on X-ray data from Rosalind Franklin.
• DNA is made of NUCLEOTIDES. Each nucleotide has THREE parts: a deoxyribose SUGAR, a PHOSPHATE group, and a NITROGENOUS BASE.
• There are FOUR nitrogenous bases in DNA: ADENINE (A), THYMINE (T), GUANINE (G), and CYTOSINE (C).
• COMPLEMENTARY BASE PAIRING RULE: A always pairs with T (A-T); G always pairs with C (G-C). This is critical — memorize it!
• Base pairing allows DNA REPLICATION — the process by which DNA makes an exact copy of itself before cell division, ensuring each daughter cell receives complete genetic information.
• GENE: A specific segment of DNA that codes for a particular protein or trait. Genes are located on chromosomes.
• CHROMOSOME: A tightly coiled structure made of DNA and protein (histones). Humans have 46 chromosomes (23 pairs) in body cells.
• FLOW OF GENETIC INFORMATION (Central Dogma): DNA → RNA → PROTEIN.
• TRANSCRIPTION: DNA is used as a template to make mRNA (messenger RNA) in the nucleus.
• TRANSLATION: mRNA is read by ribosomes to assemble a chain of amino acids into a protein.
• RNA DIFFERENCES from DNA: RNA is single-stranded; uses RIBOSE sugar (not deoxyribose); uses URACIL (U) instead of THYMINE (T) as a base. So in RNA: A pairs with U (not T).
• CODON: A sequence of three bases on mRNA that codes for one amino acid.

Definitions

Section Title

Section 4: DNA Structure and Inheritance

Common Mistakes

• Forgetting that RNA uses URACIL (U) instead of THYMINE (T) — this is a very common LET distractor.
• Confusing transcription and translation — remember: TRANScription happens in the NUCLEUS (DNA→RNA); transLATION happens at the ribosome (RNA→Protein).
• Mixing up gene and chromosome — a CHROMOSOME contains many GENES; a GENE is one segment of DNA coding for one protein.
• Getting base pairs wrong — A-T and G-C. A common LET trick is asking the complement of a given DNA strand.
• Saying DNA is single-stranded — DNA is DOUBLE-stranded (double helix). RNA is SINGLE-stranded.

Exam Tips

• PUNNETT SQUARE STEPS: (1) Identify parent genotypes; (2) Determine gametes each parent can make; (3) Fill in the grid; (4) Count genotypes and phenotypes.
• Ratios to MEMORIZE: Monohybrid Tt x Tt → phenotypic 3:1, genotypic 1:2:1. Dihybrid → phenotypic 9:3:3:1.
• For test cross questions: a 1:1 ratio means the parent was HETEROZYGOUS (Tt); a 1:0 (all dominant) ratio means the parent was HOMOZYGOUS DOMINANT (TT).
• Blood type AB is the classic example of CODOMINANCE; pink flowers (red x white) is the classic example of INCOMPLETE DOMINANCE.
• LET item: 'Why is hemophilia more common in males?' Answer: It is an X-linked recessive trait; males have only one X chromosome so there is no second allele to mask it.

Key Points

• Gregor Mendel is called the FATHER OF GENETICS. He conducted experiments on PEA PLANTS and discovered the principles of heredity.
• ALLELE: Alternate forms of a gene. For example, T (tall) and t (short) are alleles of the height gene.
• DOMINANT ALLELE: The allele that is expressed (shown) even when only one copy is present. Represented by a CAPITAL letter (e.g., T).
• RECESSIVE ALLELE: The allele that is expressed ONLY when two copies are present (homozygous recessive). Represented by a lowercase letter (e.g., t).
• GENOTYPE: The allele combination an organism has (e.g., TT, Tt, tt). This is the 'genetic makeup.'
• PHENOTYPE: The physical trait that is EXPRESSED (shown) based on the genotype (e.g., tall, short). This is what you CAN SEE.
• HOMOZYGOUS: Having TWO IDENTICAL alleles for a trait (TT = homozygous dominant; tt = homozygous recessive).
• HETEROZYGOUS: Having TWO DIFFERENT alleles for a trait (Tt = heterozygous). Often called 'carriers' or 'hybrid.'
• LAW OF SEGREGATION: During gamete formation, the two alleles for a gene SEPARATE so each gamete carries only ONE allele.
• LAW OF INDEPENDENT ASSORTMENT: Genes for DIFFERENT traits are inherited independently of each other (when on different chromosomes).
• MONOHYBRID CROSS (Tt x Tt): Genotypic ratio = 1 TT : 2 Tt : 1 tt = 1:2:1. Phenotypic ratio = 3 tall : 1 short = 3:1.
• TEST CROSS (Tt x tt): Used to determine if an organism showing the dominant phenotype is TT or Tt. Result: Genotypic ratio 1 Tt : 1 tt (1:1); Phenotypic ratio 1 tall : 1 short (1:1).
• DIHYBRID CROSS (TtYy x TtYy): Involves two traits. Classic phenotypic ratio = 9:3:3:1. MEMORIZE this ratio.
• INCOMPLETE DOMINANCE: The heterozygote (Rr) shows a BLEND of both alleles — e.g., red (RR) x white (WW) gives PINK (RW) flowers. Neither allele is fully dominant.
• CODOMINANCE: Both alleles are FULLY EXPRESSED in the heterozygote — e.g., AB blood type where both A and B antigens are present.
• MULTIPLE ALLELES: More than two alleles exist for a gene in a population. Example: ABO blood type (alleles I^A, I^B, i).
• SEX-LINKED TRAITS: Genes carried on the X CHROMOSOME. Because males (XY) have only ONE X chromosome, they are more likely to express X-linked recessive traits like COLOR BLINDNESS and HEMOPHILIA.
• BLOOD TYPE: Controlled by three alleles (I^A, I^B, i). I^A and I^B are codominant; i is recessive. Blood type O = ii; A = I^A I^A or I^A i; B = I^B I^B or I^B i; AB = I^A I^B.

Definitions

Section Title

Section 5: Mendelian Genetics

Common Mistakes

• Confusing genotype and phenotype — genotype is the ALLELE combination (Tt); phenotype is the TRAIT SHOWN (tall).
• Forgetting the 3:1 phenotypic ratio for Tt x Tt — three show the dominant trait, one shows the recessive trait.
• Mixing up incomplete dominance and codominance — incomplete = BLEND; codominance = BOTH fully expressed.
• In sex-linked trait problems, forgetting that males (XY) only have ONE X allele, so they express whatever allele is on their single X chromosome.
• Mixing up the 1:2:1 (genotypic) and 3:1 (phenotypic) ratios from a monohybrid cross — these are NOT the same ratio.

Exam Tips

• LET classic question: 'Which type of structure provides evidence of COMMON ANCESTRY?' Answer: HOMOLOGOUS structures.
• The bat wing and the human arm are HOMOLOGOUS (same bone structure, different function). The bat wing and the butterfly wing are ANALOGOUS (same function, different structure).
• Lamarck vs. Darwin: Lamarck = acquired traits are inherited (WRONG); Darwin = natural selection acts on inherited variation (CORRECT).
• Mutations are the ULTIMATE SOURCE of new genetic variation — this is a frequent LET stem.
• Fossil evidence shows gradual change over geologic time — oldest fossils in the DEEPEST rock layers (law of superposition connects to geology).

Key Points

• EVOLUTION: The change in the inherited characteristics (allele frequencies) of a population over successive generations.
• CHARLES DARWIN proposed the mechanism of NATURAL SELECTION in his book 'On the Origin of Species' (1859), based partly on observations of Galapagos finches.
• THE FOUR-STEP PROCESS OF NATURAL SELECTION: (1) OVERPRODUCTION — organisms produce more offspring than can survive; (2) VARIATION — individuals differ, and some variation is heritable; (3) SELECTION — some variants are better adapted to the environment; (4) INHERITANCE — better-adapted individuals survive and pass favorable traits to offspring (ADAPTATION).
• 'SURVIVAL OF THE FITTEST' means REPRODUCTIVE SUCCESS — the individuals best suited to their environment survive long enough to reproduce and pass on their traits.
• LAMARCK vs. DARWIN: Jean-Baptiste Lamarck proposed INHERITANCE OF ACQUIRED CHARACTERISTICS — if a giraffe stretched its neck during its lifetime, it would pass that longer neck to offspring. This is INCORRECT. Darwin explained the same outcome through NATURAL SELECTION acting on INHERITED (not acquired) variation.
• MUTATION: Changes in the DNA sequence. The ULTIMATE SOURCE of all new genetic variation. Random and heritable.
• GENE FLOW: Movement of alleles between populations through MIGRATION. Increases or decreases allele frequencies.
• GENETIC DRIFT: RANDOM changes in allele frequency, especially significant in SMALL populations. Can cause alleles to be lost or fixed by chance, not selection.
• SPECIATION: The process by which one species splits into two or more new species over time due to reproductive isolation and accumulated genetic differences.
• ASEXUAL REPRODUCTION: One parent, genetically IDENTICAL offspring, uses mitosis. Examples: binary fission (bacteria), budding (yeast, hydra), vegetative propagation (plants — runners, cuttings, tubers). Fast but no genetic variation.
• SEXUAL REPRODUCTION: Two parents, genetically VARIED offspring (from meiosis and fertilization). Slower but produces variation for natural selection to act upon.
• EVIDENCE FOR EVOLUTION — memorize all six types: (1) FOSSILS; (2) COMPARATIVE ANATOMY (homologous and analogous structures); (3) VESTIGIAL STRUCTURES; (4) EMBRYOLOGY; (5) MOLECULAR BIOLOGY / DNA; (6) BIOGEOGRAPHY.
• HOMOLOGOUS STRUCTURES: Same ANATOMICAL ORIGIN, different function. Example: human arm, whale flipper, bat wing, horse forelimb. Evidence of COMMON ANCESTRY.
• ANALOGOUS STRUCTURES: Same FUNCTION, different anatomical origin. Example: bird wings and insect wings. Evidence of CONVERGENT EVOLUTION, NOT common ancestry.
• VESTIGIAL STRUCTURES: Reduced, non-functional (or minimally functional) remnants of structures that were useful in ancestors. Example: human appendix, human coccyx (tailbone), whale pelvic bones.
• BIOGEOGRAPHY: The study of where species live. Similar environments on different continents have species that look alike but are not closely related (convergent evolution); islands near continents often have species related to nearby mainland species.

Definitions

Section Title

Section 6: Evolution — Natural Selection and Evidence

Common Mistakes

• Saying Lamarck was correct about evolution — he was WRONG about inherited acquired characteristics. Darwin's natural selection is the accepted mechanism.
• Confusing homologous and analogous structures — HOMOLOGOUS = same origin (common ancestry); ANALOGOUS = same function (convergent evolution).
• Thinking 'survival of the fittest' means the strongest or fastest animal survives — it means the individual that is BEST ADAPTED to its environment and produces the MOST OFFSPRING.
• Saying mutation is directed by the environment to produce useful traits — mutations are RANDOM and the environment then SELECTS which variants are more successful.
• Confusing vestigial and analogous structures — vestigial structures are reduced remnants in one organism; analogous structures compare structures between different species.