Cell structure and its organelles
The eukaryotic cell consists of four main components: the membrane, which performs a barrier function and consists of a phospholipid bilayer with embedded proteins; the cytoplasm, which includes the cytosol and organelles; the nucleus, which stores hereditary information; and a system of organelles that ensure biochemical processes.
- Ribosomes are non-membranous organelles responsible for translation.
- Mitochondria are double-membrane organelles that carry out ATP synthesis.
- Golgi apparatus is a single-membrane organelle for protein transport and modification, and lysosome synthesis.
- Endoplasmic reticulum: rough (protein synthesis) and smooth (lipid synthesis, detoxification, calcium regulation).
- Cytoskeleton consists of microfilaments and microtubules that maintain cell shape.
The plant cell additionally contains vacuoles, plastids, and a cellulosic cell wall. The animal cell has a cell center (centrosome) and lysosomes. Prokaryotic cells lack membrane-bound organelles; their DNA is represented by a circular nucleoid.
Membrane transport
Passive transport moves along the concentration gradient without energy expenditure: simple diffusion (O₂, CO₂ through the lipid bilayer), osmosis (water through a semipermeable membrane), and facilitated diffusion via carrier proteins (glucose).
Active transport requires ATP and moves against the concentration gradient. Examples include the sodium-potassium pump, exocytosis, and endocytosis. Depending on the direction of transport, there is uniport, symport, and antiport.
Cell cycle, mitosis, and meiosis
The cell cycle consists of interphase (G1 — growth, S — DNA replication, G2 — preparation for division) and the actual division. Checkpoints G1/S, G2/M, and the metaphase checkpoint ensure DNA integrity and proper chromosome orientation. In the G0 phase, the cell is in a resting state.
Mitosis is the division of somatic cells resulting in two identical daughter cells. Stages: prophase (chromatin condensation), metaphase (formation of the metaphase plate), anaphase (separation of sister chromatids), and telophase with cytokinesis.
Meiosis produces four haploid cells and ensures combinatorial variability. Prophase I includes five stages: leptotene, zygotene, pachytene (with crossing over), diplotene (chiasmata visible), and diakinesis. The second meiotic division is similar to mitosis.
Nucleic acids and DNA compaction
DNA stores genetic information, while RNA participates in its realization. A nucleotide consists of a nitrogenous base (A, G, C, T/U), a five-carbon sugar (deoxyribose or ribose), and a phosphate group. DNA forms a double antiparallel helix (5′→3′ complementary to 3′→5′).
Types of RNA: mRNA (template for protein synthesis), tRNA (delivery of amino acids), rRNA (structural and catalytic in ribosomes), microRNAs and small interfering RNAs (regulation of expression), and long non-coding RNAs.
Levels of DNA compaction: nucleosome (DNA on a histone octamer), 30-nm solenoid (stabilized by histone H1), loop structure (cohesins), and the condensed chromosome.
Replication, transcription, translation
Replication is semiconservative, antiparallel, and requires a primer. Stages: initiation (unwinding of the helix by helicases, formation of the replication fork), elongation (DNA synthesis by DNA polymerase 5′→3′), termination. In prokaryotes, there is one origin on the circular chromosome; in eukaryotes, there are multiple initiation sites and the ORC complex.
Transcription: RNA polymerase binds to the promoter, synthesizes mRNA, and terminates when reaching the terminator. In eukaryotes, this is followed by processing — capping, polyadenylation, and splicing with intron removal; in prokaryotes, transcription and translation occur simultaneously in the cytoplasm.
Translation takes place on the ribosome with A, P, and E sites. tRNA delivers amino acids, between which peptide bonds are formed. Termination occurs at stop codons UAA, UAG, UGA.
Genetic code and gene expression
The genetic code includes 64 codons, of which 3 are stop codons. Properties: triplet nature, universality, non-overlapping nature, unambiguousness, and redundancy (degeneracy).
In eukaryotes, expression is regulated by promoters and enhancers, activators and repressors, alternative splicing, epigenetic modifications (DNA methylation, histone acetylation), and microRNAs. In prokaryotes, the operon system operates: a classic example is the lac operon, where the presence of lactose inactivates the repressor and triggers lactase synthesis.
Mutations and DNA repair
Mutations are divided into gene, chromosomal (insertions, deletions, inversions), and genomic (changes in chromosome number, e.g., trisomy 21).
In prokaryotes, photoreactivation (photolyase), excision, post-replication, error-prone, and SOS repair are distinguished. In eukaryotes, mechanisms are similar but more precise; mismatch repair, and single- and double-strand break repair via homologous or non-homologous end joining also operate.
Biochemical foundations: proteins, fats, carbohydrates
Proteins are composed of amino acids and perform catalytic, transport, structural, protective, regulatory, energy, storage, and receptor functions. Levels of structure: primary (sequence), secondary (α-helices, β-sheets), tertiary (globule), and quaternary (e.g., hemoglobin).
Lipids are a concentrated source of energy, the basis of membranes, precursors of steroid hormones and vitamin D, and ensure the absorption of fat-soluble vitamins A, D, E, and K. They are divided into saturated and unsaturated. They are broken down into glycerol and fatty acids.
Carbohydrates are the main source of energy and are stored as glycogen. Simple carbohydrates (glucose, fructose) are absorbed quickly, while complex ones (starch, fiber) are absorbed slowly. Cell metabolism consists of anabolism (synthesis with energy expenditure) and catabolism (breakdown with energy release).
Key points
- Eukaryotic cells have membrane-bound organelles; prokaryotic cells are limited to a nucleoid and ribosomes.
- Membrane transport can be passive (diffusion, osmosis) and active (Na⁺/K⁺ pump, endo- and exocytosis).
- Mitosis ensures identical daughter cells, while meiosis produces haploid germ cells and combinatorial variability.
- DNA is compacted via nucleosomes, a solenoid, loops, and a chromosome.
- Replication is semiconservative and antiparallel; synthesis always proceeds 5′→3′.
- In eukaryotes, mRNA undergoes capping, polyadenylation, and splicing before exiting into the cytoplasm.
- The genetic code is triplet-based, universal, degenerate, and unambiguous.
- Metabolism combines anabolic and catabolic processes, ensuring the cell's energy balance.