General Biology — Cell Division, Genetics, and Evolution

The Cell Cycle and Mitosis

The cell cycle:

Interphase — about 90% of the cycle:

G1 phase: cell growth, protein synthesis
S phase: DNA replication (chromosome number = 2n, DNA amount doubles)
G2 phase: preparation for division, spindle-fiber protein synthesis

Mitosis:

Prophase: chromosomes condense, spindle fibers form
Prometaphase: the nuclear envelope breaks down, spindle fibers attach
Metaphase: chromosomes align at the metaphase plate
Anaphase: sister chromatids separate → move to opposite poles
Telophase: the nuclear envelope re-forms, chromosomes decondense

Cytokinesis:

Animals: a contractile ring pinches the cell apart
Plants: a cell plate forms (from the inside out)

Result:

One parent cell → two daughter cells
Identical genetic information (2n → 2n)
Growth, replacing damaged cells, asexual reproduction

Cell-cycle regulation:

Cyclin and CDK: drive cell-cycle progression
Checkpoints: the G1, G2, and M-phase (metaphase) checkpoints
p53: a tumor-suppressor gene (halts the cycle if damage is detected)
Cancer: results from disrupted cell-cycle control

Meiosis

The purpose of meiosis:

Forms gametes (2n → n)
Halves the chromosome number (sperm, eggs)
Generates genetic diversity

Meiosis I (no DNA replication):

Prophase I: homologous chromosomes synapse (pair up), crossing over (recombination) occurs
Metaphase I: homologous pairs align at the metaphase plate
Anaphase I: homologous chromosomes separate (sister chromatids do not separate)
Telophase I: 2n → two daughter cells with n chromosome sets (each with double the DNA)

Meiosis II:

Similar to mitosis (no DNA replication)
Sister chromatids separate
Four n daughter cells → gametes (sperm, eggs)

Sources of genetic diversity:

Crossing over: DNA exchange between non-sister chromatids
Independent assortment: the random arrangement of homologous chromosome pairs (2²³ possible combinations)
Fertilization: the union of two gametes

Mitosis vs. meiosis:

Mitosis: somatic cells, 2n → 2n, two daughter cells
Meiosis: reproductive organs, 2n → n, four daughter cells
The separation of homologous chromosomes in meiosis I is the key distinction

DNA Replication and the Central Dogma

DNA replication:

Semiconservative replication: each daughter molecule has one original strand + one new strand
Origin of replication: where replication begins
Replication fork: proceeds in both directions

Key enzymes:

Helicase: unwinds the DNA double helix
Primase: synthesizes an RNA primer
DNA polymerase III: replicates in the 5’→3’ direction
DNA polymerase I: removes the RNA primer and replaces it with DNA
DNA ligase: seals nicks

Leading strand vs. lagging strand:

Leading strand: synthesized continuously
Lagging strand: synthesized discontinuously as Okazaki fragments
- joined by ligase

The central dogma:

DNA → RNA → protein

Transcription:

DNA → mRNA
RNA polymerase binds to the promoter
In eukaryotes: occurs in the nucleus, processed before moving to the cytoplasm
A 5’ cap and poly-A tail are added; introns are removed (splicing)

Translation:

mRNA → protein (at the ribosome)
Codon: three bases = one amino acid (64 possible codons)
AUG: the start codon (methionine)
UAA, UAG, UGA: stop codons
tRNA: carries an anticodon plus an amino acid

Mendelian Genetics

Mendel’s three laws:

The law of segregation:

The allele pair for a trait separates when gametes form
Aa × Aa: 3:1 phenotype ratio, 1:2:1 genotype ratio

The law of independent assortment:

Genes on different chromosomes are inherited independently
AaBb × AaBb: 9:3:3:1 (when two traits assort independently)

The law of dominance:

A dominant allele suppresses expression of a recessive allele

Linkage and crossing over:

Genes on the same chromosome tend to be inherited together (linkage)
Crossing over: linked genes can still recombine
Crossover rate ∝ distance between genes → the basis of chromosome mapping

Sex-linked inheritance:

Genes located on a sex chromosome (X, Y)
Color blindness and hemophilia: X-linked recessive (expressed more often in males)

Non-Mendelian inheritance:

Incomplete dominance: an intermediate phenotype (pink flowers)
Codominance: both alleles are expressed (ABO blood types)
Polygenic inheritance: multiple genes contribute to one trait (height, skin color)
Environmental influence: genotype + environment = phenotype

Evolutionary Theory

Darwin’s natural selection:

Variation exists within a population
Variants favorable for survival and reproduction → produce more offspring
Repeated across generations → favorable traits increase within the population

Mechanisms of evolution:

Natural selection:

Directional selection: favors one extreme
Stabilizing selection: favors the intermediate value (reduces variance)
Disruptive selection: favors both extremes (produces polymorphism)

Genetic drift:

Random fluctuation, with a larger effect in small populations
Bottleneck effect: a sharp population decline reduces diversity
Founder effect: a small number of individuals establish a new population

Gene flow:

Exchange of genes through movement between populations
Reduces genetic differences between populations

Mutation:

The ultimate source of genetic variation
Neutral, beneficial, or harmful mutations

Gene flow (species-level):

Movement between populations exchanges genes
Reduces genetic differences between populations

The Hardy-Weinberg principle:

Conditions: random mating, no migration, no natural selection, no mutation, a large population
p² + 2pq + q² = 1 (the frequencies of AA, Aa, aa)
A population not evolving is in Hardy-Weinberg equilibrium
Real populations deviate from equilibrium → evidence of evolution

Frequently Asked Questions

Q. Is there an easy way to distinguish mitosis from meiosis? A. The key difference is whether homologous chromosomes separate. In mitosis, sister chromatids (duplicated copies of the same chromosome) separate, producing two cells with identical genetic information. In meiosis I, homologous chromosomes (the different chromosome pairs inherited from each parent) separate, halving the chromosome number. Meiosis II then separates sister chromatids, just like mitosis.

Q. Why does the Hardy-Weinberg principle matter for the study of evolution? A. Hardy-Weinberg equilibrium provides a baseline: the allele frequencies expected when evolution is not occurring. If a real population’s allele frequencies deviate from that equilibrium, it signals that natural selection, genetic drift, gene flow, or mutation is at work. In other words, it’s a mathematical tool for detecting that evolution is happening.