Foundations of Biology: Cell Division, Molecular Genetics, and Mendelian Inheritance

Comprehensive vocabulary flashcards covering DNA structure and replication, cell cycle checkpoints and regulation, mitosis and meiosis, genetic variation, Mendelian genetics, linkage, mapping, and extensions of Mendelian inheritance.

NASA Working Definition of Life

A working definition developed by NASA astrobiologists in the 1990s stating that life is "a self-sustaining chemical system capable of evolution."

Nucleotide Monomer

The structural building block of nucleic acids consisting of a five-carbon sugar (ribose or deoxyribose), a phosphate group attached to the $5'$ carbon, and a nitrogenous base (purine or pyrimidine) attached to the $1'$ carbon.

Phosphodiester Linkage

A strong covalent bond that joins the $5'$ phosphate group of one nucleotide to the $3'$ hydroxyl ($\text{OH}$) group of an adjacent nucleotide, forming the structural backbone of a nucleic acid strand.

Semiconservative Replication

The mechanism of DNA replication in which each original parent strand unwinds and serves as a template to direct the synthesis of a new complementary daughter strand, yielding two double helices that each contain one old strand and one newly synthesized strand.

Origin of Replication (ori)

A specific DNA sequence where the replication complex assembles to initiate DNA synthesis. Prokaryotic circular chromosomes have a single ori, whereas eukaryotic linear chromosomes contain hundreds to tens of thousands of ori sites.

DNA Helicase

An enzyme that separates and unwinds the two strands of the double-stranded DNA molecule at the replication fork by breaking the hydrogen bonds between complementary base pairs.

Single-Stranded Binding Proteins (SSBs)

Proteins that coat and stabilize separated single strands of DNA during replication to prevent them from prematurely re-annealing through unwanted hydrogen bonding.

Topoisomerase

An enzyme that relieves torsional strain and prevents supercoiling ahead of the replication fork as the DNA double helix is unwound by helicase.

Primase

An RNA polymerase enzyme that synthesizes a short complementary RNA primer on the DNA template to provide a free $3'\text{-OH}$ group required for DNA polymerase to begin elongation.

DNA Polymerase III

The primary enzyme responsible for synthesizing new DNA strands in the $5' \rightarrow 3'$ direction by catalyzing the addition of deoxyribonucleotide triphosphates (dNTPs) to a preexisting $3'\text{-OH}$ group.

DNA Polymerase I

An enzyme that removes RNA primers between Okazaki fragments using $5' \rightarrow 3'$ exonuclease activity and fills in the resulting gaps with DNA nucleotides using its $5' \rightarrow 3'$ polymerase activity.

DNA Ligase

An enzyme that catalyzes the formation of covalent phosphodiester bonds to seal nicks in the sugar-phosphate backbone between adjacent newly synthesized DNA fragments on the lagging strand.

Leading Strand

The new strand of DNA that is synthesized continuously in the $5' \rightarrow 3'$ direction toward the advancing replication fork.

Lagging Strand

The new strand of DNA that is synthesized discontinuously away from the replication fork in the form of short segments called Okazaki fragments, each requiring its own RNA primer.

Chromosome

A discrete, continuous molecule of DNA wrapped around organizing proteins (such as histones). Eukaryotes possess multiple linear chromosomes, whereas bacteria carry a single circular chromosome.

Sister Chromatid

One of two identical, attached copies of a DNA molecule produced during S phase that together make up a replicated chromosome.

Centromere

A specialized region of a replicated chromosome that physically holds sister chromatids together and serves as the site for kinetochore assembly.

Kinetochore

A multiprotein complex assembled on the centromeric DNA of each chromatid that binds to spindle microtubules to facilitate chromosome movement during nuclear division.

Cytokinesis

The division of the cytoplasm into two daughter cells following mitosis. In animal cells, it occurs via an actin-myosin contractile ring forming a cleavage furrow; in plant cells, it occurs via vesicle fusion constructing a cell plate.

Cyclin

A regulatory protein whose concentration fluctuates cyclically throughout the cell cycle, binding to and activating specific cyclin-dependent kinases (CDKs) to drive checkpoint progression.

Cyclin-Dependent Kinase (CDK)

A protein kinase that is constitutively present in the cell cycle but remains inactive until bound to its specific cyclin partner, after which it phosphorylates target proteins required for cycle progression.

$G_1$ Checkpoint

The primary regulatory transition point assessing whether cell size is adequate, nutrients are sufficient, growth factors are present, DNA is undamaged, and telomeres are sufficiently long before committing to S phase.

p53 Protein

A tumor suppressor protein activated by DNA damage that arrests the cell cycle in $G_1$ by inducing a CDK inhibitor, thereby allowing DNA repair, triggering senescence, or inducing apoptosis if damage is irreparable.

Telomere

A repetitive, non-protein-coding DNA sequence ($5'\text{-TTAGGG-}3'$ in humans) located at the ends of linear eukaryotic chromosomes that buffers against the progressive loss of vital genetic information during lagging-strand synthesis.

Hayflick Limit

The finite number of times a normal human somatic cell population can divide in culture before reaching critical telomere shortening and entering irreversible replicative senescence.

Telomerase

A ribonucleoprotein reverse transcriptase enzyme carrying its own internal RNA template that elongates telomeres at chromosome ends, restoring replicative capacity in germ cells and many cancer cells.

$G_2$ Checkpoint

The cell cycle control point that verifies complete and accurate DNA replication without damage and ensures the presence of activated M-phase promoting factor (MPF) before initiating mitosis.

M-Phase Promoting Factor (MPF)

An active heterodimeric protein complex composed of M-cyclin and a CDK that phosphorylates target proteins to trigger nuclear envelope breakdown, chromosome condensation, and spindle formation.

Metaphase Checkpoint

The mitotic spindle checkpoint that verifies whether all chromosomes are properly aligned at the metaphase plate and correctly attached to spindle microtubules via kinetochores before permitting anaphase to proceed.

Metastasis

The process by which malignant cancer cells detach from a primary tumor and migrate through blood or lymphatic vessels to invade and colonize distant body tissues.

Gamete

A haploid reproductive cell (sperm or egg) produced by meiosis that fuses with another gamete during fertilization to form a diploid zygote.

Homologous Chromosomes (Homologs)

A pair of chromosomes of the same type that contain the same genes in the same linear arrangement, one inherited from each parent, though they may carry different alleles.

Ploidy

The number of complete sets of chromosomes present in a cell, denoted as haploid ($n$), diploid ($2n$), or polyploid ($3n, 4n$, etc.).

Synapsis

The physical pairing and lengthwise alignment of replicated homologous chromosomes during early prophase I of meiosis.

Bivalent (Tetrad)

The structure formed during prophase I by the synapsis of two homologous chromosomes, consisting of four chromatids (two pairs of sister chromatids).

Chiasma (plural: Chiasmata)

An X-shaped physical connection between non-sister chromatids of homologous chromosomes where crossing over and genetic exchange have occurred during prophase I of meiosis.

Reduction Division

The division occurring in meiosis I where homologous chromosome pairs separate into different daughter cells, reducing the chromosome number from diploid ($2n$) to haploid ($n$).

Independent Assortment

The random and independent alignment and segregation of maternal and paternal homologous chromosome pairs at the metaphase I plate, producing $2^n$ possible chromosomal combinations in gametes.

Nondisjunction

An error in nuclear division where homologous chromosome pairs fail to separate at anaphase I, or sister chromatids fail to separate at anaphase II, leading to aneuploid gametes.

Aneuploidy

A condition in which a cell has an abnormal number of chromosomes, having either gained ($2n + 1$) or lost ($2n - 1$) one or more specific chromosomes.

Trisomy 21

An aneuploid condition characterized by the presence of three copies of chromosome 21 ($2n + 1 = 47$), which causes Down syndrome and exhibits an increased incidence with advanced maternal age.

Two-Fold Cost of Males (Paradox of Sex)

The evolutionary puzzle where asexual reproduction produces twice as many child-bearing offspring as sexual reproduction because males do not directly produce offspring, yet sexual reproduction persists broadly in eukaryotes.

Purifying Selection Hypothesis

The evolutionary hypothesis stating that sexual reproduction is maintained because it allows deleterious mutations to be segregated and eliminated from a population, whereas asexual reproduction passes deleterious mutations to all offspring.

Changing-Environment Hypothesis

The evolutionary concept that genetic variation produced by meiosis and outcrossing creates diverse offspring genotypes better equipped to survive changing environmental conditions and evolving pathogens.

Polymorphic Trait

A hereditary characteristic that appears commonly within a population in two or more distinct, easily recognizable phenotypes (such as purple versus white flower color in peas).

Cross-Fertilization

A controlled genetic cross performed by collecting pollen from the male organs of one flower and transferring it to the female organ of another flower whose male organs have been removed.

Pure-Breeding Line (True-Breeding Line)

A population of organisms that, when self-fertilized or mated with identical individuals, produces offspring that consistently exhibit the exact same phenotype across generations because they are homozygous.

Phenotype

The observable physical, physiological, or biochemical traits of an organism, determined by its genotype and modulated by environmental interactions.

Genotype

The specific genetic makeup or combination of alleles present in an individual organism at a given locus (e.g., $BB$, $Bb$, or $bb$).

Reciprocal Cross

A pair of genetic crosses in which the phenotypes of the male and female parents are reversed to determine whether the transmission pattern of a trait depends on parental sex.

Principle of Segregation (Mendel's First Law)

The principle stating that the two alleles of a gene separate during gamete formation, such that each gamete receives only one allele with equal ($50\%$) probability, explained by the physical separation of homologous chromosomes in anaphase I.

Principle of Independent Assortment (Mendel's Second Law)

The principle stating that alleles of different genes assort independently of one another during gamete formation, generating a $9:3:3:1$ phenotypic ratio in an $F_2$ dihybrid cross, provided the genes reside on different chromosomes.

Testcross

A genetic cross between an individual expressing a dominant phenotype (but possessing an unknown genotype) and a tester parent homozygous recessive for all traits examined, used to reveal the unknown genotype.

Sex Linkage

The inheritance pattern observed when a gene is physically located on a sex chromosome (either the X chromosome or Y chromosome in mammals and Drosophila).

Hemizygous

A condition in a diploid organism where only a single copy of a gene is present, such as X-linked genes in males ($XY$) who have no corresponding locus on the Y chromosome.

Genetic Linkage

The tendency of two or more genes located physically close together on the same chromosome to be co-inherited more frequently than expected by independent assortment.

Recombinant Frequency

The proportion of recombinant offspring resulting from crossing over between linked genes, calculated as the total number of recombinant individuals divided by the total number of progeny.

Centimorgan (cM)

A unit of genetic map distance equal to a $1\%$ frequency of recombination between two linked gene loci (also known as a map unit).

Genetic Map

A linear representation showing the relative arrangement and distances of genes along a chromosome, constructed using the frequency of recombination between loci.

Multiple Allelism

The existence of more than two allelic forms of a single gene within a population, such as the $I^A$, $I^B$, and $i$ alleles determining the human ABO blood groups.

Complete Dominance

An allelic relationship in which the phenotype of the heterozygote is indistinguishable from that of the homozygous dominant genotype, completely masking the presence of the recessive allele.

Incomplete Dominance

A pattern of inheritance where the heterozygous genotype displays an intermediate phenotype between the two homozygotes (e.g., pink flowers from red and white parents), producing a $1:2:1$ phenotypic ratio in the $F_2$ generation.

Codominance

A pattern of inheritance in which both alleles in a heterozygote are simultaneously and fully expressed in the phenotype without blending (e.g., $I^A I^B$ individuals exhibiting AB blood type).

Pleiotropy

The phenomenon in which a single gene or mutation influences multiple, seemingly unrelated phenotypic traits, such as seen in Marfan syndrome caused by mutations in the FBN1 gene.

Epistasis

A form of gene interaction where the phenotypic expression of alleles at one gene locus is masked, suppressed, or modified by the genotype at a second, independent gene locus (e.g., coat color in Labrador retrievers yielding a $9:3:4$ ratio).

Quantitative Trait

A continuously variable phenotypic characteristic (such as human height or skin color) controlled by the additive effects of multiple polygenic loci and influenced by environmental factors.