Cell Division, Molecular Genetics, and Inherited Traits

Comprehensive flashcard set for exam preparation covering mitosis, meiosis, chromosome anatomy, cell cycle checkpoints, Mendelian principles, sex linkage, non-Mendelian inheritance patterns, genetic disorders, and chromosome mapping based on lecture notes.

Fertilization

The union of two haploid gametes (sperm and egg) that restores the diploid number ($2n$) and forms a single-celled zygote.

Zygote

A diploid ($2n$) cell produced by the fusion of paternal and maternal gametes during fertilization that subsequently undergoes mitosis to develop into an organism.

Cytokinesis

The physical division of the cytoplasm into two daughter cells, occurring via a contractile ring and cleavage furrow in animal cells, or via Golgi-derived vesicles forming a cell plate in plant cells.

Homologous Chromosomes

Pairs of maternal and paternal chromosomes in a diploid cell that possess the same genes at the same loci, although they may carry different alleles.

Sister Chromatids

Two identical copies of a replicated chromosome produced during S phase that remain physically linked at the centromere by cohesin proteins until anaphase.

Centromere

A constricted chromosomal region that binds specific cohesin and kinetochore proteins, physically joining two sister chromatids together.

Kinetochore

A protein complex assembled onto the centromeric region of a chromosome where spindle microtubules attach to drive chromosome movement during nuclear division.

Binary Fission

The mechanism of asexual cell division in prokaryotes (Bacteria and Archaea) where the single circular double-stranded DNA genome replicates and is partitioned by an FtsZ protein ring.

FtsZ Ring

A ring structure composed of FtsZ protein filaments that forms at the midpoint of a dividing bacterial cell to facilitate septum formation and division.

$G_0$ Phase

A nondividing, quiescent state outside the replicative cell cycle into which cells such as mature neurons are shunted.

$G_1/S$ Checkpoint

The primary irreversible checkpoint of the cell cycle where the cell assesses growth factors, nutritional state, and cell size before committing to genome replication.

$G_2/M$ Checkpoint

The cell cycle control point where active Cdc2/Mitotic Cyclin complexes assess the completion of DNA replication and verify DNA integrity before mitosis begins.

Spindle Checkpoint

The cell cycle checkpoint mediated by the anaphase-promoting complex (APC) ensuring all chromosomes are properly aligned and attached under tension at the metaphase plate before chromatid separation.

Cyclin-Dependent Kinases (CDKs)

Enzymes that attach phosphate groups to specific target proteins to regulate cell cycle transitions, becoming catalytically active only when bound to their corresponding cyclins.

Cyclins

Regulatory proteins whose cellular concentrations fluctuate periodically across different phases of the cell cycle to bind and activate CDKs.

p53 Gene

A crucial tumor-suppressor gene mutated in over half of all human cancers whose normal protein arrests the cell cycle for DNA repair or triggers apoptosis when cellular damage is irreparable.

Taxol (Paclitaxel)

A chemotherapeutic drug discovered in the bark of the Pacific Yew ($\textit{Taxus brevifolia}$) that binds to the $\beta$-tubulin subunit of microtubules to stabilize them, preventing disassembly and halting mitosis.

Synapsis

The tight pairing of replicated homologous chromosomes along their entire length during prophase I of meiosis, held together by the synaptonemal complex.

Synaptonemal Complex

A protein lattice consisting of lateral and central elements that physically bridges paired homologous chromosomes during meiotic synapsis.

Crossing Over

The reciprocal exchange of genetic material between nonsister chromatids of homologous chromosomes during prophase I of meiosis, creating recombinant chromatids.

Chiasma

The X-shaped cytological site where nonsister chromatids remain in physical contact after crossing over, maintaining homologous chromosome alignment until anaphase I.

Barbara McClintock

The geneticist who won the Nobel Prize in 1983 for her cytogenetic discoveries in corn, including crossing over, the roles of centromeres and telomeres, gene regulation, and transposable elements (jumping genes).

Reduction Division

The first meiotic division (meiosis I) in which homologous pairs separate, halving the chromosome number from diploid ($2n$) to haploid ($n$).

Mendel's Principle of Segregation

The genetic principle stating that the two alleles for a gene separate during meiosis I (specifically anaphase I), with each gamete receiving only one allele.

Mendel's Principle of Independent Assortment

The genetic principle stating that alleles of different genes assort independently of one another during gamete formation due to the random alignment of homologous pairs at the metaphase I plate.

Multiplication Rule

A statistical rule stating that the probability of two independent events occurring together is the product of their individual probabilities: $P(A \text{ and } B) = P(A) \cdot P(B)$.

Testcross

A cross between an individual displaying a dominant phenotype (genotype unknown: homozygous dominant or heterozygous) and an individual with a homozygous recessive genotype to deduce the unknown genotype.

Incomplete Dominance

A non-Mendelian pattern of inheritance where the heterozygote exhibits an intermediate phenotype between both homozygotes, such as pink flowers produced from red and white snapdragons.

Codominance

A pattern of inheritance in which neither allele is dominant, and the heterozygote fully expresses distinct phenotypic traits of both homozygotes, such as the AB human blood group.

Pleiotropy

A genetic condition where a single gene or allele influences multiple distinct phenotypic traits, such as the allele in Siamese cats causing both coat pigmentation patterns and crossed eyes.

Epistasis

A gene interaction where alleles at one locus mask or alter the phenotypic expression of alleles at a second distinct gene locus, such as coat coloration in mice.

Polygenic Inheritance

A mode of inheritance in which multiple independent genes influence a single phenotypic character, producing continuous variation in traits like human height and eye color.

SRY Gene

The Sex-determining Region of the Y chromosome that encodes a master switch protein triggering undifferentiated embryonic gonads to develop into testes in mammals.

ZW Sex-Determination System

The chromosomal sex-determination system found in birds where females are heterogametic ($ZW$) and males are homogametic ($ZZ$).

Haplodiploidy

A sex-determination system found in honey bees where diploid ($2n$) females develop from fertilized eggs into workers or queens, and haploid ($n$) males develop from unfertilized eggs into drones.

Barr Body

A highly condensed, transcriptionally inactive X chromosome formed randomly in the somatic cells of female mammals to achieve dosage compensation.

Genetic Linkage

The tendency of two or more genes located physically close together on the same chromosome to be inherited together, violating the law of independent assortment.

Centimorgan ($\text{cM}$)

A unit of genetic distance along a chromosome defined as the distance that yields a recombination frequency of $1\%$.

Cystic Fibrosis

An autosomal recessive genetic disorder occurring in $1/2500$ Caucasian births caused by mutations in the CFTR gene on chromosome $7$, disrupting chloride ion transport and clogging organs with thick mucus.

Sickle Cell Anemia

An autosomal recessive disease affecting $1/600$ African Americans caused by a single amino acid substitution (valine replacing glutamic acid) in the $\beta$ chain of hemoglobin.

Tay-Sachs Disease

An autosomal recessive disorder occurring in $1/3500$ Ashkenazi Jews caused by a defective hexosaminidase A enzyme, leading to central nervous system deterioration in infancy.

Huntington Disease

A lethal autosomal dominant disorder occurring in $1/24{,}000$ individuals that causes gradual deterioration of brain tissue typically beginning in middle age.

Nondisjunction

The failure of homologous chromosomes to separate during meiosis I or sister chromatids to separate during meiosis II, generating aneuploid gametes.

Down Syndrome

A condition caused by trisomy $21$ due to meiotic nondisjunction, strongly correlated with maternal age, though $80\%$ of Down syndrome children are born to mothers under age $35$.

Patau Syndrome

A severe developmental disorder caused by trisomy $13$ ($47,XX,+13$ or partial/mosaic) resulting from meiotic nondisjunction.

Klinefelter Syndrome

A condition occurring in approximately $1$ in $500$ human males characterized by an extra X chromosome ($47,XXY$), tall stature, low testosterone, and reduced fertility.

Turner Syndrome

A condition affecting approximately $1$ in $2{,}500$ human females possessing a single X chromosome ($45,X$ or $XO$), associated with short stature, immature sexual organs, and high spontaneous abortion rates ($98\%$).

Genomic Imprinting

An epigenetic phenomenon where the phenotypic expression of an allele depends strictly on whether it was inherited from the female or male parent.

Maternal Inheritance

The non-chromosomal inheritance of organelle genomes (such as mitochondrial DNA) passed exclusively from mothers to offspring because paternal mitochondria are destroyed following fertilization.

Epigenetic Code

Heritable or persistent modifications to chromatin structure—such as DNA methylation, post-translational histone modifications, and noncoding RNAs—that alter gene expression without altering the primary nucleotide sequence.