AP Biology: Cell Division, Genetics, and Molecular Biology

Comprehensive vocabulary flashcards covering the cell cycle, mitosis, meiosis, Mendelian inheritance, chromosomal mechanics, DNA replication, and gene expression.

Cell Cycle

The life of a cell from the time it is first formed from a dividing parent cell until its own division into two daughter cells, alternating between interphase ($G_1$, $S$, and $G_2$ phases) and the mitotic (M) phase.

Genome

The entire genetic endowment of a cell or organism, consisting of all of the DNA contained within its chromosomes.

Somatic Cell

Any biological cell forming the body of an organism, excluding reproductive cells (gametes) and their precursors; human somatic cells contain 46 chromosomes.

Gamete

A haploid reproductive cell (such as sperm or egg) that carries one set of chromosomes (23 in humans) and unites with another during sexual reproduction to produce a diploid zygote.

Chromatin

The complex of DNA and associated proteins that constitutes eukaryotic chromosomes; exists in an uncoiled, dispersed form of thin fibers when the cell is not dividing.

Chromosome

A cellular structure carrying genetic material located in the nucleus of eukaryotic cells, composed of one very long DNA molecule associated with proteins.

Sister Chromatids

Two identical copies of an original duplicated chromosome that remain attached along their lengths by sister chromatid cohesion and separate during cell division.

Centromere

A specialized region of DNA on each sister chromatid where the two chromatids are most closely attached by proteins, causing a narrow waist or constriction in the condensed chromosome.

Mitosis

The division of the genetic material in the nucleus of a parent cell, typically followed immediately by cytokinesis to yield two genetically identical nuclei.

Cytokinesis

The division of the cytoplasm that immediately follows mitosis or meiosis, resulting in the separation of the parent cell into distinct daughter cells.

Mitotic Spindle

An apparatus composed of microtubules and associated proteins that coordinates the movement and separation of chromosomes during mitosis.

Centrosome

A subcellular structure functioning as the microtubule-organizing center in animal cells, consisting of a pair of centrioles, which duplicates during interphase.

Cleavage Furrow

A shallow groove that develops on the surface of an animal cell near the metaphase plate, where a contractile ring of actin microfilaments and myosin deepens until the cell pinches into two.

Cell Plate

A double membrane structure assembled in the middle of a dividing plant cell by coalescing Golgi-derived vesicles containing cell wall materials, ultimately giving rise to a new cell wall.

Binary Fission

A method of asexual reproduction by division in half in prokaryotes, wherein replication of the single circular chromosome begins at the origin of replication and the cell elongates before dividing.

Cell Cycle Checkpoint

A regulatory control point in the eukaryotic cell cycle where internal and external stop and go-ahead signals determine whether the cell advances to the next stage.

Cyclin-Dependent Kinases (Cdks)

Enzymes that regulate the cell cycle by transferring phosphate groups from ATP to target proteins, whose catalytic activity depends on binding to specific regulatory cyclin proteins.

MPF (Maturation-Promoting Factor)

A cyclin-Cdk complex that triggers passage past the $G_2$ checkpoint into M phase by phosphorylating structural targets like the nuclear lamina, facilitating nuclear envelope breakdown.

G0 Phase

A quiescent, nondividing state entered by cells that have exited the cell cycle, which may be reversible (e.g., in liver cells) or permanent.

Density-Dependent Inhibition

A regulatory phenomenon wherein external cell-surface contacts between adjacent normal animal cells halt cell division once a continuous monolayer is formed.

Metastasis

The spread of malignant cancer cells from their primary origin to distant tissues and organs through the circulatory or lymphatic systems.

Karyotype

A standardized photographic display of somatic cell chromosomes arranged in homologous pairs according to size, shape, and banding patterns, typically prepared from cells arrested in metaphase.

Homologous Chromosomes

A pair of matching chromosomes of equal length, centromere location, and staining pattern, carrying alleles for the same characters at identical loci, one inherited from each parent.

Autosome

Any chromosome that is not directly involved in determining the sex of an organism; humans possess 22 homologous pairs of autosomes.

Synapsis

The physical pairing and zippered connection of duplicated homologous chromosomes via the synaptonemal complex during prophase I of meiosis.

Crossing Over

The reciprocal exchange of genetic fragments between nonsister chromatids of homologous chromosomes during prophase I of meiosis, creating novel combinations of maternal and paternal alleles.

Chiasmata

X-shaped microscopically visible regions where nonsister chromatids of homologous chromosomes crossed over during prophase I, holding homologs together until anaphase I.

Independent Assortment

The random orientation and segregation of each homologous pair at the metaphase I plate, allowing each pair to distribute maternal and paternal homologs into gametes independently of all other pairs.

Character

A heritable feature that varies among individuals within a population, such as flower color in garden pea plants.

Trait

Each distinct phenotypic variant of a given character, such as purple or white flower coloration.

Allele

Any of the alternative versions of a gene that account for variations in inherited phenotypic characters.

Locus

The specific physical position or site along the length of a chromosome where a particular gene is located.

Law of Segregation

Mendel's principle stating that the two alleles for a heritable character separate during gamete formation and end up in different gametes.

Testcross

A breeding technique involving the cross of an individual with an unknown dominant phenotype to a homozygous recessive individual to deduce the unknown genotype.

Incomplete Dominance

A pattern of inheritance where the heterozygous phenotype is an intermediate blend between the phenotypes of the two homozygous parents (e.g., pink snapdragons produced from red and white parents).

Codominance

An inheritance pattern in which the individual effects of two distinct alleles are simultaneously expressed and distinguishable in the heterozygote (e.g., MN or AB blood groups).

Pleiotropy

The ability of a single gene to influence multiple, seemingly unrelated phenotypic traits or clinical symptoms, as seen in cystic fibrosis and sickle-cell disease.

Epistasis

A genetic interaction in which the phenotypic expression of one gene alters or masks the phenotypic expression of a second, independently inherited gene.

Polygenic Inheritance

An additive mode of inheritance where two or more independent genes exert a cumulative effect on a single quantitative character, such as human skin pigmentation.

Norm of Reaction

The phenotypic range produced by a given genotype when exposed to varying environmental conditions.

Pedigree Analysis

The study of a family tree charting traits and relationships across successive generations to determine the inheritance pattern of human phenotypes or disorders.

Sex-Determination Systems

Biological mechanisms determining genetic sex across species, including the X-Y system (mammals), X-0 system (insects), Z-W system (birds/fishes), and haplo-diploid system (bees/ants).

SRY Gene

The Sex-determining Region of the Y chromosome that initiates testis development in mammalian embryos by encoding a protein that regulates downstream genes.

Barr Body

A highly condensed, transcriptionally inactive X chromosome located along the inside of the nuclear envelope in somatic cells of female mammals.

Linked Genes

Genes positioned near each other on the same chromosome that do not assort independently during meiosis and tend to be inherited together as a unit.

Nondisjunction

An error during meiosis or mitosis in which homologous chromosome pairs or sister chromatids fail to disjoin properly, yielding abnormal chromosome numbers in daughter cells.

Aneuploidy

A condition in which a zygote or cell has an abnormal number of chromosomes, having one or more extra copies (trisomy) or deficient copies (monosomy).

Genomic Imprinting

A parental origin-specific epigenetic modification occurring during gametogenesis that causes offspring to express only one parental allele of an imprinted gene.

Bacterial Transformation

A change in cellular genotype and phenotype resulting from the direct uptake and incorporation of foreign, external naked DNA from the surrounding environment.

Chargaff's Rules

Biochemical rules stating that DNA base composition varies among species, and within any species, the percentage of adenine equals thymine ($A = T$) and guanine equals cytosine ($G = C$).

Semiconservative Replication

The mechanism of DNA duplication wherein each daughter double helix retains one intact parental strand and incorporates one newly synthesized complementary strand.

Leading Strand

The newly replicated complementary DNA strand synthesized continuously toward the replication fork in the mandatory $5' \rightarrow 3'$ direction.

Lagging Strand

The newly synthesized DNA strand elongated discontinuously away from the replication fork in the $5' \rightarrow 3'$ direction as a series of Okazaki fragments.

Okazaki Fragments

Short, disconnected segments of DNA synthesized on the lagging template strand during replication, which are subsequently ligated together by DNA ligase.

Helicase

An enzyme that untwists and unzips the parental double helix at replication forks, breaking hydrogen bonds to separate the two template strands.

Topoisomerase

A protein that relieves excessive twisting strain ahead of the replication fork by breaking, swiveling, and rejoining the parental DNA strands.

DNA Ligase

An enzyme that seals nicks in the sugar-phosphate backbone of DNA by catalyzing phosphodiester bond formation between adjacent Okazaki fragments.

Heterochromatin

Eukaryotic chromatin that remains densely packed and highly condensed during interphase, preventing access to RNA polymerases and silencing transcription.

Euchromatin

The less condensed, dispersed form of eukaryotic chromatin that is accessible to transcription machinery and readily transcribed.

Gene Expression

The overall process by which DNA directs protein synthesis (or functional non-coding RNA synthesis) through transcription and translation.

Transcription

The synthesis of an RNA molecule using a complementary DNA template strand, carried out in the nucleus of eukaryotic cells by RNA polymerase.

Translation

The ribosome-mediated synthesis of a polypeptide chain using the nucleotide sequence information carried by an mRNA transcript.

Codon

A three-nucleotide sequence in mRNA (or DNA) that specifies a specific amino acid or a termination signal during protein translation.

Reading Frame

The specific triplet grouping of consecutive ribonucleotides on mRNA established by the translation initiation complex to ensure correct amino acid sequencing.

Ribozyme

An enzymatic RNA molecule that catalyzes biochemical reactions, such as self-splicing during the excision of pre-mRNA introns.

Anticodon

A specific nucleotide triplet located at one end of a tRNA molecule that forms complementary base pairs with a corresponding codon on mRNA.

Wobble

Flexible base-pairing rules allowing the $5'$ base of a tRNA anticodon to pair with nonstandard bases at the third ($3'$) position of an mRNA codon, reducing the required number of unique tRNAs.

Release Factor

A protein shaped like an aminoacyl-tRNA that recognizes stop codons in the ribosomal A site, triggering the addition of a water molecule to hydrolyze and release the completed polypeptide.