Molecular Biology, Genetics, and Cell Division

Comprehensive vocabulary flashcards covering cell division, life cycles, Mendelian inheritance, chromosome dynamics, DNA replication, gene expression, and regulatory mechanisms.

Asymmetrical cell division

A form of cell division where mother and daughter cells can usually be distinguished by size, with the mother cell producing a daughter cell while remaining essentially unchanged.

Symmetrical cell division

A form of cell division in which a mother cell divides to produce two daughter cells that are essentially identical and cannot be distinguished from one another.

Clones

Genetically identical or virtually identical offspring resulting from asexual reproduction.

Binary fission

A type of asexual cell division in prokaryotes that produces two genetically identical daughter cells from a parent cell.

Mitosis

The type of eukaryotic cell division in which a single nucleus gives rise to two genetically identical daughter nuclei containing the same number of chromosomes as the parent nucleus.

Meiosis

A specialized type of cell division in eukaryotes involving two successive nuclear divisions that halves the amount of DNA, producing four genetically variable haploid cells.

Somatic cell

Any cell in the body of a multicellular organism that is not specialized for reproduction and is derived by mitosis.

Homologous pairs

Pairs of chromosomes that are nearly identical in length and DNA sequence, with one chromosome derived from the maternal parent and the other from the paternal parent.

Gametes

Specialized reproductive cells (such as sperm and egg) containing only a single set of non-homologous chromosomes ($n$) that fuse during fertilization.

Fertilization

The fusion of two haploid gametes, each containing $n$ chromosomes, to produce a diploid zygote with $2n$ chromosomes.

Diplontic life cycle

A sexual life cycle where haploid gametes immediately undergo fertilization, causing essentially all of the life cycle to be spent in the diploid stage.

Haplontic life cycle

A sexual life cycle where the diploid zygote immediately undergoes meiosis, causing essentially all of the life cycle to be spent in the haploid stage.

Haplo-diplontic life cycle

A sexual life cycle in which an organism spends significant time in both haploid and diploid multicellular stages.

ori

The origin of replication; the specific DNA site where replication of the circular prokaryotic chromosome begins.

ter

The terminus of replication; the specific DNA site where replication of the circular prokaryotic chromosome ends.

Z-ring

A contractile ring of protein fibers on the interior surface of the prokaryotic cell membrane that constricts like a drawstring to facilitate cytokinesis.

Interphase

The period of the eukaryotic cell cycle between cell divisions, composed of the $G_1$, $S$, and $G_2$ phases.

Chromatin

The complex of eukaryotic DNA and its associated packaging proteins (most commonly histones) located within the nucleus.

Histones

Structural packaging proteins rich in amino acids with positively charged side chains that interact electrostatically with negatively charged DNA.

Centromere

A specialized chromosomal region where sister chromatids are held together and which serves as the site for chromosome movement during cell division.

Sister chromatids

Two essentially identical daughter DNA molecules and their packaging proteins formed during DNA replication, held together at the centromere prior to anaphase.

Karyotype

The number and visual appearance (sizes and centromere positions) of the condensed chromosomes characteristic of a given organism.

Kinetochores

Specialized protein complexes assembled on each side of the centromere that serve as attachment points for spindle microtubules.

Centrosome

The primary microtubule-organizing center consisting of a pair of centrioles composed of nine microtubule triplets, acting as the poles around which the mitotic spindle forms.

Cytokinesis

The physical division of the cell cytoplasm and its organelles to form two distinct daughter cells following nuclear division.

Independent assortment

The random orientation and independent migration of maternal and paternal homologous chromosome pairs toward opposite poles during anaphase I of meiosis.

Chiasmata

X-shaped attachment points between nonsister chromatids of homologous chromosomes representing sites where crossing over has occurred.

Crossing over

The reciprocal physical breakage and rejoining of segments between nonsister chromatids of paired homologous chromosomes during prophase I of meiosis.

Nondisjunction

An error in cell division occurring when a pair of homologous chromosomes or sister chromatids fails to separate properly during anaphase.

Aneuploidy

A condition in which a cell or organism possesses an abnormal, unbalanced copy number of one or more individual chromosomes.

Polyploidy

The possession of more than two complete sets of chromosomes in a cell or organism, such as triploid ($3n$) or tetraploid ($4n$).

Polyspermy

The fertilization of a single female ovum by more than one male sperm cell, often leading to a triploid state.

Restriction point ($R$)

The primary control checkpoint at the $G_1$-to-$S$ transition where a eukaryotic cell commits to completing the cell cycle and dividing.

Cyclin-dependent kinases (CDKs)

Enzymes that regulate the eukaryotic cell cycle by phosphorylating specific target proteins; they require allosteric binding to a cyclin subunit to become active.

Cyclins

Regulatory protein subunits synthesized and degraded in a cyclic manner that allosterically activate specific cyclin-dependent kinases.

Necrosis

Uncontrolled cell death in multicellular organisms occurring when cells are damaged or starved, leading to swelling, lysis, and localized inflammation.

Apoptosis

A genetically programmed, orderly series of cellular events resulting in controlled cell death and phagocytosis without leaking intracellular contents.

Character

An observable physical feature of an organism, such as flower color or seed shape.

Trait

A particular variant or form of an observable character, such as purple flowers or wrinkled seeds.

Allele

A specific alternative DNA sequence or form of a given gene located at a particular chromosomal locus.

Genotype

The complete allelic makeup of an organism, either with respect to a single gene locus or across its entire genome.

Phenotype

The observable physical characteristics or outward expression resulting from an organism's genotype and environmental influences.

Locus

The specific physical position or address of a gene, base pair, or DNA sequence on a chromosome.

Law of segregation

Mendel's first law stating that when an individual produces gametes, the two alleles of each gene pair separate equally so that half of the gametes receive one allele and half receive the other.

Test cross

A mating between an individual expressing an unknown dominant genotype and a homozygous recessive individual to determine the dominant individual's genotype.

Law of independent assortment

Mendel's second law stating that allele pairs of different genes assort independently of one another during gamete formation.

Pedigree

A standardized family tree diagram documenting the occurrence and inheritance patterns of specific phenotypes across several generations.

Chromosome theory of inheritance

The foundational biological principle stating that genes are discrete physical units carried at specific loci on chromosomes.

Hemizygous

Describing an individual or cell possessing only a single copy of a gene in an otherwise diploid organism, such as X-linked genes in human XY males.

Sex-linked inheritance

The transmission pattern of a gene that is physically carried on a sex chromosome (such as the X chromosome).

Recombination frequency

The proportion of recombinant progeny generated in a test cross, calculated by dividing the number of recombinant offspring by the total number of offspring.

Incomplete dominance

A genetic condition in which neither allele is completely dominant, causing heterozygous individuals to display an intermediate phenotype between the two homozygotes.

Codominance

An inheritance pattern in which two different alleles of a gene in a heterozygote are both simultaneously and fully expressed.

Pleiotropic

Describing a single gene whose protein product functions in multiple systems, thereby influencing multiple distinct phenotypic characters.

Epistasis

A genetic interaction where the phenotypic expression of genotypes at one gene is modified or masked by genotypes present at a second gene.

Phenotypic plasticity

The capacity of a single genotype to produce different phenotypes when exposed to different environmental conditions.

Bacterial conjugation

The unidirectional transfer of genetic material between two bacterial cells via direct physical contact mediated by a sex pilus and a conjugation tube.

Transformation

The genetic alteration of a bacterial cell resulting from the direct uptake and incorporation of exogenous foreign DNA from its environment.

Chargaff's rules

The biochemical principle stating that in double-stranded DNA, the amount of adenine equals thymine ($A = T$) and the amount of guanine equals cytosine ($G = C$).

Semiconservative replication

The mechanism of DNA duplication in which each of the two parental strands acts as a template for synthesizing one complementary daughter strand.

DNA helicase

An enzyme that utilizes energy from ATP hydrolysis to break hydrogen bonds between bases and unwind the double helix at the replication fork.

Topoisomerase

An enzyme that relieves torsional strain ahead of the replication fork by breaking, untwisting, and rejoining the DNA sugar-phosphate backbone.

Primase

A DNA-dependent RNA polymerase that synthesizes a short RNA primer complementary to the template strand to initiate DNA synthesis.

Okazaki fragments

Short stretches of newly synthesized DNA produced discontinuously on the lagging template strand during replication, each initiated with an RNA primer.

DNA ligase

An enzyme that seals single-strand nicks in the DNA backbone by catalyzing phosphodiester bond formation between adjacent fragments.

Telomeres

Repetitive, non-protein-coding DNA sequences at eukaryotic chromosome ends that protect functional genetic material from degradation and end-shortening.

Telomerase

An RNA-dependent DNA polymerase carrying its own RNA template that synthesizes and extends telomeric repeat sequences at eukaryotic chromosome ends.

Mismatch repair

A post-replication repair mechanism where enzymatic complexes detect mismatched bases, excise a short stretch of the newly synthesized strand, and fill the gap.

Point mutation

A genetic mutation involving the substitution, deletion, or insertion of a single nucleotide base pair in a DNA sequence.

Mutagens

Chemical or physical agents that alter the chemical structure of DNA and induce mutations above the spontaneous background rate.

Central dogma of molecular biology

The foundational premise that genetic information flows from DNA through replication, to RNA through transcription, and to polypeptide chains through translation.

Promoter

A specialized DNA sequence upstream of a gene that binds RNA polymerase and regulatory factors to direct the initiation of transcription.

Introns

Noncoding intervening sequences within eukaryotic genes that are transcribed into pre-mRNA but removed during RNA splicing before translation.

Exons

Expressed protein-coding or regulatory RNA sequences in eukaryotic genes that remain in the mature mRNA molecule after splicing.

Spliceosome

A ribonucleoprotein complex composed of small nuclear ribonucleoprotein particles (snRNPs) that excises introns and ligates exons in pre-mRNA.

5′ cap

A chemically modified 7-methylguanosine triphosphate attached backward ($5'$ to $5'$) to the $5'$ end of eukaryotic pre-mRNA to protect it from degradation and facilitate ribosome binding.

Poly A tail

A stretch of $100$ to $300$ adenine nucleotides enzymatically added to the $3'$ end of eukaryotic pre-mRNA that aids nuclear export and enhances stability.

Codon

A sequential triplet of ribonucleotide bases in mRNA read in the $5'$-to-$3'$ direction that encodes a specific amino acid or acts as a stop signal.

Synonymous mutation

A base-pair substitution in a protein-coding sequence that alters a codon but does not change the specified amino acid due to genetic code degeneracy.

Missense mutation

A base-pair substitution in a coding region that changes an mRNA codon such that it specifies a different amino acid.

Nonsense mutation

A point mutation that converts a sense codon specifying an amino acid into a premature stop codon, causing translation to terminate early.

Frame-shift mutation

The insertion or deletion of a number of base pairs not divisible by three, shifting the reading frame and altering all downstream codons.

Anticodon

A triplet of nucleotide bases in a tRNA molecule that base-pairs antiparallel and complementarily with a specific mRNA codon.

Wobble

The nonstandard base pairing permitted at the third position of an mRNA codon ($3'$ end) and the first position of a tRNA anticodon ($5'$ end).

tRNA synthetases

Highly specific enzymes that catalyze the ATP-dependent covalent attachment of each amino acid to its corresponding tRNA molecule.

Polysome

A complex consisting of an mRNA molecule actively and simultaneously translated by multiple ribosomes lined up along its length.

Signal sequence

A short sequence of amino acids in a nascent polypeptide that targets the protein to a specific organelle or cellular destination.

Proteasome

A large cylindrical eukaryotic protein complex that uses ATP to unfold and degrade polyubiquitinated proteins into small peptides.

Operon

A genetic unit found in prokaryotes consisting of a cluster of structural genes transcribed together from a single shared promoter into a single continuous mRNA.

Operator

A regulatory DNA sequence situated within or near an operon's promoter that binds a repressor protein to prevent transcription.

Epigenetic changes

Reversible, heritable modifications to DNA bases or histone proteins that alter gene accessibility and expression without changing the DNA sequence.

Histone acetyltransferases (HATs)

Enzymes that add acetyl groups to positively charged lysine residues on histone tails, neutralizing their charge and opening chromatin for transcription.

CpG islands

Genomic segments rich in cytosine-phosphate-guanine dinucleotides often located near promoters that are predominantly unmethylated in transcriptionally active genes.

Barr body

The highly condensed, transcriptionally inactive heterochromatic X chromosome present in somatic cells of female mammals.

Alternative splicing

A eukaryotic post-transcriptional mechanism by which different combinations of exons are included or excluded, producing multiple functional proteins from one gene.

RNA interference (RNAi)

An evolutionary conserved eukaryotic gene-silencing process where microRNAs (miRNAs) or siRNAs bind target mRNAs to cause their degradation or inhibit translation.