Molecular Biology, Cellular Division, and Genetics

Comprehensive vocabulary flashcards covering the cell cycle, mitosis, meiosis, Central Dogma, transcription, RNA processing, the genetic code, translation, mutations, and Mendelian and non-Mendelian inheritance patterns.

Cell Division

The biological process by which a parent cell divides into two or more daughter cells; in multicellular organisms, it functions in growth, development, and tissue repair or replacement of old cells, while in unicellular organisms, it functions in reproduction.

Somatic Cells

Non-reproductive biological cells that form the body of a multicellular organism, typically maintaining a diploid ($2n$) chromosome number and dividing via mitosis.

Gametes

Specialized haploid ($n$) reproductive cells, such as sperm and eggs, produced through meiosis that unite during fertilization.

Chromatin

The structural complex of DNA (nucleic acid) and histone proteins found in eukaryotic nuclei that condenses to form discrete chromosomes during cell division.

Histones

Positively charged, globular proteins around which eukaryotic DNA winds to form nucleosomes, facilitating the compact packaging of chromatin.

Centromere

The specialized, constricted region of a chromosome where replicated sister chromatids are held together and where spindle fibers attach during cell division.

Sister Chromatids

The two identical, replicated copies of a chromosome produced during $S$ phase that are joined together at the centromere and segregated during anaphase.

Cell Cycle

The ordered sequence of phases through which a eukaryotic cell progresses to replicate its genetic material and divide, consisting of interphase ($G_1$, $S$, and $G_2$ phases) and mitotic (M) phase.

$S$ Phase

The synthesis subphase of interphase during which genomic DNA replication occurs, doubling the quantity of DNA so each chromosome consists of two sister chromatids.

Ploidy

The number of complete sets of chromosomes present in the nucleus of a biological cell, designated by the variable $n$ (e.g., haploid $n$, diploid $2n$, or triploid $3n$).

Binary Fission

The asexual reproductive division process utilized by prokaryotes (bacteria and archaea) in which a single circular DNA chromosome replicates and the cell divides into two identical daughter cells.

Mitosis

The process of eukaryotic nuclear division that segregates replicated chromosomes equally into two daughter nuclei, conserving the parent cell's ploidy (e.g., $2n \rightarrow 2n$).

Metaphase Plate

The imaginary plane equidistant between the two spindle poles where chromosomes align along their centromeres during metaphase.

Central Dogma

The foundational principle articulated by Francis Crick stating that genetic sequence information flows from DNA to RNA to protein, and that once information has entered a protein, it cannot flow back into nucleic acids.

Transcription

The enzymatic process by which an RNA polymerase enzyme synthesizes a single-stranded RNA molecule complementary and antiparallel to a DNA template strand.

RNA Polymerase

The enzyme complex responsible for synthesizing RNA from ribonucleotide triphosphates along a DNA template strand in the $5' \rightarrow 3'$ direction without requiring an initial primer.

Template Strand

The specific DNA strand that is read in the $3' \rightarrow 5'$ direction by RNA polymerase to build a complementary, antiparallel RNA molecule.

Coding Strand

The strand of DNA that is not transcribed by RNA polymerase; its nucleotide sequence matches the synthesized mRNA in the $5' \rightarrow 3'$ direction, with thymine ($T$) in DNA replaced by uracil ($U$) in RNA.

Promoter

A specific regulatory DNA sequence situated upstream of the transcription start site that binds RNA polymerase and initiation proteins to determine the site and direction of transcription.

Sigma (\sigma)

A bacterial protein that binds to core RNA polymerase to form the holoenzyme, guiding the complex to bind specific $-35$ and $-10$ promoter boxes in prokaryotes.

TATA Box

A conserved eukaryotic promoter sequence located approximately $30$ base pairs upstream of the transcription start site that is recognized by basal transcription factors.

Basal Transcription Factors

A group of eukaryotic accessory proteins that bind to promoter sequences (including the TATA box) to assemble the transcription pre-initiation complex and recruit RNA polymerase.

Phosphodiester Bond

The covalent bond that links adjacent nucleotides together in a nucleic acid chain, formed between the $3'\text{-OH}$ group of one nucleotide sugar and the $5'$ phosphate group of the next via a condensation reaction.

Pre-mRNA

The primary, unedited RNA transcript synthesized directly from eukaryotic DNA before undergoing post-transcriptional processing in the nucleus.

Exons

The protein-coding regions of eukaryotic pre-mRNA transcripts that are preserved and ligated together during RNA splicing to appear in mature mRNA.

Introns

The non-coding intervening sequences within eukaryotic pre-mRNA transcripts that are excised and removed during nuclear RNA splicing.

Splicing

The eukaryotic post-transcriptional processing event in which introns are enzymatically removed from pre-mRNA and exons are spliced together.

$5'$ Cap

A modified 7-methylguanylate nucleotide added to the $5'$ end of eukaryotic pre-mRNA that protects the molecule from enzymatic degradation and acts as a recognition signal for ribosome binding.

Poly(A) Tail

A stretch of $50\text{--}200$ adenine nucleotides added to the $3'$ end of eukaryotic pre-mRNA following cleavage at the poly(A) signal, preventing RNA degradation and facilitating nuclear export.

Codon

A three-nucleotide sequence in mRNA that specifies a single amino acid or signals the initiation or termination of polypeptide synthesis during translation.

Open Reading Frame (ORF)

A contiguous sequence of codons in DNA or RNA that starts with an initiation codon ($5'\text{-AUG-}3'$) and concludes with an in-frame stop codon, encoding a continuous polypeptide.

Redundancy of the Genetic Code

The characteristic of the genetic code wherein most of the $20$ standard amino acids are encoded by more than one distinct triplet codon.

Unambiguity of the Genetic Code

The rule of the genetic code establishing that a specific triplet codon always specifies one, and only one, particular amino acid.

Conservative Nature of the Genetic Code

The property of the genetic code in which alternative codons specifying the same amino acid almost always share the first two nucleotide bases, minimizing the impact of third-base mutations.

Mutation

A permanent, heritable alteration in the nucleotide sequence of an organism's DNA resulting from replication errors or environmental damage.

Point Mutation

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

Silent Mutation

A point mutation that changes a codon sequence in DNA and mRNA but, owing to the redundancy of the genetic code, does not alter the specified amino acid or resulting protein.

Missense Mutation

A point mutation in which a single base pair change results in the substitution of one amino acid for another in the encoded polypeptide sequence.

Nonsense Mutation

A point mutation that prematurely converts an amino acid-specifying codon into a stop codon, causing premature termination of translation and a truncated polypeptide.

Frameshift Mutation

A mutation caused by the insertion or deletion of a number of nucleotides that is not a multiple of three, shifting the reading frame of all downstream triplet codons.

Polymorphism

A common, naturally occurring DNA sequence variation present in more than $1\%$ of a population that typically does not cause overt disease.

Transfer RNA (tRNA)

An L-shaped adaptor RNA molecule containing an anticodon loop and an amino acid attachment site ($\text{CCA-3'}$) that transports specified amino acids to the ribosome during translation.

Anticodon

A sequence of three nucleotides in a tRNA loop that forms antiparallel, complementary hydrogen bonds with a specific triplet codon on mRNA.

Aminoacyl-tRNA Synthetase

An enzyme that catalyzes the covalent attachment of a specific amino acid to its cognate tRNA molecule in an ATP-dependent reaction, charging the tRNA.

Ribosome

A macromolecular ribonucleoprotein machine consisting of large and small subunits composed of ribosomal RNA (rRNA) and proteins that synthesizes polypeptides from mRNA templates.

A Site

The aminoacyl site of the ribosome that accepts and binds an incoming charged aminoacyl-tRNA carrying the next amino acid for the polypeptide chain.

P Site

The peptidyl site of the ribosome that holds the tRNA molecule linked to the growing polypeptide chain.

E Site

The exit site of the ribosome where an uncharged tRNA is positioned immediately before it dissociates and leaves the ribosome.

Translocation

The elongation step in which the ribosome shifts three nucleotides ($1$ codon) downstream along mRNA in the $5' \rightarrow 3'$ direction, moving tRNAs from the A and P sites into the P and E sites, respectively.

Wobble Base Pairing

The relaxed hydrogen-bonding flexibility at the third base ($3'$ position) of an mRNA codon, enabling approximately $40$ unique tRNAs to decode all $61$ amino acid codons.

Shine-Dalgarno Sequence

A conserved purine-rich RNA sequence situated upstream of the start codon in prokaryotic mRNA that base-pairs with the small ribosomal subunit rRNA to align the ribosome for translation.

Polyribosome

A complex of multiple ribosomes translating the same mRNA molecule simultaneously, rapidly producing multiple polypeptide copies from a single transcript.

Release Factor

A protein that enters the ribosomal A site upon encountering an in-frame stop codon, catalyzing the hydrolysis and release of the completed polypeptide from the P-site tRNA.

Meiosis

A two-stage cell division process ($\text{Meiosis I}$ and $\text{Meiosis II}$) occurring in sexually reproducing organisms that reduces chromosome ploidy from diploid ($2n$) to haploid ($n$), producing four genetically distinct gametes.

Homologous Chromosomes

A pair of chromosomes in a diploid cell that possess the same length, centromere position, and linear arrangement of gene loci, with one inherited from the mother and one from the father.

Synapsis

The physical side-by-side pairing and tight alignment of homologous chromosomes along their lengths during prophase I of meiosis.

Crossing Over

The reciprocal exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis, resulting in recombinant chromosomes with unique allele combinations.

Zygote

The initial diploid ($2n$) cell produced by the fertilization and fusion of two haploid ($n$) gametes (sperm and egg).

Allele

A specific alternative nucleotide sequence variant of a gene located at a defined chromosomal locus.

Genotype

The underlying genetic composition or combination of alleles present at specific gene loci in an organism.

Phenotype

The observable physical, physiological, metabolic, or behavioral characteristics of an organism resulting from its genotype and environmental interactions.

Homozygous

Having two identical alleles at a given locus on homologous chromosomes (e.g., $FF$ or $ff$).

Heterozygous

Having two different alleles at a given locus on homologous chromosomes (e.g., $Ff$).

Dominant Allele

An allele that determines the phenotype of a heterozygote, masking the expression of a co-occurring recessive allele.

Recessive Allele

An allele whose phenotypic effect is masked in the presence of a dominant allele and is only visibly expressed when the individual is homozygous for that allele.

Principle of Segregation

Mendel's First Law stating that the two alleles for each gene segregate from each other during gamete formation in meiosis I, so each gamete has a $50\%$ chance of carrying either allele.

Principle of Independent Assortment

Mendel's Second Law stating that alleles of two or more different genes sort into gametes independently of one another, provided the genes reside on different non-homologous chromosomes.

Monohybrid Cross

A genetic mating experiment between two individuals that are both heterozygous for a single gene of interest (e.g., $Ff \times Ff$).

Autosomes

Any chromosomes that are not sex chromosomes, comprising pairs $1\text{--}22$ in humans.

Pedigree

A diagrammatic family tree representing ancestral relationships, biological sex, and phenotypic expression used to trace the inheritance pattern of specific traits across multiple generations.

Carrier

An individual who is heterozygous for a recessive allele ($Aa$), carrying and transmitting the allele without exhibiting the associated phenotype.

Hemizygous

The condition of possessing only a single copy of a gene in a diploid organism, characteristic of genes located on the X chromosome in biological males ($XY$).

X-Linked Recessive Trait

A trait governed by a recessive allele on the X chromosome that is expressed phenotypically in homozygous females ($X^b X^b$) and hemizygous males ($X^b Y$), affecting males with significantly higher frequency.

X-Linked Dominant Trait

A trait governed by a dominant allele on the X chromosome that is expressed when present in one copy; affected fathers pass the trait to all ($100\%$) of their daughters and none ($0\%$) of their sons.

Codominance

A non-Mendelian inheritance pattern in which the distinct phenotypic traits of both alleles are simultaneously and fully expressed in a heterozygous individual without blending.

Incomplete Dominance

A non-Mendelian inheritance pattern in which neither allele is fully dominant, producing a heterozygous phenotype that is intermediate between the two homozygous phenotypes.