Biology 104: DNA Structure, Cell Division, and Gene Expression

A comprehensive vocabulary review set covering DNA structure, semiconservative replication, the eukaryotic cell cycle, mitosis, cytokinesis, and gene expression (transcription and translation).

S Strain (Streptococcus pneumoniae)

A virulent bacterial strain that produces a smooth capsule and causes fatal pneumonia infections in mice.

R Strain (Streptococcus pneumoniae)

A nonvirulent bacterial strain that lacks a protective capsule, displays a rough colony appearance, and does not cause death in mice.

Transformation

The uptake of genetic material from dead cells (such as heat-killed S strain) by recipient live cells (such as R strain), resulting in an inherited phenotypic change.

Transforming Principle

The chemical substance demonstrated by Oswald Avery and colleagues in 1944 to be DNA, which transforms R strain bacteria into virulent S strain and is degraded specifically by DNase.

Hershey-Chase Experiment

A 1950s study using radioactive $^{32}\text{P}$ (labeling DNA) and $^{35}\text{S}$ (labeling protein capsids) that proved viral DNA enters host bacteria while viral protein stays outside.

Deoxyribonucleic Acid (DNA)

The nucleic acid polymer that stores hereditary information for cell structure, development, and metabolism, capable of high-fidelity replication and transmission across generations.

Nucleotide

The basic building block of DNA and RNA, composed of a phosphate group (phosphoric acid), a pentose sugar (deoxyribose in DNA), and a nitrogen-containing base.

Purines

Nitrogenous bases characterized by a double-ring structure; specifically adenine (A) and guanine (G).

Pyrimidines

Nitrogenous bases characterized by a single-ring structure; specifically thymine (T) and cytosine (C) in DNA.

Complementary Base Pairing

The specific pairing in DNA where adenine binds to thymine via two hydrogen bonds, and guanine binds to cytosine via three hydrogen bonds, consistently pairing a purine with a pyrimidine.

Antiparallel

The opposing arrangement of the two sugar-phosphate backbones in a DNA double helix, where the $5'$ carbon is uppermost on one strand and the $3'$ carbon is uppermost on the complementary strand.

Semiconservative Replication

The mode of DNA replication where each newly synthesized double helix contains one conserved parental template strand and one newly formed complementary daughter strand.

DNA Helicase

The replication enzyme that separates double-stranded DNA by breaking the hydrogen bonds holding complementary base pairs together.

DNA Polymerase

The enzyme that positions and synthesizes new complementary DNA nucleotides along the parental template strand during replication.

Replication Fork

The Y-shaped structure formed when DNA helicase unwinds parental DNA strands so replication enzymes can access the template strands.

Leading Strand

The new DNA strand that is synthesized continuously toward the replication fork, following the directional unwinding of DNA helicase.

Lagging Strand

The new DNA strand that is synthesized discontinuously away from the replication fork, generating short pieces of DNA called Okazaki fragments.

Okazaki Fragments

Short fragments of newly synthesized DNA produced discontinuously on the lagging template strand.

DNA Ligase

The enzyme responsible for sealing nicks in the sugar-phosphate backbone and joining Okazaki fragments together on the lagging strand.

Nucleotide Analogs

Chemical compounds with structures resembling normal DNA nucleotides that halt replication and induce death in rapidly dividing cancer cells when incorporated.

Somatic Cells

Non-reproductive body cells that continuously divide to facilitate adult tissue replacement, repair, and growth.

Apoptosis

Programmed cell death marked by cell rounding, chromatin condensation, membrane blebbing, and cellular fragmentation, which sculpts tissues during development and prevents cancer.

Caspases

Enzymes that mediate the destructive events of apoptosis; typically restrained by internal inhibitors until released by specific death signals.

Cell Cycle

The orderly sequence of stages occurring from the time a cell is formed through division until it divides into two daughter cells.

Interphase

The prolonged phase of the cell cycle comprising $G_1$, $S$, and $G_2$ stages during which a cell grows, metabolizes, and replicates its chromosomes.

$G_1$ Phase

The first interphase stage where a cell doubles its organelles and accumulates the chemical precursors necessary for DNA replication.

$G_0$ Phase

A resting state entered by non-dividing cells that pause from cycle progression, continuing regular metabolic tasks without preparing for DNA synthesis.

S Phase

The interphase stage dedicated to DNA synthesis, wherein each single-chromatid chromosome is duplicated into two identical sister chromatids.

Sister Chromatids

Two identical copies of a duplicated chromosome produced during the S phase, physically held together at a region known as the centromere.

$G_2$ Phase

The interphase period occurring after DNA synthesis, designated for the production of proteins required for mitotic cell division.

Cyclin

An internal regulatory signaling protein whose fluctuating cellular levels serve as a timing control mechanism for cell cycle transitions.

Growth Factors

External signaling proteins, such as those secreted by platelets to stimulate skin fibroblasts, that prompt mammalian cells to initiate the cell cycle.

$G_1$ Checkpoint

The primary cell cycle checkpoint that verifies DNA integrity before committing the cell to division, initiating apoptosis if the DNA cannot be repaired.

$G_2$ Checkpoint

The mitotic checkpoint that ensures DNA replication has been completed properly before permitting entry into mitosis.

M Checkpoint

The spindle assembly checkpoint during mitosis that prevents entry into anaphase if chromosomes are not aligned correctly at the metaphase plate.

Proto-oncogenes

Normal cellular genes that produce growth factors, receptors, or stimulatory proteins that drive the cell cycle forward and suppress apoptosis.

Oncogenes

Mutated forms of proto-oncogenes that stimulate the cell cycle constantly and inappropriately, driving uncontrolled division.

Tumor Suppressor Genes

Regulatory genes encoding proteins that halt the cell cycle and trigger apoptosis upon detecting cellular damage; function as cellular brakes.

Carcinogenesis

The multi-step process by which normal cells transform into cancer cells due to accumulated mutations that disrupt division controls.

Chromatin

The network of DNA strands and associated proteins, largely histones, present in an uncoiled state inside non-dividing eukaryotic nuclei.

Histones

Basic proteins that bind to eukaryotic DNA, facilitating the dense coiling and condensation of chromatin into visible chromosomes.

Diploid ($2n$)

A condition in which a cell contains two chromosomes of each type, organized as homologous pairs (e.g., $2n = 46$ in humans).

Haploid ($1n$)

A condition in which a cell has only one copy of each chromosome type, representing half the diploid number (e.g., $1n = 23$ in human gametes).

Centromere

The constricted chromosomal region where sister chromatids join and where kinetochores assemble during nuclear division.

Centrosome

The main microtubule-organizing center of the cell that duplicates prior to division and organizes spindle fibers during mitosis.

Centrioles

A pair of barrel-shaped microtubule structures found within animal centrosomes that are completely absent in plant cells.

Prophase

The initial mitotic stage where chromatin condenses into distinct chromosomes, the nucleolus disappears, and the mitotic spindle begins assembling.

Prometaphase

The mitotic phase marked by the breakdown of the nuclear envelope, appearance of kinetochores on centromeres, and attachment of kinetochore spindle fibers.

Kinetochore

A specialized protein complex assembled on each side of the centromere that anchors kinetochore spindle fibers during mitosis.

Metaphase

The mitotic phase where fully formed spindle fibers align chromosomes along the equatorial center of the cell at the metaphase plate.

Anaphase

The stage of mitosis where centromeres split and sister chromatids separate into daughter chromosomes, pulled to opposite poles as kinetochore fibers shorten.

Telophase

The final stage of mitosis wherein the spindle disassembles, nuclear envelopes reform around daughter chromosomes, nucleoli reappear, and chromosomes uncoil.

Cleavage Furrow

A shallow groove that develops in the plasma membrane of animal cells during cytokinesis, constricted by an actin contractile ring to divide the cytoplasm.

Cell Plate

A newly formed disk between dividing plant daughter cells derived from fusing Golgi vesicles, giving rise to new plasma membranes and cell walls.

Meristematic Tissue

Plant tissue found in root and shoot tips that retains the capacity to divide by mitosis throughout the life of the plant.

Gene Expression

The process by which the nucleotide sequence of a gene is converted into a functional cellular product, typically through transcription and translation.

Transcription

The nuclear process wherein a designated segment of DNA functions as a template to synthesize a complementary RNA transcript.

Translation

The cytoplasmic process taking place at ribosomes where the codon sequence of an mRNA molecule directs the sequence of amino acids in a polypeptide.

Messenger RNA (mRNA)

A linear RNA transcript that carries genetic coding instructions from nuclear DNA out to cytoplasmic ribosomes for polypeptide synthesis.

RNA Polymerase

The enzyme that opens the DNA double helix at a promoter region and synthesizes a primary RNA transcript by adding complementary RNA nucleotides.

Promoter

A specific sequence of DNA nucleotides located upstream of a gene where RNA polymerase binds to initiate transcription.

Introns

Noncoding, intervening sequences within eukaryotic pre-mRNA transcripts that are removed during RNA splicing before the mRNA leaves the nucleus.

Exons

The protein-coding portions of eukaryotic genes that are preserved and spliced together to yield mature mRNA transcripts.

$5'$ Cap

A modified guanine nucleotide added to the $5'$ terminus of eukaryotic pre-mRNA to help stabilize the molecule and facilitate ribosome binding.

$3'$ Poly-A Tail

A chain of adenine nucleotides appended to the $3'$ end of eukaryotic pre-mRNA to assist in nuclear export and resist enzymatic degradation.

Spliceosome

A complex of proteins and small RNA molecules that excises introns and ligates exons together during eukaryotic mRNA processing.

Codon

A three-nucleotide sequence in mRNA that corresponds to a specific amino acid, start signal, or termination stop signal.

Degenerate Code

The structural property of the genetic code in which multiple distinct codons specify the exact same amino acid (e.g., 61 codons for 20 amino acids).

Start Codon

The specific mRNA triplet AUG that initiates translation and codes for the insertion of methionine.

Stop Codons

The three codons (UAA, UGA, and UAG) that signal the termination of translation rather than coding for an amino acid.

Transfer RNA (tRNA)

A small boot-shaped RNA molecule that carries a specific amino acid at one end and contains a complementary anticodon at the opposite end.

Anticodon

A three-base sequence located on a tRNA molecule that forms complementary base pairs with a specific mRNA codon at the ribosome.

Ribosome

A cellular structure composed of proteins and ribosomal RNA (rRNA) organized into small and large subunits, coordinating translation.

A (Amino Acid) Site

The ribosomal binding site that accepts an incoming aminoacyl-tRNA carrying the subsequent amino acid for the growing peptide chain.

P (Peptide) Site

The ribosomal binding site that retains the tRNA attached to the growing peptide chain before transfer.

E (Exit) Site

The ribosomal site where a discharged, unlinked tRNA pauses briefly before being released into the cytoplasm.

Polyribosome

A complex consisting of several ribosomes simultaneously reading and translating a single mRNA molecule to rapidly synthesize multiple polypeptides.

Translocation

The mechanical step in translation elongation where the ribosome advances one codon along the mRNA, shifting the peptidyl-tRNA from the A site to the P site.

Release Factor

A protein that binds directly to a stop codon occupying the ribosomal A site, hydrolyzing the bond between the completed polypeptide and the last tRNA at the P site.