DNA Replication, Cell Division, and Genetics Study Guide

Comprehensive vocabulary flashcards covering the enzymes and mechanisms of DNA replication, stages of the cell cycle, mitosis, meiosis, and ploidy concepts.

DNA Polymerase

An enzyme that synthesizes new DNA strands by adding nucleotides in the $5' \rightarrow 3'$ direction from an RNA primer using an original template strand; it replicates DNA during S phase, proofreads to correct errors, and participates in DNA repair.

Primase

An enzyme that synthesizes short RNA sequences called RNA primers to provide a starting point for DNA polymerase; it works once on the leading strand and repeatedly on the lagging strand to initiate each Okazaki fragment.

Topoisomerase

An enzyme that works ahead of helicase to prevent DNA from becoming overwound or supercoiled by temporarily cutting, untwisting, and rejoining DNA strands to relieve mechanical tension and prevent knots.

Helicase

An enzyme that unzips the DNA double helix near the origin of replication by breaking hydrogen bonds between complementary base pairs, separating the two strands and opening the replication fork.

Telomerase

An enzyme active in stem cells, reproductive cells, and many cancer cells that solves the end-replication problem by adding repetitive DNA sequences to chromosome ends, rebuilding lost DNA and maintaining chromosome stability.

Telomeres

Protective non-coding caps located at the ends of eukaryotic chromosomes that prevent chromosomes from losing essential genetic information or shortening excessively during successive rounds of replication.

Leading Strand

The newly synthesized DNA strand that is built continuously by DNA polymerase in the $5' \rightarrow 3'$ direction toward the replication fork, requiring only one initial RNA primer.

Lagging Strand

The newly synthesized DNA strand that is built discontinuously in short segments away from the replication fork; it requires multiple RNA primers and undergoes synthesis in the $5' \rightarrow 3'$ direction.

DNA Ligase

An enzyme that covalently joins Okazaki fragments together on the lagging strand to create an intact, continuous DNA backbone.

RNA Polymerase

An enzyme utilized during transcription that synthesizes RNA (such as mRNA, tRNA, or rRNA) from a DNA template without needing a primer, substituting uracil (U) for thymine (T).

RNA Primers

Short RNA sequences synthesized by primase that provide the necessary free $3'$ hydroxyl starting point for DNA polymerase to initiate synthesis; they are later removed and replaced with DNA nucleotides.

Okazaki Fragments

Short segments of newly synthesized DNA formed discontinuously on the lagging strand, each beginning with an RNA primer and subsequently linked by DNA ligase; named after Reiji Okazaki.

Phosphodiester Linkages

Strong covalent chemical bonds that join adjacent nucleotides by connecting a phosphate group to the $3'$ carbon of one sugar molecule and the $5'$ carbon of the next.

Single-Stranded Binding Proteins

Proteins that attach to exposed single strands of DNA after helicase unwinds them, preventing the complementary strands from rejoining or tangling and keeping them stable for replication.

Replication Fork

The Y-shaped region of unwound DNA created by helicase where the replication machinery functions to synthesize both the leading and lagging strands simultaneously.

Origin of Replication

The specific sequence or site on a DNA molecule where helicase begins unwinding the double helix and replication is initiated.

Enzymes

Biological catalysts—typically proteins often ending in the suffix '-ase'—that speed up chemical reactions, lower activation energy, and can be reused repeatedly without being consumed.

$G_1$ Phase (Gap 1)

The first phase of interphase during which the cell grows, synthesizes proteins, builds organelles, performs standard metabolic functions, and assesses the 'Should I divide?' checkpoint.

S Phase (Synthesis)

The subphase of the cell cycle where DNA replication occurs, causing single chromosomes to become duplicated sister chromatids and doubling the total DNA content of the cell.

$G_2$ Phase (Gap 2)

The final stage of interphase in which the cell grows further, synthesizes proteins required for division, repairs damaged DNA, and clears the final checkpoint prior to entering mitosis.

Microtubule Organizing Center (MTOC)

The cellular structure that organizes microtubules and builds spindle fibers to separate chromosomes during cell division; represented by the centrosome in animal cells.

Prophase (Mitosis)

The initial stage of mitosis characterized by the condensation of chromosomes, the start of nuclear envelope breakdown, the formation of mitotic spindle fibers, and the migration of centrioles to opposite poles.

Prometaphase

The mitotic phase during which the nuclear envelope completely breaks down, spindle fibers attach to the centromeres of chromosomes, and chromosomes begin directional movement.

Metaphase (Mitosis)

The phase of mitosis in which fully condensed chromosomes line up single file across the middle (metaphase plate) of the cell with spindle fibers attached from both poles.

Anaphase (Mitosis)

The mitotic phase wherein centromeres split and sister chromatids are pulled apart by spindle fibers toward opposite poles, officially becoming individual daughter chromosomes.

Telophase (Mitosis)

The concluding phase of mitosis where nuclear envelopes re-form around each set of daughter chromosomes, nucleoli reappear, chromosomes decondense/uncoil, and two separate nuclei form.

Cytokinesis

The division of the cytoplasm and its organelles into two distinct daughter cells following the completion of nuclear division.

Cleavage Furrow

The shallow groove or indentation that appears around the cell equator as the first observable physical sign of cytokinesis in animal cells.

Cell Plate

The newly forming partition along the midline of a dividing plant cell that marks the first visible sign of cytokinesis, ultimately maturing into a rigid cell wall.

Mitosis

A form of eukaryotic nuclear division that yields two genetically identical daughter cells, maintaining the parent chromosome number except in instances of replication error or mutation.

Synapsis

The pairing and physical association of homologous chromosomes side-by-side during Prophase I of meiosis, resulting in the formation of tetrads.

Crossing Over

The exchange of genetic material between non-sister chromatids of homologous chromosomes during Prophase I of meiosis, which creates new allele combinations and increases genetic variation.

Metaphase I

The stage of meiosis I where paired homologous chromosomes (tetrads) align along the middle of the cell.

Anaphase I

The stage of meiosis I where homologous chromosomes are separated and pulled to opposite poles of the cell, while sister chromatids remain attached at their centromeres.

Telophase I

The final stage of meiosis I where homologous chromosomes reach opposite poles and two haploid nuclei begin to form.

Prophase II

The opening stage of meiosis II where chromosomes condense once again and a new spindle apparatus forms in each haploid daughter cell.

Metaphase II

The stage of meiosis II where individual chromosomes, consisting of paired sister chromatids, line up along the middle of each cell.

Anaphase II

The meiotic stage where centromeres split and sister chromatids separate from one another, migrating to opposite cellular poles as daughter chromosomes.

Telophase II

The terminal stage of meiosis II where nuclear envelopes reform around four genetically diverse chromosome sets and chromosomes uncoil, producing four haploid nuclei.

Binary Fission

The method of asexual reproduction utilized by prokaryotic cells in which a single circular DNA molecule replicates and the cell divides into two identical daughter cells.

Centromere

The specialized, constricted chromosomal region that holds sister chromatids together and serves as the attachment site for spindle fibers during cell division.

Sister Chromatids

Identical copies of a single replicated chromosome synthesized during S phase that remain physically linked together at a shared centromere until anaphase.

Chromosome

A thread-like biological structure composed of tightly packaged DNA and proteins that stores and transmits genetic information.

Nondisjunction

The failure of homologous chromosomes or sister chromatids to separate properly during meiotic or mitotic anaphase, causing daughter cells to have an abnormal number of chromosomes (e.g., Down syndrome).

Diploid ($2n$)

A cell or organism possessing two complete sets of chromosomes ($2n$)—one inherited from each biological parent—characteristic of somatic body cells (e.g., humans have $2n = 46$).

Haploid ($n$)

A cell containing exactly one set of unpaired chromosomes ($n$)—half the diploid count—characteristic of reproductive gametes such as sperm and egg cells (e.g., human gametes have $n = 23$).