Molecules of Heredity, Gene Expression, and Cellular Reproduction Exam 4

Gene

A discrete unit of heritable information carried on chromosomes that encodes information for traits and proteins.

Chromosome

A cellular structure composed of deoxyribonucleic acid (DNA) and protein that bears the hereditary units (genes).

Nucleotide

The repeating subunit building block of DNA and RNA, composed of a phosphate group, a five-carbon pentose sugar, and a nitrogen-containing base.

Chargaff's Rule

A principle determined in 1940 by biochemist E. Chargaff stating that within a DNA molecule, the amount of adenine equals thymine ($A = T$) and the amount of guanine equals cytosine ($G = C$).

X-ray Diffraction Analysis of DNA

The experimental technique used by Rosalind Franklin and Maurice Wilkins in the 1940s to demonstrate that DNA is helical, twisted like a corkscrew, long, thin, and composed of repeating subunits.

Sugar-Phosphate Backbone

The alternating chain of deoxyribose sugar and phosphate groups that forms the outer structural framework of each DNA strand.

Antiparallel Strands

The orientation of the two strands in a DNA double helix, where one strand runs in the $5'$ to $3'$ direction and the opposite strand runs in the $3'$ to $5'$ direction.

Complementary Base Pairs

Specific pairs of nitrogen-containing bases held together by hydrogen bonds along the interior of the DNA double helix ($A$ pairs with $T$, and $G$ pairs with $C$).

DNA Helicase

The enzyme that initiates DNA replication by separating the two parent DNA strands, breaking the hydrogen bonds between complementary bases to form replication forks.

Replication Bubbles

Unwound and open regions of DNA where replication occurs; bacteria (prokaryotes) form a single bubble, whereas eukaryotic chromosomes possess many bubbles.

DNA Polymerase

The enzyme that synthesizes new complementary DNA strands by positioning free nucleotides opposite template bases, adding nucleotides strictly to the $3'$ end in a $5'$ to $3'$ direction.

Leading Strand

The strand of new DNA that is synthesized continuously as a single strand from the point of origin toward the advancing replication fork.

Lagging Strand

The strand of new DNA that is synthesized discontinuously in short segments away from the overall direction of replication, moving from the replication fork back toward the origin.

Okazaki Fragments

The series of short, discontinuously synthesized DNA segments formed along the lagging strand during replication.

DNA Ligase

The enzyme responsible for covalently joining Okazaki fragments together into a continuous DNA strand on the lagging strand.

Semiconservative Replication

The mechanism of DNA duplication yielding two daughter DNA molecules, each containing one conserved parental strand and one newly synthesized strand.

Point Mutation

A mutation in which a single individual nucleotide pair within a DNA sequence is altered or substituted.

Insertion Mutation

A mutation resulting from the addition of one or more nucleotide pairs into the DNA double helix.

Deletion Mutation

A mutation resulting from the removal of one or more nucleotide pairs from the DNA double helix.

Frameshift Mutation

A mutation caused by an insertion or deletion of nucleotides that alters the triplet reading frame of mRNA during translation, leading to substantial functional changes.

Inversion

A chromosomal alteration occurring when a fragment of DNA is excised, rotated $180^\text{o}$, and re-inserted into the original gap.

Translocation

A mutation in which a large segment of DNA is detached from one chromosome and attached to a non-homologous chromosome.

One Gene-One Protein Hypothesis

The concept demonstrated by George Beadle and Edward Tatum using bread mold (Neurospora crassa), proposing that each individual DNA gene encodes the information for a specific enzyme or protein.

Messenger RNA (mRNA)

A single-stranded RNA molecule that transcribes genetic instructions from nuclear DNA and carries them into the cytoplasm to direct protein synthesis at ribosomes.

Transfer RNA (tRNA)

An adaptor RNA molecule carrying a specific amino acid at one end and a complementary anticodon triplet at the other, delivering amino acids to the ribosome.

Ribosomal RNA (rRNA)

The RNA component that combines with structural proteins to form the two functional subunits of a ribosome.

Codon

A sequence of three consecutive mRNA nucleotides that specifies a particular amino acid among the $20$ standard amino acids, or signals the start or stop of translation.

Start Codon

The specific mRNA triplet $AUG$ that codes for methionine and initiates translation in all proteins.

Stop Codons

The three mRNA triplets ($UAG$, $UAA$, and $UGA$) that signal the termination of translation and cause the release of the polypeptide chain from the ribosome.

Promoter

A specialized DNA sequence located near the beginning of a gene where RNA polymerase binds and unwinds the double helix to initiate transcription.

RNA Polymerase

The enzyme that synthesizes an RNA strand complementary to a DNA template strand by linking together ribose nucleotides during transcription.

Catalytic Site

The specialized enzymatic site located on the large ribosomal subunit that catalyzes the formation of peptide bonds linking adjacent amino acids together.

Exons

The expressed nucleotide sequences within a eukaryotic gene that are retained during RNA processing and translated into protein.

Introns

Noncoding intervening nucleotide sequences in eukaryotic genes that are cut out and degraded during nuclear RNA processing before translation.

Alternate Splicing

A regulatory post-transcriptional mechanism in eukaryotes where different combinations of exons from a single pre-mRNA are joined together, allowing one gene to produce multiple distinct proteins.

Barr Body

A condensed, genetically inactivated X chromosome found in the somatic nuclei of female mammals.

X Inactivation in Calico Cats

The random inactivation of one of the two X chromosomes in different embryonic cells of a female cat, creating distinct orange and black fur patches alongside independent white coat genes.

Cell Cycle

The ordered sequence of activities and events that occurs in a cell from its formation via division until it divides again.

Binary Fission

The simplified method of asexual cellular reproduction and division characteristic of the prokaryotic cell cycle.

Interphase

The extended period of the eukaryotic cell cycle from cell formation to division, comprising the $G_1$, $S$, and $G_2$ subphases.

G1 Phase

The first growth subphase of interphase characterized by intense metabolic activity and vigorous cell enlargement.

S Phase

The synthesis subphase of interphase during which the cell's DNA is replicated and chromosomes are duplicated.

G2 Phase

The second gap subphase of interphase during which the cell completes final growth and prepares necessary components for mitotic division.

G0 Phase

A specialized non-dividing state entered by eukaryotic cells that permanently or semi-permanently cease dividing.

Chromatin

The dispersed, uncoiled mass of DNA and associated packaging proteins present in the nucleus during non-dividing interphase.

Centromere

The primary constriction region of a chromosome that joins sister chromatids together and serves as the assembly site for the kinetochore.

Kinetochore

The protein complex situated on the outer surface of a chromosome's centromere that binds spindle microtubules during cell division.

Duplicated Chromosome Structure

A mitotic chromosome consisting of two identical sister chromatids (containing two identical DNA double helices) joined together at a central centromere.

Homologous Chromosomes

Pairs of matching chromosomes that contain the same genes in the exact same linear order, though potentially bearing different forms (alleles) of those genes.

Autosomes

Non-sex chromosomes; in humans, chromosomes numbered $1$ through $22$.

Sex Chromosomes

Chromosomes that govern the genetic sex of an organism; represented by the $X$ and $Y$ chromosomes in humans.

Diploid Number

The total chromosome count representing two full sets of homologous chromosomes ($46$ in humans).

Haploid Number

A chromosome count consisting of half the diploid complement ($23$ in humans), characteristic of gametes.

Karyotype

An organized visual display or photograph of an individual's full set of metaphase chromosomes arranged in homologous pairs.

Prophase

The first phase of mitosis during which chromatin condenses into visible chromosomes, the nuclear envelope fragments, centrosomes separate, and the mitotic spindle forms.

Metaphase

The stage of mitosis during which chromosomes are aligned along the cell equator at the metaphase plate via spindle microtubules attached to their centromeres.

Anaphase

The shortest phase of mitosis, characterized by the cleavage of centromeres and the movement of individual daughter chromosomes toward opposite cell poles.

Telophase

The terminal stage of mitosis in which chromosome movement halts, chromosomes uncoil, nuclear envelopes and nucleoli reassemble, and the mitotic spindle breaks down.

Cytokinesis in Animal Cells

The physical division of the cytoplasm during late anaphase and telophase driven by a contracting ring of actin microfilaments that creates a cleavage furrow.

Cell Plate

The dividing structure synthesized during plant cytokinesis by the equatorial fusion of carbohydrate-filled Golgi vesicles to form new cell walls and plasma membranes.

Cyclin-Dependent Kinases (Cdks)

Regulatory enzyme proteins that phosphorylate target substrates to drive the sequential progression of the eukaryotic cell cycle.

p53 Protein

A critical cell cycle regulatory protein expressed when DNA is damaged; it arrests replication, stimulates repair enzyme synthesis, or triggers apoptosis if damage is irreparable.