Cell Division, Genetics, and Inheritance

Flashcards covering the eukaryotic cell cycle, mitosis, meiosis, cell cycle regulation, cancer biology, Mendelian genetics, and non-Mendelian inheritance patterns.

How is approximately $2\,\text{m}$ of DNA packaged to fit inside the $5\,\mu\text{m}$ nucleus of a human cell?

DNA is wound around histone proteins to form nucleosomes ("beads-on-a-string"), which coil into a $30\,\text{nm}$ solenoid fiber, organize into extended looped domains, and condense further into mitotic chromosomes.

What biological preparation of condensed chromosomes arranged in numerical pairs is shown here?

A karyotype.

What is the physical structure of the bacterial genome, and how do bacteria divide?

The bacterial genome consists of a single, circular, double-stranded DNA molecule; bacteria divide by binary fission.

What role does the FtsZ protein play in prokaryotic binary fission?

FtsZ proteins assemble into a ring at the midpoint of the dividing cell to form the septum that pinches the cell into two daughter cells.

If a bacterial population begins with $1000$ cells and divides every $30\,\text{minutes}$, how many cells will be present after $2\,\text{hours}$?

$16{,}000$ cells (using $N = N_0 2^n$, where $n = 4$ generations: $1000 \times 2^4 = 16{,}000$).

What is a centromere?

A constricted region of a chromosome that binds specific proteins and holds two sister chromatids together.

What protein complex physically glues sister chromatids together following DNA replication?

Cohesin proteins.

What occurs during the three phases of interphase ($G_1$, S, and $G_2$)?

$G_1$ is the primary growth phase where cellular contents (except chromosomes) duplicate; S phase replicates the DNA into sister chromatids; $G_2$ involves additional growth, organelle replication, and chromosome condensation.

What is the $G_0$ phase of the cell cycle?

A resting state of cell cycle arrest where non-dividing or mature specialized cells (such as mature neurons) reside.

What key cellular events occur during prophase of mitosis?

Chromosomes condense to become visible under a light microscope, the mitotic spindle apparatus begins to assemble, the nuclear envelope breaks down, and the Golgi and ER disperse.

What occurs during prometaphase of mitosis?

The nuclear envelope disassembles, spindle microtubules attach to kinetochores of chromosomes from opposite poles, and chromosomes begin moving toward the cell equator.

What defines the metaphase stage of mitosis?

All chromosomes align along the equator of the cell at the metaphase plate, with sister chromatids attached to opposite spindle poles and placed under tension.

What is the distinction between Anaphase A and Anaphase B during mitotic anaphase?

In Anaphase A, kinetochores are pulled toward opposite poles due to the shortening of kinetochore microtubules; in Anaphase B, the spindle poles themselves move apart as polar microtubules elongate.

Why is telophase described as essentially the reverse of prophase?

The mitotic spindle disassembles, nuclear envelopes re-form around each cluster of separated chromosomes, chromosomes decondense, and the Golgi apparatus and ER re-form.

How does cytokinesis differ between animal cells and plant cells?

Animal cells divide by forming a contractile ring that pinches the plasma membrane into a cleavage furrow, whereas plant cells fuse Golgi-derived vesicles along the equator to form a cell plate.

How do cyclins and cyclin-dependent kinases (Cdks) function together biochemically to regulate the cell cycle?

Cdks are protein kinases maintained at constant levels that require binding to fluctuating regulatory cyclins; once activated by cyclin binding and phosphorylation, Cdks phosphorylate target proteins to drive cell cycle transitions.

What are the three primary cell cycle checkpoints and what does each verify?

$G_1/S$ verifies growth factors, cell size, and nutritional state; $G_2/M$ verifies completion of replication and DNA integrity; the Spindle checkpoint verifies that all chromosomes are properly aligned at the metaphase plate.

What is Maturation-Promoting Factor (MPF), and what does it trigger?

A cyclin-Cdk complex (Cdc2/Mitotic cyclin) whose activity peaks at the onset of mitosis to trigger chromosome condensation, nuclear envelope breakdown, and spindle formation.

What are the primary cyclins and CDKs associated with the human $G_1$ and M phases?

$G_1$ phase utilizes Cyclins D and E with Cdk4, Cdk2, and Cdk6; M phase utilizes Cyclin B with Cdk1.

What is the role of the p53 tumor-suppressor protein in cellular damage response?

When DNA damage, cell cycle abnormalities, or hypoxia occur, p53 arrests the cell cycle to allow DNA repair; if repair fails, it initiates apoptosis (programmed cell death).

Using a driving analogy, how do proto-oncogenes and tumor-suppressor genes relate to cancer development?

Mutating a proto-oncogene into an oncogene is like a car's gas pedal getting stuck pushed down (stimulating unregulated division); mutating a tumor-suppressor gene is like the brakes failing entirely.

What is the cellular mechanism of the chemotherapy drug Taxol (paclitaxel), discovered in the bark of Pacific Yew (Taxus brevifolia)?

Taxol binds to the $\beta$-tubulin subunit of microtubules and stabilizes them, preventing their normal disassembly after division, thereby halting the cell cycle and inducing apoptosis.

What are the three unique features of meiosis that distinguish it from mitosis?

Synapsis (pairing of homologous chromosomes along their lengths), homologous recombination (crossing over), and reduction division (producing haploid gametes from diploid germ-line cells).

What is the synaptonemal complex?

A protein structure composed of lateral and central elements that physically joins homologous chromosome pairs during prophase I of meiosis to facilitate crossing over.

Which scientist, pictured here, won the 1983 Nobel Prize for discoveries in corn cytogenetics, including transposable elements and genetic recombination?

Barbara McClintock.

What is the primary difference in chromosome separation between Anaphase I and Anaphase II of meiosis?

In Anaphase I, homologous chromosome pairs are separated while sister chromatids remain attached at their centromeres; in Anaphase II, sister chromatids are pulled apart to opposite poles.

In an organism with a diploid number of $2n = 28$, how many chromosomes are present in each reforming nucleus after meiosis I, and how many are present in each gamete after meiosis II?

Each nucleus after meiosis I contains $14$ duplicated chromosomes (each consisting of two sister chromatids); after meiosis II and cytokinesis, each haploid gamete contains $14$ single-chromatid chromosomes.

What was the dominant idea about heredity prior to Gregor Mendel's experiments?

The blending inheritance hypothesis, which assumed that parental traits mix together irreversibly in offspring.

What is Mendel's Principle of Segregation, and during which meiotic stage does it occur?

The two alleles for a single gene on homologous chromosomes separate from each other during gamete formation so each gamete carries only one allele; it occurs during anaphase I of meiosis.

What is Mendel's Principle of Independent Assortment, and what is its meiotic basis?

Alleles of different genes assort independently of one another during gamete formation; it results from the random alignment of non-homologous chromosome pairs on the metaphase plate during metaphase I.

In Mendel's monohybrid crosses of garden peas, what phenotypic and genotypic ratios were consistently observed in the $F_2$ generation?

A $3:1$ phenotypic ratio (dominant to recessive) and a $1:2:1$ genotypic ratio ($1$ homozygous dominant : $2$ heterozygous : $1$ homozygous recessive).

In a dihybrid cross of two heterozygous individuals ($SsYy \times SsYy$) for seed shape and seed color, what is the expected phenotypic ratio among the offspring?

$9:3:3:1$ ($9$ smooth yellow, $3$ smooth green, $3$ wrinkled yellow, $1$ wrinkled green).

What is the purpose of a genetic testcross, and what ratio indicates that the parent with the dominant phenotype is heterozygous?

A testcross crosses an individual of unknown dominant genotype with a homozygous recessive individual; an offspring phenotypic ratio of $1:1$ indicates the parent is heterozygous.

How was a testcross applied to the endemic Hawaiian plant Schiedea adamantis to determine plant genotypes?

Hermaphroditic plants ($HH$ or $Hh$) were crossed with homozygous recessive females ($hh$); a $100\%$ hermaphroditic progeny confirmed an $HH$ genotype, whereas a $50\%$ hermaphrodite to $50\%$ female outcome confirmed an $Hh$ genotype.

What is the underlying molecular and physiological defect in cystic fibrosis?

An autosomal recessive mutation in the CFTR gene on Chromosome 7 disrupts the chloride ion channel protein, blocking chloride export from epithelial cells and causing thick, sticky mucus to accumulate in the lungs and pancreas.

Based on this diagram, what is the fundamental difference between epistasis and pleiotropy?

Epistasis occurs when multiple distinct genes interact to determine a single character (trait), whereas pleiotropy occurs when a single gene affects multiple distinct characters.

How does incomplete dominance differ from codominance, and what are classical examples of each?

In incomplete dominance, heterozygotes display an intermediate phenotype (e.g., pink flowers from red and white snapdragons); in codominance, heterozygotes distinctly express aspects of both alleles simultaneously (e.g., AB blood type expressing both A and B surface antigens).

How does epistasis explain coat color in mice involving locus A (pigment production) and locus B (hair banding)?

The recessive homozygous $aa$ genotype blocks all pigment production regardless of locus B, producing albino mice ($aa\_\_$); when pigment is allowed ($A\_$), $B\_$ produces agouti coat and $bb$ produces black coat, resulting in a $9:3:4$ $F_2$ ratio.

How does temperature affect coat color phenotype in Siamese cats?

The mutation responsible for pigmentation produces a temperature-sensitive tyrosinase enzyme that is inactive at normal warm body temperatures (causing light fur) but active at cooler peripheral extremities (producing dark paws, ears, tail, and face).

What is polygenic inheritance, and how does its phenotypic distribution differ from standard Mendelian single-gene traits?

Polygenic inheritance occurs when multiple independent genes influence a single trait (e.g., human height and eye color), producing continuous variation characterized by a bell-shaped normal distribution curve rather than discrete categories.