Cell Biology and Genetics: Translation, Meiosis, Mendelian Genetics, and Pedigree Analysis

Comprehensive practice flashcards reviewing translation, meiotic cell division, Mendelian principles, non-Mendelian extensions, and human pedigree genetics based on lecture series notes.

What are the subunit and ribosomal RNA (rRNA) compositions of eukaryotic $80\text{S}$ ribosomes versus prokaryotic $70\text{S}$ ribosomes?

Eukaryotes have $80\text{S}$ ribosomes composed of a large $60\text{S}$ subunit ($46$ proteins and $25\text{S}$, $5.8\text{S}$, and $5\text{S}$ rRNA) and a small $40\text{S}$ subunit ($33$ proteins and $18\text{S}$ rRNA). Prokaryotes have $70\text{S}$ ribosomes composed of a large $50\text{S}$ subunit ($23\text{S}$ and $5\text{S}$ rRNA) and a small $30\text{S}$ subunit ($16\text{S}$ rRNA).

Approximately how many ribosomes are present in a typical mammalian cell, and what percentage of total cellular energy do they consume?

A mammalian cell contains approximately $10\text{ million}$ ribosomes, which consume approximately $60\%$ of all cellular energy.

How many total codons exist in the genetic code, and what are their specific coding functions?

There are $64$ total codons: $61$ code for amino acids (including the start codon, which establishes the translation start site and codes for Methionine), and $3$ stop codons code for the translation stop site.

What enzyme family attaches specific amino acids to their corresponding transfer RNAs (tRNAs), and what are the steps of this reaction?

The $20$ aminoacyl-tRNA synthetase enzymes attach amino acids to tRNAs in two steps: first, the amino acid reacts with ATP to create an amino acid-AMP intermediate while cleaving pyrophosphate; second, the amino acid is transferred from AMP onto the $3'$ acceptor stem of the tRNA.

Which events and molecular components form the translation initiation complex in eukaryotic cells?

The small ribosomal subunit binds to the $5'$ G-cap of mRNA, the initiator tRNA carrying Methionine binds to the start codon (aided by initiation factors that scan mRNA and unwinding helicases), and the large ribosomal subunit binds, driven by energy supplied by GTP.

What specific ribozyme links amino acids together during translation elongation, and what rRNA molecule constitutes it in prokaryotes and eukaryotes?

Peptidyl transferase is the ribozyme that catalyzes peptide bond formation at a rate of $\sim 3-10$ reactions per second; it consists of the $23\text{S}$ rRNA in the prokaryotic $50\text{S}$ subunit and the $28\text{S}$ rRNA in the eukaryotic $60\text{S}$ subunit.

Based on the provided diagram, what are the specific mechanisms by which Chloramphenicol, Erythromycin, Tetracyclines, and Streptomycin inhibit the bacterial $70\text{S}$ ribosome?

Chloramphenicol binds to the $50\text{S}$ portion and inhibits peptide bond formation; Erythromycin binds to the $50\text{S}$ portion and prevents translocation along mRNA; Tetracyclines interfere with tRNA attachment to the mRNA-ribosome complex; Streptomycin changes the shape of the $30\text{S}$ portion, causing the mRNA code to be read incorrectly.

What protein terminates translation when a stop codon is encountered, and what happens to the ribosomal complex?

A release factor protein enters the ribosome, interacts with the stop codon, and stops translation, triggering the release of the polypeptide chain and disassembly of the ribosomal subunits using energy supplied by GTP.

How do newly synthesized proteins reach their required intracellular destinations or secretagogue pathways?

Proteins contain built-in amino acid sequences called signal sequences (zip codes), such as a nuclear localization signal sequence for the nucleus or a signal peptide that binds a Signal Recognition Particle (SRP) to dock the ribosome onto the rough endoplasmic reticulum.

In flowering plants, which specific floral structures undergo meiotic cell division to produce male and female haploid ($1N$) gametes?

Haploid female gametes are produced within the ovule of the ovary (part of the carpel), while haploid male gametes are produced within the anther of the stamen.

What are the four essential biological functions of meiotic cell division in diploid ($2N$) species?

Meiosis enables sexual reproduction, increases genetic variation in populations, maintains a constant chromosome number ($46$ in humans) across generations, and produces genetically variable haploid ($1N$) gametes.

How many chromosomes are present in a human germ-line cell after DNA replication, and what is the outcome of Meiosis I and Meiosis II?

Following DNA replication, a germ-line cell contains $92$ chromatids. Meiosis I separates homologous pairs into two daughter cells containing $46$ chromosomes each; Meiosis II separates sister chromatids to produce four haploid ($1N$) cells containing $23$ chromosomes each.

When does crossing over occur during meiosis, and what is its dual significance?

Crossing over occurs during Prophase I of Meiosis I; it generates recombinant chromosomes with new combinations of maternal and paternal alleles, and it ensures homologous chromosome pairs remain physically attached until spindle separation, reducing aneuploidy risk.

How many possible combinations of maternal and paternal chromosomes can occur in human gametes due to independent assortment?

Due to independent assortment, $2^n = 2^{23}$, or approximately $8.4\text{ million}$, possible chromosome combinations can occur in each gamete.

What causes nondisjunction during meiosis, and what is the chromosomal basis of Down syndrome?

Nondisjunction is caused by an M phase (spindle) checkpoint failure, yielding aneuploid gametes with abnormal chromosome numbers. Down syndrome (Trisomy 21) results from three copies of chromosome $21$, which encodes approximately $225-350$ genes.

What are the estimated risks of conception failure and age-related miscarriage in human pregnancies?

Approximately $70\%$ of conceptions miscarry before a missed period, and only $\sim 30\%$ of fertilizations in young parents result in clinical pregnancy. The estimated risk of miscarriage is $\sim 10\%-20\%$ in women $\le 35\text{ years}$ of age, rising to $\sim 45\%$ in women $\ge 40\text{ years}$ of age.

Which major model organisms are depicted in modern biological research, and what are their scientific names?

The model organisms include the mouse (Mus musculus), frog (Xenopus laevis), mustard plant (Arabidopsis thaliana), roundworm (Caenorhabditis elegans), fruit fly (Drosophila melanogaster), yeast (Saccharomyces cerevisiae), and zebrafish (Danio rerio).

Why did Gregor Mendel select the garden pea plant as a model system for investigating heredity?

Mendel chose the pea plant because it possessed agricultural relevance, readily available seeds, easily managed sexual reproduction, distinct observable traits (such as flower color), inexpensive growth in large numbers, and a rapid reproduction rate.

How does the environment influence phenotypic expression in genetically identical organisms?

Environmental conditions modify gene expression; examples include soil conditions altering flower color in identical Hydrangea clones, distinct social interactions and uterine environments affecting cloned cows, and severe neglect reducing brain size, cortical thickness, and brain activity in children.

What is the function of the SRY gene on the human Y chromosome?

The SRY gene encodes a transcription factor protein that triggers the transcription of genes coordinating male gonad development and repressing female gonad development; mutations in SRY cause Disorders of Sex Development (DSD).

What is Mendel's Law of Segregation?

The Law of Segregation states that diploid ($2N$) individuals possess two alleles for each gene, which separate from each other during meiotic cell division so that each haploid ($1N$) gamete carries only one allele for each gene.

What were the phenotypic results and ratios observed by Gregor Mendel in the $F_1$ and $F_2$ generations of his monohybrid flower-color cross?

In the $F_1$ generation, all hybrid offspring exhibited purple flowers. In the $F_2$ generation, purple and white flowers reappeared in a $3:1$ ratio ($705$ purple to $224$ white plants, or $3.15:1$).

What is the purpose of a testcross, and how does it distinguish a heterozygous genotype from a homozygous dominant genotype?

A testcross crosses an individual displaying a dominant phenotype with an individual displaying the homozygous recessive phenotype. If all offspring exhibit the dominant phenotype, the unknown parent is homozygous dominant ($PP$); if half of the offspring display the recessive phenotype, the parent is heterozygous ($Pp$).

Based on the anatomical illustration, what distinctive phenotypic features characterize individuals with Achondroplasia?

Achondroplasia features include macrocephaly, frontal bossing, midface hypoplasia, rhizomelia (shortened limbs, especially proximally), trident hand, and genu varum (bowing of the tibia).

What is the genetic cause of Achondroplasia, and why are there no homozygous dominant ($AA$) individuals in the human population?

Achondroplasia is caused by a dominant mutation in the FGFR3 gene (with $\sim 80\%$ of cases arising de novo during meiosis, more frequently in older fathers); the homozygous dominant ($AA$) genotype is embryonic lethal and does not exist in living populations.

What is pleiotropy, and how does the mutated allele in sickle-cell disease exemplify this concept?

Pleiotropy occurs when a single gene mutation influences multiple distinct phenotypic traits. In sickle-cell disease, the beta-globin mutation leads to diverse systemic consequences including blindness, liver failure, and heart attack.

What is the difference between co-dominance and incomplete dominance in heterozygous phenotypes?

In co-dominance, both alleles are fully expressed simultaneously, as seen in AB blood groups displaying both A and B glycoproteins. In incomplete dominance, heterozygotes display an intermediate phenotype, such as pink snapdragons producing $50\%$ of the red pigment synthesized by homozygous dominant parents.

Why do X-linked recessive genetic disorders appear much more frequently in human males than in females?

Human males are hemizygous for the X chromosome (possessing only one copy of X-linked genes); therefore, a single recessive mutant allele on the X chromosome is directly expressed as a dominant phenotype.

How does cytoplasmic inheritance operate in humans and plants, and how many genes do human mitochondrial and chloroplast genomes carry?

Cytoplasmic inheritance occurs because organelles in the egg cytoplasm are transmitted exclusively by the female parent to all offspring (never by the father). The human mitochondrial genome contains $37$ genes, whereas the plant chloroplast genome contains $110$ genes.

What is polygenic inheritance, and how do melanocytes determine variations in eye color, skin tone, and hair color?

Polygenic inheritance is the regulation of a single phenotypic trait by multiple genes. Melanocytes produce two forms of melanin—eumelanin and pheomelanin; phenotype varies with melanocyte cell count, melanosome number per cell, and the concentration of pigment molecules packaged in each melanosome.

Based on the nucleotide and polypeptide diagrams, what exact molecular point mutation causes sickle-cell anemia?

The template DNA changes from $3'\text{-CTT-}5'$ (coding strand $5'\text{-GAA-}3'$) to $3'\text{-CAT-}5'$ (coding strand $5'\text{-GTA-}3'$), changing the mRNA codon from $5'\text{-GAA-}3'$ to $5'\text{-GUA-}3'$ and substituting a valine (Val) residue in place of glutamic acid (Glu).

What pedigree hallmarks characterize autosomal recessive medical traits, and how does inbreeding affect their presentation?

Autosomal recessive traits often skip generations, affect males and females with equal frequency, and typically appear in offspring born to unaffected heterozygous carrier parents. Consanguinity significantly increases their prevalence because rare recessive alleles trace back to a shared ancestor.

What is the distinction between the haploinsufficiency hypothesis and the rogue protein hypothesis for autosomal dominant disorders?

Haploinsufficiency posits that a single wild-type allele cannot generate enough functional protein to meet normal physiological thresholds (yielding a mutant phenotype), whereas the rogue protein hypothesis states that the mutant allele produces an abnormal, toxic product (such as aggregated TDP43 in ALS/FTLD) that directly impairs cellular function.

What transmission patterns identify an X-linked recessive disorder in a pedigree?

The disorder primarily affects males, skips generations via unaffected female carriers, exhibits zero father-to-son transmission, and results in all daughters of an affected father becoming obligate carriers.

What pedigree characteristics distinguish an X-linked dominant disorder from an autosomal dominant disorder?

In X-linked dominant inheritance, an affected male transmits the trait to $100\%$ of his daughters and $0\%$ of his sons, whereas an autosomal dominant trait is passed by affected fathers to both sons and daughters with equal probability ($50\%$).

What is Li-Fraumeni syndrome, and what type of inheritance does it display?

Li-Fraumeni syndrome is an autosomal dominant disorder caused by germline mutations in the TP53 tumor suppressor gene, conferring a high risk of developing brain, breast, leukemia, osteosarcoma, and soft-tissue sarcomas.