Genetics and Molecular Biology Review

Comprehensive vocabulary flashcards covering classical Mendelian genetics, inheritance patterns, cytogenetics, DNA replication mechanisms, DNA repair, and prokaryotic/eukaryotic transcription and translation.

Anther (pea plant hybridization)

The male floral structure from which pollen was collected and transferred to the female ova during Gregor Mendel's hybridization experiments.

Seed texture

One of the seven physical characteristics in garden pea plants studied by Mendel, displaying round and wrinkled alternative traits.

$F_1$ generation (seed color cross)

The first filial generation resulting from crossing true-breeding yellow-seeded and green-seeded pea plants, yielding $100\%$ yellow seeds due to yellow color dominance.

$F_2$ constricted-pod expected count (from $650$ inflated pods)

Approximately $217$ plants, calculated using Mendel's $3:1$ dominant-to-recessive ratio in the $F_2$ generation ($\frac{650}{3} \approx 217$).

Dihybrid $F_2$ flower trait distribution

The independent phenotypic distribution observed in the $F_2$ generation of a dihybrid cross, yielding $75\%$ violet flowers and $75\%$ axial flowers.

Phenotype

The observable physical and biochemical traits expressed by an organism.

Homozygous (recessive expression)

The state of possessing two identical alleles at a given locus, which is required for an organism to express a recessive trait.

Incomplete dominance

An inheritance pattern in which heterozygous offspring display an intermediate phenotype blending parental characteristics, such as black and white true-breeding mice producing all gray offspring.

Multiple alleles and codominance

An inheritance pattern exemplified by human ABO blood groups, where three alleles ($I^A$, $I^B$, and $i$) exist in the population, and alleles $I^A$ and $I^B$ are expressed equally and simultaneously in heterozygotes.

Recessive lethal allele genotypic ratio

A surviving offspring genotypic ratio of $1:2:0$ (homozygous dominant:heterozygous:homozygous recessive) following a mating between two heterozygotes, caused by the prenatal lethality of homozygous recessive individuals.

Polydactyly rarity

The genetic reality that although the allele responsible for polydactyly (extra digits) is dominant, five digits remains typical because the dominant allele is very rare in the human population.

Codominant feather inheritance (chicken interbreeding)

An offspring phenotypic ratio of $50\%$ speckled, $25\%$ black, and $25\%$ white chickens produced by interbreeding heterozygous speckled chickens.

$F_1$ gamete ratio (unlinked dihybrid cross)

The $1:1:1:1$ ratio of gametes ($AB$, $aB$, $Ab$, $ab$) produced by an $AaBb$ heterozygote as a result of Mendel's Law of Independent Assortment.

Product rule

The rule of probability used by forked-line and probability calculation methods to determine the joint likelihood of independent genetic events occurring simultaneously.

Trihybrid cross progeny variety

The expectation of $27$ distinct genotypes ($3^3$) and $8$ distinct phenotypes ($2^3$) resulting from a cross between parents heterozygous for three independently assorting, completely dominant traits.

Epistasis

A genetic interaction where the alleles of one gene mask or alter the phenotypic expression of a second gene, such as the $ee$ genotype causing yellow coat color in Labrador retrievers regardless of the $B$ locus.

Sex-linked inheritance (hemophilia)

An inheritance pattern where males are disproportionately affected by a condition because the underlying recessive gene is situated on the X chromosome, violating independent assortment.

X-linked recessive traits

Genetic traits determined by alleles located on the X chromosome that manifest in more males than females because males possess only a single X chromosome.

Chiasmata

Microscopically visible X-shaped junctions between non-sister homologous chromatids that provided the earliest physical evidence of crossing over and chromosome segment exchange.

Recombination frequency of $0.50$

A recombination frequency value of $50\%$ ($0.50$) that signifies independent assortment and the absence of genetic linkage between two loci.

Recombination frequency of $0$

A recombination frequency value of $0$ that corresponds to perfect genetic linkage, in which no crossing over occurs between two linked genes.

$$13q12$$

The standard cytogenetic nomenclature denoting position or band $12$ located on the long ($q$) arm of chromosome $13$.

Polyploid crops

Agricultural plants containing three or more complete sets of chromosomes (such as coffee, strawberries, and bananas) that tend to produce larger yields.

Inversion loop

The loop structure assumed by homologous chromosomes during meiosis when one chromosome possesses an inversion and the other is normal, allowing proper point-for-point gene alignment.

Klinefelter syndrome

A human chromosomal condition corresponding to the $XXY$ sex chromosome genotype.

X inactivation

The epigenetic process that condenses all but one X chromosome into an inactive Barr body in somatic cells, resulting in milder phenotypic consequences for X chromosome aneuploidies relative to autosomal aneuploidies.

Pericentric inversion

A chromosomal structural rearrangement resulting from two breaks that invert an internal segment including the centromere.

Chargaff's rule calculation ($27\%$ Adenine)

The mathematical application of base-pairing rules establishing that if DNA contains $27\%$ adenine, thymine is also $27\%$ ($54\%$ total), leaving the remaining $46\%$ to be split equally between cytosine ($23\%$) and guanine ($23\%$).

Hershey and Chase experiment

The landmark experiment demonstrating that DNA is the hereditary material based on the observation that radioactive phosphorus ($^{32}P$) from bacteriophages entered bacterial cells, while radioactive sulfur ($^{35}S$) did not.

Bacterial transformation (clinical concern)

The horizontal gene transfer process by which bacteria internalize foreign environmental DNA, causing clinical concern through the transfer of pathogenicity, generation of multidrug-resistant superbugs, and dissemination of toxin genes.

Uracil

A pyrimidine base found in RNA but completely absent in double-stranded DNA.

Histones

Positively charged, basic proteins around which eukaryotic genomic DNA wraps to form nucleosomes for chromatin packaging.

Semi-conservative replication

The mode of DNA replication confirmed by Meselson and Stahl in which each newly formed double-stranded molecule contains one conserved parental strand and one newly synthesized strand.

Complementary DNA sequence ($5'\text{-AATGCTAC-}3'$)

The antiparallel strand sequence written as $3'\text{-TTACGATG-}5'$ that pairs complementarily with $5'\text{-AATGCTAC-}3'$.

DNA template strand function

The mechanism supported by Meselson and Stahl's findings demonstrating that each parent strand serves as a physical template directing the synthesis of a new complementary strand.

DNA ligase

An enzyme that seals single-strand nicks and joins Okazaki fragments during replication; it is not involved in the initial unwinding and formation of the replication fork.

RNA primer

A short sequence of RNA synthesized by primase that provides a free $3'\text{-OH}$ terminus required for DNA polymerases to initiate synthesis.

DNA polymerase II

A bacterial polymerase involved in DNA repair whose impairment leads to an accumulation of replication errors across daughter cells.

Telomerase

A ribonucleoprotein enzyme that maintains and elongates the repetitive terminal ends of linear eukaryotic chromosomes.

Prokaryotic replication compartment

The cytoplasm of a prokaryotic cell where DNA replication takes place, refuting the generalization that DNA replication always occurs in a membrane-bound nucleus.

DNA polymerase proofreading

The immediate, primary error-correction mechanism in which DNA polymerase uses its intrinsic $3' \rightarrow 5'$ exonuclease activity to excise and replace mismatched nucleotides during synthesis.

Exonuclease deficiency mutational load

The dramatic increase in total mutations observed in mature organisms that carry a defective DNA polymerase lacking proofreading exonuclease capability.

Degeneracy (genetic code)

The structural property of the genetic code whereby multiple distinct codons can specify the same amino acid, such as both $AUC$ and $AUA$ encoding isoleucine.

Codon nucleotide content ($12$ codons)

A total of $36$ nucleotides, calculated because every mRNA codon consists of a continuous triplet of $3$ nucleotides ($12 \times 3 = 36$).

Reverse transcriptase

An enzyme that synthesizes complementary DNA from an RNA template, operating in reverse of the canonical DNA-to-RNA pathway of the central dogma.

Sigma ($\sigma$) subunit

The dissociable component of the E. coli RNA polymerase holoenzyme that confers promoter binding specificity and initiates transcription.

Consensus sequences ($-10$ and $-35$ regions)

Conserved prokaryotic promoter regions located upstream of the transcription initiation site that exhibit high sequence similarity across diverse bacterial species.

TATA box

A conserved promoter DNA sequence element recognized by transcription factors and core RNA polymerase complexes in both prokaryotes and eukaryotes.

Pre-mRNAs ($\alpha$-amanitin sensitivity)

Transcripts synthesized by eukaryotic RNA polymerase II that are uniquely and strongly inhibited by low concentrations of the fungal toxin $\alpha$-amanitin.

Enhancers vs. promoters

Promoters are mandatory DNA regions necessary to bind RNA polymerase and initiate basal transcription, whereas enhancers are non-essential regulatory elements that augment transcription rates.

$7$-methylguanosine cap

A chemically modified guanosine structure attached to the $5'$ end of eukaryotic pre-mRNA that stabilizes the transcript and facilitates ribosome binding during translation initiation.

Methylation (pre-rRNA and pre-tRNA)

The post-transcriptional addition of methyl groups to specific nucleotide bases or sugars that protects precursor tRNAs and rRNAs from degradation and promotes structural stability.

Nucleolus

The dense subnuclear organelle in eukaryotes where ribosomal RNAs are synthesized, processed, and combined with proteins to assemble ribosomal subunits.

Aminoacyl-tRNA synthetases (species count)

A minimum set of $20$ distinct enzymes in any given organism, each responsible for charging tRNAs with one specific standard amino acid.

$60S$ ribosomal subunit dysfunction

A translational arrest where the ribosome is unable to catalyze peptide bond formation between incoming aminoacyl-tRNAs in the A and P sites because the peptidyl transferase center is housed within the large eukaryotic subunit.