Molecular Biology, DNA Replication, Genetic Code, and Genetics Terminology

Vocabulary flashcards covering RNA structures and types, DNA replication mechanism, properties of the genetic code, and basic Mendelian genetics terminology based on lecture notes.

Primary Structure of RNA

The linear sequence of nucleotides linked by phosphodiester bonds written directionally from the $5'$ end to the $3'$ end, which encodes genetic instructions.

Secondary Structure of RNA

A structure formed by short-range, intramolecular complementary base pairing resulting in double-helical segments and single-stranded local motifs, such as the cloverleaf structure of tRNA.

Tertiary Structure of RNA

A structure formed by long-range atomic interactions between distant secondary structure elements, folding the molecule into a compact, functional 3D shape like the L-shaped crystal structure of tRNA.

Ribosomal RNA (rRNA)

The most abundant cellular RNA ($70\text{--}80\%$ of total RNA), located in the ribosome, insoluble, and responsible for forming ribosomes by binding protein molecules.

Messenger RNA (mRNA)

The least abundant cellular RNA ($5\text{--}10\%$ of total RNA), known as Informational RNA, which carries genetic instructions from DNA to ribosomes in the form of codons.

Transfer RNA (tRNA)

The second most abundant cellular RNA ($10\text{--}15\%$ of total RNA), known as an Adaptor molecule or Soluble RNA, which carries amino acids from the cytoplasm to ribosomes during protein synthesis.

Acceptor Arm

An arm of tRNA consisting of $4$ unpaired and $7$ paired nucleotides that binds an amino acid at its $3'$ end.

D-arm (DHU Arm)

The dihydrouridine arm of tRNA consisting of $3\text{--}4$ paired and $7\text{--}11$ unpaired nucleotides that binds the aminoacyl synthetase enzyme to activate amino acids.

Anticodon Arm

An arm of tRNA consisting of $5$ paired and $7$ unpaired nucleotides, three of which act as an anticodon to read mRNA codons.

Extra Arm (Variable Arm)

An arm in tRNA with a variable number of nucleotides whose exact function remains unknown.

T\psi C Arm (T-arm)

The thymine pseudouridine cytosine arm of tRNA consisting of $5$ paired and $7$ unpaired nucleotides, which helps bind tRNA to the ribosome during protein synthesis.

Conservative DNA Replication

A theoretical mode of replication producing two entirely original parental DNA strands in one molecule and one entirely new DNA molecule.

Dispersive DNA Replication

A theoretical mode of replication producing two DNA molecules where each strand contains interspersed segments of old and new DNA.

Semi-Conservative DNA Replication

The accepted mode of DNA replication where two parental strands separate and each synthesizes a new complementary strand; discovered by Taylor in Vicia faba and proved by Meselson and Stahl in E. coli.

Site of Origination (ori-site)

A specific site where DNA replication begins, occurring as a single site in prokaryotes and multiple sites in eukaryotes, created by endonuclease cleavage.

Phosphorylation of Deoxyribonucleotides

The conversion of inactive free deoxyribonucleoside monophosphates (dNMP) into active deoxyribonucleoside triphosphates (dNTP) by phosphorylase enzyme using ATP.

Helicase

An enzyme that breaks hydrogen bonds between nitrogenous bases to unwind the DNA double helix during replication.

DNA Topoisomerase

An enzyme that relieves supercoiling tension during DNA unwinding by cutting one strand, rotating it in the opposite direction, and resealing it.

SSB Protein

Single-stranded DNA binding protein that stabilizes separated template strands of DNA during replication.

RNA Primer

A short strand of RNA ($7\text{--}8$ nucleotides long) synthesized by primase that initiates DNA replication at the $5'$ end of a new strand.

Leading Strand

The continuously synthesized DNA strand formed in the $5' \rightarrow 3'$ direction toward the replication fork using a single RNA primer on a $3' \rightarrow 5'$ template.

Lagging Strand

The discontinuously synthesized DNA strand formed away from the replication fork using multiple RNA primers on a $5' \rightarrow 3'$ template, forming Okazaki fragments.

Okazaki Fragments

Short, discontinuous segments of newly synthesized DNA formed on the lagging strand template.

DNA Polymerase I

An enzyme that removes RNA primers, fills gaps with deoxyribonucleotides on the lagging strand, and performs proofreading and DNA repair.

DNA Ligase

An enzyme that seals gaps between Okazaki fragments on the lagging strand to make it continuous.

Genetic Code

The relationship between the nucleotide sequence of mRNA and the amino acid sequence of a protein; term coined by George Gamow and proved by Nirenberg, Khorana, and Matthaei.

Codon

A sequence of $3$ adjacent nitrogenous bases on mRNA that codes for one specific amino acid.

Degeneracy of Genetic Code

The property of the genetic code where multiple codons (out of $61$ coding codons) can specify the same amino acid, with exceptions like Methionine and Tryptophan.

Initiation Codon

A codon that starts protein synthesis, primarily $AUG$ (coding for Methionine) and occasionally $GUG$ (coding for Valine).

Termination Codons

Nonsense codons that terminate protein chain synthesis because they do not code for any amino acid: $UAG$ (Amber), $UAA$ (Ochre), and $UGA$ (Opal).

Genome

A single set of chromosomes ($n$) present in an organism.

Alleles

Alternative forms or variants of a gene.

Homozygous

An organism possessing identical alleles for a specific gene (e.g., $TT$ or $tt$), also referred to as a pure form.

Heterozygous

An organism possessing different alleles for a specific gene (e.g., $Tt$), also referred to as a hybrid.

Dominant Allele

An allele that expresses its trait in both homozygous and heterozygous forms, represented by a capital letter.

Recessive Allele

An allele whose expression is masked or suppressed by a dominant allele in heterozygous form, represented by a lowercase letter.

Phenotype

The physical, morphological, or external observable features of an organism.

Genotype

The genetic composition or genetic makeup of an organism.

Monohybrid Cross

A genetic cross between organisms that differ in a single character feature.

Dihybrid Cross

A genetic cross between organisms that differ in two distinct character features.

Gene Pool

The total collection of all genes and their alleles within an interbreeding population.