Chapter 6: How Cells Read the Genome - Exam 3 Study Guide

Comprehensive vocabulary flashcards covering mRNA splicing consensus sequences, ribosomal subunit composition, translational start/stop signals, mutation types, ribosomal sites (A, P, E), and eukaryotic RNA polymerases from Chapter 6.

$5'$ GU Intron Junction

The consensus sequence marking the beginning of an intron (also known as the $5'$ splice site or donor site) recognized by the spliceosome during mRNA splicing.

$3'$ AG Intron Junction

The consensus sequence marking the end of an intron (also known as the $3'$ splice site or acceptor site) recognized by the spliceosome during mRNA splicing.

Branch Point Sequence (YURAC)

An internal eukaryotic intron consensus sequence containing an invariant adenine that is critical for forming the lariat structure during splicing, where Y is a pyrimidine (C or U) and R is a purine (A or G).

Spliceosome

The eukaryotic molecular complex that recognizes consensus junction sequences to excise introns and ligate exons during mRNA processing.

$40\text{S}$ Small Ribosomal Subunit

The smaller subunit of the eukaryotic ribosome that contains $18\text{S}$ rRNA.

$60\text{S}$ Large Ribosomal Subunit

The larger subunit of the eukaryotic ribosome that contains $28\text{S}$, $5.8\text{S}$, and $5\text{S}$ rRNAs.

Svedberg Unit (S)

A sedimentation unit measuring the behavior of particles during centrifugation; values do not sum additively (e.g., $40\text{S}$ and $60\text{S}$ subunits assemble to form an $80\text{S}$ eukaryotic ribosome).

AUG

The start codon for translation in eukaryotic mRNA, signaling the ribosome where to begin protein synthesis and coding for methionine.

Methionine

The first amino acid incorporated into a eukaryotic polypeptide, coded by the start codon AUG.

Sigma Factor

A bacterial transcription factor that enables RNA polymerase to recognize and bind to the $-10$ and $-35$ promoter consensus sequences to initiate transcription.

Reading Frame

The specific division of a nucleotide sequence into consecutive groups of three bases (codons) during translation, allowing three theoretical reading orientations.

Open Reading Frame (ORF)

A nucleotide reading frame that begins with an AUG start codon, spans a long coding sequence uninterrupted by premature stop codons, and terminates with a proper stop codon.

Genetic Code Degeneracy

The redundancy of the genetic code wherein most amino acids are specified by more than one codon.

Silent Mutation

A single nucleotide substitution that changes a codon to another codon coding for the exact same amino acid, causing no change in the resulting polypeptide sequence.

Missense Mutation

A mutation that changes a codon so that it specifies a different amino acid, which may alter the folding, shape, or function of the protein.

Nonsense Mutation

A mutation that converts an amino acid-specifying codon into a premature stop codon, terminating translation early and generating a truncated protein.

Frameshift Mutation

A mutation caused by the insertion or deletion of nucleotides that shifts the genetic reading frame, altering many downstream amino acids.

A-Site (Aminoacyl Site)

The ribosomal binding site that captures the incoming tRNA carrying the next amino acid to be added to the polypeptide chain.

P-Site (Peptidyl Site)

The ribosomal binding site that holds the tRNA attached to the growing polypeptide chain during translation.

E-Site (Exit Site)

The ribosomal binding site where uncharged, empty tRNAs reside before being released from the ribosome.

RNA Polymerase I

The eukaryotic enzyme responsible for synthesizing the precursor to major ribosomal RNAs ($18\text{S}$, $28\text{S}$, and $5.8\text{S}$ rRNAs).

RNA Polymerase II

The eukaryotic enzyme responsible for synthesizing messenger RNA (mRNA) from protein-coding genes as well as many non-coding RNAs.

RNA Polymerase III

The eukaryotic enzyme responsible for synthesizing transfer RNA (tRNA), $5\text{S}$ rRNA, and various other small RNAs.

N-Terminal End

The amino-terminal end of a protein, representing the first amino acid added during translation, which is encoded near the $5'$ end of the mRNA.

C-Terminal End

The carboxyl-terminal end of a protein toward which polypeptide synthesis proceeds, corresponding to the $3'$ direction of the translated mRNA.

Ribosome

The molecular machine/organelle that carries out translation by reading mRNA transcripts and assembling amino acids into polypeptides.

Stop Codons

The three codon variants in the genetic code (UAA, UAG, and UGA) that do not code for amino acids and trigger the termination of translation.

Release Factor

A termination factor protein that recognizes and binds to a stop codon positioned in the ribosomal A-site, releasing the completed polypeptide.

DNA Coding Strand

The sense strand of DNA whose sequence is identical to that of the synthesized mRNA transcript, except that thymine (T) in DNA is replaced by uracil (U) in RNA.

Uracil-Adenine Base Pairing

The complementary hydrogen bonding between uracil and adenine in RNA, stabilized by $2$ hydrogen bonds.