Gene Expression Quiz

Gene Expression and Protein Synthesis

• DNA contains specific nucleotide sequences
• Inherited DNA dictates protein synthesis, leading to specific traits
• Proteins link genotype (DNA) and phenotype (observable traits)

What is a Gene?

• A region of DNA that codes for a functional product (protein or RNA)

Gene Expression

• The process by which DNA directs protein synthesis

Central Dogma

• DNA → RNA → Protein

Types of RNA

• mRNA (messenger RNA): Produced in the nucleus from a DNA template, carries genetic information outside the nucleus to ribosomes.
• tRNA (transfer RNA): Found in cytoplasm, carries amino acids to ribosomes during translation; each carries only one type of amino acid
• rRNA (ribosomal RNA): Produced in nucleolus; joins with proteins to form ribosomes. Ribosomes can be free in the cytoplasm or bound to the endoplasmic reticulum.

The Genetic Code & Codons

• 20 amino acids, 4 nucleotide bases
• Three nucleotides (codon) code for each amino acid
• "Words" of genes are transcribed into 3-nucleotide "words" of mRNA
• The "words" are translated into an amino acid chain forming a polypeptide

Transcription

• One strand of DNA serves as a template to produce complementary RNA
• RNA polymerase untwists DNA and adds bases to the growing mRNA molecule.
• Three stages of transcription: initiation, elongation, termination

Details of Transcription

• RNA polymerase binds to a promoter region on DNA
• RNA polymerase untwists the DNA double helix
• RNA polymerase adds complementary RNA nucleotides to the growing RNA molecule
• RNA polymerase reaches a termination sequence, ending transcription

Elongation

• RNA polymerase synthesizes mRNA at a rate of 40 nucleotides per second in eukaryotic cells.
• RNA follows base-pairing rules: U instead of T is used.

Eukaryotic RNA Modification

• Pre-mRNA is modified before it leaves the nucleus
• Addition of a 5' cap and a poly-A tail
• Introns (noncoding segments) are removed by splicing, connecting exons (coding segments).

Splicing

• A spliceosome removes introns and joins exons of pre-mRNA in eukaryotes creating an mRNA molecule with a continuous protein coding region

RNA Splicing

• Spliceosomes, catalytic RNA molecules, perform RNA splicing

Alternative RNA Splicing

• Some genes can encode more than one kind of polypeptide depending on which segments are treated as exons during splicing; this produces more proteins than genes

Translation

• mRNA codons are read in the 5' to 3' direction
• Ribosomes catalyze the synthesis of proteins.
• tRNA brings amino acids to the ribosome, matching anticodons to mRNA codons.
• Three stages: initiation, elongation, termination

Ribosomes

• Two subunits made of rRNA and proteins
• Small subunit binds mRNA & initiator tRNA
• Large subunit catalyzes bonding

tRNA

• tRNA molecule contains an anticodon that matches a specific codon on mRNA

Polypeptide Chain Elongation

• During elongation, amino acids are added to the polypeptide chain. Three steps:
• codon recognition
• peptide bond formation
• translocation

Termination Stage

• A stop codon signals the end of translation; the release factor binds releasing the polypeptide

Polyribosomes

• Multiple ribosomes can translate a single mRNA molecule at the same time, enabling faster polypeptide synthesis.

Protein Folding & Post-Translational Modification

• Polypeptide chains fold spontaneously into 3D structures.
• Enzymes modify polypeptides (cleavage, adding carbohydrate).
• Proteins require post-translational modifications (phosphorylation) to function.

Mutations

• Mutations are changes in the genetic material of a cell or virus.
• Point mutations affect one base pair. Missense mutations result in a different amino acid being incorporated into the polypeptide chain during translation.
• Insertions or deletions can be disastrous for the protein that is coded by the mutated gene.