University:
Felician University
Course:
BIO-209 | Evolution
Academic year:

2022
Views:

459

Pages:

1
Author:

StellarSymphony

TRAnSCRIPTIOn & TRAnSLATIOn Transcription Types of RNA Transcription is the synthesis of an RNA sequence from a DNA template • This process occurs within the nucleus of a cell Three main types of RNA may be produced: • mRNA – Transcript used to make protein • tRNA – Transfers amino acid to ribosome • rRNA – Catalytic component of ribosome Transcription is mediated by the enzyme RNA polymerase, which: • Separates the DNA strands (breaks H bonds between base pairs) • Covalently joins free complementary RNA nucleotides together After transcription, the RNA is released to the cytoplasm (for translation) and the DNA remains within the nucleus and reforms a double helix mRNA tRNA rRNA Genetic Code The genetic code is the set of rules by which information encoded in mRNA sequences is converted into a polypeptide sequence Codons: Triplets of bases which correspond to a particular amino acid The order of the codons determines the amino acid sequence for a protein • A coding sequence always begins with a start codon (AUG) • A coding sequence is terminated with a stop codon The genetic code has two key features: • Universality – All organisms use the same genetic code • Degeneracy – Multiple codons may code for the same amino acid UUU UUC UUA UUG CUU CUC CUA CUG AUU AUC AUA AUG GUU GUC GUA GUG Phe Leu Leu Ile Met Val UCU UCC UCA UCG CCU CCC CCA CCG ACU ACC ACA ACG GCU GCC GCA GCG Ser Pro Thr Ala UAU UGU Tyr Cys UAC UGC UAA STOP UGA STOP UAG STOP UGG Trp CAU CGU His CAC CGC Arg CAA CGA Gln CAG CGG AAU AGU Asn Ser AAC AGC AAA AGA Lys Arg AAG AGG GAU GGU Asp GAC GGC Gly GAA GGA Glu GAG GGG Translation Translation is the process of polypeptide synthesis by the ribosome • • • • • • Messenger RNA (mRNA) is transported to the ribosome A ribosome reads an mRNA sequence in base triplets called codons Each codon codes for a specific amino acid (as per the genetic code) Amino acids are transported to ribosomes by transfer RNA (tRNA) Each tRNA aligns opposite a codon via a complementary anticodon The ribosome moves along the mRNA sequence (5’ → 3’) and joins amino acids together with peptide bonds (condensation reaction) • The synthesis of a polypeptide is initiated at a start codon (AUG) and is completed when the ribosome reaches a STOP codon tRNA polypeptide amino acid codon anticodon ribosome mRNA Gene → Protein A gene is a sequence of DNA which encodes a polypeptide sequence • One gene = one polypeptide (proteins may have multiple polypeptides) There are exceptions to this fundamental relationship: • Genes may be alternatively spliced (one gene = many polypeptides) • Genes encoding tRNA or rRNA are transcribed but not translated • Genes may be mutated to alter the original polypeptide product DNA (gene) transcription mRNA translation polypeptide