Unit 2 Lecture 5 Flashcards
Compare transcription and translation.
transcription: catalyzed by RNA POL reading genes on the genome
translation: protein synthesis catalyzed by ribosomes
Compare bacterial and eukaryotic transcription/translation.
bacteria: NO RNA PROCESSING, transcription and translation occur together
eukaryotes: transcription and RNA processing in nucleus, translation in cytoplasm
What is eukaryotic mRNA processing?
- introns are removed and exons are joined
What are introns?
intervening sequences
What are exons?
expressed sequences
Where is the poly A tail?
3’ end
How are introns/exons predicted?
the preRNA (RNA primary transcript) is short-lived, so they can be predicted by comparing genomic DNA to observed mRNA
genomic DNA: exons and introns
observed mRNA: open reading frame created by exons spliced together
What does mature processed mRNA have?
5’ untranslated region and 3’ untranslated region
What does alternative splicing do?
generates different (related) polypeptides from one gene/pre-mRNA
- one gene encodes one mRNA primary transcript (aka pre-mRNA)
- different exons are used or skipped to make different polypeptides encoded by the same pre-mRNA
What are the RNAs involved in translation and what are their roles?
- charged tRNA: tRNA that carries an amino acid
- ribosomes made of rRNA and protein: small and large subunits
- mRNA: from transcription of gene
What are the 3 features of the genetic code?
- universal
- redundant
- non-overlapping
What do ribosomes do?
ensure that nucleotide sequence of mRNA is read in successive, non-overlapping groups of three nucleotides called CODONS
What is an mRNA codon complementary with?
tRNA anti-codon
ie. codon 5’-GUA-3’
anti-codon 3’-CAU-5’
What are the start codons?
AUG, every protein starts with the amino acid methionine
What are the stop codons?
UAA
UAG
UGA
Describe the universal feature of the genetic code.
same genetic code applies to vast majority of organisms (64 codons total, 3 of them are stop codons, 61 of them code for amino acids)
Describe the redundant feature of the genetic code.
> 1 codon codes for a single amino acid
ie. Arg = 6 codons each
Phe = 2 codons
Pro = 4 codons
Ser = 6 codons
- in any cell, there are usually less than 61 types of tRNAs
What are the important features of mature mRNA transcript for translation?
- sequence where the ribosome will bind
- bacteria: Shine-Dalgaro sequence or Ribosome Binding Site
- eukaryotes: ribosomal binding site is the 5’ cap - start codon (first amino acid synthesized)
- stop codon (stop translation, does not code for an amino acid)
What is an open reading frame?
sequence of bases from (and including) the start codon to the stop codon
- usually only one reading frame encodes for a functional protein
How are mRNA codons read?
starting at AUG (NOT at first base (transcription start site/+1 site) of mRNA) and then in triplets
What is the function of amino acyl tRNA synthetase enzymes and why are they described as the “translators” of the genetic info.
there is a specific enzyme for each amino acid
Genes in DNA can be read to produce what?
non-protein-coding RNA (tRNA or rRNA) or protein coding mRNA
What do tRNAs do in translation?
- bind to codons on mRNA and deliver the amino acid
- interact with mRNA by non-covalent interactions (ie. H bonds)
- have anti-codon sequences that are complementary to codon on mRNA
What are tRNAs a sequence of?
RNA bases that form a 2° structure (ie. hairpin loops)
What is the Wobble Effect?
last base (3’) can vary in a codon
- there are 61 possible codons, and without wobble, a cell would need 61 tRNAs (# tRNAs depends on species)
- most species have only about 45 tRNAs and the tRNAa can have modified bases in their 5’ position to permit binding to more than one nucleotide
- non-standard base pairing at 3rd base of codon allows a single tRNA to bind to multiple codons
- structure of ribosome also permits “wobble” in this position
Describe the general events in the process of translation of mRNA by ribosomes.
- ribosome binds to 5’ untranslated region
- initiation factors recruit small ribosomal subunit and tRNA(met) and scan the mRNA for an AUG codon
- bacteria: rbs (shine-delgardo sequence in prokaryotes) is in the 5’ UTR just upstream of start codon
- eukaryotes: ribosome binds the 5’ CAP and scans 5’ UTR for AUG - translation starts at AUG
- when complex reaches an AUG, large ribosomal subunit joins, initiation factors are released, and tRNA complementary to next codon binds to A site - codons on mRNA are translated by base pairing with amino acyl-tRNAs
- peptide bond formation is catalyzed by rRNA on the ribosome
- peptide bond joining amino acids on new polypeptide
- rRNA important for catalyzing peptide bond formation (so this RNA is sometimes called ribozyme) - proteins recognize stop codon, releasing C terminus of protein by release factor
