Unit 2 Lecture 5

Question 1

Compare transcription and translation.

Answer 1

transcription: catalyzed by RNA POL reading genes on the genome
translation: protein synthesis catalyzed by ribosomes

Question 2

Compare bacterial and eukaryotic transcription/translation.

Answer 2

bacteria: NO RNA PROCESSING, transcription and translation occur together
eukaryotes: transcription and RNA processing in nucleus, translation in cytoplasm

Question 3

What is eukaryotic mRNA processing?

Answer 3

• introns are removed and exons are joined

Question 4

What are introns?

Answer 4

intervening sequences

Question 5

What are exons?

Answer 5

expressed sequences

Question 6

Where is the poly A tail?

Answer 6

3’ end

Question 7

How are introns/exons predicted?

Answer 7

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

Question 8

What does mature processed mRNA have?

Answer 8

5’ untranslated region and 3’ untranslated region

Question 9

What does alternative splicing do?

Answer 9

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

Question 10

What are the RNAs involved in translation and what are their roles?

Answer 10

• charged tRNA: tRNA that carries an amino acid
• ribosomes made of rRNA and protein: small and large subunits
• mRNA: from transcription of gene

Question 11

What are the 3 features of the genetic code?

Answer 11

• universal
• redundant
• non-overlapping

Question 12

What do ribosomes do?

Answer 12

ensure that nucleotide sequence of mRNA is read in successive, non-overlapping groups of three nucleotides called CODONS

Question 13

What is an mRNA codon complementary with?

Answer 13

tRNA anti-codon

ie. codon 5’-GUA-3’
anti-codon 3’-CAU-5’

Question 14

What are the start codons?

Answer 14

AUG, every protein starts with the amino acid methionine

Question 15

What are the stop codons?

Answer 15

UAA
UAG
UGA

Question 16

Describe the universal feature of the genetic code.

Answer 16

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)

Question 17

Describe the redundant feature of the genetic code.

Answer 17

> 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

Question 18

What are the important features of mature mRNA transcript for translation?

Answer 18

1. sequence where the ribosome will bind
   - bacteria: Shine-Dalgaro sequence or Ribosome Binding Site
   - eukaryotes: ribosomal binding site is the 5’ cap
2. start codon (first amino acid synthesized)
3. stop codon (stop translation, does not code for an amino acid)

Question 19

What is an open reading frame?

Answer 19

sequence of bases from (and including) the start codon to the stop codon

• usually only one reading frame encodes for a functional protein

Question 20

How are mRNA codons read?

Answer 20

starting at AUG (NOT at first base (transcription start site/+1 site) of mRNA) and then in triplets

Question 21

What is the function of amino acyl tRNA synthetase enzymes and why are they described as the “translators” of the genetic info.

Answer 21

there is a specific enzyme for each amino acid

Question 22

Genes in DNA can be read to produce what?

Answer 22

non-protein-coding RNA (tRNA or rRNA) or protein coding mRNA

Question 23

What do tRNAs do in translation?

Answer 23

• 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

Question 24

What are tRNAs a sequence of?

Answer 24

RNA bases that form a 2° structure (ie. hairpin loops)

Question 25

What is the Wobble Effect?

Answer 25

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

Question 26

Describe the general events in the process of translation of mRNA by ribosomes.

Answer 26

1. 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
2. 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
3. codons on mRNA are translated by base pairing with amino acyl-tRNAs
4. 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)
5. proteins recognize stop codon, releasing C terminus of protein by release factor