translation

Question 1

what is tRNA

Answer 1

enables the translation of the information in the mRNA genetic message to a polypeptide

Question 2

what does the large degree of complementarity along stretches of tRNA cause?

Answer 2

• result in stretches in hydrogen bonding between complementary nucleotide bases
• allows for the formation of four double helical segments (clover) and three characteristic loops (L)

Question 3

what is the anticodon region of the tRNA molecule

Answer 3

a specific nucleotide triplet that forms complementary base pairs with a specific mRNA codon

• anticodons are written in a 3’ to 5’ direction and aligned properly with mRNA codons in a 5’ to 3’ direction

Question 4

what is at the 3’ end of the tRNA molecule?

Answer 4

amino acid attachment site that is made up of a single stranded CCA nucleotide sequence

• the terminal adenine (A) is the actual point of attachment for an amino acid during tRNA molecule activation

Question 5

how is activation of a tRNA molecule with a specific amino acid carried out by

Answer 5

aminoacyl tRNA synthetases (a family of enzymes)

• each is specific to the type of tRNA and corresponding amino acid that it will bind

Question 6

what does the active site of aminoacyl tRNA synthetases do

Answer 6

recognizes the anticodon of the tRNA and the region of the amino acid attachment site

• Leads to the existence of 20 aminoacyl tRNA synthetases, one for each amino acid

Question 7

what happens when it is bound to the active site?

Answer 7

the aminoacyl tRNA synthetases catalyze the covalent attachment of a tRNA molecule to its amino acid using the energy from ATP hydrolysis

• results in a charged tRNA molecule or aminoacyl tRNA that is released from the enzyme which can now deliver the amino acid to the polypeptide chain on a ribosome

Question 8

what is a translation wobble

Answer 8

greater flexibility between the 3rd nucleotide of a codon and corresponding base of a tRNA anticodon (explains redundancy)

Question 9

what is the codon-anticodon base pair interaction

Answer 9

The 5’ end (first base) of the codon will pair with the 3’ end (last base) of the anti codon

Question 10

what does translation in the cytosol require

Answer 10

the assembly of ribosomes and associated molecular components along a transcribed mRNA strand

Question 11

what is the overall process of translation

Answer 11

• As mRNA molecule is shuffled through a ribosome, specific mRNA codons are translated into amino acids one by one
• These amino acids are attached to a growing polypeptide chain until translation is terminated

Question 12

what is translation initiation in eukaryotes

Answer 12

occurs when a translation initiation complex forms towards the 5’ cap of the mRNA and then scans the mRNA until an AUG start codon is encountered

Question 13

what is translation initiation in prokaryotes

Answer 13

• translation initiation complex will assemble at one or more ribosome binding sides called Shine-Dalgarno sequences
• These sequences are located few bases upstream of start codon AUG

Question 14

why do prokaryotes have specific open reading frames for more than one protein along a single mRNA strand

Answer 14

• because there is a unique ability for translation to occur along multiple regions of a polycistronic mRNA sequence
• this occurs as prokaryotes have functionally related genes grouped together along the prokaryotic DNA and these genes are transcribed as a single unit from one promoter

Question 15

when do the small and large ribosomal subunits assemble

Answer 15

they assemble to form a functional ribosome ONLY when they are attached to an mRNA molecule

Question 16

what assembly does the initiation of translation require

Answer 16

various components including both ribosomal subunits, the mRNA that requires translation, the charged tRNA methionine, and initiation factors that will help assemble the initiation complex

Question 17

in eukaryotes, what do initiation factors do at the start of translation

Answer 17

• bind to the 5’ cap of the mRNA which allows for the recruitment of the small ribosomal subunit
• at the same time, other initiation factors will bind to the tRNA that is charged with methionine

Question 18

what does the partially assembled initiation complex do (aka when the small ribosomal subunit is recruited and factors are binded to tRNA)?

Answer 18

• partially assembled initiation complex will then move along the mRNA in a 5’ to 3’ direction until an AUG start codon is encountered
• when this happens, the large ribosomal subunit binds to the rest of the initiation complex using energy from GTP hydrolysis
• the next charged tRNA molecule then joins the ribosome

Question 19

when are initiation factors released

Answer 19

once the ribosomal translation complex is completely assembled

Question 20

what direction are polypeptides synthesized

Answer 20

from the amino end to the carboxyl end

• as a result, methionine will be the first amino acid found at the amino end of a polypeptide

Question 21

at the start of translation, where is methionine

Answer 21

unlike all tRNA charged molecules, methionine is located in the peptidyl site (P site)

Question 22

when do tRNAs enter the A site

Answer 22

as ribosomes continue to process translation and scan the mRNA molecule, each tRNA enters and binds within the aminoacyl site (A site) of the large ribosomal subunit before each amino acid is added to the polypeptide chain

Question 23

what is each charged tRNA molecule delivered with

Answer 23

with a GTP-bound elongation factor

• when the correct codon-anticodon pairing has been made, the GTP is hydrolyzed and the aminoacyl end of the tRNA is released from the elongation factor

Question 24

when does a peptidyl transferase reaction occur

Answer 24

• after the binding of the charged aminoacyl tRNA, there is a conformational change induced in the ribosomal RNA that allows for the rxn
• this involves the formation of a condensation reaction as a peptide bond and the transfer of the growing polypeptide chain onto the tRNA that is in the A site

Question 25

after the binding, how does the ribosome continue to translocate along the length of the mRNA molecule

Answer 25

- by the binding of GTP elongation factors that cause the deacylated tRNA to move from the P site to the exit site (E site) - the subsequence aminoacyl tRNA that enters the A site will allow for the release of the deacylated tRNA from the E site

Question 26

what happens once the ribosome reaches a stop codon on the mRNA sequence

Answer 26

- GTP bound release factors will bind to the A site and catalyze the hydrolysis of the bond between the terminal amino acid in the polypeptide and the tRNA in the P site - further GTP hydrolysis will dissociate the translation complex (including ribosomal subunits and any remaining bound tRNA)

Question 27

what did george beadle and edward tatum establish

Answer 27

- the relationship between genes and proteins using neurospora crassa; ONE GENE- ONE ENZYME HYPOTHESIS - hypothesis is based on how neurospora can grow well on minimal medium which contains only simple sugars, inorganic salts and essential growth vitamins - as a result, neurospora must have some enzymes produced by a specific gene that convert these simple substances into the amino acids and vitamins needed for growth

Question 28

George Beadle & Edward Tatum (one gene-one enzyme): what did they find out about arginine?

Answer 28

- found that normal bread mold grows on growth medium which lacks arginine --> neurospora are able to synthesize their own arginine - synthesis of arginine occurs throw a multistep metabolic pathway - in this metabolic pathway, the transition between steps requires specific enzymes that catalyzes the formation of each subsequent intermediate compound between precursor and synthesized arginine - precursor compound can lead to synthesis of ornithine, citrulline, and finally arginine through the action of enzymes 1, 2, and 3 respectively

Question 29

what did Adrian Srb & Norman Horowitz establish

Answer 29

- based on george beadle & edward tatum, they performed a genetic screen of radiation-treated neurospora to see if there are specific genes that produced each of the three enzymes needed for arginine synthesis - conducted genetic screening by raising colonies of radiation treated cells on a medium supplemented with nothing else or had ornithine, citrulline, or arginine added

Question 30

Adrian Srb & Norman Horowitz: what did they observe on a growth medium supplemented with arginine

Answer 30

- there was continual growth of the neurospora fungus - was a positive control and indicated that with supplemental arginine, neurospora is able to undergo growth

Question 31

Adrian Srb & Norman Horowitz: what happened when the radiation-treated neurospora cells were placed on non-supplemental medium

Answer 31

- was no growth - led them to believe that the radiation produced mutations in the genes that encode for the necessary enzymes for the production of arginine by the neurospora cells

Question 32

Adrian Srb & Norman Horowitz: what happened when the neurospora cells were placed in ornithine only or citrulline only supplemented media

Answer 32

- there was also an inhibition and growth - results identified three mutants: arge 1, arge 2, arge 3, representing mutation neurospora that had mutations for enzymes 1, 2, 3 needed for arginine production

Question 33

Adrian Srb & Norman Horowitz: why did the name for the hypothesis change to ONE GENE- ONE POLYPEPTIDE

Answer 33

- further research saw that genes do not only code for enzymes in an organism but rather dictate the structures of all proteins (produced by a specific gene product) - because of this, the hypothesis is now referred to as ONE GENE - ONE POLYPEPTIDE hypothesis

Question 34

what are some exceptions to the one gene - one polypeptide hypothesis

Answer 34

- the human proteome represents full number of proteins expressed, aka the genome - when the human genome was sequenced, there was found to be 20-25,000 protein encoding genes - research provided evidence that more than one protein can be produced from a single gene - alternative splicing of genes contribute to diverse number of mRNA transcripts