Chapter 13: Translation

If a tRNA molecule carries glutamic acid, what are the two possible anticodon sequences that it could contain? Be specific about the 5' and 3' ends.

3' - CUU - 5' or 3' - CUC - 5' (however, 3' - CUU - 5' could take care of both codons due to wobble).

What is the amino acid sequence encoded by the following mRNA?3'- ACC AGU AUG UUU ACU UUA AUU UGA CGG GUA CAU CCG -5'

3'- ACC AGU AUG UUU ACU UUA AUU UGA CGG GUA CAU CCG -5' This is the same amino acid sequence as in Q7 because codons are always read 5' - 3' and amino acid sequence is always given from N-terminus to C-terminus. Met-Gly-Ser-Leu-Ile-Ser-Phe-V al

An mRNA codon has the sequence 5 - ACG - 3'. Which tRNA anticodon will bind to this codon, and which amino acid does the tRNA carry?

5' - CGU - 3' (same as 3' - UGC - 5') or 5' - UGU - 3' (same as 3' - UGU - 5') The tRNAs carry threonine (Thr) as specified by the codon.

If a tRNA has the anticodon sequence 3' - CCI - 5' what codon(s) can it recognize?

5' - GGU - 3', 5' - GGC - 3' and 5' - GGA - 3', which all encode glycine.

A tRNA has the anticodon 5'-CCG-3'. Which mRNA codon does this tRNA recognize? Which amino acid does this tRNA carry?

5'- CGG - 3'(same as 3'- GGC - 5')The tRNA carries arginine (Arg) as specified by the codon

Are codon read from 5'-3' or 3'-5'?

5'-3'

What is the amino acid sequence encoded by the following mRNA? 5'- GCC UAC AUG GGC AGU UUA AUU UCA UUU GUA UGA CCA - 3'

5'-GCCUACAUGGGCAGUUUAAUUUCAUUUGUAUGACCA-3' Met-Gly-Ser-Leu-Ile-Ser-Phe-V al

A polypeptide contains 200 amino acids. How many nucleotides are expected in the coding sequence of the gene?

600

Briefly describe the elongation stage of translation (a couple of sentences for each stage) mentioning the functional roles of the A, P and E sites.

Elongation: a tRNA with an anticodon complementary to the codon in the A site binds to the A site. A peptide bond forms between the amino acid in the A site and the amino acid at the end of the polypeptide chain held by the tRNA in the P site. The growing polypeptide chain is then transferred to the tRNA in the A site and the ribosome translocates one codon in the 3' direction. The tRNA in the E site exits the ribosome and a new tRNA comes into the A site for the cycle to start again.

Briefly describe the initiation stage of translation (a couple of sentences for each stage) mentioning the functional roles of the A, P and E sites.

Initiation: initiation factors bind to the 5' mRNA cap and facilitate binding of the mRNA to the 40S ribosomal subunit. The subunit then scans the mRNA for the start codon. After finding the start codon, the first tRNA carrying Met binds to the P site and the 60S ribosomal subunit associates with the small subunit.

What does it mean when we say the genetic code is degenerate?

It means the same amino acid can be specified by more than one codon. There are only 20 standard amino acids but there are 61 different codons that specify amino acids.

Proteins have 3 or 4 levels of structure depending on whether the protein is composed on one or more than one polypeptide. Describe the first level of protein structure (one or two sentences for each level will do).

Primary structure: Sequence of amino acids.

Proteins have 3 or 4 levels of structure depending on whether the protein is composed on one or more than one polypeptide. Describe the fourth level of protein structure (one or two sentences for each level will do).

Quaternary structure: The association of two or more polypeptides to form a multi-subunit protein.

Briefly describe the composition of eukaryotic ribosomes (how many subunits does a ribosome have, and which two types of macromolecule are ribosomes composed of)?

Ribosomes have a small 40S subunit and a large 60S subunit. The subunits are composed of ribosomal RNA (rRNA) and proteins.

Proteins have 3 or 4 levels of structure depending on whether the protein is composed on one or more than one polypeptide. Describe the second level of protein structure (one or two sentences for each level will do).

Secondary structure: Folding of local regions of the polypeptide chain. Common secondary structures are the alpha helix and beta sheet.

Which codons are synonymous with CUU (synonymous codons specify the same amino acid)?

Synonymous codons are those that specify the same amino acid. Non-synonymous codons specify different amino acids. UUA, UUG, CUC, CUA, CUG are all synonymous with CUU.

Briefly describe the termination stage of translation (a couple of sentences for each stage) mentioning the functional roles of the A, P and E sites.

Termination: translation ends when there is a stop codon in the A site. The stop codon is recognized by a release factor which promotes release of the completed polypeptide and dissociation of the mRNA and ribosome subunits.

Proteins have 3 or 4 levels of structure depending on whether the protein is composed on one or more than one polypeptide. Describe the third level of protein structure (one or two sentences for each level will do).

Tertiary structure: Overall 3D folded shape of a polypeptide. This is the final level of structure for a protein composed of a single polypeptide.

What are the 5' and 3' untranslated regions?

The 5' untranslated region (UTR) is a region upstream from the translation start site, and it does not get translated to amino acid. The 3' UTR is a region downstream from the stop codon, and it does not get translated to amino acid.

If the DNA template strand is 5' - TCA ACG TAG CCA GGT CAT- 3' what is the amino acid sequence of the polypeptide it encodes? Which amino acid is at the N-terminus of the polypeptide, and which amino acid is at the C-terminus?

The mRNA is: 5' - AUG ACC UGG CUA CGU UGA - 3' The resulting polypeptide is therefore: Met-Thr-Trp-Leu-Arg Met is at the N-terminus and Arg is at the C-terminus

If the DNA coding strand is 5' - ATG ACG TCG CCA GGT TGA- 3' what is the amino acid sequence of the polypeptide it encodes? Which amino acid is at the N-terminus of the polypeptide, and which amino acid is at the C-terminus?

The mRNA is: 5' - AUG ACG UCG CCA GGU UGA - 3' The resulting polypeptide is therefore: Met-Thr-Ser-Pro-Gly Met is at the N-terminus and Gly is at the C-terminus

What is translation?

The sequence of nucleotides in mRNA directs the linking together of amino acids in a precise order to form a polypeptide.

The diagram below depicts a gene and shows the positions that encode translational start and stop codons. Which strand (top or bottom) is the template strand for this gene? 3'------------TAC--------------------ATT------------5' 5'------------ATG--------------------TAA-------------3'

The top strand is the template strand because, when transcribed, it will give an mRNA with the start codon at the 5' end and the stop codon at the 3' end.

How many standard amino acids are there? What distinguishes the different amino acids?

There are 20 standard amino acids distinguished by the side group. The side group gives each amino acid its distinct properties.

What is the function of aminoacyl tRNA synthase enzymes?

They add the correct amino acid to the correct tRNA molecule. A different aminoacyl tRNA synthase enzyme is required for each amino acid.

According to the original wobble rules, what is the minimum number of tRNAs needed to recognize all of the codons for Leucine (Leu)?

Three 3' - AAU - 5' could recognize UUA and UUG3' - GAI - 5' could recognize CUU, CUC and CUA 3' - GAC - 5' or 3' - GAU - 5' could recognize CUG

Where does translation start and stop? Do the start and stop codons specify an amino acid?

Translation starts at a methionine (Met) codon, which specifies the amino acid methionine (Met). Translation stops at a Stop codon, which does not specify an amino acid.

What is wobble between the codon and anticodon? In which position of the codon and anticodon is the wobble base located?

Wobble means that the nucleotides located in the 3' position of the codon and the 5' position of the anticodon do not always base pair according to the complementary AU/GC base pairing rules. For example, G at the 5' end of the anticodon can pair with either C or U at the 3' end of the codon.

What is a codon?

a sequence of 3 nucleotides that specify an amino acid

Is a codon specified by DNA, mRNA or tRNA?

mRNA