Biochemistry 2280A Lecture Notes - Lecture 21: Ribosome-Binding Site, Stem-Loop, Operon

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1 Jan 2017

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Second step in central dogma key step in regulation. The genetic code (cid:498)spells(cid:499) out the amino acid sequence in 3 (cid:498)letter(cid:499) (cid:498)words(cid:499) called codons: words are 3 letters in length and there are 4 letters. 20 amino acids = have to code for all 20. Need at least 20 codons = 20 different words. 4 x 4 4 at each position. A one letter code with a u g c = 4 codons. If codons were only 1 base in length, would only have 4 possible codons (aguc: not enough because we have 20 amino acids, not enough. A two letter code would provide: 4x4 = 16 amino acids: less space consuming but doesn"t code for all amino acids. As there are 20 amino acids, a 3 letter code (64 codons) is sufficient. Could have a 4 letter code = wasteful. 3 is the minimum u could have for the fact that we use 20 amino acids.

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The genetic code in our world is a triplet code using combinations of four letters (A,U,G,C). All code words (codons) are the same size, three bases long. Proteins in our world are build using combinations of 20 different subunits (amino acids). In class, we argied tjat imder the conditions in our world (only four differebt letters in mRNA to code for 20 different amino acids in proteins) a triplet code is the smallest possible code that could be sued. A) list the other properties of our genetic code. C) what do we mean by the term degenerate code? Cam a doutblet code also be a degenerate code? (If yes, under what circumstances? If no, why not?) D) Does degeneracy in its genetic code represent 1) an advantage; 2) a disadvantage or 3) both and advantage and a disadvantage or 4) neither an advantage nor a disadvantage to an organism?

QUESTION 1. Choose the true statement about prokaryotic and eukaryotic DNA replication.

A.Eukaryotic DNA replication involves less protein factors than does prokaryotic DNA replication.

B. Prokaryotic DNA polymerases replicate DNA 5' to 3', while eukaryotic DNA polymerases replicate DNA 3' to 5'.

C. Eukaryotic DNA replication begins at multiple origins of replication, while prokaryotic DNA replication begins at a single origin of replication.

D. Prokaryotic genomes take longer to replicate than eukaryotic genomes.

QUESTION 2. Bacterial cells use DNA replication to:

A. make back-up copies of their genome in case of a mutation.

B. manifest multiple phenotypes at once.

C. copy their genetic material prior to binary fission.

D. build functional proteins.

QUESTION 3. Choose the true statement about the genetic code.

A. A cell's mRNA code is read four nucleotides at a time to specify a single amino acid.

B. The genetic code encodes 64 amino acids, one for each of the 64 corresponding codons.

C. The genetic code is redundant, with multiple codons specifying a single amino acid.

D. The genetic code varies widely among organisms, with each having a different set of codons for a corresponding amino acid.

QUESTION 4. Of the following enzymes, which are correctly matched with their function? To be marked correct, you'll need to select all correctly matched statements, as there may be more than one correct answer.

A. Helicase, relieves torsion stress as DNA unwinds

B. Ligase, covalently bonds adjacent Okazaki fragments

C. Primase, removes RNA primers

D. DNA polymerase III, builds most of the DNA on the leading and lagging strand

QUESTION 5. Which of the following contributes to DNA replication accuracy? To be marked correct, you'll need to select all true statements, as there may be more than one correct answer.

A. The capacity to proofread improves DNA replication accuracy.

B. A slow rate of DNA replication contributes to accuracy.

C. The use of primase improves DNA replication accuracy.

D. DNA replication accuracy is improved thanks to complementary base pairing rules.

36. The term used to describe the relationship between the two strands in a DNA double helix is:
A. opposite
B. an exact duplicate
C. colinear
D. complementary
E. antagonistic

37. The sequence of one strand of a DNA molecule is 5'-AGCGCT. What is the sequence of the other DNA strand?
A. 3'-AGCGCT
B. 3'-GCTGCA
C. 5'-TCGCGA
D. 5'-AGCGCT
E. none of the above

38. The elongation of the leading strand during DNA replication:
A. progresses away from the replication fork
B. occurs in the 3' --> 5' direction
C. produces many Okazaki fragments, which subsequently are joined together
D. depends primarily on the activity of DNA polymerase
E. does not require a template strand

39. Which statement about the genetic code is FALSE?
A. the genetic code is essentially identical in all living organisms
B. there are six codons for arginine (Arg)
C. amino acids are encoded by 64 different codons
D. several amino acids are encoded by more than one codon
E. the start codon is usually AUG, the only codon for methionine

40. Approximately what fraction of the human genome corresponds to coding regions of genes (exons)?
A. 100%
B. 48%
C. 19%
D. 1.5%
E. 0.1%

Biochemistry 2280A Lecture Notes - Lecture 21: Ribosome-Binding Site, Stem-Loop, Operon

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