15. Gene Expression

Genetic Code

A minimal genetic code requires only 21 codons—one for each amino acid, and one for a stop signal. Given this, what advantage might be offered by having a code with 64 codons?

 bacterial ribosomes and eukaryotic ribosomes are identical 

 No. Bacterial and eukaryotic ribosomes are not identical, but they are capable of translating each other's mRNA. 

 eukaryotes do not really need a nucleus 

 No. Eukaryotic cells require a nucleus for a number of reasons, including processing of pre-mRNA, but a properly engineered gene can be transcribed and translated by a bacterium. 

 bacterial and eukaryotic RNA polymerases are identical 

Even though these organisms are structurally different, think about what stays the same.

Bacteria can transcribe and translate human genes to produce functional human proteins because __________.

GENETIC CODE

A particular triplet of bases in the coding sequence of DNA is AAA. The anticodon on the tRNA that binds the mRNA codon is ________.

Decoding the Genetic Code from DNA to mRNA to tRNA to Amino Acid

Which of the following describes the experimental strategy that was used to decipher the genetic code?
a. comparing the amino acid sequences of proteins with the base sequence of their genes
b. analyzing the sequence of RNAs produced from known DNA sequences
c. analyzing mutants that changed the code
d. examining the polypeptides produced when RNAs with particular sequences were translated

 No. There are 20 amino acids, but not 20 genetic codes. 

 No. There are four nitrogenous bases in DNA, but there are not four genetic codes. 

 No. The genetic code is essentially universal. Nearly all organisms use the same genetic code to translate mRNA to protein. 

Why do we say that the genetic code is universal?

Generally speaking, how many genetic codes are there?

Draw a hypothetical metabolic pathway in Neurospora crassa composed of five substrates, five enzymes, and a product called nirvana. Number the substrates 1–5, and label the enzymes A–E, in order. (For instance, enzyme A catalyzes the reaction between substrates 1 and 2.) (a) Suppose a mutation made the gene for enzyme C nonfunctional. What molecule would accumulate in the affected cells?

The Genetic Code- how to translate mRNA

Decode from DNA to mRNA to tRNA to amino acids

How to Use the Genetic Code

 No. In RNA, uracil takes the place of thymine; it does not pair with thymine. 

 No. Thymine is not present in RNA. Furthermore, in DNA, thymine does not pair with guanine. 

Make sure you understand specific base pairing.

When RNA is being made, the RNA base __________ always pairs with the base __________ in DNA.

One of the possibilities considered about the genetic code was that the code was overlapping, meaning that a single base could be part of up to three codons. How many amino acids would be encoded in the sequence 5′-AUGUUACGGAAU-3′ by a non-overlapping and a maximally overlapping triplet code?
a. 4 (non-overlapping) and 16 (overlapping)
b. 4 and 12
c. 4 and 10
d. 12 and 4

Animation: Translation: RNA Folding

Draw a hypothetical metabolic pathway in Neurospora crassa composed of five substrates, five enzymes, and a product called nirvana. Number the substrates 1–5, and label the enzymes A–E, in order. (For instance, enzyme A catalyzes the reaction between substrates 1 and 2.) (b) Suppose a mutant strain can survive if substrate 5 is added to the growth medium, but it cannot grow if substrates 1, 2, 3, or 4 are added. Which enzyme in the pathway is affected in this mutant?

 No. There are enough codons to specify all 20 amino acids. 

The systems that operate the international space station have multiple backups for critical components, with the intention of increasing the reliability and functionality of each system. What is a term that describes these backup systems?

In many cases, more than one codon codes for the same amino acid. Because of this, we say that the code is __________.

Atmospheric CO2 has been increasing rapidly since the late 1800s, largely due to human activities. Recall that CO2 enters leaves through stomata and can then be used for photosynthesis. However, transpiration occurs as a result of water evaporating through stomata. How have plants responded to elevated CO2 levels?

Assuming that the CO2 level continues to increase with time, how likely are plants to be able to continue to adapt by adjusting stomatal conductance?

A particular triplet of bases in the template strand of DNA is 5′-AGT-3′. What would be the corresponding codon for the mRNA that is transcribed?

What amino acid sequence will be generated, based on the following mRNA codon sequence? 5′–AUG–UCU–UCG–UUA–UCC–UUG–3′<img src="https://static.studychannel-prd.pearsonprd.tech/courses/biology/thumbnails/images/82a30375" class="fr-fic fr-dii">

For each of these statements about the genetic code, select True or False.
a. T/F Wobble pairing accounts for the redundancy of the genetic code.
b. T/F There are 64 different tRNAs that read the 64 possible codons.
c. T/F All possible codons are used, but not all codons specify an amino acid.
d. T/F Some codons are recognized by proteins, not by tRNAs.

THE GENETIC CODE

List the order of nucleotides on the mRNA that would be transcribed from the following DNA sequence: CGATTACTTA.

Which of the following sequences of nucleotides are possible in the template strand of DNA that would code for the polypeptide sequence Phe–Leu–Ile–Val?<img src="https://static.studychannel-prd.pearsonprd.tech/courses/biology/thumbnails/images/9f934885" class="fr-fic fr-dii">

 No. A dipeptide consists of two amino acids. 

 No. A triose is a three-carbon monosaccharide. 

Consider the nature of the genetic code.

The number of nucleotide bases "read" together on the mRNA to designate each amino acid is __________; this unit is called a(n) __________.

 the tRNA with an anticodon 5&prime;-GUU-3&prime; and glutamine at its other end 

 No. Remember that antiparallel orientation applies in translation. 

 tRNA molecules that have been charged with glutamine by two different aminoacyl-tRNA synthetases 

 No. There are 20 different aminoacyl-tRNA synthetases, one for each amino acid. Each synthetase is able to recognize, bind to, and charge different tRNAs that code for a particular amino acid. 

 separate tRNA molecules with anticodons 3&prime;-GUU-5&prime; and 3&prime;-GUC-5&prime;, respectively 

 No. Because the base-pairing rule is relaxed at the third nucleotide position (i.e., wobble), one tRNA can accurately translate these codons as glutamine. 

 the same tRNA with the anticodon 3&prime;-GUU-5&prime; 

Think about how molecular recognition affects the accurate translation.

The mRNA codons 5&prime;-CAA-3&prime; or 5&prime;-CAG-3&prime; are translated as the amino acid glutamine by __________.

 Each DNA base codes for three amino acids. 

 No. Three nucleotides code for a single amino acid. 

 No. The number of proteins that a single gene codes for can vary with mRNA processing. 

 It takes three genes to code for one protein. 

 Each amino acid in a protein is coded for by three bases in the DNA. 

How does the genetic code work?

Which statement is true?

The Genetic Code

Which of the following is not true of a codon?
a. It may code for the same amino acid as another codon.
b. It never codes for more than one amino acid.
c. It extends from one end of a tRNA molecule.
d. It is the basic unit of the genetic code.

Genetic code

 No. Review which codons specify which amino acids. 

The DNA includes a template strand and a coding strand, which are antiparallel.

The codons AAA, CCC, GGG, and UUU specify the amino acids lysine, proline, glycine, and phenylalanine, respectively. If the base sequence 5&prime;-CCCAAATTTGGG-3&prime; is present in the coding strand of a stretch of DNA, what polypeptide sequence would be encoded by the corresponding template strand?

Consider how many nucleotides code for a single amino acid.

How many nucleotides are needed to code for a protein with 450 amino acids?

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