Chapter 16: Bio 151

2 (The most common number of codons that specifies a single amino acid is two. Of the 20 amino acids, nine are coded by two codons.)

The extra base pair would make it possible to have codons two bases in length rather than three (If, instead of two sets of base pairs (A-T, G-C), there were three sets of base pairs (A-T, G-C, and X-Y), the extra base pair would make it possible to have codons two bases in length rather than three. If there were six possible nucleotides, then codons two bases in length could code for over 20 amino acids. 62 = 36. They could not be only a single base long: 61 = 6.)

48 (The sequence of a polypeptide is determined by the order of codons that specify the amino acids in the polypeptide. The polypeptide sequence methionine-histidine-alanine-leucine can be specified by 48 sequences of codons. To calculate this, you need to multiply the total number of codon sets that can code for each of the amino acids in the sequence. Therefore, 1 x 2 x 4 x 6 = 48 (see figure).)

The retrovirus needs its reverse transcriptase to make DNA before it can use the cell's molecular machinery to make new reverse transcriptase. (Howard Temin and Satoshi Mizutani discovered that the Rous sarcoma retrovirus contains the enzyme reverse transcriptase. It is important for the virus to carry its own reverse transcriptase protein because the retrovirus needs its reverse transcriptase to make DNA before it can use the cell's molecular machinery to make new reverse transcriptase. Reverse transcriptase is a necessary enzyme for RNA to produce DNA. Because the host cell would not normally have a use for this enzyme, it is not likely that the cell machinery would be in place. Hence, the virus itself must contain a reverse transcriptase gene to successfully reproduce and infect a host.)

6 (Six different amino acids would be found in a polypeptide made from translation of an RNA containing only cytosines (C) and adenines (A) in random order. The six resultant codons would be the following: AAA (lysine), AAC (asparagine), ACC, ACA (both threonine), CCC, CCA (both proline), CAC (histidine), CAA (glutamine))

Nonsense

Single-stranded RNA from a virus is used as a template for producing DNA that inserts into the host genome

The existing genetic code is structured in a way that efficiently minimizes the phenotypic effects of small changes in DNA and errors during translation

4

2 (Two amino acids are represented in the genetic code by one codon. The start codon, methionine, is AUG and tryptophan is UGG. Therefore, the correct answer is two.)

N. crassa knock-out mutants that lacked the ability to synthesize certain compounds

The gene that codes for the enzyme required to produce the next intermediate in the pathway is knocked out.

The RNA polymer would contain only uracil.

RNA -> DNA -> RNA -> protein

this cannot be determined

to carry information from DNA out of the nucleus to the site of protein synthesis (The function of mRNA is to carry information from DNA out of the nucleus to the site of protein synthesis. Messenger RNA (mRNA) is used to transcribe DNA information for protein synthesis. The DNA is transcribed to the mRNA, which then moves out of the nucleus. Proteins are made at the ribosomes (outside of the nucleus).)

they irradiated N. crassa cells, grew all the mutants on arginine, and then grew each mutant on medium containing a different intermediate in the arginine pathway (To determine the metabolic pathway for arginine synthesis, they irradiated N. crassa cells, grew all the mutants on arginine, and then grew each mutant on medium containing a different intermediate in the arginine pathway. The key design in their study was that they found three different mutants and raised them with different intermediates in the arginine pathway. This allowed them to demonstrate that each mutant (or gene) was responsible for a particular enzyme along the pathway.)

transcribe DNA to RNA (The function of RNA polymerase is to transcribe DNA to RNA. The central dogma of molecular biology is that DNA codes for RNA, which codes for proteins. The first step in this is the transcription of DNA to RNA. RNA polymerase catalyzes this process, but does not serve to amplify the "message.")

a gene altered so that it no longer produces the intended proteins (A knockout allele is a gene altered so that it no longer produces the intended proteins. Knockout alleles are used by scientists to test the effects of specific genes. The modification of these genes often allows a better understanding of metabolic pathways.)

reverse transcriptase (Reverse transcriptase allows RNA to be coded into DNA. Some RNA viruses use the enzyme reverse transcriptase to integrate their genetic information into the DNA of the host. The host then uses RNA polymerase to produce mRNA molecules that that are translated into new proteins that function for the virus.)

Each mutant gene affected only one enzyme.

the coding from mRNA to proteins

A frameshift mutation resulting in a change in the amino acid sequence

Messenger RNA molecules act as the link between genes and the protein-manufacturing centers.

The encoded protein may be unchanged due to the redundancy of the genetic code.

AAA Correct. Changing the first base from A—>U creates the stop codon UAA.

Substrate 2

enzyme B

Both transcription and translation will be inhibited. Transcription is RNA synthesis by RNA polymerase and will be inhibited directly; translation is protein synthesis and will be inhibited indirectly because ribosomes contain RNA molecules essential to their function.

Information can flow from RNA back to DNA.

False A codon is a group of three bases that can specify only one amino acid.

A knock-out mutation results in a total absence of the mutated protein. A knock-out mutation refers to the loss of a protein's function but not necessarily to its complete absence.

Deletion The original sequence has lost the base C.

One addition and one deletion mutation. This combination results in no net change in the number of bases, so the reading frame would eventually be restored.

Two The second and third codons in the new sequence are different from the original codons.

An addition mutation and a deletion mutation. If the mutations occur within the same codon, only that codon (amino acid) will be altered.

transferring of information from DNA to messenger RNA Transcription is the first of the two main steps of protein synthesis.

the central dogma The central dogma is one of the basic underpinnings of biology.

a frameshift deletion at the beginning of the gene A frameshift mutation at the beginning of a gene would affect every codon after the point where the mutation occurred. During protein synthesis, incorrect amino acids would be inserted from the point where the frameshift mutation occurred on; the resulting protein would most probably be nonfunctional. For this reason, a frameshift mutation at the beginning of a gene is generally the most severe type of mutation.

False Silent mutations do not change phenotype.

a truncated gene product, as with a nonsense mutation Frameshift mutations essentially randomize all subsequent codons. A stop codon often arises soon after the frameshift by chance.

silent mutation Because introns contain no codons, mutations will not affect the gene product.

Mutations can have a positive, neutral, or negative effect on fitness. Mutations are random with respect to fitness; they can be beneficial, neutral, or deleterious, depending on the effect on phenotype and the environment of the organism.

nonsense A nonsense mutation creates an early stop codon, resulting in a shortened protein.