In double-stranded DNA, the amount of adenine equals the amount of thymine.
Correct Answer
True
True. Because adenine always pairs with thymine, the two are present in equal amounts, an observation known as one of Chargaff's rules. The same holds for guanine and cytosine.
Category: DNA Structure
Question 2
RNA contains thymine in place of the uracil found in DNA.
Correct Answer
False
False. It is the reverse: RNA contains uracil in place of the thymine found in DNA. Uracil pairs with adenine just as thymine does.
Category: DNA Structure
Question 3
The two strands of a DNA double helix are antiparallel, running in opposite 5' to 3' directions.
Correct Answer
True
True. One strand runs 5' to 3' while its partner runs 3' to 5'. This orientation is why replication must proceed differently on the leading and lagging strands.
Category: DNA Structure
Question 4
In a DNA strand, each phosphate group links the 5' carbon of one sugar to the 2' carbon of the next sugar.
Correct Answer
False
False. The phosphate links the 5' carbon of one nucleotide's sugar to the 3' carbon of the next, forming the sugar-phosphate backbone. The 2' carbon of deoxyribose carries only a hydrogen and takes no part in the backbone.
Category: DNA Structure
Question 5
During interphase, the tightly packed, darkly staining regions of chromatin usually contain genes that are not being expressed.
Correct Answer
True
True. Densely packed chromatin is hard for the transcription machinery to reach, so its genes are usually inactive. Loosely packed, lightly staining regions usually hold active genes.
Category: DNA Structure
Question 6
The circular chromosome of E. coli is replicated from many origins of replication.
Correct Answer
False
False. A bacterial chromosome such as that of E. coli has a single origin of replication, from which two forks move around the circle. Eukaryotic chromosomes are the ones with many origins.
Category: Replication and Repair
Question 7
Each origin of replication gives rise to two replication forks that move away from each other.
Correct Answer
True
True. Replication is bidirectional: once the helix opens at an origin, forks proceed in both directions. This speeds up copying of the whole chromosome.
Category: Replication and Repair
Question 8
Telomerase is highly active in most adult somatic cells.
Correct Answer
False
False. Telomerase is active mainly in germ cells, adult stem cells and some cancer cells, and is not active in most adult somatic cells. As a result, telomeres in most body cells shorten with each division, which is associated with aging.
Category: Replication and Repair
Question 9
In humans, telomeres are made of the six-base sequence TTAGGG repeated many times.
Correct Answer
True
True. Human telomeres consist of TTAGGG repeated hundreds of times. Because these repeats do not code for genes, their gradual loss during replication protects the genes nearer the middle of the chromosome.
Category: Replication and Repair
Question 10
Mismatch repair enzymes can recognize an incorrectly paired base after replication, remove it, and replace it with the correct base.
Correct Answer
True
True. Mismatch repair catches errors that slip past the proofreading activity of DNA polymerase. The wrong base is excised from the newly made strand and the correct one is filled in.
Category: Replication and Repair
Question 11
In eukaryotic cells, an mRNA molecule can be translated while it is still being transcribed.
Correct Answer
False
False. In eukaryotes, transcription happens in the nucleus and translation in the cytoplasm, so the nuclear envelope keeps the two processes apart. Simultaneous transcription and translation happens in prokaryotes, which lack a nucleus.
Category: Transcription and Processing
Question 12
Introns are removed from a eukaryotic pre-mRNA after the mRNA has been exported to the cytoplasm.
Correct Answer
False
False. Introns are spliced out and degraded inside the nucleus, before the mature mRNA is exported. Only the processed mRNA, with its exons joined, travels to the cytoplasm.
Category: Transcription and Processing
Question 13
The 5' cap on a eukaryotic mRNA helps protect it from degradation and helps ribosomes begin translation.
Correct Answer
True
True. The cap is a modified guanine nucleotide added to the 5' end early in transcription. Besides shielding the transcript, it helps the ribosome recognize where to start.
Category: Transcription and Processing
Question 14
A typical prokaryotic mRNA persists in the cell for several hours before it is broken down.
Correct Answer
False
False. Prokaryotic mRNAs are short-lived, typically lasting only seconds to a few minutes. Eukaryotic mRNAs are the ones that commonly last for several hours.
Category: Transcription and Processing
Question 15
Three of the 64 codons are stop codons that do not specify any amino acid.
Correct Answer
True
True. UAA, UAG and UGA signal the end of translation and cause the finished polypeptide to be released. The remaining 61 codons specify amino acids.
Category: Translation
Question 16
Bacteria and humans use entirely different genetic codes.
Correct Answer
False
False. Nearly all organisms, from bacteria to humans, use the same genetic code, with only a few minor exceptions. This near universality is evidence that all life shares a common origin.
Category: Translation
Question 17
Many ribosomes can translate the same mRNA molecule at the same time.
Correct Answer
True
True. Several ribosomes can move along one mRNA in the same direction, each making its own copy of the polypeptide. This lets a cell make many copies of a protein quickly.
Category: Translation
Question 18
Translation stops when the ribosome reaches an AUG codon.
Correct Answer
False
False. AUG is the start codon, which also codes for methionine. Translation stops when the ribosome reaches one of the stop codons, UAA, UAG or UGA.
Category: Translation
Question 19
Different specialized cells in the same multicellular organism express different sets of genes.
Correct Answer
True
True. Nearly every cell in the body carries the same genes, but a muscle cell and a nerve cell switch on different subsets of them. Which genes are expressed gives each cell type its own structure and function.
Category: Gene Regulation
Question 20
In the lac operon, lactose switches the genes on by binding directly to the operator.
Correct Answer
False
False. Lactose acts through the repressor: a product of lactose binds the repressor protein, which then releases the operator. With the operator free, RNA polymerase can transcribe the genes.
Category: Gene Regulation