HW Weeks 5/6/7 T/F questions

DNA repair mechanisms all depend on the existence of two copies of the genetic information, one in each of the two homologous chromosomes.

False. Repair of damage to a single strand by base excision repair or nucleotide excision repair, for example, depends on just the two copies of genetic information contained in the two strands of the DNA double helix. By contrast, precise repair of damage to both strands of a duplex—a double-strand break, for example—requires information from a second duplex, either a sister chromatid or a homolog.

How many kinetochores are there in a human cell at mitosis?

There are 46 human chromosomes, each with two kinetochores—one for each sister chromatid—thus, there are 92 kinetochores in a human cell at mitosis.

While other proteins come and go during the cell cycle, the proteins of the origin recognition complex remain bound to the DNA throughout.

True. The origin recognition complex serves as a scaffold at origins of replication in eukaryotic cells around which other proteins are assembled and activated to initiate DNA replication.

Meiosis segregates the paternal homologs into sperm and the maternal homologs into eggs.

False. At the start of meiosis, each diploid cell contains two sets of homologs: one from the mother and one from the father. During meiosis, these two sets of homologs are randomly assorted so that sperm and eggs will get one set of homologs, but each set will be a mixture of paternal and maternal homologs.

Chromosomes are positioned on the metaphase plate by equal and opposite forces that pull them toward the two poles of the spindle.

False. Equal and opposite forces that tug the chromosomes toward the two spindle poles would tend to position them at random locations between the poles. The poleward force on each chromosome is opposed by a polar ejection force that pushes the chromosome away from the pole. The ejection force is mediated by plus-end directed kinesin motors on chromosome arms that interact with interpolar microtubules and transport the chromosomes away from the spindle poles. This balance of forces tends to position the chromosomes at the midpoint between the poles—the metaphase plate.

DNA and RNA use the same four-letter alphabet.

False. The nucleotide subunits of RNA and DNA differ in two key ways. First, the backbone in RNA uses the sugar ribose instead of deoxyribose, which is used in DNA. Second, RNA uses the base uracil in place of the base thymine, which is used in DNA. Three of the four bases—A, C, and G—are the same in RNA and DNA.

Which of the following mutational changes would you predict to be the most deleterious to gene function? Explain your answers. 1. Insertion of a single nucleotide near the end of the coding sequence. 2. Removal of a single nucleotide near the beginning of the coding sequence. 3. Deletion of three consecutive nucleotides in the middle of the coding sequence. 4. Substitution of one nucleotide for another in the middle of the coding sequence.

Single-nucleotide deletions near the beginning of the gene (2) would be the most harmful since they would change the reading frame early in the coding sequence. As a result, the encoded protein would contain a nonsensical and likely truncated sequence of amino acids. In contrast, a reading frameshift that occurs toward the end of the coding sequence, as described in 1, will result in a largely correct protein that may be functional.

10. The human α-tropomyosin gene is alternatively spliced to produce different forms of α-tropomyosin mRNA in different cell types (Figure Q6-3). For all forms of the mRNA, the protein sequences encoded by exon 1 are the same, as are the protein sequences encoded by exon 10. Exons 2 and 3 are alternative exons used in different mRNAs, as are exons 7 and 8. Which of the following statements about exons 2 and 3 is the most accurate? Is that statement also the most accurate one for exons 7 and 8? Explain your answers. A. Exons 2 and 3 must have the same number of nucleotides. B. Exons 2 and 3 must each contain an integral number of codons (that is, the number of nucleotides divided by 3 must be an integer). C. Exons 2 and 3 must each contain a number of nucleotides that when divided by 3 leaves the same remainder (that is, 0, 1, or 2).

Statement C is the only one that is necessarily true for exons 2 and 3. It is also the only one true for exons 7 and 8. While the conditions given in A and B could be the case, they need not be. However, because the encoded protein sequence is the same in segments of the mRNA that correspond to exons 1 and 10, neither choice of alternative exons (2 versus 3, or 7 versus 8) can be allowed to alter the reading frame. To maintain the normal reading frame—whatever that is—the alternative exons must have a number of nucleotides that when divided by 3 (the number of nucleotides in a codon) gives the same remainder.

When bidirectional replication forks from adjacent origins meet, a leading strand always runs into a lagging strand.

True. Consider a single template strand, with its 5ʹ end on the left and its 3ʹ end on the right. No matter where the origin is, synthesis to the left on this strand will be continuous (leading strand), and synthesis to the right will be discontinuous (lagging strand). Thus, when replication forks from adjacent origins collide, a rightward-moving (lagging) strand will always meet a leftward-moving (leading) strand

The different cells in your body rarely have genomes with the identical nucleotide sequence.

True. Each time the genome is copied in preparation for cell division, there is a chance that mistakes (mutations) will be introduced. The rate of mutation for humans is estimated to be 1 nucleotide change per 1010 nucleotides each time the DNA is replicated. Since there are 6.4 × 109 nucleotides in each diploid cell, an average of 0.64 random mutations will be introduced into the genome each time it is copied. Thus, the two daughter cells from a cell division will often differ from one another and from the parent cell that gave rise to them. Even genomes that are copied perfectly, giving rise to identical daughter cells, will often be altered in subsequent replication cycles. The proportion of identical cells depends on the exact mutation rate.

1. The consequences of errors in transcription are less severe than those of errors in DNA replication.

True. Errors in DNA replication have the potential to affect future generations of cells, while errors in transcription have no genetic consequence. Errors in transcription lead to mistakes in a small fraction of RNAs, whose functions are further monitored by downstream quality control mechanisms. They are not passed on to progeny cells. In contrast, errors in DNA replication change the gene and, thereby, affect all the copies of RNA (and protein) made in the original cell and all its progeny cells.

Genes and their encoded proteins are co-linear; that is, the order of amino acids in proteins is the same as the order of the codons in the RNA and DNA.

True. Even in eukaryotes, where the coding regions of a gene are often interrupted by noncoding segments, the order of codons in the DNA is still the same as the order of amino acids in the protein

In a replication bubble, the same parental DNA strand serves as the template strand for leading-strand synthesis in one replication fork and as the template for lagging-strand synthesis in the other fork.

True. The two ends of a single parental strand of DNA will be copied in the same direction. At the fork at one end of a replication bubble, this will correspond to the leading strand; at the fork at the other end, it will correspond to the lagging strand