 Problems for Test 1 DNA
Solutions to DNA Problems 1. 5 HCN --> 1 Adenine
 3. If a DNA sample has 23% guanine, then it has to have 23% cytosine. The remaining 54% must contain equal amounts of adenine and thymine, or 27% each. 4. Sequences of DNA which need to come apart for regulatory function, such as initiation of transcription, will tend to have A=T base pairs because they have only two hydrogen bonds. Guanine-cytosine pairs have three hydrogen bonds, and hold together more strongly. 5. A molecule that slides inbetween two base pairs and stays they can cause the sequence to be misread by enzymes trying to replicate the DNA. An extra base might get inserted into newly synthesized sequence, resulting in a mutation. (More on this, next week.) 6. Some species of microbes living at extremely high temperature have evolved to maintain positively supercoiled DNA. This helps to increase stability of their DNA, which ordinarily tends to melt open more easily at high temperature.
Solution -- Bacteria Division Problem Bacterial DNA is circular. In bacteria, as in eukaryotes, replication occurs bidirectionally. In bacteria, this replication starts at one point attached to the cell wall, and continues in both directions around the cirle. If the cell is growing very
fast, it will start to replicate each daughter chromosome BEFORE COMPLETION
of the first replication. This allows the chromosome to be replicated
in one half the time that it would take if it waited till the first round
of replication was complete.
Solution -- Overall Cell Generation Time Problem Assume that all of the cells are evenly distributed throughout the different stages in the cell cycle (G1, S, G2, Mitosis). If only five percent of the cells are in mitosis, then mitosis must only take five percent of the total time that is required for overall generation. If five percent of the time is five minutes, then one hundred percent of the time is one hundred minutes. The brown-eyed fly has to be missing a functional allele for a gene B that makes scarlet pigment. The scarlet-eyed fly has to be missing a functional allele for a DIFFERENT gene locus S that makes BROWN pigment. Solution -- Transgenic Mice Problem To simplify the notation,
let's convert this to Mendel's peas.
The transgene containing human globins is dominant, because it provides a product not expressed by the standard mouse, certainly not at the location where the transgene ended up. The normal mouse alpha and beta genes, however, are dominant to the null alleles created by transgenic allele replacement (designated by hyphens.) So let's redraw the genotypes: TT AA BB X tt Aa Bb What progeny are desired?
Purebreeding for transgene (T);
TT aa bb But -- because the human-transgenic parent is pure-breeding, the closest we can get to what we want is this: Tt Aa Bb The proportion of these progeny would be: T t A a B b (1)(1) (1)(1/2) (1)(1/2) = ? These mice are hybrid for
all three genes.
T T a a b b (1/2)(1/2) (1/2)(1/2) (1/2)(1/2) = 1/64 Better breed plenty of mice! Actually, we would be helped out by the fact that all the tt aa bb, tt A- bb, and tt aa B- mice would die before birth (Why? What proportion?). But some of the sickle-cell TT aa bb mice would also die, either shortly after birth, or later before producing offspring. The moral is, if you want
to get into mammalian transgenics—plan to write a big grant to grow all
those mice.
Solution -- X-Linked Inheritance
A A
a
A a A
a a
A
A a a
A a
a
A a a a
A
a
A a
A
A A A a
A
a
Solution -- Gene Product Interaction 1. The phenotypes of the sweet peas are determined by the interaction between two gene loci. The presence of two recessive white alleles at one locus of the white flowers that were crossed made useless the presence of a dominant purple allele at the other locus (recessive epistasis). The ratio of organisms in the self-cross that have dominant purple alleles at both gene loci, to those that have a dominant allele at the first locus, to those that have a dominant allele at the second gene locus, to those that have no dominant alleles is 9 : 3 : 3 : 1. The 3 and 3 and 1 all have white flowers because they have at least one double recessive gene locus. This makes the ratio of purple to white 9:7. 2. Again, the phenotypes of the fancy fowl are determined by the interaction between two gene loci. In this case, however, the presence of a dominant feathered allele at one locus compensates for the lack of dominant feathered alleles at the other locus (dominant epistasis). The ratio of the offspring that had a dominant feathered allele at both, to those that had a dominant allele in the first locus, to those that had a dominant allele in the second locus, to those that had no dominant alleles was 9 : 3 : 3 : 1. The 9 and 3 and 3 all have at least one dominant allele and are therefore feathered. This makes the ratio of feathered to non-feathered 15:1. 3.
9 : 4 : 3 ---> AaBbcc
Hin recombinase is involved
in site-specific recombination because it catalyzes a DNA inversion in
the Salmonella chromosome. This recombination involves no
homology, but involves enzymatic recognition of a particular short DNA
sequence.
1. The trait is recessive, because two parents who don't show the trait do pass it on to their children. 2. Dominant, because two parents who show the trait have one child without it; this one child must have inherited "normal" recessive alleles from each parent. 3. More likely dominant, because if it were recessive, two many people by coinicidence would have to have married to pass on the trait. 4. Recessive, because two two parents who don't show the trait do pass it on to their children. 5. Autosomal dominant--but a trait that only affects males, such as beard hair color. It cannot be X-linked because fathers pass it on to sons. Solution: Fly Lab, Stubble Aristapedia To determine the map distance between Stubble and Aristapedia, we cross a Stubble Aristapedia (SB AR) female with a wild-type male. Remember that SB and AR are each lethal when homozygous; so the genotype of the female has to be: SB AR
SB +
The male is of course: +
+
The actual results of one cross are shown: Summary of Results
(Note: Your numbers may differ in detail, but the same statistical results should apply.) Note that the female genotype has to be: SB +
because the recombinant phenotypes are either all wild type, or both SB and AR. |
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