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Crossing over in meiosis I is required for homologous chromosomes to properly align during metaphase and segregate during the first cell division.

Some regions of a chromosome called hotspots display a higher frequency of crossing over than other regions do. Crossing over is suppressed in chromosomal regions near the centromeres. The centromere region of a duplicated chromosome includes a collection of proteins that form a structure called the kinetochore. Scientists hypothesized that one or more of these kinetochore proteins are responsible for suppressing crossing over around the centromere.

To investigate their hypothesis, scientists modified chromosome 8 in yeast such that, in each cell, one chromosome from the pair of homologous chromosome 8s contained the gene encoding red fluorescent protein (RFP), while the other chromosome from the pair contained the gene encoding green fluorescent protein (GFP). Cells expressing RFP emit (give off) red light, and cells expressing GFP emit green light. Models of the modified chromosome 8 both before and after crossing over are shown in Figure 1.

[Figure 1. Models of modified chromosome 8 used in the experiment (A) before and (B) after crossing over occurs at the hotspot. Panel A "Before Crossing Over": two homologous chromosome 8 pairs are drawn, each as a pair of sister chromatids joined at a Centromere. On the upper homolog, the region near one end is labeled "Gene Encoding RFP" (shown as a filled gray box on both sister chromatids near the tip). On the lower homolog, the corresponding region is labeled "Gene Encoding GFP" (shown as a hatched/striped box on both sister chromatids). A "Hotspot" region is marked between the centromere and the RFP/GFP genes with double-arrow brackets. Panel B "After Crossing Over": the same two homologous chromosomes are shown after a crossover has occurred at the hotspot, so that one chromatid of the upper homolog now carries the "Gene Encoding GFP" marker (hatched box) instead of RFP, and one chromatid of the lower homolog now carries the "Gene Encoding RFP" marker (filled gray box) instead of GFP — i.e., the fluorescent markers have been exchanged between one chromatid of each homolog. Centromere and Hotspot are again labeled.]

The scientists then investigated whether attaching individual kinetochore proteins to a specific DNA sequence present in a known crossing-over hotspot on chromosome 8 affected the frequency of crossing over at this location. In their first experiment, they examined three groups of yeast cells containing the modified chromosome 8. Group 1 contained no kinetochore proteins attached to the hotspot, group 2 contained the kinetochore protein CTF attached to the hotspot, and group 3 contained the kinetochore protein IML attached to the hotspot. For each group, the scientists determined the frequency of crossing over between the RFP and GFP genes. To determine the frequency, the scientists added the number of cells emitting both red and green light to the number of cells that emitted no light and divided by the total number of cells (Figure 2).

[Figure 2. Bar graph, "The frequency of crossing over in a hotspot on yeast chromosome 8 for cell groups treated with different kinetochore proteins. Error bars represent $\pm 2\text{SE}_{\bar{x}}$." Y-axis: Frequency of Crossing Over (%), scale 0 to 8. X-axis: Attached Kinetochore Protein, three bars labeled "None" (~7.2%, light gray bar, with error bars), "CTF" (~6.1%, black bar, with error bars, visibly lower than the other two), and "IML" (~7.3%, light gray bar, with error bars). The None and IML bars are similar in height and both higher than the CTF bar.]

To investigate how increases in environmental temperatures affect the metabolism of certain organisms, researchers incubated liver cells from toads at different temperatures and measured two markers of metabolic activity (Table 1): the rate of oxygen consumption and the rate of ATP synthesis.

TABLE 1. RATE OF OXYGEN CONSUMPTION AND ATP SYNTHESIS AT DIFFERENT TEMPERATURES

To investigate whether red blood cells of animals lose the ability to take in glucose from their environment as they age, scientists collected red blood cells from guinea pigs that ranged in age from one day old to seven months old. Scientists incubated an equal number of red blood cells in separate culture dishes that contained a 300 nM solution of radioactively labeled glucose. The amount of radioactively labeled glucose present inside the red blood cells of each group was measured over time.

The common wild oat is native to regions of Europe and Asia but is an invasive species in central California grasslands. In California, the common wild oat has almost completely replaced some species of native bunchgrass. Researchers found that aphids, a type of small insect that often carries plant viruses, have a much higher reproductive rate in grasslands that include the common wild oat than in grasslands composed of only native bunchgrass species. Additionally, the viruses carried by the aphids appear to affect only the native bunchgrasses and not the common wild oat. Native bunchgrasses infected by the virus have much higher death rates than do native bunchgrasses that are not infected.

Researchers study mechanisms that enable or prevent speciation.

New genes can evolve from noncoding regions of DNA. It is not until certain regulatory elements are present in the DNA that a noncoding region becomes a new, functional gene that encodes a protein. These regulatory elements include a promoter, a 5' untranslated region (UTR) followed by a start codon, and a 3' UTR following a stop codon (Figure 1).

[Figure 1. Basic structure of a functional ag gene. A linear diagram of a gene, drawn left to right as a rectangle divided into segments: a white "Promoter" segment, then a "5' UTR" segment, then a region labeled "ag Repeat Sequences" bracketed over several dark "Exons" segments alternating with white intron/gap segments, and finally a white "3' UTR" segment at the right end.]

Researchers studied the evolution of the family of antifreeze-glycoprotein (AG) encoding genes in Gadidae, a family of marine fish known as cods. When present in the fish, these glycoproteins reduce the freezing temperature of the fish. The researchers compared genomic sequences in nine cod species and one non-cod fish species, B. brosme. They recorded the presence or absence of the elements of functional ag genes as well as ag-like sequences that are similar to a functional gene but have undergone mutation and do not contain all the elements required to enable protein production (Figure 2).

[Figure 2. Phylogenetic tree showing the evolution of ag genes. Left side: a phylogenetic tree with x-axis "Time Before Present (millions of years ago)" running from 40 down to 0 in increments of 5. The tree root is labeled "Gadidae" and branches split over time; a vertical gray shaded band marks the "Period of Freezing Temperatures" roughly between 15 and 10 million years ago. Branch labels along the tree indicate "5' UTR" arising at one internal node and "ag-like" sequences arising at another (later) internal node, with the tree then splitting into the ten terminal branches (species) listed at right. Right side: a table with columns "Promoter", "ag Repeat Sequences", "3' UTR" (each showing "+" for present or "−" for absent, aligned to each species' branch tip), followed by the species name and, where relevant, a code letter "(L)" or "(P)"; then a column "Average Water Temperature of Habitat". The rows, top to bottom, are:

| Promoter | ag Repeat Sequences | 3' UTR | Species | Average Water Temp of Habitat |
|---|---|---|---|---|
| − | − | − | *B. brosme* (non-cod fish) | 5–10°C |
| − | − | − | *G. argenteus* | 5–10°C |
| − | − | − | *T. minutus* | 5–10°C |
| − | − | − | *P. virens* | 5–10°C |
| + | + | + | *M. aeglefinus* (L) | 5–10°C |
| + | + | − | *M. merlangus* (L) | 5–10°C |
| + | + | + | *A. glacialis* (P) | 1.2–6°C |
| + | + | + | *B. saida* (P) | 1.2–6°C |
| + | + | + | *G. chalcogrammus* (P) | 1.2–6°C |
| + | + | + | *G. morhua* (P) | 1.2–6°C |

A legend below the table reads: "−" Element is absent. "+" Element is present. "L" Functional ag genes are lost. "P" Functional ag-encoded proteins are present.]

Scientists can quantify the rate of translation as ribosomes move along an mRNA from one codon to the next. Using a procedure called ribosome profiling, the scientists measured how long a ribosome remains stationary at each codon of each mRNA. They determined the average translation rate across all codons is 5.2 amino acids per second but that the average translation rate for specific codons in different mRNA sequences can vary widely. These variations in translation rates are thought to facilitate correct folding of the protein being produced. The rate at which three different codons were translated was measured in 100 different mRNAs. The scientists determined the distribution of rate (number of times each rate was recorded) for each of the three codons: GAC (Figure 1A), AUU (Figure 1B), and UGG (Figure 1C).

[Figure 1. The distribution of translation rates for three different codons (A) GAC, (B) AUU, and (C) UGG. Each panel is a histogram with y-axis "Number of Times Rate Was Recorded" from 0 to 40 (gridlines every 5) and x-axis "Translation Rate of Codon [X] (ms/codon)" from 0 to 1000 (major gridlines every 100, minor labels every 50).

Panel A (codon GAC): bars rise sharply from ~3 (at ~50) to a peak of ~20 (at ~150), then ~14 (at ~200-250), ~14 (at ~250-300), drops to ~9 (at ~300-350), then ~3 (at ~350-400), and small bars of 1-2 continuing in a long low tail out past 700.

Panel B (codon AUU): bars start at ~11 (at ~50), peak sharply at ~35 (at ~100-150), then ~31 (at ~150-200), dropping to ~10 (at ~200-250), ~4 (at ~250-300), ~2-3 (at ~300-350), and near 0-1 beyond 400 — a narrow, sharply peaked distribution concentrated below 250 ms/codon.

Panel C (codon UGG): bars start near 0-1 (at ~50), rise gradually to ~3 (at ~100-150), ~5 (at ~150-200), ~11 (at ~200-250), ~11 (at ~250-300), ~11 (at ~300-350), ~12 (at ~350-400), ~9 (at ~400-450), ~9 (at ~450-500), ~8 (at ~500-550), then declining through ~3-4 (at ~550-700), ~1-2 out to 900, with a long tail extending to 1000 — a broad, flatter, right-shifted distribution compared with panels A and B.]