Models of classroom assessment for course-based research experiences

Course-based research pedagogy involves positioning students as contributors to authentic research projects as part of an engaging educational experience that promotes their learning and persistence in science. To develop a model for assessing and grading students engaged in this type of learning experience, the assessment aims and practices of a community of experienced course-based research instructors were collected and analyzed. This approach defines four aims of course-based research assessment—(1) Assessing Laboratory Work and Scientific Thinking; (2) Evaluating Mastery of Concepts, Quantitative Thinking and Skills; (3) Appraising Forms of Scientific Communication; and (4) Metacognition of Learning—along with a set of practices for each aim. These aims and practices of assessment were then integrated with previously developed models of course-based research instruction to reveal an assessment program in which instructors provide extensive feedback to support productive student engagement in research while grading those aspects of research that are necessary for the student to succeed. Assessment conducted in this way delicately balances the need to facilitate students’ ongoing research with the requirement of a final grade without undercutting the important aims of a CRE education

The essential role of the Deinococcus radiodurans ssb gene in cell survival and radiation tolerance.

Recent evidence has implicated single-stranded DNA-binding protein (SSB) expression level as an important factor in microbial radiation resistance. The genome of the extremely radiation resistant bacterium Deinococcus radiodurans contains genes for two SSB homologs: the homodimeric, canonical Ssb, encoded by the gene ssb, and a novel pentameric protein encoded by the gene ddrB. ddrB is highly induced upon exposure to radiation, and deletions result in decreased radiation-resistance, suggesting an integral role of the protein in the extreme resistance exhibited by this organism. Although expression of ssb is also induced after irradiation, Ssb is thought to be involved primarily in replication. In this study, we demonstrate that Ssb in D. radiodurans is essential for cell survival. The lethality of an ssb deletion cannot be complemented by providing ddrB in trans. In addition, the radiation-sensitive phenotype conferred by a ddrB deletion is not alleviated by providing ssb in trans. By altering expression of the ssb gene, we also show that lower levels of transcription are required for optimal growth than are necessary for high radiation resistance. When expression is reduced to that of E. coli, ionizing radiation resistance is similarly reduced. UV resistance is also decreased under low ssb transcript levels where growth is unimpaired. These results indicate that the expression of ssb is a key component of both normal cellular metabolism as well as pathways responsible for the high radiation tolerance of D. radiodurans

Effect of <i>ssb</i> depletion on growth.

<p>SL102 was initially grown in 1 mM IPTG, then sub-cultured twice in either 1 mM IPTG (○) or 0 mM IPTG (Δ). Error bars indicate ± standard error.</p

Relative <i>ssb</i> transcript levels.

<p>Semi-quantitative PCR was used to compare <i>ssb</i> expression between strains. Transcript levels of <i>ssb</i> in (A) GY10973/p11530 (<i>ssb⁺</i>/empty vector) grown in 1 mM IPTG, (B) SL102 (Δ<i>ssb/ssb</i> expression plasmid) grown in 1 mM IPTG and (C) SL102 grown in 0.04 mM IPTG are shown relative to <i>ssb</i> transcript levels in strain GY10973/pSL202 (<i>ssb</i>⁺/<i>ss</i>b expression plasmid) grown in 1 mM IPTG. All <i>ssb</i> levels were normalized with GAPDH levels. Error bars indicate ± standard error.</p

UV survival of strains with altered <i>ssb</i> levels.

<p>GY10973/p11530 (<i>ssb⁺</i>/empty vector), GY10973/pSL202 (<i>ssb⁺/ssb</i> expression plasmid) and SL102 (Δ<i>ssb/ssb</i> expression plasmid) were grown under full inducing conditions. Cultures were diluted and aliquots spotted onto plates and allowed to dry prior to exposure to UV-C. Surviving colonies were counted and used to determine surviving fractions, as described.</p

Ionizing radiation survival of <i>ddrB</i> knockout strains.

<p>Cultures were grown in 1 mM IPTG and irradiated with 23 MeV electrons at a dose rate of 30 Gy/s. (○) SL101/p11530 (Δ<i>ddrB</i>/empty expression vector); (Δ) SL101/pSL201 (Δ<i>ddrB</i>/<i>ddrB</i> expression plasmid); and (◊) SL101/pSL202 (Δ<i>ddrB</i>/<i>ssb</i> expression plasmid). Dilutions were plated, and survivors counted after 3–5 days. The dashed line indicates 50% survival. Error bars indicate ± standard error.</p

Strains and plasmids used in this study.

<p>Strains and plasmids used in this study.</p

Ionizing radiation survival of strains with altered <i>ssb</i> levels.

<p>Cultures were grown in 1 mM IPTG and irradiated with 23 MeV electrons at a dose rate of 30 Gy/s. (○) GY10973/p11530 (<i>ssb⁺</i>/empty vector); (Δ) GY10973/pSL202 (<i>ssb⁺/ssb</i> expression plasmid) and (◊) SL102 (Δ<i>ssb/ssb</i> expression plasmid). Appropriate dilutions were plated, and survivors counted after 3–5 days. The dashed line indicates 50% survival. Error bars indicate ± standard error.</p

Primers used in this study.

<p>Primers used in this study.</p