Cell Division, Development and Reproduction

Q1A flow cytometry histogram of a growing culture shows two peaks, at 2C and 4C. Cells in the 4C peak are in:

D. Cells at 4C have replicated their DNA but not yet divided, so they are in G2 or M. G1 and G0 cells have 2C, and cells in early S phase lie between the two peaks.

Q2A tumor has a mutation that prevents Rb from binding E2F. The most likely effect is:

B. Without Rb-E2F binding, E2F is free and S phase genes are active regardless of growth signals. This removes a brake, so arrest is not expected, and the mutation does not directly involve p53 or telomerase.

Q3Mitochondrial DNA is maternally inherited mainly because:

D. Sperm have mitochondria in the midpiece, but they are degraded in the zygote. The egg keeps its own. Sperm mitochondria do exist and function.

Q4Which pair of tissues derives from neural crest?

B. Neural crest forms melanocytes, adrenal medulla, Schwann cells and sensory and autonomic neurons. Hepatocytes and pancreatic cells are endoderm, muscle and bone are mesoderm, and the lens and retina arise from surface ectoderm and neuroectoderm.

Q5Embryos lacking maternal bicoid mRNA will most likely lack:

D. Bicoid is an anterior morphogen, so loss gives embryos missing head and thorax. Posterior structures depend on other maternal factors. Neural crest is not tested, and Hox genes in the abdomen are still expressed.