25. If a sample of DNA is found to have the base composition (molar ratios) of adenine, 40； thymine, 22； guanine, 19 and cytosine, 19, what conclusion can be drawn?
(A) The DNA is a circular duplex.
(B) The DNA is a linear duplex.
(C) The DNA is single stranded.
(D) The DNA has highly repetitive sequences.
(E) The DNA has a high melting temperature.

21. Below is a diagram of two DNA strands that are part of the coding region of gene. What is the sequence of bases and strand orientation in the transcribed RNA molecular?5’ --- GTACGTGAA---3’ coding strand3’ --- CATGCACTT---5’ template strand (A) 3’ GUACGUGAA 5’ (B) 5’ GTACGTGAA 3’ (C) 3’ CAUGCACUU 5’ (D) 5’ CAUGCACUU 3’ (E) 5’ GUACGUGAA 3’

22. The human genetic disease phenylketonuria (PKU) can result from_______ . (A) lack of phenylalanine decarboxylase (B) deficiency in aromatic amino acid transaminase (C) a deficiency of protein in the diet (D) overproduction of ketone bodies (E) lack of an enzyme required for converting phenylalanine to tyrosine

23. Which of the following descriptions is not correct? (A) The leucine zipper motif is important for protein-DNA interaction. (B) Ribozymes are RNA molecules with catalytic activity. (C) The poly( A ) tail can stimulate recruitment of the 40s ribosomal subunit to mRNA； therefore, it is important in initiation of protein synthesis. (D) Diphtheria toxin can inhibit mammalian RNA synthesis by reduction the elongation factor-2 activity. (E) The suppressor tRNA molecules are capable of suppressing missense mutations.

24. “The knock-out mice” is an important animal model for biomedical research. One of the most important techniques involving in this transgenic animal’s establishment is_______ . (A) receptor expression in embryonic stem cell (B) gene targeting by homologous recombination (C) gene amplification in embryonic fibroblast cell (D) technique of chromosome walking (E) single nucleotide polymorphism analysis

26. Which of the following statements concerning cyclins is not correct? (A) They catalyze the phosphorylation of cyclin-dependent protein kinases. (B) They are regulatory subunits for enzymes that catalyze the phosphorylation of proteins. (C) They are activated and degraded during the cell cycle. (D) They can become linked to ubiquitin. (E) They contain specific amino acid sequences that target them for proteolysis. 

27. That the nature of genetic material is DNA is based on_______ . (A) the transforming material is heat-unstable (B) DNase digestion has no effects but RNase digestion can destroy the activity (C) the base composition of DNA is constant from one species to another (D) only DNA can enter the cell if E. coli is infected by T2 phage (E) UV absorption maximum is at 280 nm

28. Which one of the following statements relating to the common amino acids is correct? (A) Glutamine and asparagine have acidic side chains. (B) Valine and leucine have polar side chains containing an amide group. (C) Only phenylalanine and tyrosine absorb in the ultraviolet region near 270 nm. (D) There are three amino acids whose side chains carry a net positive charge at pH 5. (E) Methionine is one of three sulfur containing amino acids.

29. Which one of the following statements relating to lipids and membrane is correct? (A) The non-polar interior of a lipid bilayer relies on salt bridges for its stability. (B) Triacylglycerols are the simplest of the phospholipids. (C) Cholesterol molecules are generally used to provide ion channels. (D) The ability of species to migrate through a lipid bilayer increases with their polarity. (E) Unsaturated fatty acyl groups used in phospholipids are normally cis isomers.

30. In a tissue that metabolizes glucose via the pentose phosphate pathway, carbon number 1 of glucose would be expected to end up principally in_______ . (A) glycogen (B) 3-phosphoglycerate (C) carbon dioxide (D) ribose-5-phosphate (E) pyruvate

31. Which one of the following statements relating to proteins is correct? (A) All proteins have quaternary structure. (B) Histidine is often found at the active site of globular proteins. (C) The primary structure of a protein describes the amino acid composition, but not the amino acid sequence. (D) Both alpha-helix and beta-sheet segments occur as secondary structural elements in all proteins. (E) When proteins are denatured, they can never be reconstituted back to a functional state.