The three-base sequence on DNA that codes for an amino acid is called a/an

In gram-negative bacteria, the cell envelope is a complex structure composed of three main layers: an inner (cytoplasmic) membrane, a periplasmic space, and an outer membrane. The inner membrane is a phospholipid bilayer that regulates transport and energy production. The periplasmic space lies between the inner and outer membranes and contains a thin layer of peptidoglycan along with enzymes involved in nutrient processing and defense. The outer membrane contains lipopolysaccharides (LPS), porins, and other proteins that provide structural support, act as a barrier to certain antibiotics, and contribute to the organism’s pathogenicity.

100 Multiple Choice 0.5 points

A. does not have thymine: DNA contains the nitrogenous base thymine, whereas RNA lacks thymine. Instead, RNA uses uracil in place of thymine during base pairing. Because thymine is absent from RNA molecules, this difference is one of the key biochemical distinctions between RNA and DNA structure.

B. has uracil: RNA contains the nitrogenous base uracil, which pairs with adenine during RNA synthesis and function. In DNA, thymine performs this role instead. The substitution of uracil for thymine is a defining molecular feature that distinguishes RNA nucleotides from DNA nucleotides.

C. All of the choices are correct: Each of the listed statements accurately describes a property unique to RNA compared with DNA. RNA lacks thymine, contains uracil, typically exists as a single-stranded molecule, and contains ribose sugar rather than deoxyribose. Because every option describes a valid structural difference, this is the most accurate answer.

D. is typically one strand of nucleotides (single stranded): RNA molecules are generally single-stranded, allowing them to fold into complex secondary and tertiary structures that support functions such as translation and gene regulation. DNA, in contrast, usually exists as a double-stranded helix with complementary base pairing between strands.

E. has ribose: RNA nucleotides contain ribose sugar, which includes a hydroxyl group on the 2′ carbon of the pentose ring. DNA contains deoxyribose, which lacks this oxygen atom. This chemical difference contributes to RNA being more reactive and less stable than DNA.