BIOB 160 Principles of Living Systems · Unit 1: The chemistry of life · on Exam 1
1.7 Nucleic acids
Study notes for Nucleic acids in BIOB 160 Principles of Living Systems at Montana State University: the big ideas, a worked example, common mistakes and an exam tip. The same topic has endless practice questions and flashcards in StudyCat.
Big ideas
- DNA stores hereditary information; RNA carries it out (mRNA, tRNA, rRNA). Genes are DNA sequences that direct protein synthesis through mRNA.
- A nucleotide = a five-carbon sugar (deoxyribose in DNA, ribose in RNA) + a nitrogenous base + one or more phosphate groups.
- Purines (adenine, guanine) have two rings; pyrimidines (cytosine, thymine, uracil) have one. DNA uses T; RNA uses U instead.
- Nucleotides are linked by phosphodiester linkages into a sugar-phosphate backbone with a 5′ end (phosphate) and a 3′ end (hydroxyl).
- DNA is a double helix of two antiparallel strands held by hydrogen bonds between complementary bases: A–T (2 hydrogen bonds) and G–C (3 hydrogen bonds). RNA is usually single-stranded but can fold on itself.
Worked example
Write the complementary strand of 5′-ATGGCTAC-3′, labeling its ends, then write it 5′ → 3′.
Show the solution
Pair A–T and G–C, and remember the strands run in opposite directions. Under the original, the complement reads 3′-TACCGATG-5′. Sequences are normally written 5′ → 3′, so read it right to left: 5′-GTAGCCAT-3′. Check: G pairs with the original’s last C, T with its A, and so on.
Common mistakes
- Forgetting that strands are antiparallel: the complement’s 5′ end pairs with the original’s 3′ end.
- Pairing A with U in DNA. U appears only in RNA.
- Mixing purines (A, G) and pyrimidines (C, T, U).
Exam tipReading: Campbell 5.5 (pp. 84–89). When writing a complementary strand, always write the 5′ and 3′ labels; the direction is part of the answer.
Keep going in StudyCat
StudyCat is free for Montana State students: an endless quizzer with worked explanations, one-question lessons, flashcards, exam prep checklists, the class calendar with every deadline, a grade calculator and a study board for BIOB 160.