Source: Practice Exam Midterm #1, Codon Learning
Tags: DNA, RNA, nucleotide, nucleic acid, phosphodiester linkage, nitrogenous base, adenine, thymine, guanine, cytosine, uracil, ribonucleotide, deoxyribonucleotide, double helix, antiparallel, complementary base pairing, 5-prime, 3-prime, replication, transcription, translation, mRNA, tRNA, rRNA, central dogma, genetic code, codon
Difficulty: Intermediate Prerequisites: Chemical bonds (covalent vs. hydrogen), basic molecular structure.
Nucleic acids are the information molecules of the cell. DNA stores genetic information; RNA reads it out and helps convert it into protein. This unit covers the structure of nucleotides, how they polymerise, the key structural differences between DNA and RNA, and the three core processes of the central dogma (replication, transcription, translation). This is the most heavily tested topic area on this practice exam, and the questions lean toward application: expect scenarios where you identify a process from its description, predict what happens to strands based on antiparallel orientation, or calculate amino acid counts from nucleotide numbers.
Nucleotides (phosphate + sugar + base) are joined by phosphodiester linkages to form DNA and RNA. DNA is double-stranded, antiparallel, and stable; RNA is single-stranded, reactive, and versatile. The central dogma runs DNA → (transcription) → mRNA → (translation) → protein, with tRNA carrying amino acids to the ribosome during translation.
Nucleotide
The monomer of nucleic acids. Composed of three parts: a phosphate group, a 5-carbon sugar (ribose or deoxyribose), and a nitrogenous base.
Think of it as a three-part building block: connector (phosphate), body (sugar), and identity tag (base).
Ribonucleotide
A nucleotide containing ribose sugar, which has an OH group on the 2ʹ carbon. Found in RNA.
In simple terms, the RNA version of a nucleotide, with an extra oxygen on the sugar.
Deoxyribonucleotide
A nucleotide containing deoxyribose sugar, which has only an H on the 2ʹ carbon (no OH). Found in DNA. The missing oxygen makes DNA more chemically stable.
In simple terms, the DNA version, with one fewer oxygen on the sugar.
Phosphodiester linkage
The covalent bond that connects one nucleotide to the next in a nucleic acid strand, linking the phosphate group of one nucleotide to the sugar of the next.
Think of it as the "backbone weld" of DNA and RNA, analogous to peptide bonds in proteins.
Nitrogenous base
The part of the nucleotide that encodes information. DNA uses adenine (A), thymine (T), guanine (G), and cytosine (C). RNA uses adenine, uracil (U), guanine, and cytosine (uracil replaces thymine).
In simple terms, the "letter" of the genetic alphabet.
Complementary base pairing
A pairs with T (in DNA) or U (in RNA); G pairs with C. Held together by hydrogen bonds between the bases on opposite strands.
Think of it as a lock-and-key rule for matching bases across a double helix.
Antiparallel
The two strands of a DNA double helix run in opposite directions: one runs 5ʹ→3ʹ and the other runs 3ʹ→5ʹ.
In simple terms, the strands point in opposite directions, like cars on a two-lane road.
Double helix
The secondary structure of DNA: two complementary, antiparallel strands wound around each other in a spiral, held together by hydrogen bonds between base pairs.
Think of it as a twisted ladder where the rungs are base pairs and the rails are sugar-phosphate backbones.
DNA replication
The process of copying the entire DNA molecule before cell division, producing two identical double-stranded DNA molecules.
In simple terms, making a copy of the genetic blueprint so both daughter cells get one.
Transcription
The process of synthesising an RNA molecule from a DNA template. RNA polymerase binds to a gene's start site and creates phosphodiester bonds between ribonucleotides, reading the DNA sequence as a guide.
In simple terms, copying one gene's information from DNA into an RNA message.
Messenger RNA (mRNA)
Any RNA molecule that can be translated into protein by ribosomes. It is a temporary, portable copy of the information stored in a gene.
In simple terms, the instruction sheet that the ribosome reads to build a protein.
Translation
The process of building a protein using the information encoded in mRNA. Ribosomes read the mRNA sequence in three-nucleotide units (codons), and tRNA molecules deliver the matching amino acids.
In simple terms, converting the RNA message into a chain of amino acids.
Transfer RNA (tRNA)
An RNA molecule that carries a specific amino acid to the ribosome during translation. Each tRNA matches a specific codon on the mRNA through its anticodon.
Think of it as the delivery truck that brings the right amino acid to the assembly line.
Ribosomal RNA (rRNA)
RNA that forms part of the ribosome's structure. It has catalytic function and helps form peptide bonds between amino acids.
In simple terms, rRNA is the structural and catalytic core of the ribosome itself.
Codon
A sequence of three consecutive nucleotides in mRNA that specifies one amino acid (or a stop signal). There are 64 possible codons.
Think of it as a three-letter word in the genetic language.
Central dogma
The flow of genetic information in cells: DNA → RNA → protein. DNA is replicated, genes are transcribed into mRNA, and mRNA is translated into protein.
In simple terms, the one-way information highway from gene to working molecule.
Every nucleotide has three components: a phosphate group, a 5-carbon sugar, and a nitrogenous base.
The sugar determines whether it is a ribonucleotide (ribose, with an OH on the 2ʹ carbon) or a deoxyribonucleotide (deoxyribose, with only an H on the 2ʹ carbon).
If you see an OH on the 2ʹ carbon, it is a ribonucleotide. If you see only an H on the 2ʹ carbon, it is a deoxyribonucleotide.
DNA contains deoxyribose; RNA contains ribose.
DNA uses thymine; RNA uses uracil.
DNA is typically double-stranded; RNA is typically single-stranded.
DNA is more chemically stable than RNA (because it lacks the reactive 2ʹ OH group), making it better suited for long-term storage of genetic information.
RNA's 2ʹ OH group makes it more chemically reactive, which allows RNA to perform diverse chemical reactions and catalytic functions.
For long-term genetic storage, DNA is the better choice because it is much more stable and less likely to change form over time.
DNA's secondary structure is the double helix: two antiparallel strands wound together.
"Antiparallel" means the strands run in opposite 5ʹ→3ʹ directions.
The strands are held together by hydrogen bonds between complementary bases: A pairs with T, G pairs with C.
The four nitrogenous bases in DNA are cytosine, thymine, guanine, and adenine (C, T, G, A).
For two strands to form a proper double helix, they must be antiparallel and complementary.
If Strand 1 is 5ʹ-ATGC-3ʹ, its complement must run 3ʹ-TACG-5ʹ (which is written 5ʹ-GCAT-3ʹ).
Two strands both written 5ʹ→3ʹ (e.g., 5ʹ-ATGC-3ʹ and 5ʹ-GCAT-3ʹ) will not pair properly because they are parallel, not antiparallel.
Phosphodiester linkages = DNA or RNA (nucleic acids).
Glycosidic linkages = carbohydrates.
Peptide bonds = proteins.
This is a frequently tested identification question.
Replication: DNA → DNA. The purpose is to make a copy of DNA prior to cell division.
Transcription: DNA → RNA. RNA polymerase binds to the start of a gene and creates phosphodiester bonds between ribonucleotides, reading the DNA template. The product is an RNA molecule (often mRNA).
Translation: mRNA → protein. Ribosomes read the mRNA, tRNA delivers amino acids, and the ribosome links them by peptide bonds.
mRNA is defined as any RNA that can be translated into protein by ribosomes.
Its role is to serve as a temporary and portable version of the information stored in a gene.
mRNA is not the permanent archive (that is DNA) and it is not a catalytic molecule (that is typically other RNA types or enzymes).
Translation requires: mRNA (the message), ribosomes (the machine), amino acids (the raw materials), and tRNA (the adapter that matches codons to amino acids).
If you have mRNA, ribosomes, and amino acids but are missing tRNA, translation cannot proceed.
Each codon is 3 nucleotides long and codes for one amino acid.
To find the number of amino acids from a coding-region nucleotide count: divide by 3.
Example: 27 nucleotides in the coding region → 27 ÷ 3 = 9 amino acids.
Not all RNA is translated into protein. Some RNA molecules have regulatory functions, controlling which genes are transcribed.
An RNA that is not translated but helps regulate gene expression is one of the additional types of RNA with regulatory functions (neither mRNA, tRNA, nor rRNA).
The DNA/RNA stability difference is why forensic scientists can recover DNA from ancient remains but struggle to find intact RNA: DNA's missing 2ʹ OH group makes it far more durable. The codon system is the basis for genetic engineering: if you know the protein sequence you want, you can design an mRNA sequence (in multiples of three) to produce it.
Students often think mRNA is defined as "any RNA transcribed from DNA." That definition is too broad; many non-coding RNAs are also transcribed from DNA. mRNA is specifically the RNA that is translated into protein by ribosomes.
Students confuse replication (DNA → DNA) with transcription (DNA → RNA). Replication copies the whole genome before cell division; transcription copies one gene into RNA.
Students sometimes think that two strands with complementary bases will automatically form a double helix. They will not pair properly unless they are also antiparallel.
Students forget that tRNA is required for translation. Having mRNA, ribosomes, and amino acids is not enough on its own.
⚠️ Nucleotide components (phosphate + 5-carbon sugar + nitrogenous base) are tested as a direct recall question.
⚠️ Ribonucleotide vs. deoxyribonucleotide: look at the 2ʹ carbon. OH = ribonucleotide; H only = deoxyribonucleotide.
⚠️ DNA bases = C, T, G, A. RNA replaces T with U. This is tested directly.
⚠️ "Antiparallel" = opposite 5ʹ–3ʹ polarity. Two strands both written 5ʹ→3ʹ are parallel and will not pair.
⚠️ Know the three processes: replication = DNA → DNA; transcription = DNA → RNA; translation = RNA → protein.
⚠️ The codon calculation (nucleotides ÷ 3 = amino acids) appears on this exam. 27 nucleotides = 9 amino acids.
⚠️ tRNA is the missing component when translation cannot proceed despite having mRNA, ribosomes, and amino acids.
⚠️ The double helix is DNA's secondary structure.
Fill in the blank: A nucleotide is made of a phosphate group, a ______, and a nitrogenous base.
True or false: RNA is more stable than DNA for long-term information storage.
Fill in the blank: "Antiparallel" means the two DNA strands run in opposite ______ polarity.
True or false: Transcription produces a protein from an mRNA template.
Fill in the blank: 27 nucleotides in a coding region produce ______ amino acids.
Answers: 1. 5-carbon sugar. 2. False (DNA is more stable). 3. 5ʹ–3ʹ. 4. False (transcription produces RNA from a DNA template; translation produces protein from mRNA). 5. 9.
Q: What three parts make up a nucleotide?
A: A phosphate group, a 5-carbon sugar, and a nitrogenous base.
Q: Jacob notices one nucleotide has an OH on the 2ʹ carbon and another has only an H. What has he identified?
A: The first is a ribonucleotide and the second is a deoxyribonucleotide.
Q: Which four nitrogenous bases are found in DNA?
A: Cytosine, thymine, guanine, and adenine.
Q: What does "antiparallel" mean in the context of DNA strands?
A: The strands line up in opposite 5ʹ–3ʹ polarity.
Q: Carson has Strand 1 (5ʹ-ATGC-3ʹ) and Strand 2 (5ʹ-GCAT-3ʹ). Will they form a proper double helix?
A: No. They will not pair properly because they are not antiparallel; both strands are written 5ʹ→3ʹ, so they would run in the same direction.
Q: Allison must choose between DNA and RNA for long-term storage of genetic information. Which should she choose, and why?
A: DNA, because it is much more stable and less likely to change form over time (it lacks the reactive 2ʹ OH group).
Q: What is the purpose of DNA replication?
A: To make a copy of DNA prior to cell division.
Q: Phoebe sees a molecular machine binding to the start of a gene and creating phosphodiester bonds between ribonucleotides based on the DNA sequence. Which process is this?
A: Transcription.
Q: What is translation?
A: The process of building a protein using information in RNA (mRNA is read by ribosomes, tRNA delivers amino acids).
Q: Ella has mRNA, ribosomes, and amino acids for an in vitro translation system. What else does she need?
A: Transfer RNA (tRNA).
Q: Keshvi counts 27 nucleotides in the coding region of an mRNA. How many amino acids will the resulting protein contain?
A: 9 amino acids (27 ÷ 3 = 9).
Q: What role does mRNA play in the central dogma?
A: It is a temporary and portable version of the information stored in a gene.
Q: Jacob discovers an RNA that is not translated into protein but regulates which genes are transcribed. What is it?
A: It is one of the additional types of RNA with regulatory functions (not mRNA, tRNA, or rRNA).
Q: Sunny identifies Polymer X (phosphodiester linkages), Polymer Y (glycosidic linkages), and Polymer Z (peptide bonds). What are they?
A: X is DNA or RNA; Y is a carbohydrate; Z is a protein.
Nucleic acids connect directly to protein structure: the primary sequence of a protein is determined by the mRNA codon sequence, which in turn is determined by the DNA gene. The phosphodiester linkage parallels peptide bonds in proteins and glycosidic linkages in carbohydrates, so being able to match bond type to macromolecule class is a cross-topic skill. RNA's chemical versatility (due to its 2ʹ OH) connects to catalytic RNA (ribozymes) and regulatory RNA covered in later units.
nucleic acid, DNA, RNA, nucleotide, ribonucleotide, deoxyribonucleotide, phosphodiester linkage, nitrogenous base, adenine, thymine, guanine, cytosine, uracil, ribose, deoxyribose, 2-prime carbon, double helix, antiparallel, complementary base pairing, base pair, replication, transcription, translation, central dogma, mRNA, messenger RNA, tRNA, transfer RNA, rRNA, ribosomal RNA, ribosome, codon, genetic code, amino acid, regulatory RNA, non-coding RNA, RNA polymerase, gene expression