Nucleic Acids: DNA, RNA & Genes
Nucleotides — The Building Blocks
Nucleic acids are polymers composed of repeating units called nucleotides. Each Nucleotide consists of three subunits: a 5-carbon monosaccharide (pentose sugar), a nitrogen-containing Nitrogenous Base, and Phosphoric Acid. In RNA, the sugar is Ribose; in DNA, it is Deoxyribose — identical except for the absence of an oxygen on carbon 2 of deoxyribose. The nitrogenous base is attached to carbon 1 of the pentose sugar, while the phosphoric acid forms an ester linkage with the hydroxyl group at carbon 5. When a base combines with a pentose sugar alone, the resulting compound is called a Nucleoside. Adding phosphoric acid to a nucleoside produces a nucleotide. Nucleotides can carry one, two, or three phosphate groups (e.g., AMP, ADP, ATP in RNA; dAMP, dADP, dATP in DNA). ATP is a particularly important nucleotide that serves as the cell's energy currency, storing energy in the high-energy phosphate bonds between the second and third phosphate groups.
Pentose Sugars: Ribose in RNA nucleotides, deoxyribose in DNA nucleotides — the only difference is the missing OH group at carbon 2 in deoxyribose
Base Attachment: The nitrogenous base connects to carbon 1 of the pentose sugar via a glycosidic bond
Phosphate Attachment: Phosphoric acid ($H_3PO_4$) links to carbon 5 of the pentose sugar through a phosphoester bond
Nucleoside vs Nucleotide: A nucleoside is base + sugar only; a nucleotide is nucleoside + phosphate group
Polymerization: Nucleotides link together through Phosphodiester Linkage between the phosphate of one nucleotide and the sugar of the next, forming polynucleotide chains
Nitrogenous Bases Classification
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Purines (double-ringed): Adenine (A) and Guanine (G) — found in both DNA and RNA
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Pyrimidines (single-ringed): Cytosine (C) and Thymine (T) in DNA; Cytosine (C) and Uracil (U) in RNA
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Key Difference: Thymine is exclusive to DNA; Uracil replaces thymine in RNA
Ribonucleotides and Deoxyribonucleotides
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Adenine: Adenosine (nucleoside) → AMP, ADP, ATP (RNA) / d-Adenosine → dAMP, dADP, dATP (DNA)
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Guanine: Guanosine → GMP, GDP, GTP (RNA) / d-Guanosine → dGMP, dGDP, dGTP (DNA)
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Cytosine: Cytidine → CMP, CDP, CTP (RNA) / d-Cytidine → dCMP, dCDP, dCTP (DNA)
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Thymine: d-Thymidine → dTMP, dTDP, dTTP (DNA only — no RNA equivalent)
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Uracil: Uridine → UMP, UDP, UTP (RNA only — no DNA equivalent)
DNA — The Double Helix
DNA is the hereditary material that controls the properties and potential activities of a cell. It was first isolated in 1869 by Friedrich Miescher from the nuclei of pus cells, earning the name 'nucleic acid' from its nuclear origin and acidic nature. DNA is constructed from four kinds of deoxyribonucleotides — dAMP, dGMP, dCMP, and dTMP — linked by Phosphodiester Linkage in specific sequences to form long polynucleotide chains. DNA occurs primarily in chromosomes within the cell nucleus, with smaller amounts in mitochondria and chloroplasts.
Miescher's Discovery: First isolated nucleic acids from pus cell nuclei in 1869, naming them for their acidic nature and nuclear location
Chromosomal Location: DNA resides mainly in chromosomes in the nucleus, with trace amounts in mitochondria and chloroplasts
Four Nucleotides: DNA uses dAMP, dGMP, dCMP, and dTMP — each containing deoxyribose sugar, a phosphate group, and one of four bases (A, G, C, T)
The structure of DNA was determined through the combined work of several scientists. In 1951, Erwin Chargaff analysed base ratios and showed that adenine equals thymine and guanine equals cytosine in any DNA sample. Maurice Wilkins and Rosalind Franklin used X-Ray Diffraction to obtain critical structural data. Building on this evidence, James Watson and Francis Crick proposed the Double Helix model in 1953. DNA consists of two polynucleotide strands coiled around each other in antiparallel orientation — one strand runs 5' to 3' while the other runs 3' to 5'. The two strands are held together by Hydrogen Bonding between complementary bases: adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three hydrogen bonds. Each complete turn of the helix spans about 34 Å and contains approximately 10 base pairs.
$$\text{A} \leftrightarrow \text{T} \;(2\;\text{H-bonds}), \quad \text{G} \leftrightarrow \text{C} \;(3\;\text{H-bonds})$$
Complementary base pairing rules: adenine always pairs with thymine (2 hydrogen bonds), and guanine always pairs with cytosine (3 hydrogen bonds)
$\text{A}$=Adenine — a purine base(—)
$\text{T}$=Thymine — a pyrimidine base (DNA only)(—)
$\text{G}$=Guanine — a purine base(—)
$\text{C}$=Cytosine — a pyrimidine base(—)
$\text{RNA}$
→Uracil (U) replaces thymine, so A pairs with U via 2 H-bonds instead
Antiparallel Strands: The two strands run in opposite directions — one 5' to 3', the other 3' to 5' — giving the helix its stable structure
Chargaff's Rule: In any DNA sample, A ≈ T and G ≈ C — this complementary ratio was key evidence for the base-pairing model
Two H-Bonds (A=T): The adenine-thymine pair is held by two hydrogen bonds, making it relatively easier to separate
Three H-Bonds (G≡C): The guanine-cytosine pair has three hydrogen bonds, providing greater stability — regions with more G-C pairs are harder to denature
Helix Dimensions: Each turn of the double helix is ~34 Å long and contains ~10 base pairs; one Ångström equals one 100-millionth of a centimetre
DNA Content is Species-Specific: The total amount of DNA is fixed for a species (depends on chromosome number) — germ cells (sperm, ova) contain half the DNA of somatic cells
Key Figures in DNA Structure Discovery
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F. Miescher (1869): First isolated nucleic acids from pus cell nuclei
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E. Chargaff (1951): Discovered base ratios — A = T, G = C in all DNA
3
M. Wilkins & R. Franklin: Used X-ray diffraction to reveal DNA's helical structure
4
J. Watson & F. Crick (1953): Built the scale model confirming the double helix
Chargaff's Data — Base Composition (% of total bases)
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Human: A = 30.9%, T = 29.4%, G = 19.9%, C = 19.8%
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Sheep: A = 29.3%, T = 28.3%, G = 21.4%, C = 21.0%
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Wheat: A = 27.3%, T = 27.1%, G = 22.7%, C = 22.8%
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Yeast: A = 31.3%, T = 32.9%, G = 18.7%, C = 17.1%
DNA Content per Nucleus (species-specific comparison)
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Chicken — somatic cells: ~2.4 pg/nucleus (red blood cells ~2.3 pg, liver & kidney ~2.4 pg)
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Chicken — germ cells (sperm): ~1.3 pg/nucleus (half of somatic)
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Carp — somatic cells: ~3.3 pg/nucleus (RBC, liver, kidney all ~3.3 pg)
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Carp — germ cells (sperm): ~1.6 pg/nucleus (half of somatic)
RNA — Structure, Types & Function
RNA is a polymer of Ribonucleotides, each containing Ribose sugar, a phosphate group, and one of four bases: adenine (A), guanine (G), cytosine (C), and uracil (U). Unlike DNA, RNA exists as a single-stranded molecule. However, this single strand can fold back on itself to form regions of double-helical character through intramolecular base pairing between complementary sequences. The base pairing follows the same principle as DNA: cytosine pairs with guanine and uracil pairs with adenine. RNA is synthesized from DNA through a process called transcription. It is found in the nucleolus, ribosomes, cytosol, and in smaller amounts throughout the cell.
Single-Stranded Polymer: RNA is a single polynucleotide chain, unlike DNA's double helix — but it can fold to create local double-stranded regions
Uracil Replaces Thymine: RNA uses uracil (U) instead of thymine (T); uracil pairs with adenine via two hydrogen bonds
Ribose Sugar: Every RNA nucleotide contains ribose (with OH at carbon 2), distinguishing it from deoxyribose in DNA
Self-Complementary Folding: The single strand folds back on itself so that C pairs with G and U pairs with A within the same molecule, giving local double-helical regions
Synthesized from DNA: RNA is made by transcription — DNA serves as the template strand
There are three main types of RNA, each with a distinct role in the cell: Messenger RNA (mRNA), Transfer RNA (tRNA), and Ribosomal RNA (rRNA). All three are synthesized from DNA in the nucleus and then transported to the cytoplasm to perform their functions.
mRNA — The Messenger (3–4% of total RNA): Carries the genetic information from DNA in the nucleus to ribosomes in the cytoplasm. It is a single-stranded molecule of variable length, determined by the size of the gene and the protein it codes for. For example, a protein of 1,000 amino acids requires an mRNA of ~3,000 nucleotides (3 nucleotides per amino acid)
tRNA — The Transfer Agent (10–20% of total RNA): Small molecules of 75–90 nucleotides each. There is at least one specific tRNA for each of the 20 amino acids. tRNA picks up amino acids in the cytoplasm and transfers them to the ribosome, where they are linked together to form proteins
rRNA — The Molecular Machinery (up to 80% of total RNA): The most abundant type of RNA, strongly associated with ribosomal proteins (40–50% of ribosome mass). rRNA forms the structural and catalytic core of the ribosome, where mRNA and tRNA interact to translate genetic information into protein
Comparison of RNA Types
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mRNA: 3–4% of cellular RNA, variable length, carries genetic code from DNA to ribosomes
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tRNA: 10–20% of cellular RNA, 75–90 nucleotides long, transfers amino acids to ribosome
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rRNA: Up to 80% of cellular RNA, structural + catalytic core of ribosomes, facilitates protein synthesis
Genes — Units of Biological Inheritance
All the information for the structure and functioning of a cell is stored in DNA. This information is organized into discrete units called Genes. A gene is a segment of DNA that contains the code for producing a specific Polypeptide chain (protein). In the bacterium Escherichia coli, each strand of DNA contains approximately 5 million bases arranged in a particular linear order, divided into units of several hundred bases each — each unit being a gene. The E. coli genome consists of 4,639,221 base pairs coding for at least 4,288 proteins. The concept of a gene as a DNA sequence coding for a polypeptide is central to understanding how genetic information flows from DNA to RNA to protein.
DNA as Information Storage: DNA stores all instructions for cell structure and function in the sequence of its bases
Gene Definition: A gene is a specific segment of DNA that codes for a particular polypeptide (protein)
Linear Organization: Genes are arranged in a linear sequence along the DNA strand, each comprising several hundred bases
E. coli Genome Example: 4,639,221 base pairs divided into at least 4,288 genes, each coding for a specific protein
Polypeptide Product: Each gene's base sequence determines the amino acid sequence of a polypeptide chain through the processes of transcription and translation
Gene Facts from E. coli
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Each DNA strand contains ~5 million bases
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Total genome: 4,639,221 base pairs
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Codes for at least 4,288 different proteins
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Information is divided into units of several hundred bases each (genes)
DNA vs RNA — Key Differences
While both DNA and RNA are nucleic acids composed of nucleotide polymers, they differ in several structural and functional aspects. Understanding these differences is fundamental to grasping how genetic information is stored and expressed in cells.
Structural Differences Between DNA and RNA
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Sugar: DNA has deoxyribose (no OH at C-2); RNA has ribose (OH at C-2)
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Strands: DNA is double-stranded (double helix); RNA is single-stranded (may fold back on itself)
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Bases: DNA uses adenine, guanine, cytosine, thymine; RNA uses adenine, guanine, cytosine, uracil
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Size: DNA is very long (millions of base pairs); RNA is shorter (75–3,000+ nucleotides depending on type)
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Location: DNA is primarily in the nucleus; RNA is in the nucleolus, ribosomes, cytosol
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Function: DNA stores genetic information long-term; RNA carries and expresses that information
Application Strategy
Worked Examples
Problem Statement
If a DNA molecule contains 28% adenine, what are the percentages of thymine, guanine, and cytosine?
Chargaff's RuleBase Pairing
Strategic Toolkit
$$A = T, \quad G = C, \quad A + T + G + C = 100\%$$
Step 1
Find thymine percentage
By Chargaff's rule, adenine and thymine are equal in any DNA molecule. Since adenine is 28%, thymine must also be 28%.
$$T = A = 28\%$$
Step 2
Calculate remaining percentage
The remaining percentage must be split between guanine and cytosine. Subtract adenine and thymine from 100%.
$$G + C = 100\% - (A + T) = 100\% - 56\% = 44\%$$
Step 3
Split between G and C
Since guanine and cytosine are always equal, divide the remaining percentage equally between them.
$$G = C = \frac{44\%}{2} = 22\%$$
Problem Statement
A protein consists of 500 amino acids. How many nucleotides long is the mRNA molecule that codes for it?
mRNACodonGene-Product Relationship
Strategic Toolkit
$$\text{mRNA nucleotides} = \text{amino acids} \times 3$$
Step 1
Identify the codon-amino acid relationship
Each amino acid in a protein is specified by a sequence of three nucleotides (a codon) on the mRNA molecule.
$$1\;\text{amino acid} = 3\;\text{nucleotides}$$
Step 2
Multiply amino acid count by 3
Multiply the total number of amino acids by 3 to get the total mRNA nucleotide length.
$$\text{mRNA length} = 500 \times 3 = 1500\;\text{nucleotides}$$
Problem Statement
If a cell has 500,000 total RNA molecules, approximately how many are rRNA, tRNA, and mRNA?
rRNAtRNAmRNARNA Proportions
Strategic Toolkit
$$\text{rRNA} \approx 80\%, \quad \text{tRNA} \approx 15\%, \quad \text{mRNA} \approx 3.5\%$$
Step 1
Calculate rRNA count
Ribosomal RNA makes up approximately 80% of total cellular RNA.
$$\text{rRNA} = 80\% \times 500{,}000 = 400{,}000$$
Step 2
Calculate tRNA count
Transfer RNA accounts for roughly 10–20% of cellular RNA. Using a midpoint of 15%.
$$\text{tRNA} = 15\% \times 500{,}000 = 75{,}000$$
Step 3
Calculate mRNA count
Messenger RNA is the least abundant type, making up only 3–4% of total cellular RNA. Using 3.5%.
$$\text{mRNA} = 3.5\% \times 500{,}000 = 17{,}500$$
Problem Statement
A somatic cell of an organism contains 6.4 picograms of DNA per nucleus. How much DNA would be present in a sperm cell of the same organism?
DNA ContentSomatic vs Germ CellsChromosome Number
Strategic Toolkit
$$\text{DNA}_{\text{germ}} = \frac{\text{DNA}_{\text{somatic}}}{2}$$
Step 1
Apply the haploidy rule
Germ cells (sperm and ova) are haploid, meaning they carry only half the chromosomes of somatic (body) cells. Therefore, they contain exactly half the DNA.
$$\text{DNA}_{\text{sperm}} = \frac{6.4\;\text{pg}}{2} = 3.2\;\text{pg}$$
Memorize formulas
Complementary Base Pairing (DNA)
Adenine always pairs with thymine via 2 H-bonds; guanine always pairs with cytosine via 3 H-bonds. In RNA, uracil replaces thymine (A=U, 2 H-bonds).
Formula
$$A \leftrightarrow T \;(2\;\text{H-bonds}), \quad G \leftrightarrow C \;(3\;\text{H-bonds})$$
Chargaff's Rule
In any DNA sample: percentage of adenine equals thymine, percentage of guanine equals cytosine. Total A + T + G + C = 100%.
Formula
$$A = T, \quad G = C, \quad A + T + G + C = 100\%$$
mRNA Length from Amino Acids
Each amino acid requires 3 nucleotides (one codon) on mRNA. Multiply the number of amino acids by 3 to get mRNA nucleotide count.
Formula
$$\text{mRNA nucleotides} = \text{number of amino acids} \times 3$$
DNA Helix Dimensions
Each turn of the double helix is ~34 Å long and contains approximately 10 base pairs. One Ångström = 10⁻⁸ cm = 0.1 nm.
Formula
$$\text{1 turn} = 34\;\text{Å} \approx 10\;\text{base pairs}$$
DNA Content — Germ vs Somatic Cells
Germ cells (haploid) contain exactly half the DNA of somatic cells (diploid) of the same organism.
Formula
$$\text{DNA}_{\text{germ}} = \frac{1}{2} \times \text{DNA}_{\text{somatic}}$$
Review
1
Which three components constitute a nucleotide?
Ans
Nitrogenous base, pentose sugar, phosphate group
Review
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Identify Nucleotide Components
A nucleotide has three components: a nitrogenous base, a five-carbon (pentose) sugar, and one or more phosphate groups.
Nitrogenous base + pentose sugar + phosphate group
Evaluate Distractors
Hexose sugars (six-carbon) are found in glucose and fructose, not nucleotides. Sulfate groups are not part of nucleotide chemistry. Amino acids belong to proteins, not nucleic acids.
Only the first option contains all three correct components
2
A nucleoside differs from a nucleotide in that the nucleoside lacks a:
Ans
Phosphate group
Review
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Define Nucleoside
A nucleoside = nitrogenous base + pentose sugar (no phosphate group).
Base--Sugar
Define Nucleotide
A nucleotide = nitrogenous base + pentose sugar + phosphate group.
Base--Sugar--Phosphate
Compare Structures
The nucleoside lacks only the phosphate group compared to the nucleotide.
Missing component = phosphate group
3
In the DNA double helix, one complete turn of the helix measures approximately 34 Å and contains how many base pairs?
Ans
10 base pairs
Review
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Recall Helix Dimensions
One complete turn of DNA = 34 Å, containing approximately 10 base pairs.
$$\text{1 turn} = 34\;\text{Å} \approx 10\;\text{base pairs}$$
Calculate Distance Per Base Pair
Dividing the helix length by the number of base pairs gives the rise per base pair.
$$\frac{34\;\text{Å}}{10} = 3.4\;\text{Å per base pair}$$
4
The adenine-thymine base pair in DNA is stabilized by how many hydrogen bonds?
Ans
Two
Review
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Recall Base Pairing Rules
In DNA, adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C).
$$A \leftrightarrow T, \quad G \leftrightarrow C$$
Identify Hydrogen Bond Count
A-T pair has 2 hydrogen bonds; G-C pair has 3 hydrogen bonds.
$$A = T \;(2\;\text{H-bonds}), \quad G = C \;(3\;\text{H-bonds})$$
5
If a DNA molecule contains 18% adenine, what is the percentage of thymine in that molecule?
Ans
18%
Review
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Apply Chargaff's Rule
Chargaff's rule states that in double-stranded DNA, the amount of adenine equals the amount of thymine.
$$A = T$$
Substitute Given Value
Given A = 18%, therefore T must also equal 18%.
$$T = 18\%$$
6
The pentose sugar that distinguishes RNA from DNA is ribose. What structural feature does deoxyribose have that ribose lacks?
Ans
A missing hydroxyl group at the 2' position
Review
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Compare Sugar Structures
Ribose has −OH groups at both the 2' and 3' carbons. Deoxyribose has −OH only at 3' and −H at 2'.
Ribose: 2'-OH, 3'-OH; Deoxyribose: 2'-H, 3'-OH
Identify the Difference
The key structural difference is the absence of the hydroxyl group at the 2' carbon in deoxyribose.
Deoxyribose lacks the 2' hydroxyl group present in ribose
7
Messenger RNA (mRNA) carries the genetic code from DNA to the ribosome. What is the structural feature of mRNA that makes this function possible?
Ans
It is a single-stranded molecule that can be read by ribosomes in the 5' to 3' direction
Review
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Identify mRNA Structure
mRNA is synthesized as a single-stranded complementary copy of the DNA template strand.
mRNA = single-stranded, linear molecule
Link Structure to Function
The single-stranded structure exposes codons for ribosomal reading during translation.
Codons read 5' → 3' by ribosomes
8
Which RNA type has a cloverleaf-shaped secondary structure and carries amino acids to the ribosome?
Ans
tRNA
Review
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Match Structure to RNA Type
The cloverleaf secondary structure is the hallmark of tRNA, formed by base pairing within the single strand.
Cloverleaf structure → tRNA
Confirm Functional Role
tRNA molecules bind specific amino acids at their 3' end and deliver them to the ribosome using anticodon-codon base pairing.
tRNA = amino acid adaptor molecule
9
Which type of RNA is the most abundant in a cell and serves as the structural and catalytic component of ribosomes?
Ans
Ribosomal RNA (rRNA)
Review
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Recall RNA Proportions
In a typical cell, rRNA constitutes approximately 80% of total RNA, tRNA about 15%, and mRNA about 5%.
rRNA ≈ 80%, tRNA ≈ 15%, mRNA ≈ 5%
Identify rRNA Function
rRNA combines with ribosomal proteins to form ribosomes and provides the catalytic site for peptide bond formation.
rRNA = structural + catalytic component of ribosomes
10
If a somatic cell of an organism contains 6 picograms of DNA, how much DNA will be present in its germ cell (sperm or egg)?
Ans
3 pg
Review
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Apply Germ Cell DNA Formula
Germ cells (haploid) contain exactly half the DNA content of somatic cells (diploid).
$$\text{DNA}_{\text{germ}} = \frac{1}{2} \times \text{DNA}_{\text{somatic}}$$
Calculate
Substitute the given somatic DNA value.
$$\text{DNA}_{\text{germ}} = \frac{1}{2} \times 6\;\text{pg} = 3\;\text{pg}$$
11
Which of the following is the most accurate definition of a gene?
Ans
A segment of DNA that contains the code for synthesizing a specific polypeptide
Review
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Evaluate Gene Definition
A gene is a discrete segment of DNA that carries the instructions for building a specific polypeptide through the processes of transcription and translation.
Gene = DNA sequence → codes for a polypeptide
Eliminate Incorrect Options
Genes are made of DNA, not RNA. They code for polypeptides, not lipids directly. Genes are nucleotide sequences, not amino acid sequences.
Only the DNA-to-polypeptide definition is accurate
12
A protein consists of 300 amino acids. How many nucleotides are present in the mRNA molecule that codes for this protein?
Ans
900
Review
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Recall the Codon Relationship
Each amino acid is coded by a codon consisting of three nucleotides in the mRNA.
$$\text{mRNA nucleotides} = \text{number of amino acids} \times 3$$
Calculate mRNA Length
Multiply the number of amino acids by 3 to find the total nucleotide count.
$$300 \times 3 = 900\;\text{nucleotides}$$
13
A DNA sample is analyzed and found to contain 22% adenine and 28% guanine. What percentage of cytosine does this DNA contain?
Ans
28%
Review
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Apply Chargaff's Rule
In double-stranded DNA, the amount of guanine equals the amount of cytosine.
$$G = C$$
Substitute and Verify
Given G = 28%, therefore C = 28%. Verify: A + T + G + C = 22 + 22 + 28 + 28 = 100%.
$$C = 28\%, \quad \text{Total} = 100\% \;\checkmark$$
14
Which statement correctly distinguishes DNA from RNA?
Ans
DNA contains deoxyribose sugar and is typically double-stranded; RNA contains ribose sugar and is typically single-stranded
Review
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Compare Sugar Components
DNA contains deoxyribose (2'-H), RNA contains ribose (2'-OH).
DNA: deoxyribose; RNA: ribose
Compare Strand Structure
DNA is double-stranded (double helix), while RNA is typically single-stranded.
DNA: double-stranded; RNA: single-stranded
Compare Nitrogenous Bases
DNA uses adenine, guanine, cytosine, and thymine. RNA uses adenine, guanine, cytosine, and uracil (U replaces T).
DNA: A, G, C, T; RNA: A, G, C, U
15
The phosphodiester linkage in a nucleic acid strand connects which two positions?
Ans
The 3' carbon of one nucleotide's sugar to the 5' carbon of the adjacent nucleotide's sugar
Review
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Identify the Phosphodiester Bond
The phosphodiester linkage connects the 3' carbon of one deoxyribose/ribose to the 5' carbon of the next, with a phosphate group in between.
3'—O—PO₂⁻—O—5'
Explain Directionality
This asymmetric linkage gives each strand a 5' end (free phosphate) and a 3' end (free hydroxyl), creating a defined direction for enzymes that read or synthesize nucleic acids.
Strand directionality: 5' → 3'
Question 1 of 15
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1
Which three components constitute a nucleotide?