Structure of DNA and RNA (2.5.1) | Biology DP IB: SL 2016 Revision Notes (2023)

Nucleic acids: structure

  • The nucleic acids DNA and RNA are polymers of nucleotides
  • Both DNA and RNA arepolymersIt consistsmany repeating unitscallednucleotide
  • Each nucleotide is made up of:
    • APentose-Zucker(a sugar with 5 carbon atoms)
    • a nitrogenousbiological basis(with 1 or 2 atomic rings)
    • APhosphatgruppe(this is acidic and negatively charged)
  • The base and the phosphate group are bothcovalently bondedfor sugar

Structure of DNA and RNA (2.5.1) | Biology DP IB: SL 2016 Revision Notes (1)

The basic structure of a nucleotide

  • Nucleotides are joined together in chains to form strands of DNA or RNA
  • Vonphosphate group of a nucleotideforms a covalent bond with thenext pentose sugar
    • This continues to form abig polymer
  • This represents asugar-phosphate structure' with a base attached to each sugar
  • The polymer of nucleotides is known asstrand
  • DNA is double-stranded, RNA is mostly single-stranded
  • there is only4 separate basesall goodany combination/order
    • because thatSugar and phosphate are the samein each nucleotide
  • This sequence is the basis of the genetic code as a unitstorage of genetic information

exam tip

A common mistake is to describe DNA or RNA as polymers of bases; more correctly, they areNucleotide polymers

DNA and RNA: Comparison

  • Wie DNSRNA nucleic acid (ribonucleic acid) is aPolynucleotide- It consists ofmany nucleotidesconnected in a chain
  • Wie DNS, RNA nucleotides contain the nitrogenous bases adenine (A), guanine (G) and cytosine (C)
  • Unlike the DNA, RNA-nucleotidenever includeda base nitrogenadathymine(T) - instead they contain the nitrogenous baseuracil(YOU)
  • Unlike the DNA, RNA nucleotides contain the pentose sugarribose(instead of deoxyribose)
(Video) IB 2.6 - DNA Structure
  • Unlike the DNA, RNA molecules consist only ofa polynucleotide strand(they aresingle strands)
  • Unlike DNA,RNA polynucleotide chains are relativeshort compared to DNA
  • like DNA,vonsugar-phosphate bonds(between different nucleotides on the same strand) are strongcovalent Link
  • like DNA,the nitrogenous basesproject laterallyof the sugar-phosphate backbone

Structure of DNA and RNA (2.5.1) | Biology DP IB: SL 2016 Revision Notes (3)

The RNA structure

Nucleotide structure summary table

(Video) Structure of DNA and RNA IB Biology Topic 2.6

Structure of DNA and RNA (2.5.1) | Biology DP IB: SL 2016 Revision Notes (4)

exam tip

You need to know the difference between DNA and RNA molecules (basic composition, number of chains, pentose sugar present).

double helix structure

  • DNA is a double helix of two antiparallel strands of nucleotides connected by hydrogen bonds between complementary base pairs.
  • Nucleic acid DNA is aPolynucleotide- It consists ofmany nucleotidestied together in onelong necklace

Structure of DNA and RNA (2.5.1) | Biology DP IB: SL 2016 Revision Notes (5)

One nucleotide of DNA

  • DNA molecules are made up oftwo polynucleotide strandslying next to each other, going in opposite directions - the wires would beantiparallel
  • Each DNA polynucleotide strand consists ofalternately linked deoxyribose sugars and phosphate groupsbuildsugar-phosphate structure
  • Each strand of DNA polynucleotide is said to have a3' end and a 5' end(These numbers refer to which carbon atom in the pentose sugar can be attached to another nucleotide)
  • As the wires run in opposite directions (they areantiparallel), one of which is known as5' to 3' wireand the other is known as3' to 5' yarn
  • The nitrogenous bases of each nucleotide project from the backboneto the interiordouble-stranded DNA molecule
(Video) IB Biology 2.6 & 2.7 - DNA, RNA, and the Central Dogma - Interactive Lecture

Structure of DNA and RNA (2.5.1) | Biology DP IB: SL 2016 Revision Notes (6)

A single strand of polynucleotide DNA containing 3 nucleotides in a sequence


  • The two antiparallel DNA polynucleotide strands that make up the DNA molecule areheld together by hydrogen bondsbetween the nitrogenous bases
  • These hydrogen bonds always occur between thethe same base pairs:
    • de purinadenine(A) always pairs with pyrimidinethymine(T) - Two hydrogen bonds are formed between these bases
    • de puringuanine(G) always pairs with pyrimidinecytosine(C) - Three hydrogen bonds are formed between these bases
    • This is known ascomplementary base pairing
    • These pairs are calledDNA-Basenparen

Structure of DNA and RNA (2.5.1) | Biology DP IB: SL 2016 Revision Notes (7)

A stretch of DNA – two antiparallel strands of DNA polynucleotides held together by hydrogen bonds


  • DNA is not two-dimensional as shown in the diagram above
  • DNA is described as aDoppelélix
  • This refers tothree dimensional shapethat make up DNA molecules

Structure of DNA and RNA (2.5.1) | Biology DP IB: SL 2016 Revision Notes (8)

DNA molecules form a three-dimensional structure known as the DNA double helix.

(Video) IB Biology 3.1 - Genes - Interactive Lecture

exam tip

Name the different parts of a DNA molecule (sugar-phosphate structure, nucleotide, complementary base pairs, hydrogen bonds) and locate them on a diagram. Remember that covalent bonds connect the nucleotides in the phosphate sugar. The backbone and hydrogen bonds connect the bases of the two complementary strands. Remember that the bases are complementary, so the number A = T and C = G. You may be asked to determine the number of bases in a DNA molecule if you only enter the number of one of the bases.

Crick & Watson

  • Francis Crick and James Watson were two Cambridge scientists who worked together to develop theDoppelélixStructure of DNA in 1953
  • They used data fromyour previous experimentsabout the composition of DNA
  • Results published byother research laboratoriesplayed a role in the development of his model
    • Rosalind Franklin, Edwin Chargaff, and Linus Pauling, all of whom led the research effort inother universities, contributed important data to the Crick & Watson discovery
    • This suggests that there was close collaboration, but there waslots of competition between groupsto make breakthrough discovery
  • PhysicallyModelingmuch of your success
  • Early versions of the model were rejectedfor many reasons
    • Eranot compact enough
    • It would have beenunstable(DNA is a very stable molecule)
    • did not allowequivalent amountsof bases A and T, as well as C and G (Chargaff's discoveries)
    • It matches much better if theThe second strand was twistedbecome antiparallel
  • Your final model was carefully constructed withclamps,metal barsfor titles andcorrect connection angles
  • your example wasthe basis of much genetic researchworld in the years to come
    • In particular, the Crick and Watson model led to proof workthe way DNA replicatesin the cells

SOE: Using models as representations of the real world; Crick and Watson used modeling to discover the structure of DNA

  • Models in science are builtrepresent conceptsand ideas in a way that can be mapped by our brain
  • models can beaccepted or rejectedbased on experimental data generated through additional research
  • Crick em Watsonphysically built, scaleDNA models to explain biological observations
    • To usesimple lab equipment(clamps, brackets, metal rods, etc.)
    • Shemodified their modelsmaking them more realistic, for example building the correct bond angles in molecules
  • They built back-to-back models withfall and riseto reach the final model
  • SheThe first DNA model was rejected, based on the discoveries of Rosalind Franklin
    • Crick em Watsonreceived the Nobel Prizefor your work
    • Franklin died at age 37, and as a result, he never received the recognition he deserved.your important rolein defining the structure of DNA
  • Today,advanced computational modelingit will be done
    • Replaces the creation of physical modelsand provides further explanation of the functions of various biomolecules
    • Reduce trial and error cyclesof modeling as experienced by Crick and Watson

exam tip

Crick and Watson's model was widely accepted as all subsequent research supported their model.

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1. IB Biology 2.1 - Molecules to Metabolism - Interactive Lecture
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