In-depth section 2.6 notes for IB Biology HL students. I took my exams in July 2022 and got a 7 in Biology HL. The notes are a combination of information from lots of different sources with a main focus on the IB Biology Textbook.
2.6 Structure of DNA and RNA
Objectives:
❏ Understanding:
❏ The nucleic acids DNA and RNA are polymers of nucleotides
❏ DNA differs from RNA in the number of strands normally present, the base composition and
the type of pentose
❏ DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen
bonding between complementary base pairs
❏ Applications:
❏ Crick and Watson’s elucidation of the structure of DNA using model-making
❏ Skills:
❏ Drawing simple diagrams of the structure of single nucleotides and of DNA and RNA, using
circles, pentagons and rectangles to represent phosphates, pentoses and bases
❏ Nature of Science
❏ Using models as representation of the real world: Crick and Watson used model-making to
discover the structure of DNA
Nucleic acids and nucleotides:
- Nucleic acids first discovered in material extracted from the nuclei of cells
- Nucleic acids = macromolecules that are constructed by linking together nucleotides to form a polymer
- Two types of nucleic acid:
1. DNA (deoxyribonucleic acid)
a. A more stable double stranded form that stores the genetic blueprint for cells
2. RNA (ribonucleic acid)
a. A more versatile single stranded form that transfers the genetic information for decoding
- Nucleotides consist of three parts:
1. A pentose sugar (5-carbon)
2. A phosphate group
a. Acidic, negatively charged part of nucleic acids
3. A nitrogenous base
a. Contains nitrogen
b. Has either one or two rings of atoms in its structure
c. ATCG in DNA and AUCG in RNA
- Base and the phosphate are both linked by covalent bonds to the pentose sugar
, - To link nucleotides together into a chain or polymer = covalent bonds are formed between the phosphate of
one nucleotide and the pentose sugar of the next nucleotide
- Creates strong backbone for molecule of alternating sugar and phosphate groups, with a base
linked to each sugar
- Four different bases in both DNA and RNA (four different nucleotides)
- Can be linked together in any sequence because the phosphate and sugar used to link them
are the same in every nucleotide
- Any base sequence is possible along a DNA or RNA molecule
- This is the key to nucleic acids acting as a store of genetic information
- The base sequence is the store of genetic information
- The sugar phosphate backbone ensures that the store is stable and secure
- Phosphodiester bond protects the more chemically reactive organic bases inside the double helix
Structure of DNA:
- Features of the structure of DNA:
- Each strand consists of a chain of nucleotides linked by covalent bonds
- The two polynucleotide strands are antiparallel (parallel but run in opposite directions)
- In order for bases to be facing each other, the two strands must run in opposite directions
(i.e. they are antiparallel) to each other
- One strand of DNA runs from 5' to 3', and the opposite strand runs from 3' to 5'
- Two strands wound together to form a double helix
- Two strands are held together by hydrogen bonds between the nitrogenous bases to form double
helix
- Hydrogen bonds between G and C (3 hydrogen bonds) and between A and T (2 hydrogen
bonds)
- Complementary base pairing = A and T complement each other by forming base pairs
and similarly G and C complement each other by forming base pairs
- Nucleic acids are composed of nucleotide monomers which are linked into a single strand via
condensation
- Phosphate group of one nucleotide attaches to the sugar of another nucleotide (at the 3’-
hydroxyl (OH) group)
- Results in phosphodiester bond forming between the two nucleotides (condensation -
water is produced as a by-product)
- Successive condensation reactions result in the formation of long polynucleotide strands
- Nitrogenous base always attached to carbon 1 of pentose sugar
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