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OCR A Level Biology:復(fù)習(xí)筆記2.3.1 Nucleotides & Phosphodiester Bonds

Category: A-level課程, 國際課程, 教材筆記, 福利干貨 Date: 2022年11月18日上午9:36

Nucleotides

DNA and RNA are nucleic acids:?polymers?that are made up of?many repeating units?(monomers) called?nucleotides
Each nucleotide is formed from:
- A?pentose sugar?(a sugar with 5 carbon atoms)
- A nitrogen-containing?organic base
- A?phosphate group

The basic structure of a nucleotide

DNA nucleotides

The components of a DNA nucleotide are:
- A?deoxyribose?sugar with?hydrogen?at the 2' position
- A phosphate group
- One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or thymine(T)

RNA nucleotides

The components of an RNA nucleotide are:
- A?ribose?sugar with a?hydroxyl (OH) group?at the 2' position
- A phosphate group
- One of four nitrogenous bases - adenine (A), cytosine(C), guanine(G) or?uracil (U)
The presence of the?2' hydroxyl group?makes?RNA more susceptible to hydrolysis
- This is why DNA is the storage molecule and RNA is the transport molecule with a shorter molecular lifespan

An RNA nucleotide (top) compared with a DNA nucleotide (bottom)

Purines & pyrimidines

The?nitrogenous base?molecules that are found in the nucleotides of DNA (A, T, C, G) and RNA (A, U, C, G) occur in?two structural forms:?purines?and?pyrimidines
The bases?adenine?and?guanine?are?purines?– they have a?double ring structure
The bases?cytosine,?thymine?and?uracil?are?pyrimidines?– they have a?single ring structure

The molecular structures of purines and pyrimidines are slightly different

Nucleotide Structure Table

Exam Tip

Although DNA and RNA nucleotides are very similar, make sure you know the?key differences?between them: unlike DNA,?RNA nucleotides never contain the nitrogenous base thymine?(in place of this they contain the nitrogenous base?uracil) and unlike DNA, RNA nucleotides contain the pentose sugar?ribose?(instead of deoxyribose).You don’t need to know the structural formulae of the bases, just which are purines and which are pyrimidines.

Phosphodiester Bond

DNA and RNA are polymers (polynucleotides), meaning that they are made up of many nucleotides joined together in long chains
Separate?nucleotides are joined together?via?condensation reactions
- These condensation reactions occur between the?phosphate group?of one nucleotide and the?pentose sugar?of the next nucleotide
A condensation reaction between two nucleotides forms a?phosphodiester bond
- It is called a phosphodiester bond because it consists of a phosphate group and?two?ester bonds
The chain of alternating phosphate groups and pentose sugars produced as a result of many phosphodiester bonds is known as the?sugar-phosphate backbone?(of the DNA or RNA molecule)
As the?synthesis?of?polynucleotides?requires the?formation?of?phosphodiester bonds, the same is true for the reverse process: the?breakdown?of polynucleotides requires the?breakage?of phosphodiester bonds

A section of a single polynucleotide strand showing a phosphodiester bond (and the positioning of the two ester bonds and the phosphate group that make up the phosphodiester bond)