AS-LEVEL OCR BIOLOGY NOTES 
TOPIC 3: NUCLEOTIDES & NUCLEIC ACIDS

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1. Nucleotides & plynucleotides

Nucleotides are monomers which are the building blocks of DNA. There are 5 different nucleotides, but they all have a basic common structure: a ribose sugar joined to a phosphate group, and a nitrogen-containing base. The base changes according to the nucleotide:

• Guanine
• Thymine (DNA only)
• Adenine
• Cytosine
• Uracil (RNA only)

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These bases are categorised into 2 groups: purines and pyrimidines:

Purine	Pyrimidine
Double rings of carbon and nitrogen atoms Adenine Guanine	Single ring of carbon and nitrogen atoms Smaller than purines Thymine Cytosine Uracil

In DNA, the ribose sugar is deoxyribose.
In RNA, the ribose sugar is ribose.

Sugar-phosphate backbone

• Nucleotides bond by condensation reactions
  
• Forming phosphodiester bonds between ribose sugars and phosphate groups
  
• This continuous chain is the sugar-phosphate backbone

EXAM TIP Be aware that RNA and DNA are built from different nucleotides DNA = G, C, A, T RNA = G, C, A, U In transcription, thymine bases in DNA are transcribed as uracil bases in RNA.

2. adp & atp

ATP and ADP are phosphorylated nucleotides - nucleotides with phosphoryl groups attached. ATP has three phosphoryl groups and ADP has two. ATP is called the universal energy currency as it is used in energy transfer inside almost all living organisms. During its use, it becomes dephosphorylated and turned into ADP.

3. deoxyribonucleic acid (dna)

Functions of DNA
Acts as an information store

• Bases projecting from backbone act as a coded sequence
• Organisms differ in their DNA only because they contain different sequences of bases in DNA
• Contains ‘information’ to inform the primary structure of a protein

Needs to be replicable

• Produce copies that preserve the base sequences
• Preserve information
• Base-pairing rules allow for this

Long molecule

• Lots of information can be stored
• Double helix provides stability

Structure

• Deoxyribonucleic Acid
• Stable Polynucleotide
• long-chain polymer of nucleotide monomers
• Usually double stranded

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Base Pairing

• DNA strands run parallel to each other
  
• Bases project inwards, running in opposite directions (‘anti-parallel’)
  
• Chains always the same distance apart as bases pair in a specific way
  
  • A-T
    
  • G-C
    
• When a purine appears on one side, a pyridimine appears on the other
  
• As the strands come together, hydrogen bonds form between the base pairs
  

• Differing structure of the bases, means that base pairing rules always apply
  
• This form of pairing is described as complementary
  
  • A is complementary to T
    
  • G is complementary to C
    
• The 2 strands twist to form double helix of final structure
  
  • Held together by hydrogen bonds which are strong enough to maintain DNA structure but weak enough to be overcome during DNA replication    ​
    

4. semi-conservative dna replication

1. DNA double helix unwinds and “unzips”; the hydrogen bonds between base pairs are broken. This process is brought about by an enzyme called DNA helicase.
   
2. The exposed nucleotide bases act as a template for assembly of the new DNA strand.
   
3. Free nucleotides move towards these exposed bases according to the base pair rule.
   
4. An enzyme called DNA polymerase binds the nucleotides together with covalent bonds, forming the new sugar-phosphate backbone.
   
5. This process occurs to both exposed strands, resulting in 2 daughter DNA strands. ​

DNA replication

• ensures the preservation of genetic information stored in DNA
  
• Important as the structure and functions of proteins coded according to DNA relies entirely upon the correct sequences being copied
  
• Some mutations occur very infrequently and randomly
  
• These might not result in a phenotypic change, but sometimes can have life-altering consequences
  

5. the genetic code

The sequence of DNA nucleotides determines the sequence of amino acids joined together.

• A sequence of 3 bases codes for one amino acid
  
• This is called a ‘triplet’
  
• Several different triplets can code for the same amino acid
  
  • This means the genetic code is degenerate
    
• One triplet sequence can only code for one amino acid
  
• One triplet is coded for one after another: there is no overlapping of triplets
  
• A ‘gene’ is one section of DNA which codes for one polypeptide
  
• A gene is a sequence of triplets
  

EXAM TIP Several different triplets can code for the same amino acid. For example, CGG and CGU both code for Arginine. This property of DNA is called degeneracy and DNA is known to be degenerate. It means that for instance, if there was an error in copying the CGU triplet and CGG was copied instead, there would actually be no difference in the primary structure of the polypeptide.

6. transcription and translation of the genes

Transcription = the process of copying RNA from DNA

1. DNA strand unwinds and unzips by DNA helicase
   
2. Instead of a new DNA strand being formed, RNA nucleotide bases pair up with the exposed DNA bases
   
3. An enzyme called RNA synthase forms covalent bonds between the nucleotide bases
   
4. Unlike DNA, RNA is single-stranded
   
5. A messenger RNA (mRNA) strand is formed and breaks away from the DNA which then re-zips itself up thanks to the natural formation of hydrogen bonds
   
6. mRNA is now free to migrate out of the nucleus through nuclear pores
   

Translation = the process of creating polypeptides based off mRNA

1. Once mRNA has left the nucleus, it attaches to a ribosome on the rough endoplasmic reticulum
   
2. Transfer RNA (tRNA) carries the corresponding amino acid to each on the mRNA
   
3. The anti-codon is a triplet of bases that form part of a tRNA molecule and ensure that the correct amino acid is joined onto the polypeptide chain
   
4. This process is active and requires ATP
   
5. The amino acid transported by the tRNA attaches to the ribosome
   
6. Adjacent amino acids join together by peptide bonds, creating a polypeptide chain
   
7. This process continues until the ribosome reaches a ‘stop’ codon (triplet) on the mRNA. At this point, the polypeptide breaks loose from the ribosome and is free.