Nucleic Acid Structure & Function (SL IB Biology)

Differences between DNA and RNA

• Unlike DNA, RNA nucleotides never contain the nitrogenous base thymine (T) – in place of this they contain the nitrogenous base uracil (U)
• Unlike DNA, RNA nucleotides contain the pentose sugar ribose (instead of deoxyribose)

Comparing DNA and RNA nucleotides diagram

An RNA nucleotide compared with a DNA nucleotide

• Unlike DNA, RNA molecules are only made up of one polynucleotide strand (they are single-stranded)
• Unlike DNA, RNA polynucleotide chains are relatively short compared to DNA

RNA structure

mRNA as an example of the structure of an RNA molecule

Nucleotide Structure Summary Table

The role of complementary base pairing

• Complementary base pairing means that the DNA bases on different strands will always pair up in a very specific way:
  • Adenine (A) will pair up with Thymine (T)
  • Cytosine (C) will pair up with Guanine (G)
• This is because the hydrogen bonds that hold the two DNA strands together can only form between these base pairs:
  • Two hydrogen bonds form between A and T
  • Three hydrogen bonds form between C and G
• Complementary base pairing means that the base sequence on one DNA strand determines the sequence of the other strand
  • We say that one strand acts as a template of the other
• This allows DNA to be copied very precisely during DNA replication which in turn ensures that the genetic code is accurately copied and expressed in newly formed cells

Complementary base pairs and hydrogen bonding diagram

A section of DNA showing nucleotide bonding and complementary base pair bonding

Diversity of DNA base sequences

• Despite the genetic code only containing four bases (A, T, C, G), they can combine to form a very diverse range of DNA base sequences in DNA molecules of different lengths
• This means that DNA has an almost limitless capacity for storing genetic information in living organisms
• One way in which this storage capacity can be measured is by the number of genes contained within the DNA of an organism
• Even the most simplistic forms of life may contain several thousand genes within their DNA

Comparing the Number of Genes between Different Organisms Table

• The storage capacity of DNA can also be measured in the number of base pairs contained within the genome of an organism
• The DNA in the nucleus of a human cell contains about 3.2 gigabases
  • That is about 10⁹ DNA base pairs
• These base pairs are contained in DNA with a length of about 2 meters, that fits within the nucleus of each human cell
  • Given the fact that a nucleus is microscopic in size, is an indication of how incredibly well packaged this amount of genetic information is
• This gives DNA an enormous capacity for storing genetic 'data' with great economy