3.1.5.1 Structure of DNA and RNA

DNA and RNA function

• Both types of nucleic acid

• DNA (deoxyribonucleic acid) is used to store genetic information - that’s all the instructions needed to grow and develop from a fertilised egg to a fully grown adult

• RNA (ribonucleic acid) transfers genetic information from the DNA to the ribosomes, which read the RNA to make polypeptides in a process called translation

Nucleotide structure

• A nucleotide is a type of biological molecule which is made from 3 components:

  • A pentose sugar

  • A nitrogenous base

  • Phosphate group

• Each DNA nucleotide has the same pentose sugar and phosphate group, but the base can vary

What are the 4 possible bases on a DNA nucleotide?

• Adenine (A)

• Thymine (T)

• Cytosine (C)

• Guanine (G)

What are the 4 possible bases on a RNA nucleotide?

• Adenine (A)

• Uracil (U)

• Cytosine (C)

• Guanine (G)

DNA nucleotide

• Deoxyribose sugar

• Nitrogenous base (A/C/T/G)

• Phosphate group

RNA nucleotide

• Ribose sugar

• Nitrogenous base (A/G/C/U)

• Phosphate group

Polynucleotide structure

• Many nucleotides join together by a condensation reaction between the phosphate group of one nucleotide and the sugar of another to form polynucleotide chains

• Phosphodiester bonds are formed (consisting of the phosphate group and 2 ester bonds

• The chain of phosphates and sugars is known as the sugar-phosphate backbone

DNA structure

• Double helix structure

• Which means that a DNA molecule is formed from 2 separate strands which wind around each other to form a spiral

• The strands are polynucleotides and they are antiparallel

• DNA molecules are really long and are coiled up very tightly, so a lot of genetic information can fit into a small space in the cell nucleus

Complementary base-pairing

• 2 polynucleotide strands join together by hydrogen bonds between the bases

• Each base can join with a specific partner- complementary base-pairing

• Adenine pairs with thymine (A-T)

• Guanine pairs with cytosine (G-C)

• So there are always equal amounts of A and T, and G and C in a DNA molecule

• 2 hydrogen bonds form between A-T and 3 hydrogen bonds form between G-C

Polynucleotide strands

• Antiparallel - run in opposite directions

• They twist to form a DNA double helix

RNA structure

• Made of a ribose sugar, a phosphate sugar and a nitrogenous base (A/U/G/C)

• The nucleotides form a polynucleotide strand with a sugar-phosphate backbone

• The nucleotide forma single polynucleotide strand

• RNA strands are shorter than most DNA

  polynucleotides

What differentiates RNA structure from DNA structure?

• The sugar in RNA is a ribose sugar and not a deoxyribose

• Uracil replaces thymine as a base. Uracil pairs with adenine in RNA (U-A)

• The nucleotides form a single polynucleotide strand and not a double one like in DNA

• RNA strands are much shorter than most DNA polynucleotides

DNA as the carrier of the genetic code

• In 1953, experiments had shown that DNA was the carrier of the genetic code

• This was also the year in which the double helix structure was determined by scientists James Watson and Francis Crick