Nucleic acids - Structure of DNA & RNA

Define nucleic acids

Group of molecules e.g DNA & RNA

Define DNA

• Hereditary molecule, Deoxyribonucleic acid, polynucleotide, double-helix structure

Define RNA

• Ribonucleic acid, polynucleotide

Describe Nucleotide structure

• Pentose sugar → Contains 5 carbon atoms → Deoxyribose

• Phosphate group

• Nitrogenous base

  

Describe DNA base pairing sequence

Adenine → Thymine

Cytosine → Guanine

• Held by individually weak hydrogen bonds complementary to each other

Describe RNA structure

• Pentose sugar = ribose

• Single, relatively short polynucleotide chain

• mRNA transfers genetic info from DNA to ribosomes

• tRNA transports specific amino acids to the ribosome during protein synthesis.

• rRNA forms the core structure of the ribosome and helps build proteins.

• Single, relatively short polynucleotide chain

Describe base-pairing sequence for RNA

• Adenine → Uracil

• Cytosine → Guanine

• Held by individually weak hydrogen bonds

Describe the discovery of DNA structure

• In 1953, Watson & Crick worked out DNA structure following pioneering work by Rosalind Franklin on X-ray diffraction patterns of DNA

Describe formation of polynucleotide

• Pentose deoxyribose sugar, phosphate group & nitrogenous base are joined in condensation reactions to form a mononucleotide

• Two mono nucleotides then join as a result of condensation reaction between deoxyribose sugar of one mononucleotide & phosphate group pf another → bond formed between them is phosphodiester bond → dinucleotide

Give 2 reasons why DNA is a stable molecule

1. Phosphodiester backbone protects more chemically reactive organic bases inside double helix

2. Hydrogen bonds link organic base pairs forming bridges between phosphodiester uprights. As there are 3 hydrogen bonds between cytosine & guanine, the higher the proportion of C-G pairings, the more stable the DNA molecule

Outline the function of DNA

• Responsible for carrying genetic information

Give 5 ways of how DNA is adapted to carry out function

1. Most mutations are repaired so persistent mutations are rare

2. Two separate strands joined only with hydrogen bonds which allow them to separate during DNA replication & protein synthesis

3. Extremely large molecule therefore can carry immense amount of genetic information

4. Base-pairing leads to DNA being able to replicate & transfer information as MRNA

5. By having the base pairs within the helical cylinder of deoxyribose-phosphate backbone, genetic information is to some extent protected from being corrupted by outside chemical & physical forces

Describe mRNA

• Messenger

• Polymer made of RNA nucleotides

• Uracil instead of thymine

• Carries copy of genetic information form nucleus to ribosomes so a polypeptide can be made

Describe tRNA

• Transfer amino acids to ribosomes to make polypeptide

• Hydrogen bonds between complementary nitrogenous organic bases

• Single-stranded molecule

• Consists of RNA nucleotides

• Phosphodiester bonds due to sugar-phosphate backbone

Describe rRNA

• Ribosomal

• Translates mRNA to build polypeptide