Nucleic Acid Structure

Imagine you had an old recipe book stored in a secure library Imagine you had an old recipe book stored in a secure library. You want to follow a recipe in the kitchen. What would you do?

Imagine you had an old recipe book stored in a library Imagine you had an old recipe book stored in a library. You want to follow a recipe in the kitchen. What would you do? Library = Nucleus Kitchen = Ribosome Old book = DNA Photocopy = RNA Process of photocopying = Transcription Process of following the recipe = Translation

The function of nucleic acids Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are important information-carrying molecules. In all living cells, DNA holds genetic information and RNA transfers genetic information from DNA to the ribosomes. Ribosomes are formed from RNA and proteins.

The structure of nucleic acids Monomer = a nucleotide Polymer = polynucleotide (DNA or RNA)

The structure of nucleic acids – what does the specification say? Both DNA and RNA are polymers of nucleotides. Each nucleotide is formed from a pentose, a nitrogen-containing organic base and a phosphate group: • The components of a DNA nucleotide are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine. • The components of an RNA nucleotide are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil.

Making a polynucleotide Mononucleotide > Dinucleotide > Polynucleotide Condensation reactions Phosphodiester bond

DNA Structure: Double stranded Long Base pairing Hydrogen bonds Specific Complementary Ratios Hydrogen bonds

The double helix:

The double helix:

How is the structure of DNA adapted to its function? Sequence of bases determines sequence of amino acids 3 bases code for one amino acid Sugar phosphate backbone for stability Many hydrogen bonds provide strength but can be broken to replicate DNA

Triplet code Degenerate code Non-overlapping code Universal code Start and stop triplet codes

Students should be able to appreciate that the relative simplicity of DNA led many scientists to doubt that it carried the genetic code. Video Read – “Unravelling the role of DNA” page 39/40

RNA – Ribonucleic Acid A nucleic acid Exists in many different forms: Messenger RNA (mRNA) – makes a complementary copy of the DNA and travels through the nuclear pores to the ribosomes Transfer RNA (tRNA) – carries amino acids from the cytoplasm to the ribosomes to form a polypeptide

RNA Structure mRNA and tRNA Polynucleotide Single stranded Made of nucleotides: Pentose sugar – Ribose Phosphate group An N-containing organic base: Adenine (A) Cytosine (C) Guanine (G) Uracil (U)

mRNA Long strand Single strand Complementary to gene (apart from U replaces T) Made in nucleus, travels out of nuclear pore to ribosomes Sequence of 3 mRNA nucleotides = a CODON

mRNA codons

tRNA Small (~80 nucleotides) Single stranded Folded into ‘clover leaf’ shape Several types – each can attach to a different amino acid Anticodon on tRNA pairs up with codon on mRNA at the ribosome, bringing the correct sequence of amino acids together to form a polypeptide