Biology: Unit F212: Molecules, Biodiversity, Food and Health
Nucleic Acids o Polynucleotides are made from individual nucleotide monomers joined together o Nucleotides have a phosphate group, a pentose sugar and an organic base
P h o s p h a t e G r o u p
N i
B a i t s r e o g e n c o n t a i i n i i n g
D e o x y r i i b o s e S u g a r o When the nucleotide is DNA, the pentose sugar is deoxyribose o When the nucleotide is RNA, the pentose sugar is ribose o There are 4 nitrogen containing bases; two small bases with a one ring structure, known as pyrimidines - cytosine and thymine; two larger bases with a two ring structure, known as purines – adenine and guanine o Nucleotide monomers are joined together in condensation reactions with enzymes. The polynucleotide formed is DNA or RNA. Bonds form between the phosphate group of one nucleotide and the 3 rd carbon of the next nucleotide – this is a phosphodiester bond, and it forms the sugar phosphate backbone. o The DNA double helix is made up of two strands of the polynucleotide. The chains run in opposite directions – they are anti-parallel o Complementary base pairing occurs, always between one purine and one
pyrimidine. This ensures that double helix is the same width all the way along.

o Adenine (purine) and Thymine (pyrimidine) bond with 2 hydrogen bonds o Guanine (purine) and Cytosine (pyrimidine) bond with 3 hydrogen bonds o DNA must control what the cell does; it stores the genetic codes to make specific proteins – the sequence of bases along the DNA molecule code for a sequence of amino acids in the primary structure of a polypeptide o DNA must also make exact copies of itself; the original DNA created at fertilisation will be copied millions of times as a human grows. It is vital that DNA copies itself exactly – if a mistake is made then a mutation could occur. o DNA replication occurs in the S phase of interphase of cell division by a method of semi-conservative replication; DNA helicase enzyme breaks hydrogen bonds between the complementary base pairs – this unzips and unfolds the DNA molecule. Free nucleotides present in the nucleus bind with exposed complementary bases; A to T and C to G. Phosphodiester bonds
(covalent bonds) form between the phosphate group of one nucleotide and the 3 rd carbon of another nucleotide, in condensation reactions.
This is done by DNA Polymerase enzyme, forming the sugar-phosphate backbone. DNA
ligase closes any gaps in the sugar-phosphate backbone. Two new helices are formed – each contains one new strand of DNA and one old strand of DNA. o There are 3 theories of DNA replication; conservative, semi conservative and dispersive.
In semi-conservative replication, both DNA helices have one new and one old strand
In conservative replication, there is one helix of entirely new DNA and one helix of entirely old DNA.
In dispersive replication, each helix is made of strands comprised of new and old DNA. o The Meselston-Stahl Experiment is used to prove semi-conservative replication.
Bacteria are grown on agar containing a heavy isotope of N-15. They are then transferred to a medium containing a lighter N-14. DNA is extracted and suspended in caesium chloride and centrifuged every 20 minutes.

o The first generation will have a single band at the centre of the tube – one strand of ‘new’
N-14 DNA and one strand of N-15 ‘old’ DNA.
This disproves the conservative theory. o The second generation will have 2 bands; a thinner band at the centre, and a wider band at the top – helices made entirely of ‘new’ N-
14 lighter DNA. This disproves the dispersive theory. o DNA to polypeptide: by protein synthesis. This occurs in 2 stages. o Stage 1 is Transcription . This occurs inside the
nucleus. One strand of DNA is copied to form a single strand messenger RNA copy of the gene.
RNA Polymerase enzyme unzips the DNA helix by breaking hydrogen bonds between the base pairs.
Free RNA nucleotides in the nucleus complementary base pair with DNA bases. The new mRNA is a copy of a gene carrying the code to make one protein. mRNA peels itself from the DNA and leaves via a nuclear pore (by attaching to the cytoskeleton and being pulled along by a protein motor) o Stage 2 is Translation.
mRNA attaches to a ribosome. tRNA brings amino acids to the ribosome. The tRNA anticodon joins witht eh mRNA codon. Amino acids build up on the ribosome, and peptide bonds form between them in condensation reactions. This forms the polypeptide primary structure. o There are 4 key differences between DNA and RNA; DNA has the deoxribose
pentose sugar; RNA has the ribose pentose sugar . DNA is double stranded; RNA is single stranded ; DNA contains a thymine base; RNA contains a uracil base ;
DNA codes for multiple genes; RNA codes for a single gene .