AS BIOLOGY - EVERYTHING on nucleic acids OCR F212

Deoxyribonucleic acid (DNA) is a nucleic acid found in the nucleus that contains your genetic information. It is a polymer made up of monomers known as nucleotides. Nucleotides are made up of 3 components; a pentose sugar (2-deoxyribose), a phosphate group and a nitrogenous base. The four possible nitrogenous bases are Adenine, Guanine, Cytosine and Thymine. Adenine and Guanine come under Purines and Cytosine and Thymine come under Pyrimidines.

Easy way to remember - Cytosine, Thymine all share a Y hence Pyrimidines.

DNA is made up of two antiparallel polynucleotide strands. The two strands are bonded together through hydrogen bonds between the nitrogenous bases with a suger-phosphate backbone bonded together by phosphodiester bonds. Adenine always binds to Thymine with 2 hydrogen bonds. Cytosine always binds to guanine with 3 hydrogen bonds. They are known as COMPLEMENTARY base pairs. This forms the DNA double helix.

Ribonucleic acid (RNA) has 3 mains forms; rRNA (ribosomal RNA), tRNA (transfer DNA) and mRNA (messenger RNA). They are usually single stranded (rRNA exception) and are composed of 3 components; a pentose sugar (Ribose), a phosphate group and an organic base (Adenine, Guanine, Cytosine and Uracil). Uracil replaces Thymine and so always bonds with Adenine as a complementary base pair.

mRNA - A single stranded polynucleotide strand approx. 1000 nucleotides long. They are small enough to fit through nuclear pores and so can transfer the genetic code of DNA in the nucleus to the ribosomes in the cytoplasm.

rRNA (don't need to know) - Large complex double helix molecules which alongside proteins, make up a ribosome.

tRNA - Small clover shaped molecules about 80 nucleotides long that bring amino acids to the ribosome for the assembly of a protein.

DNA is replicated during Interphase in the following steps;

1 - The enzyme DNA helicase breaks the hydrogen bonds between the two polynucleotide strands. The helix structure unzips to form the two single strands.

2 - The exposed DNA polynucleotide chains act as a template against which new polynucleotides can be synthesised. Complementary bases (free-floating) align themselves opposite to the exposed bases of the polynucleotide. A with T and G with C.

3 - The nucleotides on the new strand are joined together by the enzyme DNA polymerase. Hydrogen bonds form between the bases on the original and new strand.

4 - Two identical DNA molecules are produced each containing one original and one new polynucleotide strand.

It is known as semi conservative replication because half the new strands of DNA are from the original molecule.

GENES (key definition)

Genes are a sequence of polynucleotides that code for a polypeptide.

Different proteins have different number and order of amino acids. The order of nucleotide bases in a gene determines the order of amino acids in a particular protein. Each amino acid is coded for by a sequence of THREE bases (called triplets or codons) in a gene. Eg. GTC = valine or CGA = arginine. Some amino acids like arginine (mentioned) are coded for by more than one triplet (CGA, CGG, CGT AND CGC).

PROTEIN SYNTHESIS

DNA molecules found in the nucleus are too big to move out of the nucleus but the organelles for protein synthesis are found outside in the cytoplasm. So a section of DNA is copied into RNA, which then leaves the nucleus and joins with a ribosome in the cytoplasm where it can be used to synthesise proteins.