DNA vs RNA

Transcription=RNA sequence is produced from a DNA template: RNA polymerase separates the DNA strands and makes a complementary RNA copy from one DNA strand. Once the RNA sequence has been synthesized, RNA polymerase will detach from the DNA molecule and the double helix will reform The sequence of DNA that is transcribed into RNA is called a gene Transcription occurs in the nucleus and once made, the mRNA moves to the cytoplasm.

Three main types of RNA are predominantly made: Messenger RNA (mRNA): A transcript copy of a gene used to encode a polypeptide Transfer RNA (tRNA): A clover leaf shaped sequence that carries an amino acid Ribosomal RNA (rRNA): A primary component of ribosomes

Codons are a triplet of bases which encodes a particular amino acid As there are four bases, there are 64 different codon combinations (4 x 4 x 4 = 64) The order of the codons determines the amino acid sequence for a protein The coding region always starts with a START codon (AUG) and terminates with a STOP codon

The genetic code has the following features: It is universal - every living thing uses the same code (there are only a few rare and minor exceptions) 20 amino acids but 64 codons, so more than one codon may code for the same amino acid (this allows for silent mutations whereby a change in the DNA sequence does not affect the polypeptide sequence)

Translation= protein synthesis where mRNA is translated into a sequence of amino acids in a polypeptide chain Ribosomes bind to mRNA in the cell's cytoplasm and move along the mRNA molecule in a 5' - 3' direction until it reaches a start codon (AUG) Anticodons on tRNA molecules align with codons to provide complementary base pairing (e.g. UAC will align with AUG)

Each tRNA molecule carries a specific amino acid (according to the genetic code) The ribosome moves along the mRNA molecule synthezising a polypeptide chain until it reaches a stop codon, at this point translation stops and the polypeptide chain is released

A gene is a sequence of DNA which encodes a polypeptide sequence A gene sequence is converted into a polypeptide sequence via the processes of transcription (making an mRNA transcript) and translation (polypeptide synthesis) Translation uses tRNA molecules and ribosomes to join amino acids into a polypeptide chain according to the mRNA sequence (as read in codons) The universality of the genetic code means all organisms show the same relationship between genes and polypeptides (indicating a common ancestry and allowing for transgenic techniques to be employed) Some proteins may consist of a number of polypeptide chains and thus need multiple genes (e.g. haemoglobin consists of four polypeptide subunits encoded by two different genes) When a gene is mutated it may lead to the synthesis of a defective polypeptide, hence affecting protein function