DNA and proteins OBJECTIVES Explain the meaning of the term genetic code. Describe transcription

Starter Think of a protein (not an easy one to remember, preferably) Two volunteers needed to try to read everyone’s mind and name all the proteins everyone has thought of Each of these polypeptides are produced from a gene

Definition of terms Gene Genome Genetic code Polypeptide Protein Transcription Useful pub quiz fact: there are 25,000 genes in the human genome!

How are polypeptides made from DNA? DNA contains the code for all polypeptides contained in the nucleus so that the code is conserved Polypeptides are made in the cytoplasm So another molecule carries the code to the cytoplasm: messengerRNA!

Protein synthesis Two stages: Transcription (re-writing changing only one letter: T to U) Making mRNA using DNA as a template Translation (into a new language – amino acids) Making proteins using mRNA as a template Central Dogma of molecular biology: DNA mRNAprotein In the nucleus in the cytoplasm

Transcription View animations in VLE

The genetic code DNA has a triplet code mRNA has a complementary code Each set of 3 bases is called a codon The code for each amino acid is 3 bases

What are the complementary codons? DNA tripletmRNA codon ACGACG ACGACG

DNA tripletmRNA codon ACGACG UGCUGC ACGACG UGCUGC

For the mathematical mind There are FOUR bases in DNA (A G C T) Only THREE of these are needed for to code for one amino acid So how many combination of 3 bases can you make from 4 bases? Come up with a formula! BUT how many amino acids are there??

Properties of the genetic code It is degenerate All amino acids except methionine have more than one code STOP codes Widespread TCT codes for serine in all organisms but not universal (copy only one below) In mammalian mitochondria: 2 codes for methionine A STOP code = tryptophan In ciliated protoctists 2 STOP codes = glutamic acid

Homework: View the animations on transcription in the VLE and then compile a list of key terms by matching the following: Coding regionTranscribes genes into RNA Transcription start siteUnzips/unwinds DNA before transcription PromoterWhere RNA polymerase binds RNA polymeraseArea of gene from start site and further upstream (to the left); Proteins involved in transcription bind here DNA helicaseContains the sequence for the polypeptide