Central dogma of genetics Lecture 4

The Central dogma of Genetics: conversion of DNA to Proteins In the field of genetics when gene is “Expressed” [activated] it undergoes two main process or steps: 1.The DNA strand is converted into an RNA, or what is called mRNA, strand in a process called transcription 2.The mRNA is then converted into an amino acid chain in a process called translation. The process in Prokaryotic cells such as bacteria is illustrated in Fig 1 In Eukaryotic cells there are some additional steps that were discovered after the central dogma was proposed. These will be discuss later in the course. Essentially when the gene is expressed it goes from the Genotype [DNA level] to the Phenotype [physical manifestation]. Fig1: adaped from klug p. 241

Step 1: Transcription The DNA is double stranded or is in the form of a double helix. One strand goes from 5’ to 3’ and the other strand goes in the 3’ to 5’ direction The 5’ to 3’ is the coding strand (sense strand) and the 3’ to 5’ strand is the complimentary template strand. (antisense strand) The transcription process always goes in the 5’ to 3’ direction. [however this does not always mean its on the same strand or the strands can reverse roles] The RNA polymerase (the molecule/ element/enzyme) that performs the conversion unwinds the helix and moves in the 5’ to 3’ direction of the primary strand. It uses the antisense strand to produce a complementary mRNA strand. This mRNA is a complement of the complement of the primary strand; Messenger RNA (mRNA) is an exact copy of the coding ( sense strand ) sequence apart from Thymine (T) is replaced by Uracil (U) Adapted from

Translation mRNA -> AA The translation occurs in a element of the cell called the ribosome. Translation consists of three phases: Initiation, elongation and termination Initiation: The ribosome attaches to the mRNA and moves to the initiation codon, AUG Then, another version of RNA called, transfer tRNA attaches to the AUG of mRNA The tRNA has two essential elements: an anti-codon, e.g. UAC, and an attached amino acid, e.g. methonine. tRNA Anti-codon Amino acid: methonine Adapted from chapter 12 Klug

Translation mRNA -> AA Elongation: The ribosome then moves down the mRNA in 5’to 3’ to the next codon; e.g. UUC. The ribosome has three chambers; going from left to right: – the tRNA to be discarded. – the tRNA for the “current” codon,: e.g. AUG – the tRNA for the following codon in the mRNA: e.g. UUC A peptide bond is formed between the amino acids in the “current” and “following” chambers. Adapted from chapter 12 Klug

Translation mRNA -> AA Elongation continued: The ribosome then moves down the mRNA to the next codon;e.g. GGU. The “current” tRNA is moved into the discard chamber the “following” tRNA is moved to the current chamber. This tRNA has a chain of amino acids [in this case two] attached to it. Adapted from chapter 12 Klug

Translation mRNA -> AA Elongation continued: The process continues with the next tRNA moving into the empty “following” chamber A peptide bond is formed with the amino acid of the new tRNA and the current peptide chain. The ribosome then moves to the next codon Adapted from chapter 12 Klug

Translation mRNA -> AA Termination : The ribosome continues to move down the mRNA strand until it reaches a termination codon; e.g. UGA. There is no tRNA corresponding to this codon so the “following” chamber remains empty. No peptide bond is formed so when the ribosome moves again it causes the amino acid chain to break of its tRNA and so release the amino acid chain. Adapted from chapter 12 Klug

Animation of the translation process Translation Animation Translation Animation 2 Translation animation 3

Exam Question Describe, using suitable examples, the steps involved in the “Central Dogma “ of genetics: converting a DNA strand into its corresponding amino acid chain.

References [2] accessed on the 30/09/2011http:// accessed on the 30/09/2011 [5] MC / MC /