Genetic Code & Transcriptional Control

LECTURE 8

Genetic Code & Transcriptional Control

Biology, Campbell & Reece. 7th Edn. Ch 17, pp.311-314, pp. 327-331; Ch 16, pp. 305-306 By

Mohamed Abumaree Molecular Reproductive Biologist & Immunologist College of Medicine King Saud bin Abdulaziz University for Health Science 2009

Genetic Code  61 of 64 triplets code for amino acids  AUG initiation codon signals the start of mRNA translation  Codes for Met  So polypeptide chains begin with Met 2



An enzyme may subsequently remove this starter amino acid from the chain  3 termination codons (stop) signal the end of translation (don’t designate amino acids)

Gene Expression in Prokaryotes & Eukaryotes 

Prokaryotes & eukaryotes perform transcription & translation similarly, but differences exist!!  A prokaryotic cell simultaneously transcribe & translate the same gene & the newly made protein can quickly diffuse to its site of function

 In eukaryotes: 1) RNA polymerases depend on transcription factor 2) Transcription is terminated differently 3) Ribosomes are slightly different 4) Transcription (nucleus) is separated from translation (cytoplasm) & RNA processing occurs in the nucleus 5) A complicated mechanisms target proteins to the appropriate organelle

Point mutations

Mutations are chemical changes in just one base pair of a gene

Type of Point Mutations

Substitutions

Insertions & Deletions

7

Substitutions The replacement of one nucleotide & its partner with another pair of nucleotides

8

 Silent mutations: Same amino acid & protein  Missense mutation: Different amino acid but same protein function  Nonsense mutation: (Change causing translation to be terminated prematurely & the formation of a polypeptide (nonfunctional) shorter than the polypeptide encoded by the normal gene 9

2. Insertions & Deletions  Additions or losses of nucleotide pairs in a gene  Have a catastrophic effect on the protein more than substitutions  Because mRNA is read as a series of nucleotide triplets during translation, the nucleotides insertion/deletion alter the reading frame of the genetic message, such a mutation, called a frameshift mutation, which occurs whenever the number of nucleotides inserted or deleted is not a multiple of three

All nucleotides downstream of deletion or insertion will be improperly grouped into codons & the result will be extensive missense probably ending sooner or later in nonsense & premature termination

Mutagens  A number of physical & chemical agents that interact with DNA causing mutations  Such as X–rays can cause genetic changes 12

Mutagenic radiation, a physical mutagen, includes ultraviolet (UV) light, which can cause disruptive thymine dimers in DNA 13