Protein Synthesis

Protein Synthesis

1. Transcription- process by which DNA base sequence is converted to a complementary base sequence strand of RNA

   • 1)Initiation
     • RNA polymerase(enzyme) binds to the promoter sequence on DNA. The promoter is a specific nucleotide sequence along the DNA that marks where transcription of the gene begins.Promoter nucleotide sequence (rich in A and T) has a complementary shape to the RNA polymerase active site. Binding of RNA polymerase to the promoter unzips the DNA double helix to separate two strands and expose a short stretch of nucleotides on each strand. The DNA double helix structure unwinds as well. The hydrogen bonds between the complementary bases are broken in order to separate the strands. Only one of the two DNA strands is transcribed. This is known as the template strand. The template strand determines the sequence of nucleotides in an RNA transcript. Promoter determines where transcription starts and which of two DNA strands is used as a template strand. Each gene only has one promoter sequence.
   • 2)Elongation
     • RNA synthesis does not require a pre-existing primer strand to start to start synthesising RNA strands. The exposed template DNA strand will determine the order of RNA nucleotides to be added to the RNA strand being synthesised. This occurs due to complementary base pairing between incoming RNA nucleotides and the DNA nucleotides on the template strand. A base pairs with U (base sub for T in RNA), while G base pairs with C. RNA polymerase then catalyses the formation of phosphodiester bonds between two adjacent RNA nucleotides to covalently link them together. The bond is formed between the 3' hydroxyl(-OH) group of the last RNA nucleotide in the RNA strand and the 5'-phosphate group of the newly added nucleotide. Each RNA nucleotide added to the elongating RNA is from a ribonucleoside triphosphate. As each nucleoside triphosphate is added to elongating mRNA strand, two of three phosphates are lost as a pyrophosphate molecule.
     • Hydrolysis of high energy bonds between phosphate groups provides energy for phosphodiester bond formation
     • DNA template strand is read from 3' to 5' direction by RNA polymerase. New RNA strand is synthesised in the 5' to 3' direction as incoming RNA nucleotides are added to the 3' end of the growing RNA strand. This is due to RNA polymerase active site is complementary in shape to the free -OH group at the 3' end of a RNA strand. RNA strand and DNA template strand will be antiparallel to each other.
     • The nucleotide sequence on the RNA will be the same as the nucleotide sequence on the non-template strand of the gene, with exception that uracil is present in RNA instead of thymine. Hence, non-template strand is therefore known as the coding strand in some books.
   • 3)Termination

2. Translation-process by which base sequence on mRNA molecule is converted to a sequence of amino acids in a polypeptide

   • Activation
   • Initiation
   • Elongation
   • Termination

3. RNA processing

   • Prokaryotes
     • DNA is not membrane bound therefore transcription and translation occur simultaneously.Translation of mRNA begins as soon as 5' end of the mRNA peels away from the DNA template strand. There is no need for RNA processing in prokaryotes
   • Eukaryotes
     • In eukaryotes, the newly synthesised mRNA is known as pre-mRNA. It then undergoes RNA processing to produce functional mature mRNA which then get exported out of nucleus for translation. Involves 3 processes.

4. Genetic Code

   • describes the rules by which the nucleotide sequence of a gene, through the intermediary of mRNA,is translated into the amino acid sequence of a functional protein.
     • A cell cannot directly translate the base triplets on DNA into amino acids and mRNA produced during transcription has to be used.Sequences of bases on DNA act as a template for determining the sequence of bases along the mRNA strand(based on complementary base-pairing rules)
   • Each base triplet specifies for a particular amino acid. Each base triplet on the mRNA is called a codon, and each codon specifies either one amino acid or a stop to the translation process.
     • During translation, the sequence of codons on mRNA determines the order in which amino acids are incorporated in a polypeptide chain (sequence of amino acids in a polypeptide chain (sequence of amino acids in a polypeptide chain).
   • 4^3(64) mRNA codons. These 64 codons codons constitute the genetic code, specifying 20 amino acids found in proteins. 3 of which are stop codons that mark the end of translation.
   • The codon AUG has a dual function. The first AUG in an mRNA is the signal to initiate translation in most species. It also codes for methionine, so methionine always is the first amino acid in new polypeptides in most organisms.
   • Features: The code is degenerate. The code is non-overlapping. The code is punctuated. The code is (nearly) universal.