4.3 Protein Synthesis
4.3.2 TRANSLATION (Cytoplasm) - mRNA protein
4.3.1 TRANSCRIPTION (Nucleus)
Overview the roles of transcription and translation in the flow of genetic information
Importance of Protein Synthesis
the synthesis of mRNA on a DNA template which is take place in nucleus
information that has been transcribed from DNA to mRNA can then be translated and thereby expressed by the formation of specific protein.
Transcription RNA polymerase enzyme involved Specific sequences of nucleotides along the DNA mark where the transcription begin ( promoter site ) and ends ( terminator site ) The stretch of DNA that is transcribed into mRNA is called a transcription unit
mRNA elongates in its 5’ to 3’ direction
3 steps involves in transcription THE PROCESS OF TRANSCRIPTION
RNA POLYMERASE BINDING AND INITIATION
RNA polymerase binding and initiation RNA polymerase recognize and attaches to promoter site on DNA. Enzyme begins to separate the DNA strand .
Segment of DNA strand unwind.
2. Elongation Of RNA strand As RNA polymerase moves along the template of DNA, complementary RNA nucleotides pair with DNA nucleotides of the strand
RNA polymerase joins the RNA nucleotides together in the 5’ to 3’ direction
Elongation of mRNA continues until RNA polymerase reaches a terminator site on the DNA Terminator site causes the RNA polymerase to stop transcribing DNA and release the mRNA
mRNA will leave the nucleus through the nucleus pore to the cytoplasm
RNA Splicing (RNA Processing) Exon : A nucleotide sequence in a gene that codes for parts or all of the gene product and is therefore expressed in Intron : A nucleotide sequence in a gene that does not code for gene product. : It usually transcribed in eukaryotes into mRNA but subsequently removed from
transcript before translation.
Genetic code: Base triplet in DNA provides a template for ordering the complementary triplet in mRNA molecule. Every base triplet is amino acid.
Three bases of an mRNA codon are designated as first, second and third bases .
Genetic code There are only four nucleotide to specify 20 amino acids; A-adenine, C-cytosine, G-guanine, T-thymine (unique to DNA), U-uracil (unique to RNA) [pyrimidine, very similar to thymine]. Flow of information from gene to protein is based on triplet code.
A cell cannot directly translate a gene’s base triplets into amino acids.
An mRNA molecule is complementary rather than identical to its DNA template according to base-pairing rules. mRNA base triplets are called codons. Noticed that U only can be found on mRNA strand, substitute for T (only on DNA strand). U on mRNA pairs with A on DNA strand, while T on DNA strand pairs with A on mRNA.
Two important codons in protein synthesis are initiation codon (start signal) and termination codon or stop signal.
Codon AUG is a starter to the process of translation. Codon AUG has dual function, as a start signal / initiation codon and it also code for amino acid methionine (Met). Polypeptide chains begin with methionine
An enzyme may subsequently remove starter amino acid from chain.
Three triplet bases of stop signal: UAA, UAG, UGA . marking the end of a genetic code. Genetic massages begin with the mRNA codon AUG, which signals the protein- synthesizing machinery to begin translating the mRNA at the location.
Three of 64 codons function as stop signal or termination codon. They are UAA, UAG and UGA . Any one of these termination codons marks the end of a genetic massage, and the completed polypeptide chain is released from the ribosome.