Details of DNA transcription and translation
Amino acids are made of a carboxyl group and amine group and an R group.
they are the subunits for polypeptide chains which ultimately make a protein.
There are twenty one amino acids which correspond to a CODON. This forms the GENETIC CODE
Transcription: mRNA is made from the DNA strand
1. DNA unwinds and a promoter region is exposed. There is a SENSE STRAND (TEMPLATE STRAND) of DNA to make RNA.
2. RNA polymerase synthesizes the mRNA from a 5' to 3' direction. RNA nucleotides are attached only on the 3' end.
3. When the mRNA is complete it undergoes processing
a.EDITING THE mRNA STRAND: INTRONS are removed and EXONS remain.
b. A polyA tail and 5' cap are placed on the mRNA in order to protect the strand from exonuclease. Exonucleases digest and recycle mRNA. We want them to avoid digesting the mRNA while it is being read by the ribosome
4. mRNA is now ready for the ribosome. IN EUKARYOTES, it leaves the nucleus. IN PROKARYOTES, protein synthesis starts right away
Translation: mRNA gets read by the ribosome and a polypeptide chain is formed
Initiation:
1. METHIONINE is the first amino acid brought by a tRNA. tRNA has an ANTICODON which is complementary to the mRNA CODON. The START CODON IS ALWAYS AUG. Meanwhile, the small ribosomal subunit attaches to the strand.
2. Next the large ribosomal subunit arrives with its P site and A site.
AMINO ACYL T-RNA is the name of the tRNA attached to the amino acid.
the following is excerpted from mrsdaintreysonlineclassroom. retrieved
january 20, 2016:
"Elongation
more amino acids are added and connected together to form a polypeptide, as specified by the mRNA sequence.
i. an incoming amino-acyl-tRNA (lets call this AA2-tRNA2) recognizes the codon in the A site and binds there.
ii. a peptide bond is formed through dehydration synthesis between the new amino acid and the growing polypeptide chain.
iii. the amino acid is removed from tRNA1 (bond breaks between aa1 and tRNA1)
iv. the tRNA1 that was in the P site is released, and the tRNA in the A site is translocated to the P site.
v. the ribosome moves over one codon along the mRNA (to the right in our diagram, or more specifically in the 5' ----> 3' direction.)
vi. This movement shifts the tRNA2 (which is attached to the growing amino acid chain) to the P site.
vii. tRNA3 with aa3 can now move into A site and bind with the next codon on mRNA.
viii. THIS PROCESS REPEATS, and the CHAIN ELONGATES as long as there are new codons to read on the mRNA.
more amino acids are added and connected together to form a polypeptide, as specified by the mRNA sequence.
i. an incoming amino-acyl-tRNA (lets call this AA2-tRNA2) recognizes the codon in the A site and binds there.
ii. a peptide bond is formed through dehydration synthesis between the new amino acid and the growing polypeptide chain.
iii. the amino acid is removed from tRNA1 (bond breaks between aa1 and tRNA1)
iv. the tRNA1 that was in the P site is released, and the tRNA in the A site is translocated to the P site.
v. the ribosome moves over one codon along the mRNA (to the right in our diagram, or more specifically in the 5' ----> 3' direction.)
vi. This movement shifts the tRNA2 (which is attached to the growing amino acid chain) to the P site.
vii. tRNA3 with aa3 can now move into A site and bind with the next codon on mRNA.
viii. THIS PROCESS REPEATS, and the CHAIN ELONGATES as long as there are new codons to read on the mRNA.
Termination
The process above repeats until a special codon, called a STOP CODON, is reached. There are 3 Stop codons: UAA, UAG, UGA.
i. the stop codons do not code for amino acids but instead act as signals to stop translation.
ii. a protein called release factor binds directly to the stop codon in the A site. The release factor causes a water molecule to be added to the end of the polypeptide chain, and the chain then separates from the last tRNA.
The process above repeats until a special codon, called a STOP CODON, is reached. There are 3 Stop codons: UAA, UAG, UGA.
i. the stop codons do not code for amino acids but instead act as signals to stop translation.
ii. a protein called release factor binds directly to the stop codon in the A site. The release factor causes a water molecule to be added to the end of the polypeptide chain, and the chain then separates from the last tRNA.
- the protein is now complete. The mRNA is now usually broken down by exonuclease and the ribosome splits into its large and small subunits.
- the new protein is sent for final processing into the endoplasmic reticulum and golgi apparatus "
