Protein Synthesis
Biology H
Ribonucleic Acid
Sugar is ribose
Single strand
Uracil instead of thymine
Found in the nucleus and cytoplasm
3 types
mRNA: carries DNA’s message to the cytoplasm
tRNA: brings amino acids to the ribosome
rRNA: found in the ribosome
Proteins
Made of chains of amino acids (20 kinds)
The instructions to make a protein are called a gene—a sequence of nucleotides in DNA
Instructions have to get out of the nucleus and to the ribosome (protein factory)
Transcription
Makes an RNA copy of the DNA code for a protein
RNA polymerase bonds to the promoter region of the DNA molecule (initiation)
The DNA segment to be copied uncoils; only 1 strand will be copied
RNA polymerase moves along the DNA strand making a complementary mRNA strand (elongation)
mRNA strand pulls off as it grows and the DNA molecule re-coils
When RNA polymerase reaches the terminator region, transcription ends (termination)
End product: primary RNA transcript which now goes for processing
RNA processing
The primary transcript undergoes chemical modifications
Methyl guanine cap is added to the front end of the transcript; helps the mRNA attach to the ribosome
Poly-adenine tail is added to keep RNA from being degraded by enzymes in the cytoplasm
RNA is spliced: non-coding regions (introns) are removed and the coding regions (exons) are spliced
together to form the mature mRNA
Mature mRNA is now ready to leave the nucleus, travel to a ribosome and build a protein
Translation
Makes a protein from the mRNA message
It requires tRNA and ribosomes
tRNA: carries amino acids to the ribosomes, each carries only 1 kind of amino acid
Loop at one end of tRNA has a segment called the anti-codon which is complementary to the mRNA
codon
At the opposite end of the anti-codon is the amino acid attachment site
Ribosomes
Made in the nucleolus of rRNA and proteins in 2 subunits, the large and the small
Small subunit has the binding site for mRNA
Large subunit has 3 binding sites for tRNA
P site: holds tRNA carrying the growing protein
A site: holds tRNA carrying the next amino acid to be attached
E site: holds the exiting tRNA
Steps in translation
Initiation
Methyl guanine cap attaches to the small subunit of the ribosome so the start codon is at the P site
tRNA with the correct anti-codon binds to the mRNA
Large subunit binds to the small one creating a functional ribosome
Elongation
tRNA binds to the codon on the mRNA at the A site
Peptide bonds form between the 2 amino acids
tRNA at the P site moves to the E site and is released
The tRNA at the A site moves to the P site
The next codon is now at the A site, the correct tRNA binds, peptide bond forms, etc
Termination
The stop codon is reached
The mRNA and protein are released
mRNA can enter another ribosome for further copying
The protein goes for processing and shipment
The ribosome subunits separate
The genetic code
The code is nearly universal among all living organisms
Amino acids are coded for in groups of 3 nucleotides on mRNA called codons
64 existing codons
60 codons code only for amino acids
AUG codes for methionine and is the start codon
3 codons are stop signals
More than 1 codon can code for a single amino acid
Only 1 strand of the DNA is read
mRNA codon and tRNA anticodon are complementary and pair up in the ribosome to ensure the right
amino acid is being added to the growing protein