Protein synthesis consists of two phases
- Overall protein synthesis is caused by making a RNA copy of a DNA gene. This leaves the nucleus and enters cytoplasm and associates with ribosomes to manufacture protein molecules.
- Transcription is where one strand of the deoxyribonucleic acid (DNA) double helix is used as a template by ribonucleic acid (RNA) polymerase to synthesise messenger RNA from RNA nucleotides.
The mRNA then migrates into the cytoplasm maturing, for example, by the splicing of non-coding sequences.
- Translation occurs when the ribosome binds to mRNA at the start codon and transfer RNA brings amino acids into position along the mRNA template.
The ribosome moves from codon to codon along the mRNA producing a polypeptide sequence.
- Coded sequence of DNA and RNA: Each contains 4 different nucleotides with a base
- Thymine or Uracil (RNA)
- Enzymes: RNA polymerase
- Messenger RNA mRNA
- Transfer RNA tRNA
- Ribosomes have rRNA
Transfer RNA molecule
Transfer RNA is shaped like a clover leaf with three hairpin loops. It contains an amino acid attachment site on one end and a special section in the middle loop called the anticodon site.
- Occurs in the cell nucleus.
- RNA polymerase unwinds the DNA of the protein making gene
- RNA polymerase attaches to a promoter of the gene
- RNA polymerase separates the strands
- It encodes from 5' to the 3' end of the RNA on the template strand of the RNA
- The RNA copy will make a copy of the complementary side of the DNA
- The DNA double helix acts as a template for RNA.
- DNA Cytosine pairs with RNA guanine
- DNA Thymine pairs with RNA adenine
- DNA Adenine pairs with RNA Uracil NOT Thymine
- DNA Guanine pairs with RNA Cytosine
- mRNA replaces thymine with a base called uracil.
- Complementary RNA nucleotides pair with the DNA
RNA polymerase binds the RNA nucleotides together
- It stops at a terminator point and polymerase lets go
- It may be due to the mRNA forms a hairpin
- In a eukaryote this forms pre mRNA
- Pre mRNA is composed of introns and exons
- 5' cap is added which is a modified guanine
- 3' end adds a Poly A tail composed of adenines
- Introns "nonsense" are removed leaving exons
- Makes the molecule more robust
- Now called mature RNA
- The result is a molecule of messenger RNA
- This leads the nucleus to travel to the cytoplasm
- It then attaches to a ribosome
- mRNA molecule is read to make proteins
- Proteins are carried by transfer RNA
- AUG is the start codon and codes for Methionine
- 3 UAA, UAG, UGA are stop codons
- There are 4 x 4 x 4 possible sequences
- Therefore 61 coding codons for amino acids
- On the Transfer RNA is a protein attached to an anti codon
- These have three nucleotides and carry an amino acid
- Only a certain sequence will give a certain amino acid
- Codons: Each triplet encodes for a particular amino acid
- This process occurs in Ribosomes and cytosol
- Ribosomes are made up of proteins and ribosomal RNA
- Ribosomal RNA can have structural qualities
- Ribosome can look like a burger
- Acts from 5' to the 3' end of the mRNA
- End result is a polypeptide molecule: a polymer of amino acids
- The order of the amino acids determines their shape and function
- We use about 20 amino acids to build molecules
- Once the chain is complete it folds into its shape
- They are macro-molecular production units and the site of protein synthesis and may be free in cytosol and as particles attached to the membranes of the endoplasmic reticulum in eukaryotic cells.
- Ribosomes are composed of special proteins and nucleic acids. They are formed from two subunits locking together. They have rRNA - ribosomal RNA
- In the mammalian cell there can be as many as 10 million ribosomes. They can join up amino acids at a rate of 200 per minute.
- Small proteins can therefore be made fairly quickly but two to three hours are needed for larger proteins such as the massive 30,000 amino acid muscle protein titin.
- Different types
- Prokaryotes have 70S ribosomes: 30S and 50S subunits
- Eukaryotes have 80S ribosomes: 40S and 60S subunits
- Antibiotics targets
- 30 s : Aminoglycosides and Tetracyclines
- 50s: Macrolides e.g. erythromycin, azithromycin
- A site : amino acyl site for amino acid - tRNA
- P site : Polypeptide chain bound
- E site: Exit site
|DNA ||CTA ATA TAA CCC|
|mRNA ||GAU UAU AUU GGG|
|Protein||ASP TYR ILE GLY|