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PROTEIN SYNTHESIS Transcription
1. An enzyme (RNA polymerase) attaches to the DNA
2. The hydrogen bonds between paired bases break, and the DNA molecule unwinds.
3. RNA nucleotides with bases complementary to those on the template (antisense) strand of the DNA pair up and bond to form an
mRNA molecule (by condensation reactions creating phosphodiester bonds) RNA polymerase catalyses the reaction.
4. The mRNA molecule passes out of the nucleus through a pore in the nuclear envelope.
5. It then attaches to a ribosome, in eukaryotic cells ribosomes are usually attached to the endoplasmic reticulum. Translation can now
Ribosomes are made of two subunits. The mRNA attaches to the smaller subunit so that 2 mRNA codons face the 2 binding sites of the larger
1. A Transfer RNA molecule carries the amino acids to the ribosomes.
2. A tRNA molecule, with an anticodon that's complementary to the first codon on the mRNA, attaches itself to the mRNA by
complementary base pairing.
3. Amino acids attached to the tRNA molecules are joined by a peptide bond this is created due to a condensation reaction between
the amine group of one amino acid and the carboxylic acid group of the next, and forms a dipeptide.
4. The whole process is repeated (creating a polypeptide), and translation continues until the ribosome reaches a stop signal.
THE NATURE OF THE GENETIC CODE
The triplet code: There are 20 amino acids and start and stop signals to code for and only four bases to do it. Using three bases gives 64
Non-overlapping: Each set of three bases forms a triplet they don't overlap, so no base from one triplet is part of another triplet,
avoiding confusion about which amino acid is being coded for.
Degenerate: Some amino acids have more than one codon, which offers some protection against mutation.
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Between transcription and translation, mRNA is often edited, with some sections being removed and other sections spliced together.
Non-coding introns are removed. The sequences that are expressed are called extrons. Therefore this means that several proteins can be
formed from one length of mRNA if it is spliced in different ways.
The discovery of mRNA splicing means that `one gene = several proteins'.…read more