Protein Synthesis Flashcards
Summarise the process of protein synthesis in four key steps
1) DNA provides the instructions in the form of a long sequence of bases.
2) A complementary section of part of this sequence is made in the form of a molecule called pre-mRNA which is a process called transcription.
3) The pre-mRNA is spliced to form mRNA
4) The mRNA is used as a template to which complementary tRNA molecules attach and the amino acid molecules they carry are linked to form a polypeptide- a process called translation.
What is transcription
The formation of pre-mRNA in eukaryotes and mRNA in prokaryotes from a section of the template strand of DNA. It is the first stage in protein synthesis.
Describe the process of transcription
1) An enzyme acts on a specific region of the DNA causing the two strands to separate and expose the nucleotide bases in that region.
2) The nucleotide bases on one of the two DNA strands, known as the template strand, pair with their complementary nucleotides from the pool which is present in the nucleus.
3) The enzyme RNA polymerase then moves along the strand and joins the nucleotides together to form a pre mRNA molecule.
4) In this way an exposed guanine base on the DNA binds to the cytosine base of a free nucleotide. Similarly, cytosine links to guanine, and thymine joins to adenine. The exception is adenine, which links to uracil rather than thymine.
5) As the RNA polymerase adds the nucleotides one at a time to build a strand of pre-mRNA, the DNA strands rejoin behind it.
6) As a result, only about 12 base pairs on the DNA are exposed at any one time.
7) When the RNA polymerase reaches a particular sequence of bases on the DNA (called a terminator sequence), it detaches and the production of pre-mRNA is complete.
What is splicing
The process following transcription in eukaryotic cells in which introns are removed from pre-mRNA and exons are joined together to form mRNA
Why does splicing not take place in prokaryotes
Splicing does not take place in prokaryotes because introns are uncommon in prokaryotic cells
What is pre-mRNA spliced to form
mRNA
Why does splicing take place in eukaryotic organisms
The DNA of a gene in eukaryotic cells is made up of exons which code for a protein and introns which do not. These intervening introns would prevent the synthesis of a polypeptide. In splicing, the introns are removed and the exons are joined together to form mRNA.
What is translation
- The second phase of protein synthesis that takes place in the ribosomes.
- mRNA is used as a template for the attachment of tRNA molecules with complementary anticodons.
- The amino acids carried on adjacent tRNA molecules are joined to form a polypeptide chain.
Describe the process of translation (how a polypeptide is synthesised after the mRNA passes out of a nuclear pore)
1) A ribosome becomes attached to the starting codon at one end of the mRNA molecule.
2) The tRNA molecule with the complementary anticodon sequence moves towards the ribosome and pairs up with the codon on the mRNA. This tRNA carriers a specific amino acid
3) A tRNA molecule with a complementary anticodon pairs with the next codon on the mRNA. This tRNA carries another amino acid.
4) The ribosome moves along the mRNA, bringing together two tRNA molecules at any one time, each pairing up with the corresponding two codons on the mRNA.
5) The two amino acids on the tRNA are joined by a peptide bond using an enzyme and ATP which is hydrolysed to provide the required energy.
6) The ribosome moves onto the third codon in the sequence on the mRNA, thereby linking the amino acids on the second and third tRNA molecules.
7) As this happens, the first tRNA is released from its amino acid and is free to collect another amino acid from the amino acid pool in the cell.
8) The process continues this way until a ribosome hits a stop codon.
9) At this point the ribosome, mRNA, and the last tRNA molecule separate and the polypeptide chain is complete.
Describe how protein synthesis happens quickly
- Up to 15 amino acids can be added to a polypeptide chain in a second
- Up to 50 ribosomes can pass immediately behind the first, so that many identical polypeptides can be assembled simultaneously.
Briefly summarise the process of protein synthesis
- The DNA sequence of triplets that make up a gene determine the sequence of codons on mRNA.
- The sequence of codons on mRNA determine the order in which the tRNA molecules line up.
- They, in turn, determine the sequence of amino acids in the polypeptide.
- In this way genes precisely determine which proteins a cell manufactures.
- As many of these proteins are enzymes, genes effectively control the activities of cells.
