6.2.2
Polypeptide Synthesis
Transcription
Transcription
Transcription is the first step in protein synthesis. As a result of this process, mRNA that is a copy of the target gene is produced in the nucleus. The stages involved are:


1) Binding of RNA polymerase
1) Binding of RNA polymerase
- RNA polymerase is the enzyme that allows transcription to take place.
- RNA polymerase binds to the locus of the gene to be transcribed (the target gene).


2) Separation of DNA strands
2) Separation of DNA strands
- When RNA polymerase binds to DNA, DNA helicase unwinds the double strands and the hydrogen bonds that bind the two strands together break.
- The DNA strands separate.
- The bases of the target gene are exposed.


3) Binding to template strand
3) Binding to template strand
- RNA polymerase binds free-floating RNA nucleotides to the template strand.
- The template strand is the DNA strand that is complementary to the base sequence of the target gene.
- The RNA nucleotides form a strand of mRNA that is complementary to the template strand.
- The template strand is complementary to the gene so this means mRNA is a copy of the gene.


4) Joining the nucleotides
4) Joining the nucleotides
- The free-floating nucleotides are joined together by RNA polymerase.
- Phosphodiester bonds form between the nucleotides to form the completed strand of mRNA.


5) STOP codon
5) STOP codon
- RNA polymerase eventually reaches the triplet of bases that signal 'stop'.
- E.g. UAG encodes a STOP codon.
- The RNA polymerase stops separating the DNA and producing mRNA.


6) Removal of the mRNA
6) Removal of the mRNA
- The mRNA strand is separated from the template strand by RNA polymerase.
- The hydrogen bonds between the two strands of DNA form again and the strands join together.


7) mRNA leaves the nucleus
7) mRNA leaves the nucleus
- The completed mRNA strand leaves the nucleus and enters the cytoplasm.
- mRNA is used in translation, the next step in protein synthesis.
Translation
Translation
Translation is the second step in protein synthesis. Translation takes place in the cytoplasm. The steps involved are:


1) Attachment to the ribosome
1) Attachment to the ribosome
- mRNA that has been produced during transcription binds to a ribosome in the cytoplasm.
- The ribosome is the site of protein synthesis.


2) Binding of tRNA
2) Binding of tRNA
- Six bases (two codons) can fit inside the ribosome at one time.
- One molecule of tRNA binds to the first codon in the ribosome.
- The tRNA molecule has an anticodon that is complementary to a specific codon.
- The anticodon allows the correct tRNA molecule to bind to the correct codon.


3) Bringing in amino acids
3) Bringing in amino acids
- Each tRNA molecule carries a specific amino acid into the ribosome.
- The amino acid is bound to tRNA using ATP.


4) Binding of the second tRNA
4) Binding of the second tRNA
- A tRNA molecule binds to the second codon in the ribosome.
- When a tRNA molecule binds to mRNA, the corresponding amino acid is brought into the ribosome.
- The two amino acids in the ribosome form a peptide bond.


5) Movement of the ribosome
5) Movement of the ribosome
- When the two amino acids bind together, the ribosome moves along the mRNA strand so that a new codon enters the ribosome.
- A complementary tRNA molecule binds to the new codon.
- A new amino acid is brought into the ribosome.
- A peptide bond forms between the new amino acid and the existing chain of amino acids (a polypeptide chain).


6) STOP codon
6) STOP codon
- When the ribosome reaches a STOP codon (e.g. UAG) there is no corresponding tRNA molecule.
- The polypeptide chain is released from the ribosome.


7) Completion of the polypeptide
7) Completion of the polypeptide
- The polypeptide chain has been formed and is ready to complete its function (e.g. as a protein channel).
- Some polypeptide chains are joined to other chains or a prosthetic group is added.
- E.g. Each haemoglobin molecule is made up of four polypeptide chains and each polypeptide has an iron prosthetic group.
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
Jump to other topics
1Cell Structure
1.1Cell Structure
1.1.1Studying Cells - Microscopes
1.1.2Introduction to Eukaryotic & Prokaryotic Cells
1.1.3Ultrastructure of Eukaryotic Cells
1.1.4Ultrastructure of Eukaryotic Cells 2
1.1.5Ultrastructure of Eukaryotic Cells 3
1.1.6Prokaryotic Cells
1.1.7Viruses
1.1.8End of Topic Test - Cell Structure
1.1.9Exam-Style Question - Microscopes
1.1.10A-A* (AO2/3) - Cell Structure
2Biological Molecules
2.1Testing for Biological Modules
2.2Carbohydrates & Lipids
2.3Proteins
3Enzymes
4Cell Membranes & Transport
4.1Biological Membranes
5The Mitotic Cell Cycle
6Nucleic Acids & Protein Synthesis
6.1Nucleic Acids
7Transport in Plants
8Transport in Mammals
8.1Circulatory System
8.2Transport of Oxygen & Carbon Dioxide
9Gas Exchange
9.1Gas Exchange System
10Infectious Diseases
10.1Infectious Diseases
10.2Antibiotics
11Immunity
12Energy & Respiration (A2 Only)
13Photosynthesis (A2 Only)
14Homeostasis (A2 Only)
14.1Homeostasis
14.2The Kidney
14.3Cell Signalling
14.4Blood Glucose Concentration
14.5Homeostasis in Plants
15Control & Coordination (A2 Only)
15.1Control & Coordination in Mammals
15.1.1Neurones
15.1.2Receptors
15.1.3Taste
15.1.4Reflexes
15.1.5Action Potentials
15.1.6Saltatory Conduction
15.1.7Synapses
15.1.8Cholinergic Synnapses
15.1.9Neuromuscular Junction
15.1.10Skeletal Muscle
15.1.11Sliding Filament Theory Contraction
15.1.12Sliding Filament Theory Contraction 2
15.1.13Menstruation
15.1.14Contraceptive Pill
15.2Control & Co-Ordination in Plants
16Inherited Change (A2 Only)
16.1Passage of Information to Offspring
16.2Genes & Phenotype
17Selection & Evolution (A2 Only)
17.2Natural & Artificial Selection
18Classification & Conservation (A2 Only)
18.1Biodiversity
18.2Classification
19Genetic Technology (A2 Only)
19.1Manipulating Genomes
19.2Genetic Technology Applied to Medicine
19.3Genetically Modified Organisms in Agriculture
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