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Transcription & Translation

Transcription & Translation. Assessment Statements. 3.5.1 Compare the structure of RNA and DNA. 3.5.2 Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase.

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Transcription & Translation

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  1. Transcription & Translation

  2. Assessment Statements • 3.5.1 Compare the structure of RNA and DNA. • 3.5.2 Outline DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase. • 3.5.3 Describe the genetic code in terms of codons composed of triplets of bases. • 3.5.4 Explain the process of translation, leading to polypeptide formation. • 3.5.5 Discuss the relationship between one gene and one polypeptide. • 7.3.1 State that transcription is carried out in a direction. • 7.3.2 Distinguish between the sense and antisense strands of DNA. • 7.3.3 Explain the process of transcription in prokaryotes, including the role of the promoter region, RNA polymerase, nucleoside triphosphates and the terminator. • 7.3.4 State that eukaryotic RNA needs the removal of introns to form mature mRNA. • 7.4.1 Explain that each tRNA molecule is recognized by a tRNA-activating enzyme that binds a specific amino acid to the tRNA, using ATP for energy. • 7.4.2 Outline the structure of ribosomes, including protein and RNA composition, large and small subunits, three tRNA binding sites and mRNA binding sites. • 7.4.3 State that translation consists of initiation, elongation, translocation and termination. • 7.4.4 State that translation occurs in a direction. • 7.4.5 Draw and label a diagram showing the structure of a peptide bond between two amino acids. • 7.4.6 Explain the process of translation, including ribosomes, polysomes, start codons and stop codons. • 7.4.7 State that free ribosomes synthesize proteins for use primarily within the cell, and that bound ribosomes synthesize proteins primarily for secretion or for lysosomes.

  3. Compare the structure of RNA and DNA DNA RNA RNA is single-stranded molecule DNA is double-stranded molecule DNAcontainsdeoxyribose sugar RNA contains ribose sugar There is one form of DNA There are three forms of RNA: tRNA; mRNA and rRNA DNA has nitrogenous base thymine RNA has nitrogenous base uracil DNA & RNA are polymers of nucleotides i.e. both are nucleic acids DNA & RNAcontain four nitrogenous bases: A, G, C, T for DNA & A, G, C, U for RNA

  4. Types of RNA and their functions

  5. The genetic code – triplets of bases

  6. The genetic code • it is composed of mRNA base triplets called codons • there are 64 different codons each coding for the addition of an amino acid to a growing polypeptide chain • the genetic code is degenerate i.e.more than one codon can code for a particular amino acid • the genetic code is universal i.e.it is the same in almost all organisms • AUGcodes for Methionine & is the start codon • some nonsense codons code for the end of translation i.e. act as stop codons

  7. Relationship between genes and polypeptides • genes code for the synthesis of proteins (polypeptides) • one gene codes for one polypeptide chain • one gene is transcribed into one mRNA • mRNA is translated by a ribosome to synthesize a polypeptide • if the information on a gene is changed (i.e. mutated), this may alter the structure of a protein • genetic information transcribed by eukaryotes is edited before it is translated • polypeptides may be altered before they become fully functional proteins

  8. Transcription & Translation • Transcription;- the synthesis of RNA from a DNA template, DNA base sequence of the gene is copied into messenger RNA (mRNA) • Translation;- base sequence on mRNA is translated into an amino acid sequence in a polypeptide chain • One gene is transcribed and translated to produce one polypeptide chain.

  9. Transcription • RNA polymerase is the enzyme that controls transcription process • RNA polymerase binds to a promoter region on the DNA • RNA polymerase unwinds the DNA strands & splits it into two strands • RNA polymerase binds free nucleoside triphosphates to the antisense (template) strand of DNA • as it moves along in a 5'-> 3' direction • using complementary pairing (A with U& C with G) between template strand and mRNA nucleotides • nucleoside triphosphates loses two phosphates to release the energy required for transcription process • transcription continues until RNA polymerase reaches a terminator signal • mRNA detaches from the template strand and DNA rewinds • RNA polymerase detaches from the DNA • many RNA polymerases can follow each other during transcription process • intronsare removed & exons spliced (in eukaryotes) to form mature mRNA

  10. How Spliceosomes Process RNA

  11. Structure of ribosomes • ribosomes are composed of ≈ 60 % ribosomal RNA (rRNA) & 40 % protein • ribosomes consists of two sub-units; small sub-unit (30S) & large sub-unit (50S) • small sub-unit has binding site for mRNA • large sub-unit has three tRNA binding sites: Aminacyl(A), Peptidyl (P) and Exit (E) site • in eukaryotes, ribosomes can be free or bound to rough ER

  12. Computer generated model of a ribosome

  13. Protein Synthesis - Animation

  14. Translation • consists of initiation, elongation and termination • mRNA translated in a 5' -> 3' direction • mRNA binds to small ribosome sub-unit then to large ribosome sub-unit • tRNA activating enzymes link correct amino acid to each tRNA activated tRNA has an anticodon and the corresponding amino acid attached • First, initiator tRNA binds to start codon (AUG )on the small subunit of ribosome • Second, tRNA binds to large ribosome subunit • ribosome moves down mRNA after a second tRNA binds • amino acid/ polypeptide on first tRNA is transferred & bonded to amino acid on second tRNA • peptide bonds between amino acids catalysed by peptidyl transferase • Translation requires Guanosine-5'-triphosphate (GTP) a source of energy & (or) substrates • movement of ribosome down the mRNA continues , loss of tRNA and new tRNA binds • Ribosome reaches a stop codon - termination • polypeptide released, the ribosome sub-units detach from the mRNA

  15. Formation of a dipeptide from two amino acids

  16. A diagram showing the structure of a peptide bond between two amino acids

  17. Free and membrane bound ribosomes • Free ribosomes in the cytoplasm are associated with the synthesis of proteins for internal use in the cell • Ribosomes which are attached to the wall of the endoplasmic reticulum are associated with proteins which will be placed into vesicles and secreted form the cell

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