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Cellular Control(F215) How DNA codes for proteins Transcription of DNA

Cellular Control(F215) How DNA codes for proteins Transcription of DNA. Learning objectives (a) state that genes code for polypeptides, including enzymes; (b) explain the meaning of the term genetic code ;

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Cellular Control(F215) How DNA codes for proteins Transcription of DNA

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  1. Cellular Control(F215) How DNA codes for proteinsTranscription of DNA Learning objectives (a) state that genes code for polypeptides, including enzymes; (b) explain the meaning of the term genetic code; (c) describe, with the aid of diagrams, the way in which a nucleotide sequence codes for the amino acid sequence in a polypeptide;

  2. What is a gene? • Genes are lengths of DNA, made from a sequence of nucleotide bases. • These code for one or more polypeptides. • Genes are units of hereditary – they can be passed on. • The majority of genes are found inside the nucleus of a cell, although some are found in the mitochondria. • The specific space where a gene is found is known as the locus.

  3. One molecule of DNA Histone proteins

  4. The genetic code • 4 different bases (A, T, C and G) • Triplet codes make 64 different combinations (43) of amino acid (there are only 20 so this is plenty) • A degenerate code (several codes with the same meaning) except methionine • Some codes do not code for an amino acid, but for a “stop” or end of the polypeptide chain sequence. • Some code is universal e.g. serine is the same in all organisms (TCT) – this is useful in genetic engineering as gene transfer will still result in production of the same protein Degenerate code

  5. This is a second type of nucleic acid called ribonucleic acid RNA is also made up of smaller subunits called nucleotides Phosphate group Ribose Sugar Base RNA Structure

  6. Unlike DNA, RNA only has one nucleotide strand. RNA has one base that is different from DNA – the base Uracil replaces Thymine and so Uracil now becomes the complementary base partner of Adenine. RNA Structure

  7. Messenger RNA • DNA is the “instruction manual” for making proteins, but protein synthesis itself occurs in the cytoplasm. • Messenger RNA (mRNA) a type of RNAcopies information from a particular strand of DNA and carries it out into the cytoplasm. • This process is called transcription.

  8. Transcription • In order to copy the set of instructions needed to make a particular protein, the section of the DNA helix where the gene is located needs to unwind and unzip to expose the code (series of bases). • The unwound and unzipped part of DNA that is to be copied is known as the DNA template. (Note: bases in DNA easily break apart because the bonding here is weak)

  9. Transcription • Free RNA nucleotides start to pair up with their complementary base of DNA on the strand opposite them and form weak hydrogen bonds. • The RNA nucleotides are activated – this means that they have two extra phosphoryl groups attached (ATP, UTP, GTP and CTP) • The RNA nucleotides start to link together by forming strong chemical bonds between the phosphate of one nucleotide linking on to the sugar of another to make a single RNA strand. This is called messenger RNA or mRNA. • The two extra phosphoryl groups are released, which releases energy for bonding between adjacent nucleotides • The enzyme RNA polymerase controls this process.

  10. Weak hydrogen bonds forming between bases Strong chemical bonds forming between sugar and phosphate groups. Energy for this comes from the release of phosphoryl groups

  11. Bonding between ribose sugar and phosphate group of the next nucleotide

  12. Once the set of instructions have been copied from the DNA strand onto mRNA, the weak hydrogen bonds binding the bases of DNA and mRNA together start to break. Transcription

  13. Transcription • The newly transcribed section of mRNA now leaves the nucleus via the nuclear pores and enters the cytoplasm. • Hydrogen bonds between DNA bases re-form and DNA winds back up into a double helix.

  14. A group of three adjacent bases in mRNA are known as a codon. Different sequences of bases code for different amino acids – this is the genetic code Codons, starting and stopping

  15. Some of these codons tell the cell when to start and stop production of protein These are known as start and stop codons. They are found at the beginning and end of the gene e.g TAG is a stop codon Codons, starting and stopping

  16. Transcription Summary • Occurs in the nucleus • A copy of the DNA code (gene) for a protein is made – in the form of m-RNA • To copy the DNA code, the DNA must unwind and then unzip to expose its base sequence • A selection of RNA nucleotides must then be available to lock into the DNA strand • These nucleotides pair up with the exposed bases of the DNA • The base pairing is – A with U, T with A, C with G, G with C • They then join to each other forming a single strand of mRNA • 3 nucleotides in mRNA make a codon. Some codons are found at the start and the end of a protein code to start and stop protein synthesis • The mRNA leaves the nucleus via a nuclear pore to travel to a ribosome

  17. A T C G G C C G A T C G A T T A

  18. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  19. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T the double stranded DNA starts to unwind...

  20. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  21. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  22. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  23. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T exposing the sense and antisense (DNA) strands

  24. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  25. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T antisense strand (DNA) sense strand (mRNA)

  26. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  27. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  28. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  29. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T RNA polymerase enzyme

  30. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  31. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  32. T G CG A T T A G C A T C G T A C G T A A C G C T A A T C G T A G C A T G C A T

  33. T G CG A T T A G C A T C G T A C G T A G U C A A C G C T A A T C G T A G C A T G C A T

  34. T G CG A T T A G C A T C G T A C G T A G U A C A C G C T A A T C G T A G C A T G C A T free mRNA nucleotides

  35. T G CG A T T A G C A T C G T A C G T A U G A C A C G C T A A T C G T A G C A T G C A T

  36. T G CG A T T A G C A T C G T A C G T A U G A C A C G C T A A T C G T A G C A T G C A T

  37. T G CG A T T A G C A T C G T A C G T A U G A C A C G C T A A T C G T A G C A T G C A T

  38. T G CG A T T A G C A T C G T A C G T A U A G C A C G C T A A T C G T A G C A T G C A T

  39. T G CG A T T A G C A T C G T A C G T A A U G C A C G C T A A T C G T A G C A T G C A T

  40. T G CG A T T A G C A T C G T A C G T A A U G C A C G C T A A T C G T A G C A T G C A T the free nucleotides bind with their complementary base pairs

  41. T G CG A T T A G C A T C G T A C G T A A U G C A C G C T A A T C G T A G C A T G C A T

  42. T G CG A T T A G C A T C G T A C G T A A U G C G A C G C T A A T C G T A G C A T G C A T

  43. T G CG A T T A G C A T C G T A C G T A U G C G A A C G C T A A T C G T A G C A T G C A T the RNA polymerase moves along the sense strand of DNA

  44. U G C G A A C G C T A A T C G T A G C A T G C A T T G CG A T T A G C A T C G T A C G T A U U A

  45. U G C G A A C G C T A A T C G T A G C A T G C A T T G CG A T T A G C A T C G T A C G T A U U A G C A

  46. U G C G A A C G C T A A T C G T A G C A T G C A T T G CG A T T A G C A T C G T A C G T A U U A G C A U C G

  47. U G C G A A C G C T A A T C G T A G C A T G C A T T G CG A T T A G C A T C G T A C G T A U U A G C A U C G U A C

  48. U G C G A A C G C T A A T C G T A G C A T G C A T T G CG A T T A G C A T C G T A C G T A U U A G C A U C G U A C G U A

  49. U G C G A A C G C T A A T C G T A G C A T G C A T T G CG A T T A G C A T C G T A C G T A when a “stop” sequence is reached the enzyme becomes detached U U A G C A U C G U A C G U A

  50. U G C G A A C G C T A A T C G T A G C A T G C A T T G CG A T T A G C A T C G T A C G T A U U A G C A U C G U A C G U A

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