Transcription Translation and the Effects of Radiation

1. Transcription Translation and the Effects of Radiation Protein Synthesis in eukaryotes and How proteins are damaged / destroyed by exposure to radiation

2. Protein Synthesis • The process by which amino acids are linked together to form proteins • Transcription of the gene in the nucleus is the first step in the process • Translation is the second step which occurs on the ribosomes in the cytoplasm of the cell Protein Molecule

3. RNA • mRNA – encodes the amino acid sequence from the DNA • rRNA – with ribosomal proteins makes up the ribosomes (the site of translation) • tRNA – brings the amino acids to the ribosome during translation • snRNA (small nuclear RNA) – short RNA sequences that associate with proteins together which participate in RNA processing.

4. Nucleotides DNA Nucleotide Pairing Adenine – Thymine Cytosine – Guanine RNA Nucleotide Pairing Adenine – Uracil Cytosine - Guanine

5. Transcription • The synthesis of a single-stranded mRNA copy of a segment of DNA • RNA nucleotides pair with DNA nucleotides of the template • Occurs in the nucleus • Occurs throughout the cell cycle

6. Transcription • Transcription begins at the promoter (a sequence just prior to the actual coding sequence), proceeds through the coding region, and ends at the terminator.

7. Transcription • The enzyme, RNA Polymerase II along with other proteins called GTFs (general transcription factors) together form a Complete Transcription Initiation Complex which binds to the promoter and unwinds the DNA molecule. • The nucleotides pair with each other ensuring that the information in DNA is transcribed correctly to the mRNA. • The RNA nucleotides combine through dehydration reactions catalyzed by RNA polymerase II to form long mRNA segments.

8. Transcription • Upon completion of the transcription mRNA leaves the nucleus and goes to a ribosome • The information on the mRNA is carried in groups of three nucleotides called codons. • Each codon codes for a specific amino acid • For Example: GAU is the codon for Aspartic Acid • There are 64 codons but only 20 amino acids, so the code is redundant.

9. Codons Codons and the Amino Acids they code for:

10. Translation Translation occurs in 3 stages: Initiation Elongation Termination • The mRNA binds to a ribosome • The ribosome consists of rRNA and proteins

11. Translation Initiation The ribosome consists of rRNA (a small and a large subunit) and proteins. • A complex of the initiator tRNA (which has an anticodon), the small ribosomal subunit, and proteins move along the mRNA until if finds the AUG start codon, and then binds to it • The large ribosomal subunit then binds to the small subunit to form the initiation complex

12. Translation Elongation • Attached to the end opposite the anticodon of the tRNA is an amino acid • The initiator tRNA positions itself in the first binding site (P site) • Another tRNA moves into the site after the AUG codon, this is the second binding site (A site) • We now have two tRNA anticodons in the binding sites to the mRNA strand

13. Translation • After the initiator tRNA binds and the following tRNA binds, enzyme action breaks the bond between the amino acid and the tRNA. • Another enzyme action catalyzes the formation of a peptide bond between the first and second amino acid • The initiator tRNA is then released from the ribosome

14. Translation • The ribosome moves along the mRNA strand so that the P site is at the codon after the initiation codon and the A site is moved to the third codon of the mRNA strand so that a tRNA (with an amino acid attached) can be placed into the third site • Enzyme action then breaks the bond between the tRNA and the amino acid of the second site • Enzyme action catalyzes a peptide bond between the amino acids of the second and third site

15. Translation Elongation

16. Termination • The process continues until a stop codon (UAA, UAG, or UGA) is reached • The polypeptide is then released from the tRNA • The tRNA is released from the ribosome and can be reused • The subunits of the ribosome dissociate

17. Effects of Radiation UV Radiation – measured in wavelength Visible light: 400-700nm • Radiation at wavelengths of 290-320 nm, designated as UV-B, causes damage at the molecular level • DNA bases (Adenine, Thymine, Guanine & Cytosine) absorb UV-B radiation, causing a mismatch of pairs. Abnormal bonds can also form between adjacent molecules on the same strand. • Changes in the DNA molecule often mean that transcription cannot be completed and translation will not occur.

18. Effects of Radiation Abnormal bonds formed between adjacent molecules

19. Effects of Radiation X-rays • X-rays are a form of ionizing radiation which collide with molecules and by releasing electrons create ions • The ions break the covalent bonds of the sugar-phosphate backbone of DNA • This causes gross chromosomal mutations • This is also an effective form of cancer treatment

20. Credits • Russell, P. (2006) iGenetics: A Molecular Approach. Pearson: New York. • Seeley, R., Stephens, T., Tate, P. (2003) Anatomy & Physiology: Sixth Edition. McGraw-Hill: New York.

21. Image Credits • http://genome.imim.es/courses/Madrid04/exercises/ensembl/images/Transcription.01.GIF • http://www.fas.org/irp/imint/docs/rst/Sect20/aminoacida_1.GIF • http://www.scq.ubc.ca/wp-content/dna.gif • http://www.phschool.com/science/biology_place/biocoach/transcription/complete.html • http://sps.k12.ar.us/massengale/images/584ELONGATION3.gif • http://earthobservatory.nasa.gov/Library/UVB/Images/dna_mutation.gif