Genes and how they work!

1. Genes and how they work!

2. Genetic Code • How does the order of nucleotides in DNA encode information to specify the order of amino acids?

3. Genetic Code • Crick 1961 – elucidated the genetic code • Logic used - How many bases (nucleotides) are needed to code for 20 amino acids? • One base can code for 4 amino acids (41) • Two bases can code for 16 amino acids (42) • Three bases can code for 64 amino acids (43) • Therefore a sequence of three bases is the most reasonable number for a coden!

5. 3 bases constitutes a codon (codes for an amino acid) with no space/markers between codons.

6. Codons and their amino acids • Nirenberg – used synthetic mRNA • Eg. UUUUUUU  phenylalanine • Did not take long to determine amino acids and the corresponding 3 nucleotide sequence

8. Codon/amino acid relationship almost universal • e.g. Codon AGA  arginine in Bacteria, Humans and all other organisms • except for Mitochondria and Chloroplasts and a few ciliates • What does this tell you?

9. How does DNA make Proteins? • Central dogma: • DNA  RNA  ProteinTranscription Translation

11. RNA • Ribosomal RNA (rRNA) – made of several RNA molecules and over 50 proteins • Messenger RNA (mRNA) • Transfer RNA (tRNA)‏

12. Transcription (making mRNA) • Promotor – short sequence on DNA template strand where RNA polymerase binds. • Initiation – binding by RNA polymerase and starts unwinding DNA (17 base pairs long) • Elongation – 50 nucleotides added per second, no proof reading by RNA polymerase, therefore errors may occur. • Why is this not a big problem?

16. Transcription (cont’d)‏ • Termination – stop sequences (series of GC forms a GC hairpin, slows down transcription. • Followed by 4 A which attaches 4 U, which are weak bonds, strand disassociation occurs

18. mRNA • mRNA now needs to travel out into cytoplasm • mRNA modified to prevent degradation by nucleases and phosphatases • Terminal 5’ end (usually A or G) is removed and is replaced with an unusual 5’-5’ linkage with GTP forming a 5’ cap. Protects end from degradation by nucleases and phosphotases. • 3’ end contain AAUAAA, poly A polymerase adds about 250 A’s to 3’ end  long A tail. Needed to prevent degradation.

20. Structure of tRNA

23. Translation • Making polypeptides

30. Advantage • In humans 1 to 1.5% of genome is exons • 24% are introns, rest of genome (75%) is non-incoding • Spliceosomes are large proteins that splice the exons together. • Human genes can be spliced together differently by spliceosomes. • Therefore 30,000 genes in humans can encode 120,000 different mRNA’s

31. Differences between Eukaryotic and Prokaryotic cells

32. Differences between Prokaryotes and Eukaryotes • Most eukaryotes posses Introns, Prokaryotes mostly do not! • Eukaryote mRNA contain transcripts of one gene. Prokaryote mRNA transcripts of several genes. • mRNA of eukaryotes must exit nucleus before translation can take place • Prokaryotes – translation starts at AUG codon Eukaryotic,start is also AUG, mRNA has a 5’ cap where translation is initiated. • Eukaryotic mRNA are modified, cap, tail and introns cut out • Eukaryotic rRNA are larger than those of Bacteria