genes and how they work!. genetic code how does the order of nucleotides in dna encode information...
TRANSCRIPT
Genes and how they work!
Genetic Code
• How does the order of nucleotides in DNA encode information to specify the order of amino acids?
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!
• 3 bases constitutes a codon (codes for an amino acid) with no space/markers between codons.
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
• 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?
How does DNA make Proteins?
• Central dogma:
• DNA RNA Protein Transcription Translation
RNA
• Ribosomal RNA (rRNA) – made of several RNA molecules and over 50 proteins
• Messenger RNA (mRNA)
• Transfer RNA (tRNA)
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?
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
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.
Structure of tRNA
Translation
• Making polypeptides
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
Differences between Eukaryotic and Prokaryotic cells
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