Transcript Genes and How they work!

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 nonincoding
• 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