Genes and how they work!. Genetic Code How does the order of nucleotides in DNA encode information...

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