Chemical Biology 03 Oct 13, 2010

Central Dogma -II temporary intermediate (“messenger”)

Translation

cytoplasm

flow of information in ALL cellular organisms = “the Central Dogma” DNA Polymerase copies everything; RNA polymerase does NOT

How does RNA polymerase decide which DNA regions to transcribe?

RNA

gene A

gene B gene DNA

RNA DNA Polymerase copies everything; RNA polymerase does NOT

How does RNA polymerase decide which DNA regions to transcribe?

Answer: DNA sequences serve as sign posts: start and stop

RNA

gene A

gene B gene DNA

RNA terminator sequence sequence DNA sequences dictate: WHERE to start transcription WHERE to stop transcription WHICH strand to use as a template

HOW do they do that? Promoter Sequences and Termination Sequences serve as “Recognition Sequences” for Proteins that collaborate with RNA Polymerase.

Specific DNA sequence = Specific Protein Binding Site example: TATA-Binding Protein: (eukaryotic)

DNA Movie Vault: 7.4 “Transcription Factors”

Accessory proteins associate with the promoter sequence (and other sequences)

RNA polymerase and other proteins join in to form the

“Transcription Initiation Complex”

proper position and orientation of RNA polymerase is determined Transcription Initiation Complex

oriented to move in one direction

given this direction of movement, there is only one possible template strand to choose (the one that will allow movement towards its 5’ end)

RNA grows at its ____ end Movie Vault: 7.2 BIG PICTURE: each gene is demarcated by important DNA sequences that orient RNA polymerase; each gene uses only one template strand

DNA double helix •rate of RNA polymerization is constant (~30 nte /sec)

•BUT frequency of initiation can vary

(what might determine that?) in Eukaryotes

1o transcript (“pre-mRNA”)

Processing of messenger RNA • at the 5’ end • at the 3’end • in the middle

mature mRNA ready for CAP is added to 5’ end and TAIL is added to 3’ end of each mRNA

mRNA 5’ 3’

Necessary for message stability And identification as a messenger RNA DNA

transcripon

protein translaon intervening sequences

Splicing expressed sequences - remove introns exons - rejoin exons

protein translation β Globin primary transcript is 2000 nucleotides long

Splicing

protein translation

β Globin polypeptide is 147 amino acids long Arrangement of exons and introns in two human blood genes The GENETIC CODE Translate from RNA to PROTEIN RNA sequence dictates order of amino acid assembly Triplet code: set of 3 nucleotides (“codon”) encodes 1 amino acid

nucleotides

With 3 “letters” to a word, and 4 different letters to choose from, how many possible “words” would this make?

amino acids The Genetic Code: (codons in 5’->3’) A note about sequence conventions

template strand = “sense” strand

RNA sequence ~ sequence of NONsense strand (=complement of template strand)

so when referring to a gene sequence, we often use the “nonsense” strand sequence of nucleotides