Transcription and Translation

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DNA vs. RNA

DNA RNADeoxyribose sugar (pentose) Ribose sugar (pentose)

Bases: A, T, G, C Bases: A, U, G, C

Double-strand helix Single-strand helix

Very long Relatively short

What is transcription? Since our DNA molecules are too large to

leave the nucleus, a smaller mRNA molecule is formed.

Takes place in the nucleus – 5i to 3i direction

HL Stuff to know Starting point is the “promoter region” Nucleotides are attached by RNA polymerase Covalent bonds between nucleotides Moves along the antisense strand Stops when it reaches the “terminator

sequence”

Introns are removed through splicing and broken down in the nucleus – makes mature mRNA

DNA helicase – RNA polymerase

Codons Triplets of bases on the mRNA strand Some are start codons and stop codons

Translation Takes place in the cytoplasm – 5i to 3i

Ribosomes are the enzymes – small subunit and large subunit

Activation of tRNA The corresponding amino acid binds to the

tRNA at the 3i end Uses tRNA-activating enzyme The code is degenerate – one amino acid may

bind to different types of tRNA

Code:

A = BlueU = RedG = YellowC = Green

Initiation First tRNA molecules binds to the start codon

AUG

Elongation More tRNA bind, polypeptide bonds form between

amino acids

Translocation The tRNA has detached from its amino acid

and is released from the E (exit) site

Termination Ribosome reaches stop codon (no matching tRNA),

mRNA is released and ribosome separates Polypeptide folds into secondary, tertiary structure

Gene = Polypeptide??? Most of the time, yes BUT! – some genes do not code for mRNA that

produces polypeptides Some just control the expression of other genes

Does it matter where the ribosome is?? YES!

Free in the cytoplasm = protein used in the cell Attached to ER = secreted outside the cell

Peptide bond