Transcription and Translation
DNA vs. RNA
Deoxyribose 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
First tRNA molecules binds to the start codon AUG
More tRNA bind, polypeptide bonds form between amino
The tRNA has detached from its amino acid and is
released from the E (exit) site
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
Some just control the expression of other genes
Does it matter where the ribosome is??
Free in the cytoplasm = protein used in the cell
Attached to ER = secreted outside the cell