Information Transfer and Protein Synthesis

“Central Dogma”DNA

mRNA

Protein(polypeptide)

• DNA stores/transmits a “code”

• 4 nitrogen bases A, T, C, G

• 20 essential amino acids

Molecular Code

A colored scanning electron micrograph ofa group of human chromosomes (x6,875)

Transcription

Transcription is the process by which one strand of the DNA is copied to form a

complimentary strand of messenger RNA (mRNA)

Transcription

Transcription

initiation

elongation

termination

initiation

• RNA polymerase binds to the DNA at the promoter region

• DNA unwinds exposing the coding strand

elongation

• DNA acts as a template (pattern) for the mRNA

• RNA polymerase moves along DNA elongating (making longer) the mRNA strand

termination

• RNA polymerase reaches the terminator region on the DNA

• RNA is release RNA polymerase falls off

A transmission electron micrographof an unidentified operon of thebacterium Escherichia coli, x72,600.Ribosomes attach to mRNA, andprotein synthesis begins even beforetranscription is complete.

RNA Processing

• the primary mRNA transcript may be 200,000 nucleotides in length

• mRNA in the cytoplasm is only a few 1000 nucleotides in length

• mRNA is modified (processed) before leaving the nucleus

Enzymes attach a cap of chemically modified guanine

nucleotides (methyl-guanine, or mG) to the starting end of

the mRNA molecule

Other enzymes then replace part of the

opposite end with a tail of 100–200 adenine

nucleotides called a poly-A tail

The final step in mRNA processing involves

removal of some internal segments of the RNA that do not code for protein

called introns

The parts of the transcript that remain (and code for protein) are called exons. They are joined together in a process called splicing.