Biology AP - DNA & Protein Synthesis

RNA Polymerase

catalyzes RNA synthesis which pries the DNA strand apart and joins together the RNA nucleotides

RNA is complementary to the DNA template strand (A-T, U-A, C-G, G-C)

does not need any primer

follows the same base pairing rules as DNA except uracil instead of thymine

Promoters

signal transcription start point

usually extend several dozen nucleotide pairs upstream of the start point

Transcription factors

mediate the binding of RNA polymerase and the initiation of transcription

Transcription initiation complex

completed assembly of transcription factors and RNA polymerase II bound to promoter

TATA box

promoter

crucial in forming the initiation complex in eukaryotes

Elongation of the RNA strand

as RNA polymerase moves along the DNA, it untwists the double helix (10-20 bases at a time)

transcription progresses at a rate of 40 nucleotides per second in eukaryotes

a gene can be transcribed simultaneously by several RNA polymerases

nucleotides are added to the 3’ end of the growing RNA molecule

Termination of transcription

termination is different in bacteria and eukaryotes

Bacteria termination

polymerase stops transcription at the end of the terminator

mRNA can be translated without further modification

Eukaryotes termination

RNA polymerase II transcribes the polyadenylation signal sequence

RNA transcript is released 10-35 nucleotides past this polyadenylation sequence

RNA processing

enzymes in the eukaryotic nucleus modify pre-mRNA before the genetic messages are dispatched to the cytoplasm

during RNA processing, both ends of the primary transcript are usually altered

certain interior sections of the molecule are cut out → remaining parts spliced together

mRNA Alteration Ends

each end of a pre-mRNA molecule is modified in a particular way

• 5’ end receives a modified nucleotide 5’ cap

• 3’ end gets a poly-A tail

Function of mRNA Alteration

facilitate the export of mRNA to the cytoplasm

protect mRNA from hydrolytic enzymes

help ribosomes attach to the 5’ end

Introns

noncoding regions

intervening sequences

Exons

eventually expressed

usually translated into amino acid sequences

RNA splicing

removes introns and joins exons → creating an mRNA molecule with a continuous coding sequence

some cases → carried out by spliceosomes

Spliceosomes

consist of a variety of proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites

RNAs catalyze the splicing reaction

Three binding sites on ribosome

P site → holds the tRNA that carries the growing polypeptide chain

A site → holds the tRNA that carries the next amino acid to be added to the chain

E site → exit site where discharged tRNA leave the ribosome