Protein Synthesis

The Central Dogma

the flow of genetic info from DNA (via replication) → RNA (via transcription) → proteins (via translation)

Protein synthesis is

the process of translating genetic information into functional proteins through transcription and translation

begins in the nucleus & completes in the cytoplasam

1. Transcription

2. DNA Splicing/DNA Processing

3. Translation

Transcription

first step of protein synthesis: process of copying a segment of DNA into COMPLEMENTARY (opposite) mRNA, which occurs in the nucleus

initiation

1st of transcription: RNA polymerase attaches to the prompter of a gene unwinding the double helix

• a transcription bubble: is formed

• prompter: mRNA start codon, AUG. methionine

elongation

step 2 of transcription: RNA polymerase reads the DNA to synthesize a complementary RNA copy from 5’→3’

termination

when RNA polymerase/prompter reach the termination site the mRNA is complete

DNA splicing/DNA processing

2nd step of protein synthesis: the mRNA copy from transcription is processed so its ready to leave the nucleus

• mRNA has exons & introns that are not needed for protein synthesis → spliceosomes remove introns

• 5’ Guanine cap added to allow mRNA to exit nucleus

• 3’ Poly(A) tail added to allow mRNA to exit nucleus

exons & introns

segments of mRNA → exons are needed codes, introns are not needed

spliceosomes

remove the introns in loops (excised introns) to splice the exons together

• spliceosome component: snRNPs + other proteins

• snRNP: small nuclear ribonucleoprotein

  • snRNA: small nuclear ribonucleic acid

  • protein

5’ end of mRNA

receives a Guanine cap to protect it and facilitate export from the nucleus.

3’ end of mRNA

receives a Poly(A) cap to protect it and facilitate export from the nucleus.

Translation

3rd step of protein synthesis: uses mRNA copy as blueprint for amino acid string/polypeptide (tRNA is attracted bc OPPOSITE& hydrogen bonds)→ protein

step 1 of translation

ribosomes attach to the AUG/start codon of the mRNA

step 2 of translation

Codon Recognition: A site (Aminoacyl-tRNA binding site)

• ribosomes reads mRNA in groups of 3 bases (codons) & binds complementary anti-codon/tRNA to mRNA

  • uses energy: 2GTP → 2GDP

step 3 of translation

Peptide bond formation: P site (Peptidyl-tRNA binding site)

• ribosome forms peptide bonds between amino acids at end of tRNA to growing polypeptide chain

step 4 of translation

Translocation: E site (Exit site)

• ribosome moves down the mRNA, shifting the tRNA to the E site (exits), so the ribosome is ready for the next tRNA/codon

• uses energy: 1 GTP → GDP

end of translation

when ribosome reads stop codon it detaches from the mRNA

• polypeptide chain folds into final protein form!

translation stop codons

UGA, UAG, UAA

codon

series of 3 bases which code for 1 amino acid

• Transcription & Translation start @ 3’ so it synthesizes 5’→3’

messenger RNA

mRNA: carries/contains genetic code from DNA from nucleus → cytoplasm

• synthesized in nucleus via transcription

• unstable

ribosomal RNA

rRNA: part of ribosome that ensures proper placement of mRNA & tRNA

• synthesized in nucleus, exists in ribosome, assist in translation

• stable

transfer RNA

tRNA: brings amino acids to ribosome during protein synthesis, matching its anticodon end to complementary mRNA codons

• synthesized in nucleus, exists in cytoplasm

• stable: hydrogen bonding RNA (sometimes double helix)

  • folds in on self

mRNA strand → corresponding DNA template