Protein synthesis:

Transcription and translation

Goals

1. To understand how the processes of transcription and translation are necessary for the expression of genes

2. To be able to compare and contrast DNA replication and transcription

3. To understand the importance of the universal genetic "language" of the codons used during translation

Outline:

Gene expression: The DNA in our chromosomes somehow gives rise to our traits

Genetic information is stored in DNA as a sequence of nucleotides

It is information on how to make proteins (all kinds)

The proteins will cause us to have various traits

The flow of information has two steps:

The central dogma: DNA Æ RNA Æ proteins

Step 1: Transcription -- making a working copy of the genetic information

Step 2: Translation -- from the language of nucleic acids to the language of proteins

Transcription

Contrasts between DNA and RNA

• different sugar
• different base (uracil)

The process -- carried on by RNA polymerase

initiation

RNA polymerase recognizes and binds to the promoter sequence

transcription begins when it reaches the initiation site

elongation

RNA polymerization as directed by the template strand of DNA

the entire transcription unit is transcribed

termination

transcription stops at the terminator sequence

Prokaryotes vs. eukaryotes

prokaryotes

no nucleus, mRNA short-lived

eukaryotes

5' cap added (G with 3Ps)

poly-A tail added to 3' end

introns are cut out, exons are spliced together (often by snRNPs)

Translation

mRNA -- the message

The genetic code: triplets of nucleotides (codons) specify amino acids

• nearly universal
• redundant, especially in 3rd position
• start and stop codons
• correct reading frame is vital

tRNA -- the translator

amino acid on one end, anticodon on the other

correct amino acid put on tRNA with corresponding anticodon by aminoacyl-tRNA synthetases

ribosomes -- the site of translation

function:

• facilitates pairing of mRNA codon with correct tRNA anticodon
• catalyses formation of peptide bond

structure:

• made of rRNA and proteins
• has two subunits (large and small)

 

The process

initiation

• small ribosomal subunit binds to 5' leader of mRNA
• methione tRNA bids to start codon
• large ribosomal subunits binds

elongation

• codon recognition and addition of next tRNA
• peptide bond formation -- creates polypeptide
• translocation from S to P site
• repeat

termination

• release factor binds to stop codon
• bond between last tRNA and polypeptide is broken
• ribosomal subunits and mRNA dissociate

Notes

• Polyribosomes can make lots of polypeptides from single mRNA
• Polypeptides are usually modified before final protein is produced

Mutations

3 kinds:

• substitutions
• insertions
• deletions

Results may vary:

• neutral (no difference)
• missense (different amino acids)
• nonsense (early stop or no start

frameshift mutations are very bad

Vocabulary

click here to go to Transcription and translation vocabulary