Transcription and Translation

From DNA to RNA to protein

Overview

• Genes in DNA contain information to make proteins.

• The cell makes mRNA copies of genes that are needed.

• The mRNA is read at the ribosomes in the rough ER.

• Protein is produced.

Key Players

• mRNA carries the information from a gene in DNA.

• Ribosomes, made of rRNA, consist of subunits and carry out an enzyme-like role.

• tRNA carries specific amino acids to the ribosome.

Transcription

RNA polymerase is the enzyme responsible for making mRNA copies of genes. DNA unzips at the

site of the gene that is needed.

Transcription

RNA polymerase matches bases in the sense strand with RNA bases, building a strand of mRNA that

carries the information encoded in the DNA.

Transcription

Encoded in DNA is a signal telling RNA polymerase where to stop. Transcription ends at that point.

Transcription

The completed mRNA molecule then moves from the nucleus to the rough ER for translation.

Translation

• Initiation begins with a tRNA bearing methionine (met) attaching to one of the ribosomal units. The codon for methionine is a universal “start” codon for “reading” the mRNA strand.

Translation

• The ribosomal unit binds to mRNA where the code for met is located (AUG). The anticodon (UAC) of the tRNA matches the “start” codon on mRNA (AUG).

Translation

• The larger ribosomal subunit now binds to the smaller unit, forming a ribosomal complex. The tRNA binds to the first active site on the ribosome. Translation may now begin.

Translation

• The second codon in mRNA (GUU) matches the anticodon of a tRNA carrying the amino acid valine (CAA). The second tRNA binds to the second active site on the large subunit.

Translation

• A catalytic site on the larger subunit binds the two amino acids together using dehydration synthesis, forming a peptide bond between them.

Translation

• The first tRNA now detaches and goes of to find another met in the cytoplasm. The mRNA chain shifts over one codon, placing the second codon (CAU) over the second active site.

Translation

• A tRNA with an anticodon (GUA) matching the exposed codon (CAU) moves onto the ribosome. This tRNA carries histidine (his).

Translation

• A new peptide bond forms between val and his on the catalytic site. The tRNA that carried val will detach and find another val in the cytoplasm. The mRNA strand will then shift over one more codon.

Translation

• The process continues until the ribosome finds a “stop” codon. The subunits detach from one another, the mRNA is released, and the polypeptide chain moves down the ER for further processing. The initial met is removed and the chain is folded into its final shape.

Summary

Transcription Animation

• http://learn.genetics.utah.edu/units/basics/transcribe/

Mutations

• Any mistakes in the DNA code can result in a “broken” (non-functional) protein.

• A mutation affecting only a few somatic cells (body cells) might not have any effect, unless the mutation turns the cell cancerous.

• A mutation affecting a sex cell can be passed on to the offspring.

Types of Mutations

• Point mutation: base substitution that may or may not code for a different amino acid.

• Insertion mutation: one or more bases is inserted into the DNA strand.

• Deletion: one or more bases is deleted from the DNA strand.

Effects of mutations

• Original: THE FAT CAT SAT

• Point mutation: THA FAT CAT SAT

• Insertion: THE FTT ATC ATS AT

• Deletion: THE FTC ATS AT

Hemoglobin mutations

DNA mRNA Amino AcidProperties of

AAEffect on protein

Disease

Original codon 6

CTC GAG Glutamic Acid Hydrophilic Normal None

Mutation 1 CTT GAA Glutamic Acid Hydrophilic Neutral None

Mutation 2 GTC CAG Glutamine Hydrophilic Neutral None

Mutation 3 CAC GUG Valine HydrophobicLoses water

solubilitySickle Cell

Anemia