Protein Synthesis Chapter 10

From DNA to RNA Single strand Contains ribose sugar Double helix Contains deoxyribose sugar Contains the bases: Guanine Cytosine Adenine Thymine Single strand Contains ribose sugar Contains the bases: Guanine Cytosine Adenine Uracil

Three Types of RNA Messenger RNA (mRNA) Transfer RNA (tRNA) Copies the DNA nucleotides from a gene and carries this “message” to the ribosomes Transfer RNA (tRNA) Transports amino acids to the mRNA once it is attached to the ribosomes Ribosomal RNA (rRNA) Used to manufacture ribosomes Contains enzymes for forming peptide bonds between the amino acids brought by the tRNA to the ribosomes

Protein Synthesis Protein synthesis is also known as “gene expression” This is because DNA contains nucleotide bases in a specific order that serves as a recipe for putting amino acids together in the correct sequence for each individual A segment of DNA that tells the cell how to make a polypeptide (small protein) is a gene. The function of a protein is controlled from the beginning by the order of the amino acids that it contains.

Protein Synthesis Protein synthesis is divided into two groups of reactions: Transcription The process by which the mRNA copies the nucleotide sequence of one gene on the DNA Translation The process by which tRNA delivers amino acids to the mRNA attached to a ribosome in order to assemble the protein (polypeptide)

Protein Synthesis: Transcription Step 1 RNA polymerase binds to a promoter, a specific sequence on the DNA that marks the beginning of a gene The promoter acts as a “start” signal for transcription Step 2 The enzyme, RNA polymerase, unwinds the DNA and breaks the hydrogen bonds between the base pairs of the gene to be copied

Transcription continued Step 3 RNA polymerase adds mRNA nucleotides in a 5’  3’ direction along the DNA according to the base-pairing rules Exception: mRNA contains no thymine; therefore, uracil from mRNA bonds to the adenine in DNA RNA polymerase reaches a sequence at the end of the gene that signals it to stop transcribing the DNA The mRNA is now released from the DNA The DNA goes back together and rewinds

Transcription vs. Replication Only one side of DNA is transcribed Purpose is to copy a gene in order to make a protein Accomplished by RNA polymerase Both sides of DNA are replicated Purpose is to make a complete copy of the DNA for cell division Accomplished by DNA polymerase and others

mRNA Processing Before mRNA leaves the nucleus, it must be protected from enzymes in the cytosol that will destroy it A small string of nucleotides that all contain the base guanine is attached to the 5’ end This is known as the G-cap A long string of nucleotides that all contain the base adenine is attached to the 3’ end This is known as the poly-A tail

mRNA Processing In addition to protecting the mRNA from enzymatic destruction, the G-cap and poly-A tail also help to: Pull the mRNA transcript out of the nucleus & Attach the mRNA transcript to a ribosome

mRNA Processing Not all of our DNA contains codes for amino acids Some of our DNA contains sequences known as introns (interfering sequences) that are not part of the gene These introns are unique to each person There are enzymes that must cut out the introns before the mRNA leaves the nucleus These enzymes that remove the introns from the DNA form a spliceosome

mRNA Processing: Spliceosome http://www.phschool.com/science/biology_place/biocoach/images/transcription/eusplice.gif

mRNA Processing The remaining mRNA transcript consists of exons Exons are the portions of the mRNA that must be EXPRESSED (translated into amino acids) When the introns are removed, the exons are spliced together to make a complete mRNA transcript of the gene that is to be translated Once the introns are removed and the exons are spliced together, the mRNA is ready to leave the nucleus

mRNA Processing http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pre-mRNA.gif