RNA, & Protein Synthesis 7th Grade, Week 4, Day 1 Monday, July 15, 2013

The Central Dogma

RNA vs. DNA Ask them if they can point out some of the differences already?

Ribonucleic Acid RNA is required for translation of genetic information stored in DNA into protein products Transcribed from DNA Contains ribose sugar instead of deoxyribose Single stranded instead of double stranded Uracil instead of thymine

Types of RNA Precursor mRNA (pre-mRNA): an immature form of messenger RNA that contains introns and exons Messenger RNA (mRNA): contains only exons that form the code for the sequence of amino acids that makes up a protein Transfer RNA (tRNA): decodes the message contained in mRNA and allows for the synthesis of proteins Ribosomal RNA (rRNA): forms part of the structure of ribosome

Transcription Making RNA from DNA Occurs in the nucleus RNA Polymerase synthesizes pre-mRNA from DNA in the 5’  3’ direction Happens in much the same way that DNA polymerase synthesizes new strands of DNA during replication

Transcription Template (antisense) strand is transcribed, while the other (sense) strand remains inactive Uracil bonds with Adenosine in place of Thymine Initial pre-mRNA made contains both exons and introns So which strand is going to be identical to the new pre-mRNA? The non-template strand (which is why it’s also called the sense strand).

3 Steps of Transcription Initiation: occurs when the RNA polymerase binds to promoter and forms the transcription bubble Elongation: the RNA chain is lengthened by the addition of bases in the 5’ to 3’ direction Termination: RNA polymerase runs into a termination region.

Splicing In order for the pre-mRNA to leave the nucleus and travel to the ribosomes in the cytoplasm for translation, it must be made into mature mRNA This happens by removing the introns so that only exons are left An enzyme complex called a spliceosome performs this task

Alternative Splicing Not all exons are always left – sometimes they get spliced out as well Therefore, the same pre-mRNA can be spliced differently to get different gene products (proteins) and create diversity

RNA Processing The cap: is a modified guanine (G) Addition of a poly-adenosine (poly-A) tail and a 5’ cap before the mRNA can exit the nucleus and move into the cytoplasm The cap: is a modified guanine (G) protects the RNA from being degraded by enzymes that degrade RNA from the 5′ end; serves as an assembly point for the proteins needed to recruit the small subunit of the ribosome to begin translation. Don’t worry about the un-translated regions.

Proteins DNA acts like a blueprint that determines the structure of every protein made in your body Every protein is made up of amino acids There are 20 amino acids: Essential: must be supplied in the diet Non – essential: synthesized de novo We obtain most of our amino acids by digesting proteins taken in with our food. The digestive process breaks the protein chains down into individual amino acid molecules which are then absorbed by the blood and transported to the individual body cells.

Proteins During protein synthesis, the separate amino acids are reassembled into new chains. Each kind of protein has its own particular sequence of amino acids, which differs from the sequence in every other kind of protein. Just the way the order of letters in a word give it its own specific form and meaning, it is the order of the amino acids in the chain that determines the protein's structure and function. The code for ordering the amino acids of a protein is written as a sequence of bases in the DNA in the nucleus.

The Genetic Code Triplet code: codons are made of 3 nucleotide bases which are non-overlapping The system is redundant – amino acids are encoded by more than one codon Practice – Translate: 5’ – AUG ACU AAU GCU UAA – 3’ Answer: Met Thr Asn Ala Stop

Translation mRNA is translated into amino acids using the genetic code, which are then assembled into a protein This process takes place in the cytoplasm on ribosomes Both mRNA and tRNA are necessary for this process

Steps in Translation Ribosomes bind mature mRNA There are about 32 different tRNA molecules Each tRNA molecule has an anticodon that is complementary to a codon on mRNA coding for a certain amino acid (so most amino acids have more than one tRNA that will code for them) The tRNA will then retrieve that amino acid and bring it to the ribosome for protein assembly

Ribosomes Made up of rRNA Composed of two subunits – one large and one small Ribosomes can be free in the cytosol or membrane bound to the endoplasmic reticulum (ER) called the "rough ER“ Where the process of protein assembly is carried out

Amino acid U A C tRNA molecule A transfer RNA molecule arrives. It brings an amino acid to the first three bases (codon) on the mRNA. anticodon The three unpaired bases (anticodon) on the tRNA link up with the codon. A U G G G C U U A A A G C A G U G C A C G U U

U A C C C G Another tRNA molecule comes into place, bringing a second amino acid. Its anticodon links up with the second codon on the mRNA. A U G G G C U U A A A G C A G U G C A C G U U

U A C C C G Peptide bond A peptide bond forms between the two amino acids. A U G G G C U U A A A G C A G U G C A C G U U

U A C C C G The first tRNA molecule releases its amino acid and moves off into the cytoplasm. A U G G G C U U A A A G C A G U G C A C G U U

C C G The ribosome moves along the mRNA to the next codon. A U G G G C U U A A A G C A G U G C A C G U U

C C G A A U Another tRNA molecule brings the next amino acid into place. A U G G G C U U A A A G C A G U G C A C G U U

C C G C C G A peptide bond joins the second and third amino acids to form a polypeptide chain. A U G G G C U U A A A G C A G U G C A C G U U

A C G G U C The process continues. The polypeptide chain gets longer. This continues until a termination (stop) codon is reached. The polypeptide is then complete. A U G G G C U U A A A G C A G U G C A C G U U

Rules of Translation The start codon is AUG which codes for methionine What does this mean about the first amino acid in every protein? Chain elongation continues on the ribosome as it reads the mRNA strand Translation continues until a stop codon is reached The stop codons, unlike the start codon, do not encode amino acids Completed protein is then folded with help from chaperones

But what happens when this process goes wrong?

Mutations Frame shift mutation: adding or deleting one base causes a change in the reading frame…why?

Missense mutation: a base change that results in substituting one amino acid for another

Nonsense mutation: a base change that results in substituting a stop codon in place of an amino acid This results in early termination of the protein

Silent mutation: a base change that results in no change in the encoded amino acid (or stop codon) Why could this happen? Are these still dangerous? Why or why not? REDUNDANCY! Not really – still makes the same exact protein product