Protein Synthesis

DNA  RNA  Protein

2 Major Steps Transcription – DNA is transcribed (copied!) into single stranded mRNA (DNA code transcribed into RNA code) Translation – mRNA is translated into protein (amino acids)

Transcription Occurs in nucleus (where the DNA is located!) Generally divided into three steps – initiation, elongation, termination

A. Initiation RNA polymerase (enzyme) attaches to promoter regions of DNA at TATA box RNAP requires TATA box and other transcription factors to be present

B. Elongation RNAP can only add nucleotides to the 3’ of the growing mRNA strand; thus mRNA is synthesized in the 5’ 3’ direction

C. Termination Transcription ends when mRNA polymerase reaches a specific STOP sequence: ATT ATC ACT

mRNA processing After transcription, mRNA must be modified so that it can exit the nucleus 1. GTP cap added to 5’ end of mRNA (stability & attachment point for ribosome down the road) 2. Poly-A tail added to 3’ end of mRNA (stability & guidance so mRNA goes from nucleus to ribosome) 3. Removal of introns

Introns vs. Exons Exons – sequences that contain the code for a polypeptide (protein); exons are expressed Introns – non-coding sequences of mRNA

Original, unprocessed mRNA contains both introns and exons Before mRNA exits nucleus for translation at ribosome, snRNPs (small nuclear ribonucleoproteins) remove introns and splice exons together

Translation Occurs in cytoplasm where processed mRNA meets ribosome and is translated into protein Steps: Initiation Elongation Termination

For translation to occur, you need… Activated tRNA -tRNA in cytoplasm finds the correct amino acid -Attachment (activation) requires 1 ATP and enzyme

Ribosomes consist of 2 subunits (small and large) 2. Ribosomal subunits Ribosomes consist of 2 subunits (small and large) Each subunit is made of rRNA and protein Each is built separately in nucleolus The two subunits exit nucleolus and form the ribosome in the cytoplasm when translation starts

The Genetic Code Codon – triplets of bases mRNA makes a template strand (from DNA) that is translated into protein via a triplet code Every three base pairs codes for a particular aa

1. Initiation Brings together mRNA (from nucleus), tRNA (in cytoplasm), and ribosomal subunits (from nucleolus) Small ribosomal subunit binds to mRNA and a specific tRNA (methionine) Small subunit scans mRNA until it finds AUG (start codon) Large subunit attaches the complex

2. Elongation Begins with the next tRNA arriving at the P site of the ribosome

Polyribosome

3. Termination Occurs when the ribosome encounters one of three STOP codons located on mRNA At this point, polypeptide synthesis is complete and the ribosome detaches from the polypeptide

Point Mutations Chemical changes in just one base pair of a gene 2 general types: Base pair substitutions Base pair insertions or deletions

1. Base pair substitutions Substitution of one base pair with another Often silent mutations because they don’t have an effect on encoded protein due to redundancy of genetic code: DNA: CCG  CCA mRNA: GGC  GGU AA: gly  gly

Example: Sickle Cell Anemia Results from a single point mutation in a gene that codes for one of the polypeptide chains that form hemoglobin Fatigue, paleness, rapid heart rate, shortness of breath, jaundice

2. Insertions & Deletions Additions or deletions of nucleotide base pairs BAD NEWS… these are often called frameshift mutations because they literally shift the reading frame of the mRNA  protein

Example: Tay-Sachs Disease Inherited frameshift mutation on chromosome 15 Mutation results in malformation of hexoaminidase A, a protein that breaks down a particular chemical in gangliosides (nerve tissue) Deafness, blindness, dementia, paralysis, slow growth, mental retardation etc

Mutagens Spontaneous mutations – errors that result from cellular machinery malfunction Environmental mutations – damage to DNA caused by environment Mutagens – chemical agents that interact with DNA and cause problems X-rays, UV light, various drugs (seriously)