Protein Synthesis Biology 11 preAP Overview and Application

Try This! 1) One strand of DNA has the following sequence: T A C C G A C G G What is the corresponding sequence of RNA? 2) If this new RNA is transported out of the nucleus directly to a ribosome, which of the three types of RNA is it?

Overview Molecular Biology’s central dogma: replication, transcription, translation, assembly: Howard Hugh’s Medical It is expected that you will know: The key terms gene expression, transcription, and translation. How to explain the process of transcription How eukaryotic cells modify RNA after transcription The steps to translation How point mutations can change the amino acid sequence

Overview What are the stages of Protein Synthesis? 1) Transcription 2) mRNA Processing and Modification 3) Translation

Transcription DNA is used as a template to form a molecule of single- stranded mRNA (messenger RNA) ns/transcription/movie.htm ns/transcription/movie.htm

Transcription Three steps: 1) DNA unzips 2) Complimentary base pairing of ribonucleotides 3) Termination

Transcription – 1) Unzip One section of DNA is unzipped (H-bond between nitrogen bases are broken) when RNA polymerase attaches to the promotor region of the DNA with the assistance of transcription factors. – Transcription Initation Complex The DNA continues to be unzipped by RNA polymerase

Transcription – 2) Complimentary RNA polymerase attatches free-floating ribonucleotides in the 5’-3’ direction – temporarily hydrogen bond with their complimentary nucleotides on the DNA transcription unit – Covalent bond to the 3’ end of the previous ribonucleotide Only one of the DNA strands is used as the template. Ex: DNA – A T C G RNA –

Transcription - Termination RNA polymerase reaches terminator sequence RNA polymerase detaches from DNA, releasing single-stranded RNA strand DNA rewinds into its double helix

mRNA Processing Addition of 5’ cap Addition of poly-A tail RNA splicing – Introns are removes, exons are spliced together by an enzyme called spliceosome This enzyme has small nuclear RNA (snRNA) to help it work and is therefore referred to as a riboenzyme Modified mRNA is transported out of the nucleus

Significance of mRNA splicing… We have fewer than genes to make ~ proteins! – One gene can make different proteins due to alternative mRNA splicing.

Translation mRNA goes to the ribosome and is used as a template to synthesize a protein molecule. ns/translation/movie.htm ns/translation/movie.htm

Translation Three Steps: 1) Initiation 2) Elongation 3) Termination

Translation – 1) Initiation need start codon AUG The first tRNA, with anticodon UAC and amino acid methionine temporarily binds to the first site

Translation – 2) Elongation The second tRNA molecule attaches to the second site of translation.

Translation – 2) Elongation The amino acid from the first tRNA is transferred to the amino acid on the second tRNA.

Translation – 2) Elongation The first tRNA exits, the ribosome moves, a new tRNA enters, and the process is repeated.

Translation – 2) Elongation The process is repeated many times, and a peptide, or strand of amino acids, is formed.

Translation – 3) Termination The release factor enters. Translation, or protein synthesis, ends.

Translation – 3) Termination The completed polypeptide is released.

Translation - Application What does this diagram represent? - Translation - ELONGATION

Polyribosomes Many ribosomes transcribing the same mRNA Very efficient!

Destination of Proteins How does a protein “know” whether it to be used for a function in the cell, or bound for secretion? – Signal peptide: the first ~20 amino acids directs the final destination

Application – table of mRNA codons (p 511)

Application Consider the following DNA base sequence read from left to right: T A C G C A A A A T G G Determine the amino acid sequence that results from this DNA strand.

Application DNA sequence: T A C G C A A A A T G G mRNA sequence: A U G C G U U U U A C C Animo acid sequence: methionine – arginine – phenylalanine – threonine

Application Which of the following is an anticodon of a molecule of tRNA carrying glycine? A. G G A B. C C U C. C C T D. C G C

Application Which of the following is an anticodon of a molecule of tRNA carrying glycine? Amino acid: glycine mRNA codon: GGU/GGC/GGA/GGG tRNA anticodon: CCA/CCG/CCU/CCC

Application Which of the following is an anticodon of a molecule of tRNA carrying glycine? A. G G A B. C C U < ANSWER! C. C C T D. C G C

Mutation Definitions point mutation: – A change in the sequence of a base pair within a gene 1) Base-pair substitution A. Missense mutation – still amino acid B. Nonsense Mutation – code for stop 2) Insertion and deletion – can cause frameshift mutation!

Definitions – Gene Mutations frameshift mutations THE CAT ATE THE RAT – what if the “C” was deleted? – The original DNA coded for which amino acids? DNA: TAC GGT TAG mRNA: AUG CCA AUC amino acids: methionine – proline - isoleucine – The mutated DNA now codes for which amino acids? DNA: TAC CGG TTA G mRNA: AUG GCC AAU C animo acids: methionine – alanine - asparagine

Definitions - Gene Mutation What effect does changing the amino acids have on the protein product? – Changes the shape. Since shape determines function, it alters how the protein will function.

Proteins: Shape determines Function!

Definitions - Gene Mutation Which has the potential for the most damage, deletions or subsitituations? – Deletions! Frameshift

Definitions Mutagens – Environmental influences that cause mutations – Examples: Radiaton: radioactive elements, ultraviolet (UV) light, X rays Organic chemicals: cigarette smoke, pesticides

Any Questions?

Closing Molecular Biology’s central dogma: replication, transcription, translation, assembly: Howard Hugh’s Medical Epic: