From Gene to Protein Chapter 17

Purpose of Genes Inherited instructions in DNA direct protein synthesis; proteins are the link between genotype and phenotype. Beadle and Tatum (1941) found that mutations in specific genes resulted in specific defective enzymes -- one gene-one enzyme hypothesis. Later restated as the one gene-one polypeptide hypothesis. Polypeptides produced in cells determine an organism’s traits.

RNA Ribonucleic acid (RNA) is the “middle-man” between DNA's genetic instructions and the resultant traits that are expressed. RNA is a single-stranded polymer of nucleotides. Its five-carbon sugar is ribose. Nitrogenous base uracil replaces thymine. Three types: rRNA, mRNA, and tRNA.

Types of RNA Ribosomal RNA (rRNA) – component of ribosomes, sites of protein contruction. Messenger RNA (mRNA) – carries DNA code to from nucleus to ribosome; contains 3-letter codons.

Types of RNA cont. Transfer RNA (tRNA) – brings amino acids to ribosome for addition to polypeptide; contains 3-letter anticodons.

Protein Synthesis Sequence of nucleotides in DNA ultimately determines the sequence of amino acids in a protein. Proteins are made of twenty types of amino acids linked in specific order (primary structure). 1. Transcription -- RNA is made using DNA as a template (in nucleus). 2. Translation – mRNA code is used to construct polypeptides (on ribosomes). Prokaryotes lack nuclei, so DNA is not segregated from ribosomes; transcription and translation occur in rapid succession. In eukaryotes, the processes are separated; mRNA is modified before translation.

Steps in Transcription 1) Polymerase binding: RNA polymerase binds to DNA at regions called promoters (region of DNA where transcription begins; initiation site). RNA polymerase cannot recognize the promoter without the help of transcription factors (proteins that bind to specific DNA nucleotide sequences). TATA box -- short nucleotide sequence on the promoter, rich in thymine (T) and adenine (A), located about 15-25 nucleotides before the initiation site. The two DNA strands separate, and transcription begins, forming mRNA.

Steps in Transcription (cont) 2) Elongation: RNA polymerase moves along DNA and untwists and opens a segment of DNA; one of the exposed DNA strands is the template for base-pairing with RNA nucleotides. RNA grows one nucleotide at a time in the 5' to 3' direction (T-A; A-U; C-G). Several molecules of RNA polymerase can simultaneously transcribe the same gene; production of proteins in large amounts. 3) Termination: Transcription proceeds until RNA polymerase reaches a termination site on the DNA; most commonly AATAAA; mRNA is released. Prokaryotic mRNA is ready for translation as soon as it leaves the DNA template. Eukaryotic mRNA must be processed before it leaves the nucleus.

Transcription Animations http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__mrna_synthesis__transcription___quiz_1_.html http://www.dnalc.org/resources/3d/13-transcription-advanced.html

Steps in Translation During translation, proteins are synthesized according to a genetic message of codons along mRNA. Initiation brings together the mRNA, the first amino acid attached to its tRNA, and the two ribosomal subunits. Small ribosomal subunit binds first to an initiator tRNA with the anti-codon UAC; carries methionine, the first amino acid to be added. Small ribosomal subunit next binds to the 5' end of mRNA; initiator tRNA base pairs with the start codon on mRNA. Then large ribosomal subunit binds to the small one to form a functional ribosome. The initiator tRNA fits into the P site on the ribosome. The vacant A site is ready for the next amino acid/tRNA.

The Code Molecules of tRNA are specific for only one particular amino acid. Each type of tRNA associates a distinct mRNA codon with one of the 20 amino acids used to make proteins. DNA genetic code: AAA mRNA codon: UUU tRNA anticodon: AAA (“phenylalanine”) As tRNAs deposit amino acids in the correct order, ribosomal enzymes link them into a chain.

Translation Animations http://www.biostudio.com/demo_freeman_protein_synthesis.htm http://www.dnalc.org/resources/3d/16-translation-advanced.html

Modifying mRNA after transcription Before eukaryotic mRNA leaves the nucleus, it is processed in two ways: a) both ends are altered, b) sequences are removed and the rest spliced together. Modified guanine is added to the 5’ end (GTP cap). Poly-A tail is added to the 3’ end (about 200 adenine nucleotides). These modifications protect mRNA from breaking down and act as signals in the translation process.

Modifying mRNA after transcription (cont) The original transcript is called heterogeneous nuclear RNA (hnRNA) or pre-mRNA. Some segments on RNA are removed and remaining pieces are linked together during RNA splicing. Introns -- Noncoding sequences in DNA that are transcribed, but not translated. Exons -- Coding sequences of a gene that are transcribed and expressed. Allows cells to make different proteins from same gene. Some introns may be exons in different cells.

RNA Splicing Animation http://www.dnalc.org/resources/3d/index.html