From Gene to Protein Chapter 17

Ch. 17.1 The Flow Of genetic information Proteins are the link between genotype and phenotype Gene expression is the process by which DNA directs the synthesis of proteins Expression of genes that code for proteins includes both TRANSCRIPTION and TRANSLATION DNA transcribed into mRNA translated into tRNA synthesizes a protein which expresses your genes.

One gene – one polypeptide One gene – one enzyme (NO) Because not all proteins are enzymes. One gene – one protein (NO) Because many proteins are constructed from two or more different polypeptides. Example Hemoglobin – the oxygen transporting protein of vertebrate red blood cells, is built from two kinds of polypeptides. So there are two genes that code for this protein. One gene – one polypeptide (YES)….. For now!

Transcription and translation mRNA is the bridge between DNA and protein synthesis DNA – A,T,C, and G (double helix) and RNA – A, U, C, and G (single strand) Nucleic acids and proteins are polymers with specific sequences of monomers. In DNA or RNA the monomers are the 4 types of nucleotides. Each polypeptide of a protein is arranged in a particular linear order (the protein’s primary structure) The monomers are amino acids Nucleic acids and proteins contain information written in two different chemical languages. Getting from DNA to protein requires both transcription and translation.

Transcription and translation Transcription is the synthesis of RNA under the direction of the DNA. DNA is a template to assemble a strand of RNA (just like DNA replication) This type of RNA molecule is mRNA (messengerRNA) because it carries the instructions of DNA to the ribosome where proteins are synthesized. Transcription of a eukaryotic gene results in “pre-mRNA” – primary transcript Transcription of a prokaryotic gene translation begins while transcription is still in progress. There is a change in “language”. The cell must translate the base sequence of mRNA into the amino acid sequence of a polypeptide.

Codons: triplets of bases The flow of information from gene to protein is based on a triplet code. During transcription the gene determines the sequence of nitrogenous bases Template Strand : DNA is transcribed along only one of its double helix strands (for certain genes, the complementary strand is transcribed) mRNA is complimentary, not identical. It is based on the rules of base pairing. T to A, C to G, G to C, BUT A to U (uracil). During translation the sequence of codons is decoded into a sequence of amino acids that make up the polypeptide chain. 300 nucleotides would code for 100 amino acids.

Cracking the code AAA, UUU, GGG, and CCC were determined in the early 1960s. All 64 codes were determined by the mid- 1960s 61 code for amino acids, the other 3 code for ”stop” signals. This marks the end of translation. AUG has a dual function. It codes for methionine and also functions as the “start” or initiation codon. There is redundancy (GAG and GAA both code for glutamic acid) but there is no ambiguity (neither code for any other amino acid).

Evolution of the genetic code The genetic code is shared by all organisms : from the simplest bacteria to the most complex plants and animals. RNA codon CCG is translated to proline in all organisms whose genetic code has been examined. Transformation : because of this common genetic code, one species can be programmed to produce proteins characteristic of a second species by introducing DNA from the second species into the first.

Ch. 17.2 Molecular components of Transcription RNA polymerase pries the two strands of DNA apart and joins the mRNA nucleotides as they base pair along the DNA in the 5 prime to 3 prime direction. (unlike DNA, RNA polymerase does not need a primer). RNA polymerase initiates transcription at a PROMOTER – DNA sequence TATA The stretch of DNA that is transcribed is called the TRANSCRIPTION UNIT The RNA sequence AAUAAA signals the termination of transcription 3 Stages of Transcription: Initiation – RNA polymerase binds to promoter, DNA unwinds, RNA synthesis begins Elongation – polymerase moves “downstream” Termination – transcription is released and the double helix reforms

Ch. 17.3 Modifying RNA after transcription Prior to leaving the nucleus and entering the cytoplasm, both ends of the mRNA are modified… RNA PROCESSING. Some interior sections are cut out and the remaining parts are spliced together. 50-250 adenine nucleotides are added to the termination signal forming a poly – A tail. (AAAAAAAAA)

Split genes and rna splicing Average length of a transcription unit is 27,000 base pairs, but it only takes about 1,200 nucleotides to code for an average protein of 400 amino acids. Introns – non-coding segments between coding regions Exons – all other regions of the sequence will be expressed (EXONS EXIT) snRPS – “snurps” are located in the nucleus and recognize the splicing sites. The RNA in a snRPS is about 150 nucleotides long. Several snRPS form a larger assembly called a spliceosome. Spliceosomes release the introns and splice together the exons