Under$tJncl]ng Inhentance

11+~ ANATOMY OF A EELJKARYC)’TICPROTEIN ~

- (TATA box) Downstream -1 Stop site Start site Upstream Poly A site enhancer ~ ATG r r TAA1 1] Sense 5’ [ II strand Exon Exon Intron Exon Template 3’ I Ill ~ strand

Upstream Downstream region region region

~ach eukaryotic gene is placed in one of stream of the poly A site. Within the poly A The region upstream of the promoter and, three classes according to which of the site are sequences that, when transcribed, less frequently, the downstream region or three eukaryotic RNA polymerases is in- signal the location at which the primary RNA the transcription region itself contain se- volved in its transcription. The for transcript is cleaved and equipped with a quences that control the rate of initiation of RNAs are transcribed by RNA polymerases “tail” composed of a succession of ribo- transcription. Although expression of a pro- I and Ill. The genes for , the class containing the base A. (The tein gene is regulated at a number of stages first brought to mind by the word “gene” and poly A tail is thought to aid the transport of in the pathway from gene to , control the class focused on here, are transcribed messenger RNA from the nucleus of a of replication initiation is the dominant regu- by RNA polymerase II (po/ 11). to the cytoplasm,) Note that the poly A site latory mechanism, (Primary among the other lies downstream of the DNA codon (here regulatory mechanisms is control of splic- Shown above are the components of a TAA) corresponding to one of the RNA ing.) The regulated expression of a gene prototypic protein gene. By convention the codons (UAA) that signals the end of trans- (the when, where, and degree of expres- sense strand of the gene, the strand with the lation of the transcribed RNA. sion) is the key to phenotypic differences sequence of DNA bases corresponding to between the various cells of a multicellular the sequence of RNA bases in the primary Within the transcription region are exons organism and also between organisms that RNA transcript, is depicted with its 5’-to-3’ and . Exons tend to be about 300 possess similar genotypes, direction coincident with the left-to-right di- base pairs long; each is a succession of rection. (Often only the sense strand of a codons uninterrupted by stop codons. ln- Initiation of transcription is controlled mainly gene is displayed.) The left-to-right direc- trons, on the other hand, are not uninterrupted by DNA sequences (cis elements) and by tion thus coincides with the direction in successions of codons, and the RNA seg- certain proteins, many but not all of which which the template strand is transcribed. ments transcribed from introns are spliced are sequence-specific DNA-binding proteins The terms “upstream” and “downstream” out of the primary RNA transcript before (Vans-acting transcription factors). Thus describe the location of one feature of a , A few protein genes contain no both temporal and cellular specificities of gene relative to that of another, Their mean- introns (the human a–interferon gene is an transcription control are governed by the ings in that context are based on regarding example), most contain at least one, and availability of the different tram-acting tran- transcription as a directional process analo- some contain a large number (the human scription factors. Interactions of transcrip- gous to the flow of water in a stream, thyroglobulin gene contains about forty). tion factors with Ciselements and with each Generally the amount of DNA composing other lead to formation of complex protein The start site is the location of the first the introns of a protein gene is far greater assemblies that control the ability of po/ II to deoxyribonucleotide in the template strand than the amount composing its exons. initiate transcription. Most of the complexes that happens to be transcribed, It defines enhance transcription initiation, but some the beginning of the transcription region of Close upstream of the start site is a pro- act as . Enhancers and repres- the gene. Note that the start site lies up- moter sequence, where po/ II binds and sors can be located as far as 10,000 base stream of the DNA codon (ATG) corre- initiates transcription. A common promoter pairs away from the transcription region, sponding to the RNA codon (AUG) that sequence in eukaryotic genes is the so- signals the start of translation of the tran- called TATA box, which has the consensus Class Iand class Ill genes differ from protein scribed RNA, The transcription region ends sequence 5 ‘-TATAAA and is located at a genes not only in their anatomies but also in at sclme nonspecific deoxyribonucleotide variable short distance (about 30 base pairs) the promoters, cis elements, and trarw- between 500 and 2000 base pairs down- upstream of the start site. acting factors involved in their transcription,

64 L~)\,4/~/~~]oTS(icn((, Number 20 19(12