ch 17 (002) transcription, RNA processing, and translation

adding caps and tails to transcripts

-5' cap: as soon as the 5' end of eukaryotic pre-mRNA emerges from RNA polymerase, enzymes add 5'cap structure, which consists of a modified guanine (7-methylguanylate) nucleotide with 3 phosphate groups -poly(A) tail: an enzyme cleaves the 3' end of the pre-mRNA downstream of the poly(A) signal. another enzyme adds a long row of 100-250 adenine nucleotides that are not encoded on the DNA template strand, known as poly(A) tail. -with the addition of the cap and tail and completion of splicing, processing of the pre-mRNA is complete. product is mature mRNA. -caps and tails protect mRNAs from degradation by ribonucleaves (enzymes that degrade RNA) and enhance the efficiency of translation.

RNA processing

-any of the modifications, such as splicing or poly(A) tail addition, needed to convert a primary transcript into a mature RNA. ---experimental mRNAs that have a cap and a tail last longer when they are introduced into cells than do experimental mRNAs that lack a cap or tail ----experimental mRNAs with caps and tails produce more proteins than do experimental mRNAs without caps and tails

RNA processing in eukaryotes summary

-eukaryotic genes consist of exons, which are parts of the primary transcript that remain in mature RNA, and introns, which are regions of the primary transcript that are removed in forming mature RNA. -macromolecular machines, called spliceosomes, splice introns out of pre-mRNAs. -enzymes add a 5' cap and a poly(A) tail to spliced transcripts, producing a mature mRNA that is ready to be translated.

genes-in-pieces hypothesis

-exons: regions of eukaryotic genes that are part of the final mRNA are exons (b/c they are expressed) -introns: sections of primary transcript not in mRNA are introns (b/c they are intervening) ; are not represented in the final product. -because of introns, eukaryotic genes are much larger than their corresponding mature RNAs.

how does an mRNA triplet specify an amino acid

-hypothesis 1: amino acids interact directly with mRNA codons -hypothesis 2: adapter molecules hold amino acids and interact with mRNA codons

RNA processing in eukaryotes

-in bacteria, when transcription terminates, the results is a mature mRNA that's ready to be translated into a protein. __translation often begins while mRNA is still being transcribed. -when eukaryotic genes of any type are transcribed, the initial product is termed a primary transcript. this RNA must undergo multiple processing before it is functional. __for protein-coding genes, the primary transcript is called a pre-mRNA

translation in bacteria and eukaryotes

-multiple ribosomes attach to each mRNA, forming a polyribosome; in this way, many copies of a protein can be produced from a single mRNA. -transcription and translation occur concurrently in bacteria b/c there is no nuclear envelope to separate the 2 processes. -RNA polymerase: bacteria (1), eukaryotes (3, each produces a different class of RNA) -promoter structure: bacteria (typically contains a -35 box & a -10 box), eukaryotes (more variable; often includes a TATA box about -30 from the transcription start site) -proteins involved in recognizing promoter: bacteria (none), eukaryotes (extensive; several processing steps occur in the nucleus before RNA is exported to the cytoplasm: (1) enzyme-catalyzed addition of 5' cap on mRNAs, (2) splicing (intron removal); by spliceosome to produce mRNA, (3) enzyme-catalyzed addition of 3' poly(A) tail on mRNAs

splice process

-snRNPs splice RNA within the nucleus process -1) snRNPs bind to start of intron and an A base within the intron. -process begins when snRNPs bind to the 5' exon-intron boudary, which is marked by the bases GU. and to a ke adenine ribonucleotide (A) near the end the intron. -2) snRNPs assemble to form the spliceosome -once the initial snRNPs are in place, other snRNPs arrive to form a multipart complex spliceosome. spliceosome found in human cells contain about 145 different proteins and RNAs, making them the most complex macromolecular machines known. -3) intron is cut; loop forms. -intron forms a loop plus a single-stranded stem (a lariat) with the adenine at its connecting point. -4) intron is released as a lariat; exons are joined together. -lariat is cut out, and a phosphodiester linkage links the exons on either side, producing a continuous coding sequence--the mRNA -splicing is complete. in most cases, the excised intron is degraded to ribonucleoside monophosphates.

RNA splicing

-the transcription of eukaryotic genes by RNA polymerase generates a primary transcript that contains both exons and introns. ---as transcription proceeds, the introns are removed from the growing RNA strand by a splicing process. --during info processing phase, pieces of the primary transcript are removed and the remaining segments are joined together. --splicing occurs within the nucleus while transcription is still under way and results in an RNA that contains an uninterupted genetic message. ---splicing of primary transcripts are catalyzed by RNAs called small nuclear RNAs (snRNAs) working with a complex of proteins. these protein-plus-RNA macromolecular machines are known as small nuclear ribonucleoproteins, or snRNPs.

introduction to translation

-there is a strong correlation btw the number of ribosomes in a given type of cell and the rate at which that cell synthesizes proteins. ribosomes are site of protein synthesis. -transcription and translation occur simultaneously in bacteria. in bacteria, ribosomes attach to mRNA transcripts and begin translation while RNA polymerase is still transcribing the DNA template strand. -transcription and translation are separated in space and time in eukaryotes