Chapter 7

Question 1

How Cells Read the Genome

Answer 1

• even before DNA code was uncovered, we knew that the info contained in genes directs the synthesis of proteins
  
  • proteins determine cell structure and function
  • each type of protein has its own unique amino acid sequence, which dictates how the chain will fold to form a molecule with a distinctive shape and chemistry
  • The genetic instructions carried by DNA must therefore specify the amino acid sequences of proteins

Question 2

gene

Answer 2

• When a particular protein is needed by the cell, the nucleotide sequence of the appropriate segment of a DNA molecule is first copied into another type of nucleic acid—RNA (ribonucleic acid)
  
  • the segment of DNA is the gene
• resulting RNA copies are used to direct synthesis of the protein
• genetic info in cells: DNA to RNA to protein
  
  • occurs in ALL cells(central dogma)

Question 3

transcription and translation

Answer 3

• how cells read out/express the instructions in their genes
• many identical RNA copies come from the same gene
• each RNA molec can direct the synthesis of many identical protein molecs
  
  • cells can rapidly synthesize large amts of protein
• BUT each gene can be transcribed, and its RNA translated, at different rates, providing the cell with a way to make vast quantities of some proteins and tiny quantities of others
• a cell can change (or regulate) the expression of each of its genes according to the needs of the moment

Question 4

transcription

Answer 4

• The first step a cell takes in expressing one of its many thousands of genes is to copy the nucleotide sequence of that gene into RNA
  
  • called transcription bc language(nucleotides) is same
• Steps:
  
  • small portion of DNA double helix opens and unwindes to expose bases on each strand
  • one of 2 strands is template for RNA synthesis
  • Ribonucleotides are added, one by one, to the growing RNA chain(complementary base pairing)
  • enzyme RNA polymerase links ribonucleotides to RNA chain using covalent bonds
  • RNA chain produced by transcription (RNA transcript)
    
    • exactly complementary to DNA strand

Question 5

RNA

Answer 5

• a linear polymer made of four different nucleotide subunits, linked together by phosphodiester bonds.
• all RNA is made transcription

Question 6

DNA vs RNA

Answer 6

1. the nucleotides in RNA are ribonulcleotides(they contain ribose) while DNA contains deoxyribose
2. DNA has bases ACTG; RNA has ACUG
3. DNA is double stranded(can’t fold), RNA single stranded(can fold into shapes like polypeptide chains fold up to form final protein shapes)
   
   1. SO, DNA functions as an info store, but some RNAs have structural, regulatory, or catalytic roles

*Both have their nucleotides linked by a phosphodiester bond

Question 7

DNA replication vs transcription

Answer 7

• newly formed DNA strands remain H-bonded to DNA template strand, but just behind where the ribonucleotides are added, the RNA chain is displaced and double helix re-forms
  
  • this is why RNA is single stranded
  • RNA are copied from a limited region of DNA so RNA is much shorter than DNA
    
    • DNA in human chromosomes is up to 250 million nucleotide pairs long, RNA is roughly a couple thousand nucleotides or less

Question 8

RNA polymerases

Answer 8

• catalyze the formation of the phosphodiester bonds that link the nucleotides together and form the sugar-phosphate backbone of the RNA chain
• The RNA polymerase moves stepwise along the DNA, unwinding the DNA helix just ahead to expose a new region of the template strand for complementary base-pairing
• RNA chain is extended one nucleotide at a time from 5’ to 3’
• The incoming ribonucleoside triphosphates (ATP, CTP, UTP, and GTP) provide the energy needed to drive the reaction forward
• the synthesis of the next RNA is usually started before the first RNA has been completed b/c RNA is released from DNA almost immediately

Question 9

DNA polymerase vs. RNA polymerase

Answer 9

1. RNA polymerase uses ribonucleoside for phosphates as substrates, so it catalyzes the linkage of ribonucleotides, not deoxyribonucleotides
2. unlike DNA polymerase, RNA polymerases can start an RNA chain w/o a primer
   
   1. This difference likely evolved because transcription need not be as accurate as DNA replication; unlike DNA, RNA is not used as the permanent storage form of genetic information in cells, so mistakes in RNA transcripts have relatively minor consequences for a cell
   2. RNA polymerase makes one mistake every 10⁴ nucleotides copied into RNA, while DNA makes only one mistake for every 10⁷ nucleotides copied

Question 10

message RNA(mRNA)

Answer 10

• The vast majority of genes carried in a cell’s DNA specify the amino acid sequences of proteins.
• The RNA molecules encoded by these genes—which ultimately direct the synthesis of proteins
  
  • In eukaryotes, each mRNA typically carries information transcribed from just one gene, which codes for a single protein
  • in bacteria, a set of adjacent genes is often transcribed as a single mRNA, which therefore carries the information for several different proteins.

Question 11

non messeneger RNA

Answer 11

• the final product of other genes is RNA itself
• these non messenger RNAs, like proteins, have various roles: reulatory, structural, and catalytic cell components
• they play ket parts in translating genetic message into protein
  
  • ribosomal RNA(rRNA) form the structural and catalytic core of the ribosomesm which translate mRNAs into protein
  • transfer RNA(tRNA) act as adaptors that select specific amino acids and hold them in place on a ribosome for their incorporation into protein
  • microRNAs(miRNA) serve as kept regulators of eukaryotic gene expression

Question 12

gene expression

Answer 12

• the process by which the information encoded in a DNA sequence is translated into a product that has some effect on a cell or organism.
• When final product of the gene is a protein, gene expression includes both transcription and translation.
• When an RNA molecule is the gene’s final product, however, gene expression only requires transcription.

Question 13

signals in DNA tell rNA polymerase Where to start and finish transcription

Answer 13

• is the main point at which the cell selects which proteins or RNAs are to be produced. To begin transcription, RNA polymerase must be able to recognize the start of a gene and bind firmly to the DNA at this site
• In Bacteria:
  
  • When an RNA polymerase collides randomly with a DNA molecule, the enzyme sticks weakly to the double helix and then slides rapidly along its lengt
  • RNA polymerase latches on tightly only after it has encountered a gene region called a promoter(specific sequence of nucleotides that lies immediately upstream of the starting point for RNA synthesis)
  • RNAp opens up double helix in front of promoter
  • one strand is template for complementary base pairing with incoming ribonucleoside triphosphates, two of which are joined together by the p to begin synthesis of RNA chain
  • elongation continues until enzyme encounters the terminator(stop site)(terminator sequence is contained within the gene and is transcriber into the 3’ end of new RNA)

Question 14

sigma(σ)factor

Answer 14

• used in bacteria
• a subunit of RNAp that recognizes promoter on DNA
• It turns out that each base presents unique features to the outside of the double helix, allowing the sigma factor to find the promoter sequence without having to separate the entwined DNA strands

Question 15

Choosing which of the 2 DNA strands is template

Answer 15

• each strand has a different nucleotide sequence and would produce a different RNA transcript
• Every promoter has a certain polarity: it contains two different nucleotide sequences upstream of the transcriptional start site that position the RNA polymerase, ensuring that it binds to the promoter in only one orientation
• Because the polymerase can only synthesize RNA in the 5′-to-3′ direction once the enzyme is bound it must use the DNA strand oriented in the 3′-to-5′ direction as its template
  
  • on a given chromosome, transcription DOESN’T always proceeds in the same direction. With respect to the chromosome as a whole, the direction of transcription varies from gene to gene

Question 16

how Eukaryotic gene transcription differs from bacterial

Answer 16

• bacteria has 1 type of RNAp, eukaryotic has 3: RNAp I,II,III
  
  • these RNAps each transcribe diff types of genes
  • I and III transcribe the genes encoding tRNA, rRNA, and other RNAs that play structural and catalytic role
  • II transcribes majority of eukaryotic genes, including those that encode proteins and miRNAs
• bacterial RNA polymerase w/ sigma subunit is able to initiate transcription on its own, BUT eukaryotic RNAp requires help from accessory proteins(mainly general transcription factors which assemble at each promoter w/ RNAp to begin transcription)
• in bacteria, genes are close together in DNA, but in eukaryotes, genes are spread out w/ stretches of up to 100,000 nucleotides btwn each gene which allows a single gene to be controlled by a large variety of regulatory DNA sequences scattered along the DNA
  
  • eukaryotic is more complex than bacteria
• eukaryotic transcription initiation must take into account the packing of DNA into nucleosomes and more compact forms of chromatin structur

Question 17

general transcription factors

Answer 17

• These accessory proteins assemble on the promoter, where they position the RNAp and pull apart the DNA double helix to expose the template strand, allowing the p to begin transcription
  
  • similar to sigma factor in bacterial transcription
• TFIIB, TFIID, etc.

Question 18

assembly process of general transcription factors

Answer 18

• general transcription factor TFIID binds to a short segment of DNA double helix of mostly T and As
  
  • (this part of promoter is TATA box)
  • TATA box is usually 25 nucleotides upstream of start site
• local distortion of DNA double helix occurs, which other proteins use as a landmark
• other factors assemble w/ RNAp to form a complete transcription complex
  
  • order of assembly differs from 1 promoter to next
• phosphate groups are added to RNAp “tail” so RNAp can be released and transcription can start
  
  • TFIIH(contains protein kinase as a subunit) initiates liberation
• when transcription begins, general transcription factors release from DNA
• when RNApII finishes transcription it is released from DNA, the phosphates on tail are stripped off by protein phosphates, and RNApII finds a new promoter
  
  • Only the dephosphorylated form of RNA polymerase II can initiate RNA synthesis

Question 19

eukaryotic mrNAs Are processed in the Nucleus

Answer 19

• bacterial DNA lies exposed to the cytoplasm, which contains ribosomes on which protein synthesis occurs
  
  • as mRNA in bacterium are being synthesized, ribosomes attach to free 5’ end of RNA transcript and begin translating it into protein
• in eukaryotic cells, DNA is within nucleus
  
  • transcription=in nucleus
  • protein synthesis=on ribosomes in cytoplasm
  • mRNA must be transported out of nucleus throguh small pores in nuclear envelope
  • RNA processing occurs(capping, splicing, polyadenylation)
  • enzymes responsible for RNA processing ride on phosphorylated tail of RNApII as it synthesizes RNA and they process the transcript as it emerges from the polymerase

Question 20

RNA capping

Answer 20

• occurs only on RNA transcripts destined to become mRNA molec(precursor mRNAs/pre-mRNAs)
• capping modifies 5’end which is synthesized first
  
  • an atypical nueclotide(guanine bearing a methyl group) is attached to 5’ end by triphosphate bridge
• occurs after RNApII produced about 25 nucleotides of RNA, long before the whole gene has been transcribed

Question 21

Polyadenylation

Answer 21

• adds special structure on newly transcribed mRNAs 3’
• trimmed by an enzyme that cuts RNA chain at a particular sequence of nucleotides
• transcript is then finished by a 2nd enzyme that adds a series of repeated adenines to cut end
  
  • poly-A tail is usually a few 100 nucleotides long

Question 22

Effect of capping and polyadenylation

Answer 22

• increase stability of eukaryotic mRNA molec
• facilitate its export from nucleus to cytoplasm
• generally mark the RNA molec as an mRNA
• used by protein-synthesis machinery to make sure both ends of mRNA are present and that message is complete before protein synthesis begins

Question 23

Introns

Answer 23

• pre-mRNAs undergo extra processing before they are functioning mRNAs
• far more drastic than capping+polyadenylation
• in bacteria, most proteins are encoded by uninterrupted stretches of DNA sequence that is transcribed into an mRNA that, w/o further processing, can b translated into protein
• in eukaryotes, most protein coding genes are interrupted by introns(long, noncoding, intervening sequences~range from 1-10,000 nucleotides long)
  
  • exons: expressed sequences(scattered pieces of coding sequence) are shorter than introns and are a small fraction of whole gene
    
    • some protein-coding eukaryotic genes lack introns, some have few, but most have many

Question 24

RNA splicing

Answer 24

• introns and exons are transcribed into RNA
• after capping, as RNApII continues transcription, RNA splicing beings
  
  • introns are removed from newly synthesized RNA and exons are stitched together
  • each transcript receives poly-A tail
    
    • either after splicing OR before final splicing rxns have been completed
    • after splicing and 5’ and 3’ ends are modified, RNA is a functional mRNA that can exit the nucleotides and be translated into protein
• How dooes cell determine what parts of RNA transcript to remove during splicing:
  
  • each intron’s end/almost end contains a few short nucleotide sequences(the same) that act as cues for its removal from the pre-mRNA
  • elaborate splicing machine cuts out intron in form of a “lairat” structure formed by rxn of A nucleotide
• carried out by snRNAs instead of proteins

Question 26

small nuclear RNAs(snRNAs)

Answer 26

* carry out RNA splicing * are packaged with additional proteins to form small nuclear ribonucleoproteins(snRNPs) * snRNPs recognize splice-site sequences through complementary base-pairing btwn their RNA components and sequences in pre-mRNA * together snRNPs form core of the **spliceosome** * large assembly of RNA and protein molecs that carry out RNA splicing in nucleus

Question 27

benefits of intron-exon type of gene arrangement

Answer 27

* **1) alternative splicing**: allows many different proteins to be produced from the same gene * ab 95% of human genes do this * RNA splicing allows eukaryotes to expand genome * 2) speeds up emergence of new and useful proteins: novel proteins have arisen by mixing and matching diff econs of preexisting genes * many proteins in present-day cells resemble patchworkds composed from a common set of protein pieces(called protein domains)

Question 28

translation

Answer 28

* the conversion of the information in RNA into protein * **genetic code**: the rules by which the nucleotide sequence of a gene, through an intermediary mRNA molecule, is translated into the amino acid sequence of a protein * b/c RNA is made of 4 diff nucleotides, so there are 4x4x4=64 diff combos of 3 nucleotides * each group of 3 consec nucleotides in RNA=codon * each codon specific 1 amino acid * The same genetic code is used in nearly all present-day organisms * slight differences found in mRNA of mitochondria and of some fungi and protozoa * mitochondria have their own DNA replication, transcription, and protein synthesis

Question 29

reading frames

Answer 29

* mRNA sequences can be translated in any 1 of 3 diff reading frames depending on where the decoding process(translation) begins

Question 30

transfer RNA(tRNA)

Answer 30

* codons in mRNA don't directly recognize aa's they specify * group of 3 nucleotides doesn't bind directly to aa * **adapter molecules** recognize and bind to a codon at one sit on their surface and to an aa at another site * tRNA, 80 nucleotides * EX: a 5′-GCUC-3′ sequence in one part of a polynucleotide chain can base-pair with a 5′-GAGC-3′ sequence in another region of the same molecule * 2 regions of unpaired nucleotides situated at either end of the L-shaped tRNA molecule are crucial to the function of tRNAs in protein synthesis

Question 31

anticodon

Answer 31

* formed by one region of unpaired nucleotides at one end of L-shaped tRNA * anticodon: a set of 3 consecutive nucleotides that bind, through base pairing, to the complementary codon in an mRNA molec * other region is a short single-stranded region at 3' end * this is where the aa that matches the codon is covalently attached to the tRNA

Question 32

the genetic code is redundant

Answer 32

* several diff codons specify a single aa * tells us 2 things: * there is more than one tRNA for many aa's * some tRNA molec can base-pair w/ more than one codon * some tRNA require accuracy at first 2 positions of codon but can tolerate a mismatch(wobble) at 3rd position

Question 33

aminoacyl-tRNA synthetases

Answer 33

* Responsible for recognition and attachment of correct aa * **aminoacyl-tRNA synthetases** covalently couple each aa to its appropriate set of tRNA molecs * most organisms have diff synthetase enzymes for each aa * means there are 20 synthetases in all: * one attaches glycine to all tRNAs that recognize codons for glycine, etc with all 19 other aa's * each synthetase enzyme recognizes specific nucleotides in both anticodon and aa-accepting arm of correct tRNA * overall, allows each codon in mRNA molec to specify its proper aa

Question 34

ribosome

Answer 34

* in both prokaryotes and eukaryotes * a molecular machine that can move along the mRNA, capture complementary tRNA molecules, hold the tRNAs in position, and then covalently link the amino acids that they carry to form a polypeptide chain * ribsomes are large complexes made from dozens of small ribosomal proteins and several curcial RNA molec(rRNA) * eukaryotic cells contain millions of ribosomes in cytoplasm

Question 35

Eukaryotic vs. Prokaryotic ribosomes

Answer 35

* both are composed of 1 large subunit(catalyzes the formation of the peptide bonds that covalently link the aa together in a polypeptide chain) and 1 small subunit(matches the tRNAs to the codons of the mRNA) which fit together to form a complete ribosome * come together near mRNA 5' end to start protein synthesis * mRNA is pulled thru ribosome * as mRNA goes forward 5' to 3', the ribosome translates its nucleotide sequence into aa sequence, one codon at a time, using tRNA as adaptors * when synthesis is over, 2 subunits separate * Efficient!: a eukaryotic ribosome adds 2 aa's to polypeptide chain each second, while bacterial does 20 each second

Question 36

How ribosomes add aa's to growing peptide chain

Answer 36

* each ribosome has 3 binding sites for tRNA(A,P,E) * to add aa's to polypeptide chain: * appropriate charged tRNA enters A site by base pairing w/ complementary codon on mRNA molec * it's aa is then linked to peptide chain and held by tRNA in P site * Next, large ribosomal subunit shifts forward, moving the spent tRNA to E site and ejecting it * new proteins grow from amino to carboxyl end until stop codon in mRNA is encountered

Question 37

ribosomal proteins

Answer 37

* ribosomes are 2/3 RNA and 1/3 protein * 3D structure of ribosomes proves that rRNA are responsible for ribosome's overall structure and its ability to choreograph and catalyze protein synthesis * ribosomal proteins * located on surface and fill in gaps of folded RNA * help fold and stabilize RNA core * permit the changes in rRNA conformation that are necessary for this RNA to catalyze efficient protein synthesis * form tRNA binding sites(APE)

Question 39

ribozymes

Answer 39

* RNA molecules that possess catalytic activity * there is good reason to suspect that RNA rather than protein molecules served as the first catalysts for living cells * so, in the past cells were run almost entirely by ribozymes

Question 40

initiator tRNA

Answer 40

* a special charged tRNA that initiates translation * mRNA begins with codon AUG * this initiator tRNA always carries aa methionine, so newly made proteins all have methionine as first aa at their N-terminal end(which is synthesized first) * methionine is usually removed later by a specific protease

Question 41

translation initiation factors in eukaryotes

Answer 41

* In eukaryotes, an initiator tRNA, charged with methionine, is first loaded into the P site of the small ribosomal subunit, along with additional proteins called **translation initiation factors** * diff from tRNA that carry methionine * of all tRNA, only a charged initiator tRNA molec can bind tightly to P site in absense of the large ribosomal subunit * next, small ribosomal subunit loaded w/ initiator tRNA bind to 5' end of an mRNA molec(marked by 5'cap) * small ribosomal subunit moves forward(5'to3') along mRNA searching for AUG * when encountered and recognized by initiator tRNA, several initiation factors dissociate from small ribosomal subunit to make way for large subunit * b/c initiator tRNA is bound to the P site, protein synthesis is ready to begin with the addition of the next charged tRNA to the A site

Question 42

selecting start codons in bacteria

Answer 42

* bacterial mRNA have no 5' caps to tell ribosome where to begin searching for start of translation * instead, they have specific **ribosome binding sequences**(up to 6 nucleotides) that are located a few nucleotides upstream of the AUGs at which translation begins * unlike eukaryotic ribosomes, prokaryotic ribosomes can bind directly to start codon as long as ribosome binding site precedes it

Question 43

polycistronic

Answer 43

* mRNA that encode several diff proteins, each of which is translated from same mRNA molec * EX: prokaryotic mRNA, NOT eukaryotic which only carry info for a single protein

Question 44

Singaling the end of translation in eukaryotes and prokaryotes

Answer 44

* stop codons in mRNA(UAA, UAG, and UGA) * are not recognized by tRNA, and do not specify an aa, but instead signal to the ribosome to stop translation * release factors(proteinS) bind to any stop codon that reaches A site on ribosome, causing it to catalyze the addition of a water molec instead of an aa to the peptidyl-tRNA * this rxn frees carboxyl end of polypeptide chain from its attachment to a tRNA molec, b/c this is the only attachment that holds growing polypep * complete protein chain is released along with mRNA(release by ribosome) which then dissociates into its two separate subunits for the next molec

Question 45

polyribosomes/polysomes

Answer 45

* if mRNA is being translated efficiently, a new ribosome attaches to 5' end of mRNA almost as soon as preceding ribosome has translate enough of sequence to move out of the way * polyribosomes(polysomes): large cytoplasmic assemblies made up of many ribosomes spaced as close as 80 nucleotides apart along a single mRNA molec * many ribosomes working at once inc. efficiency * found in both bacteria and eukaryotes * BUT bacteria speed up process even further b/c bacterial mRNA don't need to be processed and is also physically accessible to ribosomes * SO ribosomes attach to free end of mRNA and start translating even before transcription is complete

Question 46

Inhibitors of prokaryotic protein synthesis as antibiotics

Answer 46

* most effective anitbiotics inhibit baterial, but not eukaryotic RNA and protein synthesis * diff antibiotics bind to diff regions of bacterial ribosomes, so they often inhibit digg steps in protein synthesis

Question 47

Controlled protein Breakdown

Answer 47

* helps regulate the Amount of each protein in a Cell * # of copies of a protein depend on how quick they are made and how long they survive * controlling breakdown of proteins into constituent aa's helps cells regulate the amt of each particular protein * structural proteins(bone/muscle) last for months/years whie other proteins(EX metabolic enzymes that regulate cell growth and division) last for seconds, hours, or days

Question 48

proteases

Answer 48

* cells possess specialized pathways that enzymatically break proteins down into their constituent aa's(proteolysis) * the enzymes that degrade proteins, first to short peptides and finally to individual aa's, are called **proteases** * proteases act by cutting(hydrolyzing) peptide bonds btwn aa's * proteolytic pathways: * rapidly degrade proteins that must have short lifespans * recognize and remove proteins that are damaged/misfolded * misfolded porteins usually aggregate and cause damage/cell death * eventually ALL proteins accumulate damage and are degraded by proteolysis

Question 49

proteasomes

Answer 49

* **proteasomes:** large protein machines that breaks down protein in eukaryotic cells * present in both cytosol and nucleus * contain central cylinder formed from proteases whose active sites face into an inner chamber * each end of cylinder is stoppered by a large protein complex formed from \>=10 types of protein subunits * these stoppers bind the proteins destined for degradation and then(using ATP hydrolysis as fuel) unfold the doomed proteins and thread them into inner chamber * proteases chop them into short peptides, which are ejected from either end of proteasome

Question 50

How do proteasomes select which proteins in the cell should be degraded?

Answer 50

* Eukaryotes: * special enzymes covalently attach a small protein chain of ubiquitin * ubiquitylated proteins are recognized, unfolded, and fed into proteasomes by proteins in stopper * short-lived proteins often contain short aa sequences that identify them as one to be ubiquitylated

Question 52

post-translational modifications

Answer 52

* **post-translational modifications**: further attention given to proteins once they leave ribosome, but before they become useful to the cell * EX: * covalent modification(phosphorylation) * binding of small-molecule cofactors * association w/ other protein subunits(needed for a newly synthesized protein to become fully functional

Question 53

RNA world

Answer 53

* hypothesis: RNA both stored genetic info and catalyzed chemical rxns in primitive cells * overtime DNA took over * if this idea is correct, the transition out of RNA world was never completed * b/c RNA still catalyzes several fundamental rxns in modern cells

Question 54

life requires autocatalysis

Answer 54

* autocatalytic system: an increasingly complex chemical system of organic monomers and polymers that function together to generate more molecules of the same types, fueled by a supply of simple raw materials in the primitive environment on Earth * would contain a far from random selection of interacting molecs * would reproduce itself * would compete with other systems dependent on same raw materials * would decay toward chem equil and die if deprived of its raw materials/maintained at a temp that upsets balance * our options: * protein are most versatile catalysts, but can't reproduce themselves directly * RNA molecs, however, could, in principle, catalyze their own synthesis

Question 55

RNA Can Both store information and Catalyze Chemical reactions

Answer 55

* RNAs do not have the same structural and functional diversity as do protein enzymes(b/c they're built from only four diff subunits) * BUT, ribozymes can catalyze many types of chemical reactions

Question 56

RNA and DNA in evolution

Answer 56

* RNA is thought to predate DNA in evolution * chemical evidence: * ribose is readily formed from formaldehyde which is one of the principal products of experiments simulating conditions on primitive Earth * deoxyribose is harder to make * DNA appeared later and proved more suited * double helix makes DNA more stable * deamination(most common, unwanted nucleotide injury) is easier to detect and repair in DNA * DNA took over the primary genetic function, and proteins became the major catalysts, while RNA remained primarily as the intermediary connecting the 2

Question 57

TBP

Answer 57

tata binding protein a subunit of the general transcription factyor TFIID

Question 58

peptidyl transferase

Answer 58

* catalytic site in rRNA * highly structured pocket that precisely orients the two reactants—the elongating polypeptide and the charged tRNA— * thereby greatly increasing the probability of a productive reaction.

Question 59

export of mature mRNAs from nucleus

Answer 59

* the cap and poly-a tail of a mature mRNA molec are “marked” by proteins that recognize these modifications. * In addition, a group of proteins called the exon junction complex is deposited on the pre-mRNA after each successful splice has occurred * Once the mRNA is deemed “export ready,” a nuclear transport receptor associates with the mRNA and guides it through the nuclear pore