chapter 8 part 3 Flashcards
do exons or introns become part of the mature mRNA?
exons
what do exons do?
they encode protein segments
are introns common in bacterial genes?
no, but they are common in eukaryotic genes and occasionally found in archael genes
what are introns most common in?
eukaryotes
what are the most common type of intron?
pre-mRNA transcript introns
what are the most common type of introns removed by?
the spliceosome complex
what are other types of introns removed by?
self-splicing or a different enzymatic process
does intron splicing require a lot of precision or not a lot of percision?
it requires a lot of precision since it has to remove the intron nucleotides accurately
what is the presence of intron sequences demonstrated by?
a technique called R-looping
do regions of the DNA where introns are present have a complementary region within the mRNA?
No, they loop out visibly since they dont have any complementary regions
what does the 5’ splice site contain?
a conensus sequence with an invariant GU dinucleotide at the 5’- most end of the intron
the 3’ splice site at the oppostite end of the intron has a how many long nucleotide consensus sequence with a what region?
11, pyramadine rich and a nearly invariant AG at the 3’ most end
where is the branch site?
20 ti 40 nucleotides upstream of the 3’ end of the intron
facts about the branch site
it is pyrimidine-rich and contains an invariant adenine called the branch point adenine near the 3’ end of consensus
what are introns removed from the pre-mRNA by?
an snRNP-protein complex called the spliceosome
what is cleaved first in introns and what is formed?
the 5’ splice site
lariant intron structure is formed when the 5’ intron end binds to the branch point adenine
what is cleaved second in intreons and what happens?
the 3’ splice site and the exon ends are ligated together
are introns removed one by one?
yes, but they arent really removed in order
composition of the spliceosome
a large complex of multiple snRNPs (small nucleur ribonucleoprotein particles)
-snRNPs u1 through u6
-composition is dynamic -it changes throughout the steps
what re the spliceosome components recruited by?
the SR proteins
what do SR proteins bind to and what do they do?
they bind to a sequence in exons called exonic splicing enhancers (ESEs) and ensure accurate splicing
what splice sites are spliceosomes recruited to?
5’ to 3’ sites by the SR proteins
what functions as an assembly platform and regulator of pre-mRNA processing machinery?
the carboxyl terminal domain (CTD) of RNA pol II
what does the the carboxyl terminal domain (CTD) of RNA pol II
functions as an assembly platform and regulator of pre-mRNA processing machinery
do large eukaryotic genomes express more proteins than there are genes in the genome?
yes
how many polypeptides do human cells produce and how many genes are contained?
100,000
about 22,000 genes
what explains the facts that eukaryotic genomes express more proteins than there are genes in the genome?
-pre-mRNA can be spliced in alternative patterns in different cell types
-alternative promoters can initiate transcription at distinct start points
-alternative sites for polyadenylation can produce different mature mRNAs
what do the three transcription-associated mechanisms that explain why more proteins are expressed than genes in the genome compromise?
alternative pre-mrna processing
what percent of human genes are thought to undergo alternative splicing?
70%
what gene has one of the most complex patterns of alternative splicing?
Drosophila Dscam gene
how many exons in the Drosophila Dscam gene?
24 exons
how many different polypeptides can be produced through alternative splicing in the Drosophila Dscam gene?
38,000
when do we use alternative promoters?
when more than one sequence upstream of a gene can initiate transcription
what does alternative polyadenylation require?
more than one polyadenylation signal in a gene
what are alternative promoters of polyadenylation controlled by?
variable expression of regulatory proteins in specific cell types
what are group 1 introns
large, self splicing ribozymes that catalyze their own excision from some mRNAs
takes place via transesterification reactions that remove intron and ligate the exon ends together
group II introns
these are also self-splicing ribozymes
form very complex secondary structures
self-splicing takes place in a lariant-like manner
are bacteria and eukaryotes transcribed in large precursor molecules?
yes
what happens to the large precursor molecules that are transcribed from bacteria and eukaryotic rRNAs?
they are cleaved into smaller molecules by removal of spacer sequences
what forms the ribosome subunits?
the rRNAS folded up into complex secondary structures which join ribosomal proteins
what chemical modifications occur after transcription is complete?
methylation
is processing of tRNA the same in bacteria and eukaryotes?
no
Some _________ tRNAs are produced
simultaneously with rRNAs, others are
transcribed as part of a large pre-tRNA
transcript and then cleaved into individual tRNA molecules
bacterial
in prokaryotes or eukaryotes is each tRNA gene individually transcribed
in eukaryotes
how many tRNAs do most organisms produce?
most produce about 30 to 40, but fewer than 61
what happens due to a third-base wobble?
no more than 61 tRNAs
do some eukaryotic genomes contain all 61 possible tRNA genes?
yes, some do. one for each codon
explain bacterial tRNA Processing?
-bacterial trnas require processing before assuming their functional role
-many are cleaved from the large precursor transcript
-nucleotides are trimmed
=some nucleotides are chemically modified
-trans fold into a precise three-dimensional structures
-postranscriptional addition of bases
what do eukaryotic tRNAS contain that are removed?
small introns
RNA editing
responsible for post-transcriptional modifications to the nucleotide sequence of some mRNAS
in one kind of RNA editing, uracils are added. what are they added with the assistance of?
guide RNA or gRNA
what is a second type of RNA editing?
base substitution
what is frequently replaced with base substitution?
cytosine with uracil
what is base substitution seen in?
apolipoprotein B proteins from one gene in human liver and intestinal cells
what does base substitution of the mRNA in intestinal cells produce?
a premature stop codon
what protein multisubunit contains the TATA-binding protein
TFIID
