BIOL 2112 Chapter Notes - Chapter 17: Nuclear Membrane, Category 3 Cable, Polynucleotide
3 May 2018
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Lecture 26 - Transcription
1) How many different RNA polymerases are there in eukaryotes and what types of genes
do they transcribe? RNA polymerase I – ribosomal RNA (rRNA), RNA polymerase II –
messenger RNA (mRNA) protein coding, micro RNA (miRNA), RNA polymerase III –
transfer RNA (tRNA) 2) In prokaryotes transcription and translation are coupled. How does
this differ from eukaryotes? Bacteria do not have nuclei, therefore nuclear membranes do
not separate bacterial DNA and mRNA from ribosomes and other protein synthesizing
equipment. Lack of compartmentalization allows translation of an mRNA to begin while
its transcription is still in process. Eukaryotic cells have nuclei. The presence of a nuclear
envelope separates transcription from translation in space and time. Transcription occurs
in the nucleus, but the mRNA must be transported to the cytoplasm for translation. Before
eukaryotic RNA transcripts can leave the nucleus, they are modified in various ways to
produce the final, functional mRNA. The transcription of a protein-coding eukaryotic gene
results in pre-mRNA, and further processing yields finished mRNA. The initial RNA
transcript from any gene including those specifying RNA that is not translated into
protein, is more generally called a primary transcript. 3) A. Given the following DNA
template, what is the sequence and orientation of the messenger RNA gene product?
Include the direction of transcription along the DNA template.
5’-TTA ATT TTG ATG ATA CGC TGT GGG AGA GAC AAT AAG AAA CAT-3’
3’-AAT TAAAACTAC TAT GCG ACA CCC TCT CTG TTA TTC TTT GTA-5’
Transcription
5’-UUA AUUUUGAUG AUA CGC UGU GGG AGA GAC AAU AAG AAA CAU-3’
B. What is the translated product of this messenger RNA?
LEU – ILE – LEU – Met – ILE – ARG – CYS – GLY – ARG – ASP – ASN – LYS – LYS – HIS
4) What is a readig frae? Our ability to extract the intended message from a written
language depends on reading the symbols in the correct groupings. The reading frame is
also important in the molecular language of cells. The polypeptide can be made correctly
ol if the RNA uleotides are read fro left to right 5’
3’
5) List the stages of transcription and describe the events of each stage in ~1 sentence.
mRNA, a carrier of information from the DNA to the ell’s protei-synthesizing machinery,
is transcribes from the template strand of a gene. An enzyme called RNA polymerase pries
the two strands of DNA apart and joins together the RNA nucleotides complementary to
the DNA template strand, thus elongating the RNA polynucleotide. RNA polymerases can
assemble a polynucleotide only in its 5’
3’ diretio, addig oto its 3’ ed. RNA
polerases are ale to ake a hai fro srath & do’t eed to add the first
nucleotide onto a pre-existing primer. Specific sequences along the DNA mark where the
transcription begins and ends. The DNA sequence where RNA polymerase attaches and
initiates transcription is known as the promoter. In bacteria, the sequence that signals the
end of transcription is called the terminator. The promoter is said to be upstream from the
terminator. The stretch of DNA downstream from the promoter that is transcribed into an
RNA molecule is called a transcription unit. Bacteria have a single type of RNA polymerase
that synthesizes not only mRNA but other types of RNA that function in protein synthesis
such as rRNA. Eukaryotes have at least 3 types of RNA polymerase in their nuclei: one
used for the synthesis of pre-mRNA called RNA polymerase II. The other polymerases
Gene
promoter
region
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transcribe RNA molecules that are not translated into a protein. The promoter of a gene
includes the transcription start point – the nucleotide where RNA polymerase begins
synthesis of mRNA – and typically extends several dozen or so nucleotide pairs upstream
from the start point. Based on interactions with proteins, RNA polymerase binds in a
precise location and orientation on the promoter. This determines where transcription
starts and which two strands of the DNA helix is used as the template. In bacteria, part of
the RNA polymerase itself specifically recognizes and binds to the promoter. In
eukaryotes, a collection of proteins called transcription factors mediate binding of RNA
polymerase and the initiation of transcription. Only after transcription factors are
attached to promoter does RNA polymerase bind to it. The whole complex of transcription
factors and RNA polymerase II bound to the promoter is called a transcription initiation
complex. Once the appropriate transcription factors are firmly attached to the promoter
DNA and polymerase is bound to them in the correct orientation on the DNA, the enzyme
unwinds the two DNA strands and begins transcribing the template strand at the start
point. As RNA polymerase moves along the DNA, it untwists the double helix, exposing
about 10-20 nucleotides at a time for pairing with RNA nucleotides. The enzyme adds
uleotides to the 3’ ed of the groig RNA oleule as it otiues alog the doule
helix. The new RNA molecule peels away from the DNA template, and the DNA double
helix re-forms. Transcription progresses at about 40 nucleotides per second in eukaryotes.
Bacteria and eukaryotes differ in the way they terminate transcription. In bacteria,
transcription proceeds through a terminator sequence of DNA. The transcribed terminator
(an RNA sequence) functions as the termination signal causing the polymerase to detach
from the DNA and release the transcript which requires no further modification before
translation. In eukaryotes, RNA polymerase II transcribes a sequence on the DNA called
the polyadenylation signal sequence which specifies a polyadenylation signal (AAUAAA) in
the pre-mRNA. Once this stretch of RNA nucleotides appears, it is immediately bound by
certain proteins in the nucleus. Then at a point around 10-35 nucleotides downstream
from the AAUAAA, these proteins cut the RNA transcript free from the polymerase,
releasing pre-mRNA. The pre-mRNA undergoes processing. The cleavage marks the end of
mRNA but the polymerase continues to transcribe. Enzymes begin to degrade the RNA
startig at its el eposed 5’ ed. The polerase otiues trasriig, pursued
the enzymes, until they catch up to the polymerase and it dissociates from the DNA. 6) A.
What is the role of the TATA box during the initiation of transcription? A eukaryotic
promoter commonly includes a TATA box (a nucleotide sequence containing TATA) about
25 nucleotides upstream from the transcriptional start point. Several transcription factors,
once recognizing the TATA box, must bind to the DNA before mRNA polymerase can bind
in the correct position and orientation. B. What is the role of general transcription factors
when they bind to the promoter of a gene? In eukaryotes, a collection of proteins called
transcription factors mediate binding of RNA polymerase and the initiation of
transcription. Only after transcription factors are attached to promoter does RNA
polymerase bind to it. C. What makes up the transcription initiation complex? The whole
complex of transcription factors and RNA polymerase II bound to the promoter is called a
transcription initiation complex. 7) What is the advantage of using a messenger RNA
intermediate to synthesize protein in the cell? The congregation of many polymerase
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Document Summary
Rna polymerase i ribosomal rna (rrna), rna polymerase ii messenger rna (mrna) protein coding, micro rna (mirna), rna polymerase iii transfer rna (trna) 2) in prokaryotes transcription and translation are coupled. Bacteria do not have nuclei, therefore nuclear membranes do not separate bacterial dna and mrna from ribosomes and other protein synthesizing equipment. Lack of compartmentalization allows translation of an mrna to begin while its transcription is still in process. The presence of a nuclear envelope separates transcription from translation in space and time. Transcription occurs in the nucleus, but the mrna must be transported to the cytoplasm for translation. Before eukaryotic rna transcripts can leave the nucleus, they are modified in various ways to produce the final, functional mrna. The transcription of a protein-coding eukaryotic gene results in pre-mrna, and further processing yields finished mrna. The initial rna transcript from any gene including those specifying rna that is not translated into protein, is more generally called a primary transcript.
