Chapter five RNA synthesis and RNA synthesis and processing
(RNA合成与加工）
(A
RNA synthesis is
ll d
Overview
called transcription (转
)
录).Transcription is a
process in which
process in which one DNA strand is
used as template to p
synthesize a
complementary
RNA.
I t li ll l1%f th DNA i
In most mammalian cells, only 1% of the DNA sequence is copied into a functional RNA (mRNA). Only one part of the DNA is transcribed to produce nuclear RNA, and only a minor portion is transcribed to produce nuclear RNA and only a minor portion of the nuclear RNA survives the RNA processing steps.
One of the most important stages in RNA processing is RNA splicing (RNA剪接). In many genes, the DNA sequence coding splicing(RNA In many genes,the DNA sequence coding for proteins, or "exons", may be interrupted by "introns".
In the cell nucleus, the DNA that includes all the exons and
In the cell nucleus the DNA that includes all the exons and introns of the gene is first transcribed into a complementary RNA copy called "nuclear RNA," or nRNA. In a second step, introns py,p,
are removed from nRNA by RNA splicing. The edited sequence is called "messenger RNA," or mRNA.
T l t t d d di t d
Template strand and coding strand
To distinguish the two strands of DNA –
p
?The template strand refers to the strand of DNA that serves as a template for the synthesis of mRNA.
The coding strand (编码链)or the mRNA like strand ?The coding strand(or the mRNA-like strand refers to the strand with a base sequence directly corresponding to the mRNA sequence.
“Template strand (模板链) ”may be called “minus strand”, or “antisense strand (反意义链) ”.
The "coding strand"may be termed "non-template strand", p,
"plus strand", or "sense strand".
RNA structure
1.Covalent structure: like DNA, RNA is a long polymer consisting of nucleotides joined by 3, 5phosphodiester consisting of nucleotides joined by3’5’phosphodiester bonds. However, there are some differences:
?The bases in RNA are Adenine, Guanine, Uracil and
Th b i RNA Ad i G i U il d Cytosine. Thymine in DNA is replaced by Uracil in RNA. The uracil can form base pairs with adenine
The uracil can form base pairs with adenine.
?Sugar in RNA is ribose rather than deoxyribose as in DNA
2.As with DNA, the nucleotide sequence of RNA is also written as a base sequence in the 5’→3’direction. Thus
w tte as a base seque ce t e53d ect https://www.doczj.com/doc/9b11090857.html, GUCAAGCCGGAC is the sequence of one short RNA molecule.
3. RNA secondary structure
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Most RNA molecules are single-stranded but an RNA molecule may contain regions which can form complementary base paring may contain regions which can form complementary base-paring where the RNA strand loops back on itself.
Classes of RNA （RNA的种类）
Classes of RNA
1) Messenger RNAs (mRNAs, 信使RNA):
This class of RNAs is the genetic coding templates used by This class of RNAs is the genetic coding templates used by the translational machinery.
2) Transfer RNAs (tRNAs, 转运RNA):
This class of small RNAs forms covalent attachments to individual amino acids and recognizes the encoded sequences of the mRNAs.
f h
3)Ribosomal RNAs (rRNAs, 核糖体RNA):
This class of RNAs is assembled, together with numerous ribosomal proteins, to form the ribosomes (核糖体).
4) Small nuclear RNAs (snRNA,小核RNA):
4)Small nuclear RNAs(snRNA RNA):
This class of RNA is part of the machinery for splicing precursor mRNA. It is only present in eukaryote. precursor mRNA It is only present in eukaryote
Unit one: RNA Polymerases(RNA
y(聚合酶) 1.The function of RNA polymerases
? A single RNA polymerase catalyzes the synthesis of all three
E. coli RNA classes--mRNA, rRNA, and tRNA.
?RNA polymerases can initiate a new strand.
?The nucleotides used to extend a growing RNA chain Th l id d d i RNA h i are ribonucleoside triphosphates(NTPs).
①Two phosphate groups are released as pyrophosphate(PP)during the reaction.
焦磷酸) during the reaction. pyrophosphate (PP
i
②Strand growth is always in the 5' to 3'direction.
③The first nucleotide at the 5' end retains its triphosphate group.
p p g p
The chemical reaction catalyzed by RNA polymerases. The chemical reaction catalyzed by RNA polymerases
2. Transcription exhibits several features that
are distinct from replication.
）Transcription initiates, both in prokaryotes and
1Transcription initiates both in prokaryotes and eukaryotes, from many more sites than replication.
2）There are many more molecules of RNA polymerase per cell than DNA polymerase.
3）RNA polymerase proceeds at a rate much slower than DNA polymerase (approximately 50-100 bases/sec for RNA DNA polymerase(approximately50-100bases/sec for RNA
versus near 1000 bases/sec for DNA).
4）The fidelity of RNA polymerization is much lower than DNA.
3Classes of RNA polymerases
3. Classes of RNA polymerases
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1)Prokaryotic RNA Polymerase（E.coli）:
The σsubunit (σfactor) plays an important role in recognizing
the transcriptional initiation site, and also possesses the helicase
th t i ti l i iti ti it d l th h li activity to unwind the DNA double helix.Several different forms of σsubunits have been identified, with molecular weights ranging
,g g g from 28 kDa to 70 kDa.
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Nucleotide synthesis is carried out by core polymerase.
The "holoenzyme"refers to a complete and fully functional
e y e.e o oe y e c udes e co e po y e se d enzyme.The holoenzyme includes the core polymerase and
the σfactor.
Once the "holoenzyme"
recognizes the correct
recognizes the correct
promoter, the sigma factor
d ssoc es o
e N
dissociates from the RNA
polymerase.
The core polymerase
The core polymerase
begins to unwind the helix
of the DNA creating a
region of non-paired
deoxyribonucleotides that
serve as a template for
t l t f
RNA synthesis.
2)Eukaryotes
In eukaryotes, the genetic information carried by DNA is contained
within the nucleus. This information is transcribed into RNA which must
then be carried across the membrane into the cytosol prior to translation. then be carried across the membrane into the cytosol prior to translation This spatial and temporal separation of transcription and translation is
one of many differences between eukaryotic and prokaryotic
one of many differences between eukaryotic and prokaryotic
transcription.
Eukaryotic cells have three kinds of RNA
1RNA Polymerase I (Pol I)
Polymerases
1.RNA Polymerase I (Pol I)rRNA
Promoters vary greatly
2RNA Polymerase II (Pol II)
2. RNA Polymerase II (Pol II)mRNA
Promotor -TATA box (TATAAA) and Inr Initiator
3. RNA Polymerase III (Pol III)y ()
tRNA, 5S rRNA
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