replication/transcription question - looking for quick answer!?

Question

Why does replication occur on both strands of DNA, but transcription occur only on one strand? Why not just replicate using one strand of DNA to produce one new helix rather than two new helices, or why not take a okazaki - fragment - type approach to transcription and use the complimentary strand?

i think i understand - if only one DNA strand is copied you end up with 1.5 helices..

Answer 1

DNA replication happens on both strands because you need a pair of chromosomes.

DNA transcription takes place in one strand is because you need FUNCTIONAL protein and that strand is the anti-sense because your mRNA must be a sensed strand. You can transcibe the sense strand to give you an anti-sense strand mRNA and your protein will be non-functional.

Add: If only one strand is copied, the strand does not form a helix. It is just a strand known as a single-stranded DNA. You need double-stranded for other stuffs such as proof reading.

Hope I am clear.

Answer 2

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In principle: DNA replication is semi-conservative
H - bonds 'unzip', strands unwind,
complementary nucleotides are added to existing strands (MGA2 04-04)
After replication, each double-helix has one "old" & one "new" strand
[note alternative conservative & dispersive models: Homework #3 ]

DNA is not the "Genetic Code" for proteins
The information in DNA must first be transcribed into RNA
A messenger RNA transcript is base-complementary to the template strand of DNA
& therefore co-linear with the sense strand of DNA





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DNA synthesis in prokaryotes:
Nucleotides are added simultaneously to both strands, but
DNA grows in the 5' 3' direction ONLY (Gr08-21)

Distinguish:
Replication: duplication of a double-stranded DNA (dsDNA) molecule
an exact copy of the existing molecule (cf. xerox copy)
Synthesis: biochemical creation of a new single-stranded DNA (ssdNA) molecule
a base-complementary 'copy' of an existing strand (cf. silly putty copy)
Homework #4


DNA Synthesis in prokaryotes (review; see also Gr08-20) (MGA2 04-5,6,7)
(1) Formation of replication fork (Gr08-27)
provides two single-stranded DNA template (ssDNA)
(2) Synthesis of RNA primer
(3) Addition of dNTPs by DNAPol III at 3' end only (Gr08-21)
continuous synthesis on leading strand
(4) discontinuous synthesis on lagging strand (Gr08-28)
Okazaki fragments
proof-reading by 3'5' exonuclease activity
(5) Excision of RNA primer by DNAPol I
ligation (connection) of fragment ends at gaps by DNA ligase (Gr08-30)

A talkie animation of DNA synthesis (requires MoviePlayer) `[onlineMGA2 animation]

DNA synthesis occurs at multiple replications forks (replicons) (Gr08-22)

DNA synthesis occurs on leading & lagging strands simultaneously
A single, dimeric DNAPol III replicates both strands (Gr08-31)

DNA synthesis in eukaryotes
Eukaryotic genomes are much larger [MGA2_02-10]
=> eukaryotic DNA synthesis is more "efficient":
More DNAPol molecules, more replicons, slower rate of synthesis
E. coli: 15 DNAPol molecules at 3,500 replicons add 100,000 bases/min
=> 4.2 x 106 bp genome replicated in 20 ~ 40 min
Drosophila: 50,000 DNAPol molecules at 25,000 replicons add 500 ~ 5,000 bases/min
=> 330 x 106 bp diploid genome replicated in < 3 min : net 600x faster


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Transcription: synthesis of messenger RNA (mRNA) (online MGA2 animation)


DNA is transcribed by DNA-dependent RNA Polymerase (RNAPol I) (Gr08-37) / (Gr10-11)
(1) Recognition (Gr10-09a)


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In principle: DNA replication is semi-conservative
H - bonds 'unzip', strands unwind,
complementary nucleotides are added to existing strands (MGA2 04-04)
After replication, each double-helix has one "old" & one "new" strand
[note alternative conservative & dispersive models: Homework #3 ]

DNA is not the "Genetic Code" for proteins
The information in DNA must first be transcribed into RNA
A messenger RNA transcript is base-complementary to the template strand of DNA
& therefore co-linear with the sense strand of DNA





--------------------------------------------------------------------------------


--------------------------------------------------------------------------------

DNA synthesis in prokaryotes:
Nucleotides are added simultaneously to both strands, but
DNA grows in the 5' 3' direction ONLY (Gr08-21)

Distinguish:
Replication: duplication of a double-stranded DNA (dsDNA) molecule
an exact copy of the existing molecule (cf. xerox copy)
Synthesis: biochemical creation of a new single-stranded DNA (ssdNA) molecule
a base-complementary 'copy' of an existing strand (cf. silly putty copy)
Homework #4


DNA Synthesis in prokaryotes (review; see also Gr08-20) (MGA2 04-5,6,7)
(1) Formation of replication fork (Gr08-27)
provides two single-stranded DNA template (ssDNA)
(2) Synthesis of RNA primer
(3) Addition of dNTPs by DNAPol III at 3' end only (Gr08-21)
continuous synthesis on leading strand
(4) discontinuous synthesis on lagging strand (Gr08-28)
Okazaki fragments
proof-reading by 3'5' exonuclease activity
(5) Excision of RNA primer by DNAPol I
ligation (connection) of fragment ends at gaps by DNA ligase (Gr08-30)

A talkie animation of DNA synthesis (requires MoviePlayer) `[onlineMGA2 animation]

DNA synthesis occurs at multiple replications forks (replicons) (Gr08-22)

DNA synthesis occurs on leading & lagging strands simultaneously
A single, dimeric DNAPol III replicates both strands (Gr08-31)

DNA synthesis in eukaryotes
Eukaryotic genomes are much larger [MGA2_02-10]
=> eukaryotic DNA synthesis is more "efficient":
More DNAPol molecules, more replicons, slower rate of synthesis
E. coli: 15 DNAPol molecules at 3,500 replicons add 100,000 bases/min
=> 4.2 x 106 bp genome replicated in 20 ~ 40 min
Drosophila: 50,000 DNAPol molecules at 25,000 replicons add 500 ~ 5,000 bases/min
=> 330 x 106 bp diploid genome replicated in < 3 min : net 600x faster


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Transcription: synthesis of messenger RNA (mRNA) (online MGA2 animation)


DNA is transcribed by DNA-dependent RNA Polymerase (RNAPol I) (Gr08-37) / (Gr10-11)
(1) Recognition (Gr10-09a)

Promoters - short DNA sequences that regulate transcription [MGA2_03-09]
typically 'upstream' = ' leftward' from 5' end of sense strand
(2) Initiation & Elongation (Gr10-09c)
mRNA synthesized 5'3' from template strand
mRNA sequence therefore homologous to sense strand

Colinear: mRNA and DNA sense strand have “same” sequence [MGA2_03-07]

(Except substitute U for T)
Process similar to DNA replication, except
No primer is required (Gr10-10)
Transcription may occur in opposite orientations on the two strands [MGA2_03-05]
Not all of the DNA will be transcribed [MGA2_03-04]
(3) Termination

Regulation of transcription
In prokaryotes, transcription & translation may occur simultaneously
In eukaryotes, transcription occurs in nucleus [MGA2_03-06]
translation occurs in cytoplasm (see next section):
=> RNA must cross nuclear membrane (Gr10-02)
transcription & translation are physically separate
primary RNA transcript is extensively processed
heterogeneous nuclear RNA (hnRNA) mRNA (Gr10-14)

Post-transcriptional processing of eukaryotic RNA transcripts is complex [MGA2_03-11]
promoters & enhancers determine initiation & control rate
'cap' (7-methyl guanosine, 7mG) added to 5' end (Gr10-15b)
'tail' of poly-A (5'-AAAAAAAAAA~~~-3') added to 3' end (Gr10-15e)
'splicing' of hnRNA : eukaryotic genes are "split" (MGA2 03-12,14,15,16)
intron sequences in DNA are removed from hnRNA : "intervening" sequences
exon sequences in DNA are represented in mRNA: "expressed" in protein (MGA2 3-16)
1 ~ 16+ exons / 'gene'
>90% of transcript may be removed [MGA2_02-28]
visualized as heteroduplexes (Gr10-16) / (Gr10-17)
DNA introns 'loop out'
DNA exons pair with mRNA



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