Is Translation Genetics Continuous or Discontinuous
DNA Replication & Transcription
In principle : DNA replication is semi-conservative [ HOMEWORK #4 ]
H - bonds 'unzip', strands unwind,
complementary nucleotides added to existing strands
After replication, each double-helix has one "old" & one "new" strand
DNA is not the "Genetic Code" for proteins
information in DNA must first be transcribed into RNA
messenger RNA transcript is base-complementary to template strand of DNA
& therefore co-linear with sense strand of DNA
DNA & RNA syntheses occur only in the 5'
3' direction
DNA synthesis in prokaryotes:
Nucleotides are added simultaneously to both strands, but
DNA grows in the 5' 3' direction ONLY [iG1 10.10]
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)
occurs only in the 5' 3' direction
Homework #5
DNA Synthesis in prokaryotes
Formation of replication fork at Origin of Replication [iG1 10.16, 19]
provides two single-stranded DNA template ( ssDNA )
multiple replications forks ( replicons ) [ HOMEWORK #6 ]
Synthesis of RNA primer [iG1 10.15]
Addition of dNTPs by DNA Polymerase ( DNAPol III ) at 3' end only
continuous synthesis on leading strand [ iG1 10.13 ]
discontinuous synthesis on lagging strand [ iG1 10.20 ]
Okazaki fragments
leading & lagging strand synthesis simultaneously [iG1 10.21 ]
A single, dimeric DNAPol III replicates both strands
Proof-reading by 3' 
Excision of RNA primer by DNAPol I
ligation (connection) of fragment ends at gaps by DNA ligase [iG1 10.22 ]
DNA synthesis in eukaryotes
Eukaryotic genomes are much larger [the "C-value Paradox"]
eukaryotic DNA synthesis is more efficient:
More DNAPol molecules, slower rate of synthesis, more replicons on multiple chromosomes
Transcription : synthesis of messenger RNA (mRNA) (online MGA2 animation)
What is a " Gene " [iGen3 05-03] [Structure of a Eukaryotic Gene]
RNA transcribed from DNA by RNA Polymerase ( RNAPol I ) [iG1 4.17 ]
(1) Recognition of transcriptional unit: ~ 'gene' [iG1 4.18 ]
Promoters - short DNA sequences that regulate transcription
typically 'upstream' = 'leftward' from 5' end of sense strand [iG1 4.12 ]
(2) Initiation & Elongation [iG1 4.22, 23, 24 ]
mRNA synthesized 5'3' from DNA template strand
mRNA sequence therefore homologous to DNA sense strand
Colinear : mRNA and DNA sense strand "line up"
(in prokaryotes, but not eukaryotes: see below)
Process similar to DNA replication [iG1 4.25 ], except
No primer is required
Transcription may occur from either strand
Most DNA is not transcribed into RNA
(3) Termination [iG1 4.27, 28, 29 ]
Regulation of transcription
In prokaryotes, transcription & translation may occur simultaneously
In eukaryotes, transcription occurs in nucleus [ ex.: Lampbrush chromosomes]
translation occurs in cytoplasm (see next section):
RNA must cross nuclear membrane
transcription & translation are physically separated
primary RNA transcript is extensively processed
heterogeneous nuclear RNA ( hnRNA ) mRNA
Post-transcriptional processing of eukaryotic RNA is complex [iG1 4.9]
promoters [iG1 4.19] & enhancers determine initiation & control rate
'cap' ( 7-methyl guanosine , 7mG) added to 5' end [iG1 4.26 ]
'tail' of poly-A (5'-~~~AAAAAAAAAA-3') added to 3' end [iG1 4.33 ]
'splicing' of hnRNA : eukaryotic genes are "split"
intron DNA sequence equivalents removed from hnRNA : " int ervening"
exon DNA sequence equivalents represented in mRNA: " ex pressed" in protein
1 ~ 12's of exons / 'gene'
>90% of transcript may be 'spliced out'
[An important note on terminology] [or, to put it another way]
Splicing mechanism uses donor and acceptor sites [iG1 5.18, 19, 20]
Eukaryotic genes & mRNA are not colinear!
DNA / RNA hybridization produces heteroduplexes
DNA introns 'loop out'
DNA exons pair with mRNA
Eukaryotic exons may be widely separated
Alternative splicing of the same transcript produces different products [iG1 4.16 ]
Different exon regions are combined as different mRNAs [iG1 5.01]
Alternative exon combinations differ functionally [iG1 5.22]
Homework #7: Problem-solving with DNA & RNA
Ongoing Homework problem:
What is a 'gene'? How do the discoveries of (1) introns and exons and (2) alternative splicing in eukaryotic genomes modify the concept?
schweitzerroire1998.blogspot.com
Source: https://www.mun.ca/biology/scarr/2250_DNA_replication_&_transcription_2016.html
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