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
Nucleotides are added simultaneously to the 3' ends of
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
Transcription: synthesis of messenger RNA (mRNA)
What is a "Gene"? [Structure
of a Eukaryotic Gene]
RNA transcribed from DNA by RNA Polymerase (RNAPol
I)
(1) Recognition of transcriptional unit: ~ 'gene'
Promoters - short DNA sequences
that regulate
transcription
typically 'upstream'
= 'leftward' from 5' end of sense strand
(2) Initiation & Elongation
mRNA synthesized 5'3' from DNA template strand
mRNA sequence therefore homologous to DNA sense strand
Co-linear: mRNA and DNA
sense strand "line up"
(in
prokaryotes, but not
eukaryotes: see below)
Process similar to DNA replication, except
No primer required
Transcription may occur from either or both strands
Most DNA is not
transcribed into RNA
(3) Termination
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 [Summary]
promoters [iG1 4.19] &
enhancers determine initiation &
control rate
'cap' (7-methyl guanosine, 7mG) added to 5'
end
'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 : "intervening"
exon DNA
sequence equivalents represented
in mRNA: "expressed" 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
co-linear!
DNA / RNA hybridization
produces heteroduplexes
DNA
introns
'loop out'
DNA exons pair
with mRNA
Eukaryotic exons may be widely separated
Generalized structure of a eukaryotic
transcription complex
Alternative splicing of the same transcript produces
different products [iG1 4.16]
Different exon
transcripts are combined as different mRNAs [iG1 5.01]
Alternative transcript
combinations differ functionally [iG1 5.22]
What is a 'gene'? How do introns and exons and alternative
splicing in eukaryotic genomes modify the concept?
