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
Homework
#5
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: Prokaryotic mRNA and DNA sense strand "line up"
Process similar to DNA replication, except
Transcription may occur from either and (or) both strands
Most DNA 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
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]
'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 ~ 99% of transcript may be 'spliced out'
[An
important note on terminology] [or, to
put it another way]
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
Different exon
transcripts are combined as different mRNAs
Alternative transcript
combinations differ functionally
What is a 'gene'? How do introns and exons and alternative
splicing in eukaryotic genomes modify the concept?
