DNA Replication & Transcription
In principle: DNA replication semi-conservative [HOMEWORK
#4]
H-bonds 'unzip', strands unwind,
complementary nucleotides added to existing strands
each "daughter" double-helix has one "old"
& one "new" strand
DNA not the "Genetic
Code" for proteins
Information in DNA must
first be transcribed into RNA
messenger
RNA
transcript
base-complementary to template strand of DNA
DNA & RNA syntheses occur only in
the 5' 
3' direction
DNA synthesis
Nucleotides added simultaneously
to 3' ends of both strands, but
DNA grows in 5' 3'
direction ONLY
Distinguish:
Replication:
duplication of double-stranded
DNA (dsDNA) molecule
makes exact 'copy' of existing molecule (cf.
'xerox' copy)
Synthesis:
biochemical creation of new single-stranded DNA (ssdNA) molecule
base-complementary 'copy' of existing strand (cf. 'silly putty' copy)
occurs only
in 5'3' direction
Homework
#5
Transcription: synthesis of messenger RNA (mRNA)
RNA transcribed from DNA by RNA Polymerase (RNAPol
I)
(1) Recognition of transcriptional unit:
Promoters - short DNA sequences
that regulate
transcription
typically 'upstream'
from 5' end of sense strand
(2) Initiation & Elongation
mRNA synthesized 5'3'
from DNA template strand
mRNA sequence therefore homologous to (co-linear with) DNA sense strand
Transcription may occur from either and (or) both strands
Most DNA not
transcribed into mRNA
(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 physically
separated
primary RNA
transcript extensively processed
heterogeneous
nuclear RNA (hnRNA) mRNA
Post-transcriptional processing of eukaryotic RNA [Summary]
'splicing' of hnRNA
intron DNA sequence
equivalents removed from hnRNA: "intron
segments intervening"
in hnRNA
exon DNA
sequence equivalents represented
in mRNA: "exon segments "expressed" in protein
1 ~ 12's of exons
/ 'gene'
90 ~ 99% of transcript may be 'spliced out' : retained RNA 'spliced
together'
[An
important note on terminology , or to
put it another way]
Eukaryotic genes & mRNA are not 'co-linear' !
DNA / RNA hybridization
produces heteroduplexes
DNA
intron
equivalents 'loop
out'
DNA exon equivalents pair
with mRNA exons
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?
