Chapter 5 Macromolecules and Molecular Genetics
DNA Replication
?Complementary base pairing
?Semiconservative
?Template and primer
?From 5’-phosphate to the 3’-hydroxyl end
?DNA polymerase and ligase
?A short stretch of RNA serves as a primer
?Primase,a specific RNA-polymerizing enzyme
Initiation of DNA synthesis
1,Origin of replication:
300 bases with specific
sequence;
2,DNA double helix
opens up at the origin of
replication
3,Replication fork (left)
4,Bidirectional replication
5.Theta structure in
circular DNA
6,Single eukaryotic
chromosome has many
origins
7,Replication is carefully
regulated.
DNA replication
process
1,Leading strands,5’-P 3’-OH
2,Lagging strands,5’-P RNA 3’-OH
3,DNA polymerase I,II,III
Leading and Lagging Strands
Leading strand,DNA synthesis occurs
continuously from the 5’-phosphate to
the 3’-hydroxyl;
Lagging strand,DNA synthesis occurs
discontinuously.
Enzymes in lagging strand synthesis:
DNA primase,DNA polymerase III
and I,DNA ligase
Okazaki
Fragment
Enzymes in lagging strand
synthesis:
DNA primase,
DNA polymerase III and I,
DNA ligase
Proofreading,DNA polymease III
has 3’ 5’ exonuclease activity
Replication of Linear Genetic
Elements:Protein Primer and
Telomerase
Questions on DNA replication
?Describe the DNA replication process.
?What proteins are involved in initiation of DNA
synthesis (DNA replication fork)?
?How do leading- and lagging strands replicate?
?What are enzymes involved in replicating the
lagging strand?
?What are the differences between replicating
circular DNA and linear DNA
RNA Structure and Function?
?RNA acts at two levels,Genetic and Functional:
?Genetically,carry the genetic information from DNA;
?Functionally,Structural role
? Messenger RNA (mRNA)
? Ribosomal RNA (rRNA)
? Transfer RNA (tRNA)
? Catalytic RNA,Ribozyme
Transcription
A stem-loop results
in termination of
transcription
Promoter
sequence
Promoters are specific DNA
sequences at which RNA polymerase
attach.
Few Key Points:
?Only regions of DNA that are preceded
by a promoter can be transcribed into
RNA;
?The more effective promoters are called
strong promoter;
?Eukaryotes have three different types of
RNA polymerase:
– one for synthesizing mRNA,
– one for synthesizing tRNA
– one for synthesizing rRNA
Important Concepts
?Polygenic mRNA or polycistronic mRNA:
– a single mRNA molecule often codes for more
than on protein in prokaryotes.
?Operon,a series of genes all regulated by
one operator is called operon.
RNA processing,the conversion of a
precursor RNA into a mature RNA
Methylated guanine
Ribozymes,
Catalytic
RNAs
Self-splicing
ribozymal intron
of the protozoan
Tetrahymena
Questions on RNA
?What structural roles has RNA played?
?Describe the transcription process?
?How is transcription terminated?
?What is a promotor and what is its function?
?Polygenic or polycistronic mRNA?
?What is an Operon?
?Describe the RNA processing?
?What is a ribozyme? For example?
Translation,the process of protein synthesis
? Initiation
? Elongation
? Termination-release
? Post-modification
? Ribosomes are the site
of protein synthesis
The size of the ribosome
is expressed in Svedberg unit (S)
a measure of rate at which the
component sediments in the
centrifuge
Ribosome Structure
Initiation of protein synthesis
?Initiation complex
– 30 S ribosome subunit
– mRNA
– formylmethionine tRNA (Start codon AUG)
– initiation factors
– Guanosine triphosphate
?Shine-Dalgarno sequence,Just proceding the
initiation codon on the mRNA is a sequence of from three
to nine nucleotides that is involved in the binding of the
mRNA to the ribosome.
Polysome,Translation by several
ribosomes on a single messenger RNA
?Polysomes increase the speed and efficiency
of the mRNA translation
Molecular
chaperones
Many proteins
require the
assistance of
other proteins
called
Molecular
Chaperones
for proper
folding or for
assembly into
large
complexes
Open Reading Frames (ORF)
?ORF,A mRNA starts with a start
codon AUG and stops with a stop
codons UGA,or UAG,UAA.
?Software can be developed to scan
ORF.
?Bioinformatics,combination of
computer and molcular biology
Questions to Translation
? Describe the translation process.
? What are the differences between prokaryotic and
eukaryotic ribosomes?
? What are involved in initiation o protein synthesis?
? What are polysomes and its function?
? What is the function of molecular chaperones?
? Start codon and stop codons?
? Universal codons?
? Open Reading Frame (ORF)?
? Bioinformatics?