GENA RELICATION
Review of NUCLEIC ACIDS DNA Genetic material In the nucleus
Double stranded A,T C,G
RNA Carries information from the nucleus to the site where
proteins are made Single stranded A,U C,G
DNA is found in the nucleus Humans have 23 pairs of chromosomes
Chromosomes are made of DNA
DNA moleculThe sides of the ladder are sugar and phosphates; the
rungs of the ladder are the nitrogen bases A,T,C,G
How DNA is the Master Control
DNA Replication The structure of DNA provided an insight to
Watson and Crick for how DNA replicates each strand in a duplex of
DNA is complementary to each other, each can form a template when
separated. The order of bases on one strand can be used to add in
complementary bases and therefore duplicate the pairs of bases
exactly. Model for DNA Replication Watson and Cricks model:
semiconservative replication
Three models of DNA replication
Experiment which supports the Semiconservative Model Matthew
Meselson and Franklin Stahl experiments supported the
semiconservative model labeled the nucleotides of the old strands
with a heavy isotope of nitrogen (15N) while any new nucleotides
would be indicated by a lighter isotope (14N). Replicated strands
could be separated by density in a centrifuge. Each model:
Meselson-Stahl experiment
Conclusion: DNA replication follows semiconservative model
Conservative and Dispersive models were disproven in their
experiment.
DNA Replication More than a dozen enzymes and other proteins
participate E. coli can replicate 4.5 x 106 base pairs bp) in less
than an hour human cells can replicate 6 x 109 bp in only a few
hours DNA replication is very accurate less than 1 error per
billion nucleotides!! DNA Replication Start Sites Where does DNA
replication start? special sites termed origins of replication
single site in bacterial chromosome multiple sites in eukaryotic
chromosome Enzymes (helicases) separate the two strands forms a
replication bubble other proteins (single strand binding proteins -
ssb) bind to keep strands separated Origins of Replication
Enzymes of DNA ReplicationDNA polymerases synthesize DNA by
adding a nucleotide that is complementary to the base in the
template strand Rate of synthesis Bacteria - 500 nucleotides / sec
Human cells - 50 nucleotides / sec Incorporation of a
nucleotide
DNA Synthesis leading strand is synthesized continuously lagging
strand is synthesized discontinuousl in short segments called
Okazaki fragments DNA ligase joins the fragments
Initiating DNA SynthesisAfter separation of the DNA strands DNA
Polymerase cannot initiate DNA syn. Needs a 3 OH to add nucleotide
to. synthesizing a new chain requires a primer, a short segment of
RNA Primase (an RNA Polymerase) adds about 10 nucleotides
complementary to template Priming DNA SynthesisNote: RNA primer is
removed from DNA by another DNA Polymerase
Bacterial DNA Replication Proteins
Bacterial DNA Replication Proteins Helicase Unwinds parental
double helix at replication forks DNA double helix are tightly
coupled. High temperature is needed to break them (95oC
ssb proteinbinds to and stabilizes ssDNA Topoisomerase Corrects
overwinding ahead of replication forks breaks, swivels, and rejoins
DNA strands Primase synthesizes single primer for leading strand
synthesizes RNA primer for each lagging strand DNA pol III
continuous synthesis of leading strand discontinuous synthesis of
lagging strand DNA pol Iremoves primer (RNA) from DNA strand and
replaces it with DNA DNA Ligase joins 3 end of fragment with 5 end
of adjacent fragment DNA replication Fork
Summary of DNA Replication
Mutation Mutation: A change in the base sequence of the DNA
Mutations are changes in the genotype which may or may not affect
the phenotype Causes of mutations Spontaneous mutations Occur in
the absence of mutation causing agents Due to occasional mistakes
in DNA replication Induced mutationsCaused by mutagens, agents such
as chemicals and radiation which induce mutations Types of Repair
of DNA Bases may be damaged by chemical and/or physical agentsUV
light, reactive chemicals, radiation, etc. Some mismatched bases
may be missed by proofreading activity of DNA pol Must be corrected
to ensure high fidelity of DNA sequence Types of mutation Base
substitutions The most common type of mutation A single base pair
is replaced by another Frame shift mutations One or more base pairs
are inserted or deleted in the DNA Results in a change in the
reading of codons mutagen Chemical mutagensExample: Nitrous acid
alters adenine such that it pairs with cytosine instead of
thymine
Radiation Ionizing radiation e.g., Xrays and gamma rays Causes
the formation of ions that can react with nucleotides (causing base
changes) and the deoxyribose-phosphate backbone (causes chromosomes
to break). UV radiationInduces formation of covalent bonds between
adjacent thymines to form thymine dimers which can not be
replicated
Consequences of base substitutions Silent mutation: base change
results in no change of the amino acid sequence of the translated
protein Silent mutations have no effect on phenotype A result of
the fact that multiple codons can code for the same amino acidE.g.,
AGU and AGC both code for Serine Missense mutation: base change
results in the change of an amino acid in the translated protein
The amino acid substitution induced by the missense mutation may
have no effect on the function of the protein OR It may abolish the
activity of the protein or alter its function having an effect on
phenotype Example: sickle cell disease in humans is due to a
missense mutation in the gene for globin. As a result the shape of
red blood cells is altered affecting their movement through
capillaries.
Nonsense mutation: base change generates a stop codon in place
of that coding for an amino acid Results in production of a
truncated protein. Usually results in a non-functional protein
Frameshift mutation: addition or deletion of one or more bases
Results in misreading of the codons (changed reading frame) Almost
always results in long stretches of altered amino acids and the
production of inactive protein
Nucleotide Excision Repair In nucleotide excision repair, a
nuclease cuts out a segment of a damaged strand. The gap is filled
in by DNA polymerase and ligase.
Penyakit mutasi Xeroderma PigmentosumIndividuals with this
genetic disease have defective repair enzymes cant remove thymine
dimers caused by UV light very sensitive to sunlight and often get
skin cancers
Royal Hemophilia X linked Queen Victoria (Great Britain) was the
carrier Gene passed on to many other royal families in Europe
Russian, Prussian and Spanish affected But not the British royal
family Genetic warfare? Translation: Making Proteins A group of
three nucleotides in messenger RNA codes for a certain amino acid
to be placed in a protein. Each group of three nucleotides is
called a CODON. Stop kodon UAG, UGA, UAAStart kodon AUG DNA Repair
Spontaneous DNA damage Pathways to remove DNA damage Damage
detection The repair of Double-strand break DNA repair enzymes
Now you know---How DNA is the Master Control
TAC CGA TCGATG GCT AGC transcripsiTAC CGA TCGAUG GCU AGC new RNA
translasiTAC CGA TCGAUG GCU AGC met, ala, ser So how do you
remember the difference? Replication just makes a repeat copy of
DNA Transcription rips a new strand of RNA Translation starts with
a new slate- the slate of amino acids to make a protein
How DNA is the Master ControlDNAnucleotidesREPLICATIONREpeats
DNA can make copies of itself=REPLICATION
5DNA is the master control of the cell, dictating which RNAs and
proteins are made. Every cell in an organism has the same DNA. For
that reason, there must be a way to make copies of the DNA in a
cell, so all new cells made are identical to the old ones. The
process by which copies of DNA are made is called
REPLICATION.Meselson-Stahl Experiment supports the Semiconservative
ModelEach model: the semi-conservative model, the conservative
model, and the dispersive model, made specific predictions on the
density of replicated DNA
strands.ConservativeSemi-ConservativeDispersiveone heavyone
lightboth intermediate
Mutation Generation passed on to daughter DNAs48
Emergency DNA Repair for Double helix break49Now you know---How
DNA is the Master
ControlDNARNAPROTEINTRANSCRIPTIONTRANSLATIONnucleotidesnucleotidesamino
acidsREPLICATIONREpeatsRemember you are translating from the
language of nucleotides to the language of amino acids
39Now you know that DNA dictates how other DNA will be made, how
RNA will be made and ultimately how proteins are made.