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Inheritance (DNA is the genetic material (Building a structural model of…
Inheritance
DNA is the genetic material
chromosomal components (DNA and protein)
are
genetic material
Griffith's research (1928)
mixed heat-killed pathogenic with living harmless cells
some living cells became pathogenic
Transformation
- change in genotype and phenotype due to assimilation of foreign DNA
(Later work showed DNA as the transforming substance
Viral DNA
Bacterio(phages)
- viruses that infect bacteria - widely used in molecular genetics research
Virus
- DNA enclosed in protein coat (capsid)
Alfred Hershey and Martha Chase showed that DNA is genetic material of phages (T2)
only one of the two components of T2 enters during infection
proves that the injected DNA provides the genetic information
Additional Evidence
DNA is a polymer of nucleotides, all having a nitrogenous base, sugar, and a phosphate group
Nitrogenous bases are adenine (A), thymine (T), guanine (G), or cytosine (C)
Erwin Chargass reported that DNA comp. changes with species
Chargaff's rules
base composition of DNA varies between species
A and T bases are always equal
G and C are always equal
(Not understood until the discovery of the double helix)
Building a structural model of DNA
Maurice Wilkins and Rosalind Franklin's X-Ray crystallography to study molecular structure
picture was produced from this led to many deductions
DNA is helical (double-stranded helix), with width of the helix and N-bases spacing
Ten base pairs make a full turn
bases are .34 nm apart
Franklin concluded that there were two outer sugar-phosphate backbones, with the bases on the interior of the molecule
Watson built a model in which the backbones were antiparallel
pairing purines (A/G) with pyrimidines (C/T) gave a consistent X-ray width
Watson-Crick model explains Chargaff's rules: A=T and G=C
Proteins in DNA replication/repair
structure and function relationship begin in the double helix
Base pairing to a Template Strand
Each strand acts as a template for building a new strand in replication
Parent molecule winds and two new daughter strands are built based on base-pairing rules
semiconservative
model states that each daughter strand take one of the parent strands to use as its own
Conservative
model states the parent strands rejoin after splitting
Dispersive
model shows the parent and daughter strands being mixed together
DNA Replication
more than a dozen enzymes and other proteins participate in DNA replication
replication begins at
origins of replication
where DNA strands are separated
eukaryotic chromosomes have hundreds to thousands of origins of replication
replication happens in both directions from each origin until entire molecule is copied
each replication bubble has a replication fork
replication fork
- Y-shaped region where new DNA strands are elongating
helicases
are enzymes that untwist the double helix at the replication forks
single-strand binding proteins
bind and stabilize single-stranded DNA
Topoisomerase
relieves the strain of twisting of the double helix by breaking/rejoining DNA strands
synthesizing new DNA strands
DNA polymerases require a primer to which they can add nucleotides
The initial nucleotide strand is a short RNA primer
synthesized by an enzyme primase
Primase starts an RNA chain and adds nucleotides one at a time
uses the parental DNA as a template
primer is short (5-10 nucleotides long)
3' end serves as the starting point for the new DNA strand
DNA polymerases catalyze the synthesis of new DNA at a replication fork
the rate of elongation is about 500 nucleotides per second in bacteria and 50 per sec in human cells
Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate
dATP supplies adenine to DNA and is similar to the ATP of energy metabolism
the difference is in their sugars
dATP has deoxyribose while ATO has ribose