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Ch.16: The Molecular Basis of Inheritance - Coggle Diagram
Ch.16: The Molecular Basis of Inheritance
DNA Is The Genetic Material
Additional Evidence That DNA Is the Genetic
Material
DNA
is a polymer of nucleotides
each consisting of
a nitrogenous base
a sugar
a phosphate
group
The nitrogenous bases can be adenine (A),
thymine (T), guanine (G), or cytosine (C)
This evidence of molecular diversity among
organisms made DNA a more credible candidate
for the genetic material
Chargaff’s rules
The base composition of DNA varies between
species
In any species the number of A and T bases is equal
and the number of G and C bases is equal
The basis for these rules was not understood until
the discovery of the double helix
Building a Structural Model of DNA
The pattern in the photo suggested that the DNA
molecule was made up of two strands, forming a
double helix
Franklin’s X-ray crystallographic images of DNA
allowed James Watson to deduce that DNA was
helical
Watson built a model in which the backbones were
antiparallel
(their subunits run in opposite
directions)
They determined that adenine (A) paired only with
thymine (T), and guanine (G) paired only with
cytosine (C)
The Watson-Crick model explains Chargaff’s rules:
in any organism the amount of A = T, and the
amount of G = C
Evidence That Viral DNA Can Program Cells
Bacteriophages
-viruses that infect bacteria
A
virus
is DNA (sometimes RNA) enclosed by a
protective coat, often simply protein
Alfred Hershey and Martha Chase concluded that the injected DNA of the phage
provides the genetic information
More evidence for DNA as the genetic material
came from studies of viruses that infect bacteria
Evidence That DNA Can Transform Bacteria
The discovery of the genetic role of DNA began
with research by Frederick Griffith in 1928
When he mixed heat-killed remains of the
pathogenic strain with living cells of the harmless
strain, some living cells became pathogenic
He called this phenomenon transformation, now
defined as a change in genotype and phenotype
due to assimilation of foreign DNA
Later work by Oswald Avery, Maclyn McCarty, and
Colin MacLeod identified the transforming
substance as DNA
Chromosome Consists of a
DNA Molecule Packed Together with Proteins
In a bacterium, the DNA is “supercoiled” and found
in a region of the cell called the nucleoid
In the eukaryotic cell, DNA is precisely combined
with proteins in a complex called
chromatin
Chromosomes fit into the nucleus through an
elaborate, multilevel system of packing
Proteins called
histones
are responsible for the
main level of DNA packing in interphase chromatin
In a 10-nm chromatin fiber, the unfolded chromatin
resembles beads on a string, with each “bead”
being a
nucleosome
Types of Chromatin
Loosely packed chromatin is called
euchromatin
During interphase a few regions of chromatin
(centromeres and telomeres) are highly condensed
into
heterochromatin
Most chromatin is loosely packed in the nucleus
during interphase and condenses prior to mitosis
Summary of Key Concepts
Experiments with bacteria and with
phages
provided the first strong evidence that the genetic material is DNA.
Watson and Crick deduced that DNA is a
double helix
and built a structural model. Two
antiparallel
sugar-phosphate chains wind around the outside of the molecule; the nitrogenous bases project into the interior, where they hydrogen-bond in specific pairs, A with T, G with C.
The Meselson-Stahl experiment showed that
DNA replication
is
semiconservative
: The parental molecule unwinds, and each strand then serves as a template for the synthesis of a new strand according to base-pairing rules
DNA polymerases
proofread new DNA, replacing incorrect nucleotides. In
mismatch repair
, enzymes correct errors that persist.
Nucleotide excision repair
is a process by which nucleases cut out and other enzymes replace damaged stretches of DNA.
The ends of eukaryotic chromosomal DNA get shorter with each round of replication. The presence of
telomeres
, repetitive sequences at the ends of linear DNA molecules, postpones the erosion of genes.
Telomerase
catalyzes the lengthening of telomeres in germ cells.
The
chromatin
making up a eukaryotic chromosome is composed of DNA, histones, and other proteins. The
histones
bind to each other and to the DNA to form
nucleosomes
, the most basic units of DNA packing, which exist as part of the fiber. Histone tails extend outward from each bead-like nucleosome core. Additional coiling and folding lead ultimately to the highly condensed chromatin of the metaphase chromosome
Chromosomes occupy restricted areas in the interphase nucleus. In interphase cells, most fiber chromatin is loosely arranged (
euchromatin
), but some is more densely arranged (
heterochromatin
). Euchromatin, but not heterochromatin, is generally accessible for gene transcription
Proteins Work Together in
DNA Replication and Repair
Watson and Crick noted that the specific base
pairing suggested a possible copying mechanism
for genetic material
The copying of DNA is called
DNA replication
Base Pairing to a Template
Strand
Watson and Crick’s
semiconservative model of
replication
predicts that when a double helix
replicates, each daughter molecule will have one
old strand (derived or “conserved” from the parent
molecule) and one newly made strand
Competing models were the conservative model
(the two parent strands rejoin) and the dispersive
model (each strand is a mix of old and new)
DNA Replication: A Closer Look
Getting Started
Replication begins at particular sites called
origins
of replication
, where the two DNA strands are
separated, opening up a replication “bubble”
Replication proceeds in both directions from each
origin, until the entire molecule is copied
At the end of each replication bubble is a
replication fork
, a Y-shaped region where parental
DNA strands are being unwound
Helicases
are enzymes that untwist the double
helix at the replication forks
Topoisomerase
relieves the strain of twisting of
the double helix by breaking, swiveling, and
rejoining DNA strands
Synthesizing a New DNA Strand
DNA polymerases require a primer to which they
can add nucleotides
The initial nucleotide chain is a
short RNA primer
This is synthesized by the enzyme
primase
Enzymes called
DNA polymerases
catalyze the
synthesis of new DNA at a replication fork
Antiparallel Elongation
DNA polymerases add nucleotides only to the free
3′ end of a growing strand; therefore, a new DNA
strand can elongate only in the 5′ → 3′ direction
Along one template strand of DNA, the DNA
polymerase synthesizes a
leading strand
continuously, moving toward the replication fork
To elongate the other new strand, called the
lagging strand
, DNA polymerase must work in the
direction away from the replication fork
The lagging strand is synthesized as a series of
segments called
Okazaki fragments
, which are
joined together by
DNA ligase
Proofreading and Repairing DNA
DNA polymerases proofread newly made DNA,
replacing any incorrect nucleotides
In
mismatch repair
of DNA, repair enzymes
replace incorrectly paired nucleotides that have
evaded the proofreading process
In
nucleotide excision repair
, a
nuclease
cuts out
and replaces damaged stretches of DNA
Evolutionary Significance of Altered DNA
Nucleotides
The error rate after proofreading and repair is low
but not zero
Sequence changes may become permanent and
can be passed on to the next generation
These changes (mutations) are the source of the
genetic variation upon which natural selection
operates and are ultimately responsible for the
appearance of new species
Replicating the Ends of DNA Molecules
Eukaryotic chromosomal DNA molecules have
special nucleotide sequences at their ends called
telomeres
Telomeres do not prevent the shortening of DNA
molecules, but they do postpone the erosion of
genes near the ends of DNA molecules
An enzyme called
telomerase
catalyzes the
lengthening of telomeres in germ cells
The shortening of telomeres might protect cells
from cancerous growth by limiting the number of
cell divisions
There is evidence of telomerase activity in cancer
cells, which may allow cancer cells to persist