Chapter 16 Flashcards
In 1953, James Watson and Francis Crick introduced an
elegant double-helical model for the structure of deoxyribonucleic acid, or DNA
DNA, the substance of inheritance, is
the most celebrate molecule of our time
Hereditary information is encoded in
DNA and reproduced in all cells of the body
This DNA program directs the
development of biochemical, anatomical, physiological, and (to some extent) behavioral traits
DNA is the
genetic material
Early in the 20th century,
the identification of the molecules of inheritance loomed as a major challenge to biologists
When T.H. Morgan’s group showed that genes are located on chromosomes,
the two components of chromosomes—DNA and protein— became candidates for the genetic material
The key factor in determining the genetic material was
choosing appropriate experimental organisms
The role of DNA in heredity was first discovered by
studying bacteria and the viruses that infect them
The discovery of the genetic role of DNA began with research by
Frederick Griffith in 1928
the mouse guy
Frederick Griffith worked with
two strains of a bacterium, one pathogenic (bad) and one harmless
When Griffith mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain,
some living cells became pathogenic
Griffith called this phenomenon
transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA
In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that
the transforming substance was DNA
they figured out Griffith (the mouse guys) experiment.
((DNA is transforming bacteria causing mice to die??))
Their (Oswald Avery, Maclyn McCarty, and Colin MacLeod) conclusion was based on experimental evidence that
only DNA worked in transforming harmless bacteria into pathogenic bacteria.
Many biologists remained skeptical, mainly because little was known about DNA.
Evidence that viral DNA can
program cells
More evidence for DNA as the genetic material came from
studies of viruses that infect bacteria
Such viruses, called bacteriophages (or phages), are
widely used in molecular genetics research
Bacteria is only made of
DNA and protein
In 1952, Alfred Hershey and Martha Chase performed experiments showing that
DNA is the genetic material of a phage known as T2.
the blender experiment
To determine this, Alfred Hershey and Martha Chase designed an experiment showing that
only one of the two components of T2 (DNA or protein) enters an E. coli cell during infection
Alfred Hershey and Martha Chase concluded that
the injected DNA of the phage provides the genetic information
It was known that DNA is a
polymer of nucleotides, each consisting of a nitrogenous base, a sugar, and a phosphate group
In 1950, Edwin Chargaff reported that
DNA composition varies from one species to the next.
This evidence of diversity made DNA a more credible candidate for the genetic material
((DNA Rules))
Two findings became known as Chargaff’s Rules:
- The base composition of DNA varies between species
- -Humans have 30.3% A (adenine)
- -E. coli has 26% A (adenine)
-In any species the number of A and T bases are equal and the number of G and C bases are equal
The bases for Chargaff’s rules was not understood until
the discovery of the double helix
After DNA was accepted as the genetic material, the challenge was to
determine how its structure accounts for its role in heredity
Maurice Wilkins and Rosalind Franklin were using a technique called
X-ray Crystallography to study molecule structure
Rosalind Franklin produced a picture of the
DNA molecule using this X-ray Crystallography technique
Scientists use X-ray crystallography to
determine a protein’s structure
Another method is
nuclear magnetic resonance (NMR) spectroscopy, which does not require protein crystallization
Bioinformatics uses computer programs to
predict protein structure from amino acid sequences
Rosalind Franklin’s X-ray crystallographic images of DNA enabled James Watson to
deduce that DNA was helical
The X-ray images also enabled James Watson to deduce the
width of the helix and the spacing of the nitrogenous bases
The pattern in the photo suggested that the DNA molecule was made up of
two strands, forming a double helix
James Watson and Francis Crick built models of a
double helix to conform to the X-rays and chemistry of DNA
Rosalind Franklin had concluded that there were
two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior
James Watson built a model in which the
backbones were antiparallel (their subunits run in opposite directions)
At first, James Watson and Francis Crick thought the
bases paired like with like (A with A, and so on), but such pairings did not result in a uniform width
Instead, pairing a purine with a pyrimidine resulted in a
uniform width consistent with the X-ray data
James Watson and Francis Crick reasoned that the
pairing was more specific, dictated by the base structures
James Watson and Francis Crick 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
Many proteins work together in
DNA replication and repair
The relationship between structure and function is
manifest in the double helix
James Watson and Francis Crick noted that
the specific base pairing suggested a possible copying mechanism for genetic material.
The Basic Principle:
Base pairing to a template strand
Since the two strands of DNA are complementary,
each strand acts as a template for building a new strand in replication
In DNA replication,
the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules
DNA replication occurs in the
S phase of interphase
James Watson and Francis 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
((half old stuff and half new stuff))
Competing models were the
conservative model (the two parent strands rejoin) and the dispersive model (each strand is a mix of old and new)
Experiments by Matthew Meselson and Franklin Stahl supported the
semiconservative model ((half old stuff and half new stuff))
The copying of DNA is remarkable in its
speed and accuracy.
There is only 1 mistake in 10 billion nucleotides
More than a dozen enzymes and other proteins participate in
DNA replication
Replication begins at particular sites called
origins of replication, where the two DNA strands are separated, opening up a replication “bubble”
A eukaryotic chromosome may have
hundreds or even thousands of origins of replication
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 new DNA strands are elongating
Helicases are
enzymes that untwist the double helix at the replication forks
Single-strand binding proteins
bind to and stabilize single-stranded DNA (keeps the strands apart)
Topoisomerase
corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands
DNA polymerases cannot initiate synthesis of a
polynucleotide; they can only add nucleotides to the 3’ end
The initial nucleotide strand is a short
RNA primer
An enzyme called primase can start an
RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template
The primer is short (5-10 nucleotides long), and the
3’ end serves as the starting point for the new DNA strand
Enzymes called DNA polymerases III catalyze the
elongation of new DNA at a replication fork
Most DNA polymerases III require
a primer and a DNA template strand
The rate of elongation is about
500 nucleotides per second in bacteria and 50 per second 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 ATP has ribose
As each monomer of dATP joins the DNA strand,
it loses two phosphate groups as molecule of pyrophosphate
The antiparallel structure of the double helix affects
replication
DNA polymerases (III??) add
nucleotides only to the free 3’ end of a growing strand; therefore, a new DNA strand can elongate only in the 5’ to 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 fragments called
Okazaki fragments, which are joined together by DNA ligase
The proteins that participate in DNA replication form a
large complex, a “DNA replication machine”
The DNA replication machine may be
stationary during the replication process
Recent studies support a model in which
DNA polymerase molecules “reel in” parental DNA and “extrude” newly made daughter DNA molecules
DNA polymerases I proofread newly made
DNA, replacing any incorrect nucleotides
In mismatch repair of DNA,
repair enzymes correct errors in base pairing
(during duplication)
DNA can be damaged by
exposure to harmful chemical or physical agents such as cigarette smoke and X-rays; it can also undergo spontaneous changes
In nucleotide excision repair,
a nuclease cuts out and replaces damaged stretches of DNA
(not during duplication. this is when DNA is damaged by exposure to harmful stuff)
Error rate after proofreading 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
Limitations of DNA polymerase create problems for the
linear DNA of eukaryotic chromosomes
The usual replication machinery provides no way to
complete the 5’ ends, so repeated rounds of replication produce shorter DNA molecules with uneven ends.
This is not a problem for prokaryotes, most of which have circular chromosomes.
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
It has been proposed that the shortening of telomeres is
connected to aging
If chromosomes of germ cells became shorter in every cell cycle,
essential genes would eventually be missing from the gametes they produce
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
(once it gets the shortest, it can signal apoptosis)
There is evidence of telomerase activity in
cancer cells, which may allow cancer cells to persist
A chromosome consists of a
DNA molecule packed together with proteins
The bacterial chromosome is a
double-stranded, circular DNA molecule associated with a small amount of protein
Eukaryotic chromosomes have
linear DNA molecules associated with a large amount of protein
In a bacterium,
the DNA is “supercoiled” and found in a region of the cell called the nucleoid
(prokaryote)
Chromatin, a complex of DNA and protein, is found in
the nucleus of eukaryotic cells
Chromosomes fit into the nucleus through an
elaborate, multilevel system of packing
Histones are
proteins that are responsible for the first level of DNA packing in chromatin
DNA winds around
histones to form nucleosome “beads”
Nucleosomes are
strung together like beads on a string by linker DNA
Chromatin undergoes changes in
packing during the cell cycle
At interphase, some chromatin is organized into a
10-nm fiber, but much is compacted into a 30-nm fiber, through folding and looping
(don’t need to know the sizes and lengths of these)
Though interphase chromosomes are not highly condensed,
they still occupy specific restricted regions in the nucleus
Most chromatin is loosely packed in the
nucleus during interphase and condenses prior to mitosis
Loosely packed chromatin is called
euchromatin
During interphase a few regions of chromatin (centromeres and telomeres) are highly condensed into
heterochromatin
Dense packing of the heterochromatin makes it
difficult for the cell to express genetic information coded in these regions
Histones can undergo chemical modifications that result in
changes in chromatin organization
