Chapter 16
• Hereditary information is encoded in DNA
(deoxyribonucleic acid)
– Reproduced in all cells of the body
– Transmitted to offspring by chromosomes in
gametes
• DNA directs the development of biochemical,
anatomical, physiological,
and behavioral traits
• Nucleic Acid
• Located in nucleus of cell
– Genetic material inherited from
parents
• Genes code for specific proteins with unique code of
nucleotides
• Monomers= nucleotides
– 3 parts to nucleotide
• 5 carbon sugar (deoxyribose)
• Phosphate group
• Nitrogenous base
• Nucleotides form polynucleotides
• Double helix structure
– 2 polynucleotides wrap around each other
– Nitrogenous bases pair in center between 2
backbones
• Strands are complementary
– 4 nitrogenous bases
• Adenine-Thymine
• Cytosine-Guanine
• Strands are anti-parallel
• Two ends of strand are
different from each other
– One end has a phosphate
attached to a 5’ carbon
– Other end has a hydroxyl
group attached to a 3’ carbon
5’
3’
3’
5’
• Bacterial chromosome= double-stranded,
circular DNA molecule associated with a small
amount of protein
– In bacteria, the DNA is “supercoiled” and found in
a region of the cell called the nucleoid
• Eukaryotic chromosomes have linear DNA
molecules associated with a large amount of
protein
– Chromatin, a complex of DNA and protein, is
found in the nucleus of eukaryotic cells
• In humans, each cell has DNA comprised of ~6
billion base pairs
• Each diploid cell contains ~2 m of DNA
• In total, humans contain ~100 trillion m of
DNA
– Enough to circle equator of Earth 2.5 million
times!
• 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
• DNA fits into the nucleus through an elaborate,
multilevel system of packing
Nucleosome
(10 nm in diameter)
DNA double helix
(2 nm in diameter)
Histones
DNA, the double helix
Histones
Histone
tail
H1
Nucleosomes, or “beads on
a string” (10-nm fiber)
The DNA molecule binds with proteins known as histones, due to a
negative charge on the strands of the DNA molecule and positive
charges on histones
Nucleosome
(10 nm in diameter)
DNA double helix
(2 nm in diameter)
Histones
DNA, the double helix
Histones
Histone
tail
H1
Nucleosomes, or “beads on
a string” (10-nm fiber)
Nucleosome is a histone complex with the DNA molecule wrapped around
twice. The histone tails (amino end of protein) extend outward. The
strands of DNA between the nucleosomes are called “linker DNA”.
Chromatid
(700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
30-nm fiber
Interactions between histone
tails and linker DNA result in
further compaction into 30-nm
fiber
Replicated
chromosome
(1,400 nm)
Looped domains
(300-nm fiber)
Metaphase
chromosome
Chromatid
(700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
30-nm fiber
This fiber forms loops called
looped domains attached to a
protein scaffold, compacting
material into 300 nm fiber
Replicated
chromosome
(1,400 nm)
Looped domains
(300-nm fiber)
Metaphase
chromosome
Chromatid
(700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
30-nm fiber
The looped domains condense
further into the chromosomes
visible during the stages of
mitosis
Replicated
chromosome
(1,400 nm)
Looped domains
(300-nm fiber)
Metaphase
chromosome
• Early in the 20th century, the identification of
the molecules of inheritance loomed as a
major challenge to biologists
• T. H. Morgan’s group showed genes are
located on chromosomes
– 2 components of chromosomes—DNA and
protein—became candidates for the genetic
material
• Frederick Griffith (1928)
• Experiments with two strains of a bacteria
causing pneumonia
– one pathogenic and one harmless
• When he mixed heat-killed remains of the
pathogenic strain with living cells of the harmless
strain, some living cells became pathogenic
– Transformation= a change in genotype and phenotype due
to assimilation of foreign DNA
• Studies in 1944 by Oswald Avery, Maclyn
McCarty, and Colin MacLeod provided
experimental evidence that only DNA worked
in transforming harmless bacteria into
pathogenic bacteria
• In 1950, Erwin Chargaff reported that DNA
composition varies from one species to the
next
– Made DNA a more credible candidate for the
genetic material
• At this time, it was known that
DNA is a polymer of
nucleotides, each consisting of
a nitrogenous base, a sugar,
and a phosphate group
• Two findings became known as
Chargaff’s rules
– The base composition of DNA
varies between species
– In any species the number of A and
T bases are equal and the number
of G and C bases are equal
• 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
• In 1952, Alfred Hershey and Martha Chase
designed an experiment using a phage known
as T2 and E. coli cells
EXPERIMENT
Phage
Radioactive
protein
Empty
protein
shell
Radioactivity
(phage protein)
in liquid
Bacterial cell
Batch 1:
Radioactive
sulfur
(35S)
DNA
Phage
DNA
Centrifuge
Pellet (bacterial
cells and contents)
Radioactive
DNA
Batch 2:
Radioactive
phosphorus
(32P)
Centrifuge
Pellet
Radioactivity
(phage DNA)
in pellet
• Experiment with T2 and E. coli cells
– Results showed only one of the two components
of T2 (DNA or protein) enters an E. coli cell during
infection
– Concluded that the injected DNA of the phage
provides the genetic information
• 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 Xray crystallography to study
molecular structure
– Franklin produced a picture
of the DNA molecule using
this technique
• Franklin’s picture was used by James Watson
and Francis Crick to model the structure of
DNA
– DNA was helical
– Width of the helix and spacing of nitrogenous
bases
• The pattern in the photo suggested that the
DNA molecule was made up of two strands,
forming a double helix
Nitrogenous bases
C
5 end
G
C
Hydrogen bond
G
C
G
3 end
C
G
A
T
3.4 nm
A
T
C
G
C
G
A
T
1 nm
A
G
C
G
A
G
A
T
3 end
T
A
T
G
C
T
C
C
C
G
T
A
(a) Key features of
DNA structure
0.34 nm
5 end
(b) Partial chemical structure
Sugar-phosphate backbone
(c) Space-filling
model
Strands are anti-parallel
Purine  purine: too wide
Pyrimidine  pyrimidine: too narrow
Purine  pyrimidine: width
consistent with X-ray data
• Watson and Crick reasoned that the pairing was more specific
– 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
Sugar
Sugar
Adenine (A)
Thymine (T)
Sugar
Sugar
Guanine (G)
Cytosine (C)
• Watson and Crick noted that the specific base
pairing suggested a possible copying mechanism
for genetic material
• 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
• Watson and Crick’s semiconservative model of replication
– After replication, each daughter molecule will have one old strand
(from the parent molecule) and one newly made strand
A
T
A
T
A
T
A
T
C
G
C
G
C
G
C
G
T
A
T
A
T
A
T
A
A
T
A
T
A
T
A
T
G
C
G
C
G
C
G
C
(a) Parent molecule
(b) Separation of
strands
(c) “Daughter” DNA molecules,
each consisting of one
parental strand and one
new 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)
– Rejected by later experiments by Matthew Meselson and Franklin Stahl
• Replication begins at origins of replication, where
the two DNA strands are separated, opening up a
replication “bubble”
– Bacterial DNA has one origin of replication for its
circular DNA
– A eukaryotic chromosome may have hundreds or even
thousands of origins of replication, increasing speed
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
At the replication forks, both strands
replicated at same time in the 5’ to 3’
direction
Helicase- unwinds double helix
Topoisomerase- prevents overwinding at
replication fork by breaking, swiveling, and
rejoining DNA strands
Single-strand binding proteins bind to and
stabilize single-stranded DNA
DNA primase- start an RNA chain from scratch and
adds RNA nucleotides one at a time using the
parental DNA as a template
RNA primer- short (5–10 nucleotides long)
RNA molecule that serves as the starting
point for the new DNA strand
DNA polymerases- catalyze the elongation of new
DNA at a replication fork by adding nucleotides only
to the free 3’ end of a growing strand
Template strand
3
New strand
5
Sugar
Phosphate
5
3
A
Base
T
A
T
C
G
C
G
G
C
G
C
T
A
DNA
polymerase
OH
3
A
P
C
Nucleoside
triphosphate
Pi
Pyrophosphate
OH
3
C
2Pi
5
5
Leading strand: Template strand is 3’ to 5’
Lagging strand: Template strand is 5’ to 3’
DNA polymerase synthesizes the leading strand
continuously, moving toward the replication fork in 5’
to 3’ direction
To elongate the lagging strand, DNA polymerase must
work in the direction away from the replication fork
Lagging Strand made by Okazaki fragments
-small sections of DNA made in 5’ to 3’ direction
DNA ligase- joins the Okazaki
fragments
• DNA polymerases proofread newly made DNA,
replacing any incorrect nucleotides
– Mismatch repair= repair enzymes correct errors in
base pairing
– Nucleotide excision repair= a nuclease cuts out and
replaces damaged stretches of DNA
• 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 mutations are the source of the genetic
variation upon which natural selection operates
• Replication mechanism in eukaryotic cells provides no
way to complete the 5 ends
– Repeated rounds of replication produce shorter DNA
molecules with uneven ends
– Not a problem for prokaryotes with circular chromosomes
• Eukaryotic chromosomal DNA molecules have special
nucleotide sequences at their ends called telomeres
– Repetitive DNA sequences
• 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,
preventing this problem
• 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