The Molecular Basis of Inheritance
Chapter 16
Life’s Operating Instructions• 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
DNA= Deoxyribonucleic acid• 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
DNA• 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
DNA • 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
3’5’
5’3’
Chromosome Structure• 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
Chromosome Structure
• 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!
Chromosome Structure• 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
Figure 16.22a
DNA double helix(2 nm in diameter)
DNA, the double helix
Nucleosome(10 nm in diameter)
Histones
Histones
Histonetail
H1
Nucleosomes, or “beads ona 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
Figure 16.22a
DNA double helix(2 nm in diameter)
DNA, the double helix
Nucleosome(10 nm in diameter)
Histones
Histones
Histonetail
H1
Nucleosomes, or “beads ona 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”.
Figure 16.22b
30-nm fiber
30-nm fiber
Loops Scaffold
300-nm fiber
Chromatid(700 nm)
Replicatedchromosome(1,400 nm)
Looped domains(300-nm fiber) Metaphase
chromosome
Interactions between histone tails and linker DNA result in further compaction into 30-nm fiber
Figure 16.22b
30-nm fiber
30-nm fiber
Loops Scaffold
300-nm fiber
Chromatid(700 nm)
Replicatedchromosome(1,400 nm)
Looped domains(300-nm fiber) Metaphase
chromosome
This fiber forms loops called looped domains attached to a protein scaffold, compacting material into 300 nm fiber
Figure 16.22b
30-nm fiber
30-nm fiber
Loops Scaffold
300-nm fiber
Chromatid(700 nm)
Replicatedchromosome(1,400 nm)
Looped domains(300-nm fiber) Metaphase
chromosome
The looped domains condense further into the chromosomes visible during the stages of mitosis
Genetic Material
• 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
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
Genetic Material
• 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
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
Genetic Material
• 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
Figure 16.4-3
Bacterial cell
Phage
Batch 1:Radioactivesulfur(35S)
Radioactiveprotein
DNA
Batch 2:Radioactivephosphorus(32P)
RadioactiveDNA
Emptyproteinshell
PhageDNA
Centrifuge
Centrifuge
Radioactivity(phage protein)in liquid
Pellet (bacterialcells and contents)
PelletRadioactivity(phage DNA)in pellet
EXPERIMENT
Genetic Material
• 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
DNA• 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 molecular structure– Franklin produced a picture of
the DNA molecule using this technique
DNA
• 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
Figure 16.7
3.4 nm
1 nm
0.34 nm
Hydrogen bond
(a) Key features ofDNA structure
Space-fillingmodel
(c)(b) Partial chemical structure
3 end
5 end
3 end
5 end
T
T
A
A
G
G
C
C
C
C
C
C
C
C
C
C
C
G
G
G
G
G
G
G
G
G
T
T
T
T
T
T
A
A
A
A
A
A
Sugar-phosphate backbone
Nitrogenous bases
Strands are anti-parallel
Purine purine: too wide
Pyrimidine pyrimidine: too narrow
Purine pyrimidine: widthconsistent 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
Base Pairing
Sugar
Sugar
Sugar
Sugar
Adenine (A) Thymine (T)
Guanine (G) Cytosine (C)
Form=Function
• 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
(a) Parent molecule (b) Separation ofstrands
(c) “Daughter” DNA molecules,each consisting of oneparental strand and onenew strand
A
A
A
A
A
A
A
A
A
A
A
A
T
T
T
T
T
T
T
T
T
T
T
T
C
C
C
C
C
C
C
C
G
G
G
G
G
G
G
G
• 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
• 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
DNA Replication• 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
DNA Replication: Duplicating the code
At the replication forks, both strands replicated at same time in the 5’ to 3’ direction
DNA Replication: Duplicating the code
Helicase- unwinds double helix
DNA Replication: Duplicating the code
Topoisomerase- prevents overwinding at replication fork by breaking, swiveling, and rejoining DNA strands
DNA Replication: Duplicating the code
Single-strand binding proteins bind to and stabilize single-stranded DNA
DNA Replication: Duplicating the code
DNA primase- start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template
DNA Replication: Duplicating the code
RNA primer- short (5–10 nucleotides long) RNA molecule that serves as the starting point for the new DNA strand
DNA Replication: Duplicating the code
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
Figure 16.14
New strand Template strand
Sugar
Phosphate Base
Nucleosidetriphosphate
DNApolymerase
Pyrophosphate
5
5
5
5
3
3
3
3
OH
OH
OH
P P i
2 P i
PP
P
A
A
A
A
T T
TT
C
C
C
C
C
C
G
G
G
G
DNA Replication: Duplicating the code
Leading strand: Template strand is 3’ to 5’Lagging strand: Template strand is 5’ to 3’
DNA Replication: Duplicating the code
DNA polymerase synthesizes the leading strand continuously, moving toward the replication fork in 5’ to 3’ direction
DNA Replication: Duplicating the code
To elongate the lagging strand, DNA polymerase must work in the direction away from the replication fork
DNA Replication: Duplicating the code
Lagging Strand made by Okazaki fragments-small sections of DNA made in 5’ to 3’ direction
DNA Replication: Duplicating the code
DNA ligase- joins the Okazaki fragments
Preventing Mistakes• 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
Replicating the Ends of DNA Molecules
• 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
Replicating the Ends of DNA Molecules• 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