Download 12) Inheritance, genes and chromosomes • 13) DNA

7.0 Inheritance and DNA
Related Sadava’s chapters:
•  12) Inheritance, genes and
chromosomes
•  13) DNA and its role in Heredity
7.1 Inheritance, Genes and Chromosomes
Early study of inheritance worked under
two assumptions about how inheritance
works:
• Each parent contributes equally to
offspring in reciprocal crosses
(supported by experiments)
• Hereditary determinants blend in
offspring (not supported by
experiments)
7.1 Inheritance, Genes and Chromosomes
•  Mendel’s new theory of inheritance
was published in 1866, but was largely
ignored.
•  Most biologists at the time were not
used to thinking in mathematical terms.
•  Even Darwin missed the significance of
Mendel’s work.
7.1 Inheritance, Genes and Chromosomes
• Character: observable physical feature
(e.g., flower color)
• Trait: form of a character (e.g., purple
flowers or white flowers)
• A heritable trait is passed from parent
to offspring
7.1 Inheritance, Genes and Chromosomes
Mendel’s crosses:
• Pollen from one parent was transferred
to the stigma of the other parent.
Parental generation = P
• Resulting offspring = first filial
generation or F1
• If F1 plants self pollinate, produce
second filial generation or F2
7.1 Inheritance, Genes and Chromosomes
• One trait of each pair disappeared in
the F1 generation and reappeared in the
F2—these traits are recessive.
• The trait that appears in the F1 is the
dominant trait.
• The ratio of dominant to recessive in the
F2 was about 3:1.
7.1 Inheritance, Genes and Chromosomes
• Reciprocal crosses yielded the same
results: it made no difference which
parent contributed pollen.
• The idea that each parent contributes
equally was supported.
7.1 Inheritance, Genes and Chromosomes
• The blending theory was not supported
by Mendel’s crosses.
• Mendel proposed that the heritable
units were discrete particles—the
particulate theory.
• Each plant has two particles for each
character, one from each parent.
7.1 Inheritance, Genes and Chromosomes
• Diploid: The two copies of heritable unit
in an organism.
• During gamete production, only one
copy is given to the gamete—this single
set is called haploid.
7.1 Inheritance, Genes and Chromosomes
• Mendel also concluded that each
gamete contains only one particle (or
unit), but the zygote contains two—
because it is produced from the fusion
of two gametes.
• The “particles” are now called genes.
• The totality of all genes in an organism
is the genome.
7.1 Inheritance, Genes and Chromosomes
• Alleles: Different forms of a gene
• Homozygous individuals have two
copies of the same allele (e.g., ss).
• Heterozygous individuals have two
different alleles (e.g., Ss).
7.1 Inheritance, Genes and Chromosomes
• Phenotype: Physical appearance of an
organism (e.g., spherical seeds).
• Genotype: The genetic makeup (e.g.,
Ss).
• Spherical seeds can be the result of two
different genotypes—SS or Ss.
7.1 Inheritance, Genes and Chromosomes
• Mendel’s first law
• The law of segregation: The two copies
of a gene separate when an individual
makes gametes.
7.1 Inheritance, Genes and Chromosomes
7.1 Inheritance, Genes and Chromosomes
Mendel’s second law
The law of independent assortment:
• Alleles of different genes assort
independently during gamete formation.
• Doesn’t always apply to genes on the
same chromosome; but chromosomes
do segregate independently.
7.1 Inheritance, Genes and Chromosomes
7.1 Inheritance, Genes and Chromosomes
• One of Mendel’s contributions to
genetics was the use of mathematical
analyses—the rules of statistics and
probability.
• His analyses revealed patterns that
allowed him to formulate his
hypotheses.
7.1 Inheritance, Genes and Chromosomes
• Different alleles arise through mutation:
rare, stable, inherited changes in the
genetic material.
• Wild type: allele present in most of the
population. Other alleles are mutant
alleles.
• Locus with wild-type allele present less
than 99 percent of the time is
polymorphic.
7.1 Inheritance, Genes and Chromosomes
7.1 Inheritance, Genes and Chromosomes
Some alleles are neither dominant nor recessive—a
heterozygote has an intermediate phenotype:
Incomplete dominance.
7.1 Inheritance, Genes and Chromosomes
Codominance: Two alleles at one locus
produce phenotypes that are both
present in the heterozygote.
Example: ABO blood group system—
three alleles at one locus.
7.1 Inheritance, Genes and Chromosomes
Epistasis: Phenotypic expression of one gene is influenced by another
gene.
Example: Coat color in Labrador retrievers
Allele B (black) dominant to b (brown)
Allele E (pigment deposition) is dominant to e (no pigment deposition—
yellow).
7.1 Inheritance, Genes and Chromosomes
• In 1909, Thomas Hunt Morgan and
students at Columbia University pioneered
the study of the fruit fly Drosophila
melanogaster.
• Much genetic research has been done
with Drosophila, because of its size, ease
of breeding, and short generation time.
7.1 Inheritance, Genes and Chromosomes
• Some crosses performed with
Drosophila did not yield expected ratios
according to the law of independent
assortment.
• Some genes were inherited together;
the two loci were on the same
chromosome, or linked.
• All of the loci on a chromosome form a
linkage group.
7.1 Inheritance, Genes and Chromosomes
7.1 Inheritance, Genes and Chromosomes
• Recombinant frequencies can be used
to make genetic maps showing the
arrangement of genes along a
chromosome.
• Distance between genes = map unit =
recombinant frequency of 0.01.
• Map unit also called a centimorgan
(cM).
7.1 Inheritance, Genes and Chromosomes
Mammals:
• Female has two X chromosomes (XX).
• Male has one X and one Y (XY).
• Male mammals produce two kinds of
gametes—half carry a Y and half carry
an X.
• The sex of the offspring depends on
which chromosome fertilizes the egg.
7.1 Inheritance, Genes and Chromosomes
In other animals, sex determination by
chromosomes is different from mammals.
Insert Table 12 .2
7.1 Inheritance, Genes and Chromosomes
X-linked recessive phenotypes:
• Appear much more often in males than
females
• Daughters who are heterozygous are
carriers
• Mutant phenotype can skip a generation
if it passes from a male to his daughter
7.1 Inheritance, Genes and Chromosomes
Bacteria exchange genes by conjugation:
•  Sex pilus—a projection that initiates contact between
bacterial cells
•  Conjugation tube—cytoplasmic bridge that forms between
cells
The donor chromosome fragments and some material enters
the recipient cell.
7.2 DNA
By the 1920s, it was known that
chromosomes consisted of DNA and
proteins.
A new dye stained DNA and provided
circumstantial evidence that DNA was the
genetic material:
• It was in the right place
• It varied among species
• It was present in the right amount
7.2 DNA
Frederick Griffith, working with two
strains of Streptococcus pneumoniae
determined that a transforming
principle from dead cells of one strain
produced a heritable change in the other
strain.
7.2 DNA
7.2 DNA
To identify the transforming principle:
Oswald Avery treated samples to destroy
different molecules; if DNA was
destroyed, the transforming activity was
lost.
There was no loss of activity with
destruction of proteins, carbohydrates,
or lipids.
7.2 DNA
Hershey-Chase experiment:
•  Used bacteriophage T2 virus to determine whether
DNA, or protein, is the genetic material
•  Bacteriophage proteins were labeled with 35S; the
DNA was labeled with 32P
7.2 DNA
Rosalind Franklin:
•  Prepared crystallographs from
uniformly oriented DNA fibers
•  Her images suggested a spiral/helical
structure
7.2 DNA
In 1950 Erwin Chargaff found in the DNA
from many different species:
Amount of A = amount of T
Amount of C = amount of G
Or, the abundance of purines = the
abundance of pyrimidines—Chargaff’s
rule.
7.2 DNA
Model building started by Linus Pauling—building 3-D
models of possible molecular structures.
Francis Crick and James Watson used model building
and combined all the knowledge of DNA to determine
its structure.
7.2 DNA
7.2 DNA
Complementary base pairing:
• Adenine (A) pairs with thymine (T) by
two hydrogen bonds
• Cytosine (C) pairs with guanine (G) by
three hydrogen bonds
• Every base pair consists of one purine
and one pyrimidine
« It has not escaped our notice that the specific pairing we have postulated immediately
suggests a possible copying mechanisms for the genetic material »
7.2 DNA
7.2 DNA
Four key features of DNA structure:
• It is a double-stranded helix of uniform
diameter
• It is right-handed
• It is antiparallel
• Outer edges of nitrogenous bases are
exposed in the major and minor grooves
7.2 DNA
DNA has four important functions—doublehelical structure is essential:
•  Genetic material stores genetic information—millions
of nucleotides; base sequence encodes huge
amounts of information.
•  Genetic material is susceptible to mutation—a
change in information— possibly a simple alteration
to a sequence.
•  Genetic material is precisely replicated in cell division
—by complementary base pairing.
•  Genetic material is expressed as the phenotype—
nucleotide sequence determines sequence of amino
acids in proteins.
7.2 DNA
7.2 DNA
Meselson and Stahl showed that
semiconservative replication was the
correct model.
They used density labeling to distinguish
parent DNA strands from new DNA
strands.
DNA was labeled with 15N, making it
more dense.
7.2 DNA
The Meselson–Stahl Experiment
n–Stahl Experiment
7.2 DNA
Two steps in DNA replication:
• The double helix is unwound, making
two template strands
• New nucleotides are added to the new
strand at the 3′ end and joined by
phosphodiester linkages. Sequence is
determined by complementary base
pairing
7.2 DNA
7.2 DNA
• A large protein complex—the
replication complex—interacts with
the template strands.
• All chromosomes have a region called
origin of replication (ori).
• Proteins in the replication complex bind
to a DNA sequence in ori.
7.2 DNA
7.2 DNA
7.2 DNA
13.3 How Is DNA Replicated?
• The replication fork is the site where
DNA unwinds to expose bases.
• One new strand, the leading strand, is
oriented to grow at its 3′ end as the fork
opens.
• The lagging strand is oriented so that
its exposed 3′ end gets farther from the
fork.
7.2 DNA
•  Synthesis of the lagging strand occurs in small,
discontinuous stretches—Okazaki fragments.
•  Each Okazaki fragment requires its own primer,
synthesized by the primase.
•  DNA polymerase III adds nucleotides to the 3′ end,
until reaching the primer of the previous fragment.
•  DNA polymerase I then replaces the primer with
DNA.
•  The final phosphodiester linkage between fragments
is catalyzed by DNA ligase.
7.2 DNA
7.2 DNA
Rate: 1000 bp/s
Errors < 10-6
7.2 DNA
7.2 DNA
•  The sliding DNA clamp was recognized in
dividing cells—called the proliferating cell
nuclear antigen (PCNA).
•  PCNA also helps to orient the polymerase
for substrate binding, binds other proteins,
and removes the prereplication complex
from ori
•  DNA is threaded through the replication
complex
7.2 DNA
• Small, circular chromosomes have a
single origin of replication.
• As DNA moves through the replication
complex, two interlocking circular
chromosomes are formed.
• DNA topoisomerase separates the two
chromosomes.
Figure 13.19 Replication of Small Circular and Large Linear Chromosomes (A)
7.2 DNA
7.2 DNA
• Large linear chromosomes have many
hundreds of origins of replication.
• Replication complexes bind to the sites
at the same time and catalyze
simultaneous replication.
Figure 13.19 Replication of Small Circular and Large Linear Chromosomes (B)
7.2 DNA
7.2 DNA
• Eukaryote chromosomes have repetitive
sequences at the ends called
telomeres.
• These repeats are protective and
prolong cell division, especially in
rapidly-dividing cells, like bone marrow.
• Telomerase contains an RNA
sequence—acts as template for
telomeric DNA sequences.
Figure 13.20 Telomeres and Telomerase (A)
7.2 DNA
7.2 DNA
DNA polymerases make mistakes in
replication, and DNA can be damaged
in living cells.
Cells have three repair mechanisms:
• Proofreading (error rate 10-4
• Mismatch repair
• Excision repair
10-7)
7.2 DNA
7.2 DNA
7.2 DNA
•  PCR results in many copies of the DNA fragment—referred to as
amplifying the sequence.
•  The base sequence ends of the fragment to be amplified must be
known.
•  Complementary primers, about 15–30 bases long, are made in the
laboratory.
•  An initial problem with PCR was its temperature requirements.
•  The heat needed to denature the DNA destroyed most DNA
polymerases.
•  A DNA polymerase that does not denature at high temperatures
(90°C) was taken from a hot springs bacterium, Thermus
aquaticus.