Related Sadava’s chapters - ENS - Départment de biologiemthomas/L3/intro_biologie/7-Heredity-DNA.pdf · 7.0 Inheritance and DNA Related Sadava’s chapters: • 12) Inheritance,

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)


•  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.


• 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


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


• 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.


• Reciprocal crosses yielded the same results: it made no difference which parent contributed pollen.

• The idea that each parent contributes equally was supported.


• 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.


• 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.


• 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.


• 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).


• 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.


• Mendel’s first law

• The law of segregation: The two copies of a gene separate when an individual makes gametes.



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.



• 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.


• 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.



Some alleles are neither dominant nor recessive—a heterozygote has an intermediate phenotype: Incomplete dominance.


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).


• 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.


• 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.



• 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).


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.


In other animals, sex determination by chromosomes is different from mammals.

Insert Table 12 .2


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


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—double-helical 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 10-7)

• Mismatch repair

• Excision repair

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.

Documents

Related Sadava’s chapters - ENS - Départment de biologiemthomas/L3/intro_biologie/7-Heredity-DNA.pdf · 7.0 Inheritance and DNA Related Sadava’s chapters: • 12) Inheritance,