Zoology 3300, Exam 3

Cytological Techniques

Geneticists use stains to identify specific chromosomes and to analyze their structures.

Quinacrine Banding

helps distinguish different chromosomes

Giemsa Banding

Nonfluorescent staining

The Ideogram of Human Chromosome 5

p= short arm

q= long arm

Ideogram is the pattern of bands within the chromosome

Euploid

organisms have complete sets of chromosomes (diploid = 2n; triploid = 3n; tetraploid = 4n); > than 2n called polyploidy

Aneuploid

organisms have particular chromosomes or parts of chromosomes under- or over-represented.

Aneuploidy implies a genetic imbalance

polyploidy does not.

Rearrangements

are changes in chromosome structure.

Polyploidy

Extra sets of chromosomes in an organism can affect the organism's appearance and fertility.

Effects of Polypoidy

One general effect of polyploidy is an increase in cell size

Polyploid species tend to be larger and more robust than diploid counterparts

Allopolyploids

are created by hybridization between different species.

Autopolyploids

are created by chromosome duplication within a species.

Chromosome doubling

is a key event in the formation of polyploids.

Endomitosis

involves chromosome replication and separation of sister chromatids without cell division. This produces polyploid tissues.

If sister chromatids do not separate, the resulting chromosomes are

Polytene

Polypoids contain extra sets of?

Chromosomes

Many polyploids are sterile because their multiple sets of chromosomes

segregate irregularly in meiosis.

Polyploids produced by chromosome doubling in interspecific hybrids may be fertile if their constituent genomes

segregate independently

In some somatic tissues—for example,

the salivary glands of Drosophila larvae—successive rounds of chromosome replication occur without intervening cell divisions and produce large polytene chromosomes that are ideal for cytogenetic analysis.

Aneuploidy

The under- or overrepresentation of a chromosome or a chromosome segment can affect a phenotype.

Aneuploidy

a numerical change in part of the genome

Trisomy

triplication of one chromosome

Hypoploid

an organism in which a chromosome or chromosome segment is underrepresented

Hyperploid

an organism in which a chromosome or chromosome segment is overrepresented

Monosomy

the absence of one chromosome in an otherwise diploid individual

Down Syndrome in Humans

Occurs in the 21st chromosome Short in stature, loose-jointed, broad skulls, wide nostrils, large tongue, stubby hands with a crease in the palm, shorter life span with early development of dementia. Simian crease.

Patau Syndrome Trisomy 13

Mental deficiency and deafness, minor muscle seizures, cleft lip, palate, cardiac anomalies, posterier hell prominence.

Edwards Syndrome Trisomy 18

Congenial malformation of many organs, low-set, malformed ears, receding mandible, small mouth and nose with general elfin appearance,mental deficiency, horseshoe or double kidney, short sternum, 90% die within 6 month of birth.

Turners Syndrome XO

hypoploidy (XO), in a monosomy, such as Turner Syndrome in humans, only one copy of a chromosome is present.

Amniocentesis

To check for genetic abnormalities in the womb. Checking amniotic fluid, fetal cells.

A deletion or deficiency

is a missing chromosome segment.

A duplication

is an extra chromosome segment.

Cri-du-chat Syndrome Karyotype 46, XY (5p14)

Deletion of the tip of the 5th chromosome.

Inversions

segment detached,

flipped, reattached

Pericentric Inversion

Includes centromere

Paracentric Inversion

Excludes centromere

Pairing Between Normal and Inverted Chromosomes

Chromosomes stretched and tend to de-synapsis near the end of the inversion

Can you think of any advantage of having one of your chromosomes inverted?

Inversions can be adaptive and can contain specific combinations of alleles that work well in different environments. Different combination of genes in D. subobscura formed clines adaptive to wet environments (lots of inversions)

Translocations

occur when a segment from one chromosome is detached and reattached to a different (nonhomologous) chromosomes

Reciprocal translocation

pieces of two nonhomologous chromosomes are exchanged without any net loss of genetic material.

Compound chromosomes

are formed by the fusion of homologous chromosomes, sister chromatids, or homologous chromosome segments.

How can the formation of a compound chromosome help explain the following?

Lillian Morgan crossed a homozygous recessive X-linked female mutant with a wild type male

All daughters were mutant and all sons were wild type

Robertsonian translocations

are formed by the fusion of two nonhomologous chromosomes at their centromeres.

In 1911 one of Morgan's UNDERGRADUATE students put aside his algebra homework and developed the procedure for mapping chromosomes

Alfred Sturtevant

Alfred Sturtevant

1) Genes physically connected 2) New combinations of alleles

could be formed

3) Seemed there was some

physical exchange of material between homologous chromosomes

Linkage, Recombination, and Crossing Over

Genes that are on the same chromosome travel through meiosis together; however, alleles of chromosomally linked genes can be recombined by crossing over.

Chiasma

Where chromosomes touch

The frequency of recombination

measures the intensity of linkage. In the absence of linkage, this frequency is 50 percent; for very tight linkage, it is close to zero.

Recombination is caused by a physical exchange between

paired homologous chromosomes early in prophase of the first meiotic division after chromosomes have duplicated.

At any one point along a chromosome

the process of exchange (crossing over) involves only two of the four chromatids in a meiotic tetrad.

Chromosomal Mapping

Linked genes can be mapped on a chromosome by studying how often their alleles recombine

Recombination between genes on opposite side of crossover point can only be observed in the next generation

Another way of looking at genetic map distances

The distance between two points on the genetic map of a chromosome is the average number of crossovers between them.

Interference

A crossover in one region inhibited a crossover nearby.

The extent of the interference in measured by the

coefficient of coincidence. The coefficient of coincidence (c) is the ratio of

observed double crossovers expected double crossovers

Cytogenetic Mapping

Geneticists have developed techniques to localize genes on the cytological maps of chromosomes.

We are looking for chromosomal rearrangements that can be correlated to mutations from a particular region

Deletion Mapping

How could we localize the X-linked white eye mutation in Drosophila?

Produce female flies that are heterozygous for the white eye mutation and a cytologically defined deletion

Cross white eyed males with homozygous wild type females that have defined deletions in one of their X chromosomes

Duplication Mapping

White eyed females crossed with red eyes males that have chunks of X chromosome attached to another chromosome

Females = red eyes Males = duplicated chunk contains red eye gene

A deletion will reveal

The phenotype of a recessive mutation located between its endpoints, whereas a duplication will conceal the mutant phenotype.

Genetic and cytological maps are colinear;

owever, genetic distances are not proportional to cytological distances.

Linkage Analysis in Humans

Pedigree analysis provides ways of localizing genes on human chromosomes.

Evolutionary Significance of Recombination

Meiotic recombination is a way of shuffling genetic variation to potentiate evolutionary change.

In sexually reproducing species, recombination can allow favorable alleles of different genes to come together in the same organism.

Classic genetic studies of the first part of the 20th century demonstrated that genetic material must:

Be able to replicate Control the growth and development Allow the organism to adapt to

changes in the environment

Functions of the Genetic Material

Genotypic Function: Replication Phenotypic Function: Gene Expression Evolutionary Function: Mutation

Chromosomes

Genes are located on chromosomes.

Chromosomes contain proteins and nucleic acids.

The nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

Proof that Genetic Information is Stored in DNA

In most organisms, the genetic information is encoded in DNA. Exceptions In some viruses, RNA is the genetic material

Viroids are infectious naked RNA molecules Prions are infectious, heritable proteins.

First indirect evidence suggesting DNA harbored the genetic information

Most of the DNA of the cell is located in the chromosome whereas RNA and proteins are also abundant in cytoplasm

Correlation between amount of DNA in cell and number of chromosomes

Molecular composition of DNA same in all cells of an organism, RNA and protein highly variable between cells

DNA more stable

In the late 1920s British medical officer named Frederick Griffith was trying to make a vaccine for bacterial pneumonia (Streptococcus)

Griffith never discovered an effective vaccine but experiment marked a turning point

In 1931, Richard Sia and Martin Dawson performed the same experiment in vitro showing that the mice played no role in the transforming process

The Structures of DNA and RNA

DNA is usually double-stranded, with adenine paired with thymine and guanine paired with cytosine. RNA is usually single-stranded and contains uracil in place of thymine.

Nature of the Chemical Subunits

1) Phosphate group 2) Five carbon sugar (pentose) 3) A cyclic, nitrogen containing

compound called a base

Chargaff's Rules (1940s at Colombia)

Concentration of thymine was always equal to adenine

Concentration of cytosine was always equal to guanine

Concentration purines always equal to concentration pyrimidines

In the late 1940s two pairs of scientists began to try to figure out the exact structure of the DNA molecule

Rosalind Franklin and Maurice Wilkins bombarded crystal of purified DNA with X-rays and recorded how they bounced off (diffraction)

James Watson and Francis Crick

Used these X-ray patterns and what they knew about how organic molecules bind to each other and proposed Double Helix model of DNA

DNA is

a double helix of two nucleotide strands

In 1957 it was found that RNA could also serve as genetic material

Heinz Fraenkel-Conrat

Viroids

Viroids are infectious, naked RNA molecules. No protein coat. Potato spindle tuber viroid is a circular RNA molecule consisting of 359 nucleotides

Prions

Prions are heritable, infectious proteins that do not contain nucleic acids.

Altered forms of normal cellular proteins in mammals

Transmits a mis-folded protein state-pathogenic form acts as template that converts normal proteins into infectious prion form. Mis-folded proteins tend to accumulate in extracellular tissue of the central nervous system (brain) causing spongy degeneration that eventually causing death.

Prions in Animals

Mad Cow Disease and Chronic Wasting Disease

Kuru

Kuru is related to Creutzfeldt-Jakob disease. It is best known for the epidemic that occurred in Papua New Guinea in the middle of the twentieth century.It is also known as the laughing sickness due to the pathologic bursts of laughter people would display when afflicted with the disease. Can have an incubation time of 30-50 years. 9 or 10 times more prevalent in females and children of Fore tribe

DNA

DNA usually exists as a double helix, with the two strands held together by hydrogen bonds between the complementary base pairs: adenine paired with thymine and guanine paired with cytosine. The complementarity of the two strands of a double helix makes DNA uniquely suited to store and transmit genetic information. The two strands of a DNA double helix have opposite chemical polarity.

RNA

Usually exists as a single-stranded molecule containing uracil instead of thymine

The genetic information of most living organisms is stored in

deoxyribonucleic acid (DNA).

In some viruses, the genetic information is present in

ribonucleic acid (RNA).

Viroids and prions are

infectious naked molecules of RNA and protein, respectively.

Do identical twins share the same genes?

Human genome contains 25-35k genes

Human life emerges from a single cell

Average human has 65 trillion cells

Basic Features of DNA Replication In Vivo

DNA replication occurs semiconservatively, is initiated at unique origins, and usually proceeds bidirectionally from each origin of replication.

DNA Replication is Semiconservative

Each strand serves as a template

Complementary base pairing determines the sequence of the new strand

Each strand of the parental helix is conserved in the new DNA molecule

Replication in E. coli

John Cairns (1963) grew E. coli in the presence of 3H-thymidine, autoradiographs showed unwinding and semiconservative replication were closely coupled

DNA replicates by a semiconservative mechanism

as the two complementary strands of a parental double helix unwind and separate, each serves as a template for the synthesis of a new complementary strand.

The hydrogen-bonding potentials

of the bases in the template strands specify complementary base sequences in the nascent DNA strands.

Replication is initiated

at unique origins and usually proceeds bidirectionally from each origin.

DNA Polymerases and DNA Synthesis In Vitro

Much of what we know about DNA synthesis was deduced from in vitro studies.

Discovery of DNA Polymerase I in E. coli

Arthur Kornberg first in vitro synthesis of DNA in 1957

Isolated an enzyme in E. coli originally called Kornberg's enzyme that would synthesize DNA under certain circumstances

Requirements of DNA Polymerases

Primer DNA with free 3'-OH

Template DNA to specify the sequence of the new strand

Substrates: dNTPs Mg2+

DNA Polymerases

Polymerases in E. coli

-DNA Replication: DNA Polymerases III and I -DNA Repair: DNA Polymerases II, IV, and V

Polymerases in Eukaryotes (13)

-Replication of Nuclear DNA: Polymerase alpha, beta,and/or epslon

-Replication of Mitochondrial DNA: Polymerase -DNA Repair: Polymerases beta, epslon, etc.

All of these enzymes synthesize DNA 5' to 3' and require a free 3'-OH at the end of a primer

Proofreading- in DNA duplication only one error/billion base pairs

3,000/min in Humans; 30,000/min in bacteria Error rate in Taq one in every 9,000

DNA synthesis is catalyzed by enzymes called

DNA polymerases.

All DNA polymerases

require a primer strand, which is extended, and a template strand, which is copied.

All DNA polymerases have an absolute requirement for

a free 3'-OH on the primer strand, and all DNA synthesis occurs in the 5' to 3' direction.

The 3'5' exonuclease activities of DNA polymerases proofread nascent strands as they are synthesized

removing any mispaired nucleotides at the 3' termini of primer strands.