Download Biochemistry 6: Model Organisms

Biochemistry 6:
Model Organisms
Ulrike Gaul
Boris Pfander
Eckhard Wolf
Barbara Conradt
Nina Uhlenhaut
Lucas Jae
Topics and schedule of the course
1
25.4.
History and methods of genetics
UG
2
2.5.
Yeast I
BP
3
9.5.
Yeast II
BP
4
16.5.
Fly I: embryonic axis formation
UG
5
23.5.
FlyII: imaginal development
UG
6
30.5.
Mammals- early development
EW
7
6.6.
Pentecoste
8
13.6.
Mammals- metabolic control
EW
9
20.6.
Fly III - Glial function in NS
UG
10
27.6.
Worm - Cell death
BC
11
4.7.
CRISPR/Cas9
NU
12
11.7.
haploid screens
LJ
13
18.7.
Q&A
SB
14
4.8.
Exam 10:00 Lynen auditorium
SB
Introduction to genetics – essentials
Transmission genetics is the general process by which traits controlled by factors
(genes) are transmitted through gametes from generation to generation. Its
fundamental principles were first put forward by Gregor Mendel in the midnineteenth century.
Later work by others showed that genes are on chromosomes and that mutant
strains can be used to map genes on chromosomes.
The recognition that DNA encodes genetic information, the discovery of DNA’s
structure, and elucidation of the mechanism of gene expression form the
foundation of molecular genetics.
Recombinant DNA technology, which allows scientists to prepare large quantities
of specific DNA sequences, has revolutionized genetics.
Some of the model organisms used in genetics research since the early part of
the twentieth century are now used in combination with recombinant DNA
technology to study human diseases.
From genotype to phenotype
The central dogma of molecular biology
– DNA makes RNA makes protein –
explains how genes control phenotypes
Chromosomes
All somatic cells of a given species contain an identical number of chromosomes.
The number of chromosomes varies between species:
Yeast (16) – Garden pea (7) – Worm (6) – Drosophila (4)
Mouse (20) – Cattle (30) – Human (23)
Chromosomes
Some eukaryotic species live as haploids (yeast), most live as diploids. In
diploids, nearly all chromosomes occur in pairs.
The members of each pair are called homologous chromosomes.
One copy comes from the mother, the other from the father (biparental
heritance).
Chromosomes
Chromosomes are best visualized during mitosis.
They differ in length, shape and position of their centromeres.
In many species, the sex-determining chromosomes (X and Y) are not
similar in shape and size, but behave as homologs during meiosis.
Cell cycle
Interphase (G/0/1 – S – G2) – Mitosis
2n chromosomes are duplicated  4n,
then distributed between two daughter cells  each 2n
Mitosis
Prophase: Chromosome condensation, centrioles divide and move apart
Metaphase: Chromosomes align on metaphase plate, perpendicular to axis of
spindle fibers
Anaphase: Sister chromatids of each chromosome disjoin/centromeres split;
daughter chromosomes migrate to opposite poles
Telophase: Two complete sets of chromosomes, one set arriving at each pole;
cytokinesis divides the cytoplasm into two halves.
Mitosis
Prophase: Chromosome condensation, centrioles divide and move apart
Metaphase: Chromosomes align on metaphase plate, perpendicular to axis of
spindle fibers
Anaphase: Sister chromatids of each chromosome disjoin/centromeres split;
daughter chromosomes migrate to opposite poles
Telophase: Two complete sets of chromosomes, one set arriving at each pole;
cytokinesis divides the cytoplasm into two halves.
Life cycle of diploids – the need for meiosis
Meiosis converts a diploid cell into a haploid gamete or spore, making sexual
reproduction possible. During sexual reproduction, gametes then combine at
fertilization to reconstitute the diploid complement found in the parents.
In addition to reducing the chromosome set (2n  n), genetic exchange occurs
between the maternal and paternal homologs during ‘crossing over’, which
results in mosaic chromosomes. Meiosis is the major form of genetic
recombination.
Overview of meiosis
As in mitosis, cells entering meiosis
have duplicated their chromosomes in
the preceding S-phase and start out
with 4n. Therefore, two meiotic
divisions are necessary to reach 1n per
gamete.
First meiotic division
Prophase I: chromosomes condense,
each homologous pair undergoes
synapsis, and crossing over
occurs between synapsed
homologs.
Metaphase I: tetrads are aligned at
metaphase plate; half of each
tetrad is pulled randomly to one or
the other pole; sister chromatids
stay together.
Telophase I:
separation of cells.
Overview of meiosis
Second meiotic division
Each gamete receives only one
chromatid from the original tetrad.
Prophase II: Each dyad is composed
of one pair of sister chromatids
attached by a common
centromere
Metaphase II: Centromeres are
positioned on the metaphase
plate
Anaphase II: Sister chromatids are
pulled to opposite poles.
Telophase II: 1n set of chromosomes.
End result: 4 haploid gametes, with
each monad a combination of
maternal and paternal genetic
information
Development of gametes differs
between spermatogenesis and
oogenesis and varies between species.
Overview of meiosis
As in mitosis, cells entering meiosis have duplicated their chromosomes in the preceding S-phase
and start out with 4n. Therefore, two meiotic divisions are necessary to reach 1n per gamete.
First meiotic division
Prophase I:
chromosomes condense, each homologous pair undergoes synapsis, and crossing
over occurs between synapsed homologs.
Metaphase I: tetrads are aligned at metaphase plate; half of each tetrad is pulled randomly to one
or the other pole; sister chromatids stay together.
Telophase I: separation of cells.
Overview of meiosis
Second meiotic division
Each gamete receives only one chromatid from the original tetrad.
Prophase II:
Each dyad is composed of one pair of sister chromatids attached by a common
centromere
Metaphase II: Centromeres are positioned on the metaphase plate
Anaphase II: Sister chromatids are pulled to opposite poles.
Telophase II: 1n set of chromosomes.
End result :
4 haploid gametes, with each monad a combination of maternal and paternal genetic
information
Development of gametes differs between spermatogenesis and oogenesis and varies between species.
Transmission Genetics I – monohybrid cross
Three postulates are necessary to explain the data:
1) Genetic traits are controlled by ‘unit factors’ (genes on chromosomes) that exist in
pairs in individual organisms
2) Unit factors can be dominant or recessive
3) Unit factors segregate randomly
Transmission Genetics I – monohybrid cross
Transmission Genetics I – monohybrid cross
Transmission Genetics II – dihybrid (two-factor) cross
Mendel’s 9:3:3:1 dihybrid ratio
One postulate is necessary to explain the data:
The two traits can assort independently
Transmission Genetics II – dihybrid (two-factor) cross
Unit factors, genes and homologous chromosomes
Independent assortment leads to extensive genetic variation
Number of possible gametes is 2n (n=haploid number)
Fly (4)
Human (23)


16 gamete combinations
8x106 gamete combinations
Modification of Mendelian ratios
While alleles are transmitted from parent to offspring according to Mendelian
principles, they sometimes fail to display the clear-cut dominant/recessive
relationship observed by Mendel.
In many cases, in contrast to Mendelian genetics, two or more genes are known
to influence the phenotype of a single characteristic.
Another exception to Mendelian inheritance is the presence of genes on X
chromosomes, whereby one of the sexes contains only a single member of that
chromosome.
Phenotypes are often the combined result of both genetics and the environment
within which genes are expressed.
The result of the various exceptions to Mendelian principles is the occurrence
of phenotypic ratios that differ from those resulting from standard monohybrid,
dihybrid and trihybrid crosses.
Extranuclear inheritance, resulting from the expression of genes present in the
DNA found in mitochondria and chloroplasts, modifies Mendelian inheritance
patterns. Such genes are often transmitted through the female gamete.
Alleles alter phenotypes in different ways
Alleles are alternative forms of the same gene.
The allele most frequently occurring in a population is called the wild-type allele.
This is often, but not always, dominant.
Different alleles of genes are created by mutations.
Loss-of-function mutations (often recessive)
Partial loss-of function
= hypomorphic allele
Complete loss-of-function = null allele
Gain-of-function mutations (often dominant)
Overexpression
= hypermorph
Ectopic expression
= neomorph
Codominance of alleles
Codominant inheritance is characterized by distinct expression
of the gene products of both alleles.
Example: ABO blood groups in humans with 4 phenotypes
Complementation analysis
Test whether two independently isolated mutations, which cause a similar phenotype, are alleles
of the same gene or of two different genes
X-linkage describes genes on the X chromosome
Unlike the outcome of typical
monohybrid cross, reciprocal
crosses of X-linked mutants do
not yield identical results.
Fly/Human: male is XY, female
is XX.
X-linked genes:
Fly: Eye color gene white
Human: Color blindness
(deutan type, protan type);
Hemophilia (A and B)
X-linkage describes genes on the X chromosome
Phenotypic traits controlled by recessive X-linked genes are passed from
homozygous mothers to all sons, since sons receive their (one and only) X
chromosome from the mother and the Y chromosome from the father.
Genetic background and the environment can affect phenotypic expression
Penetrance – percentage of individuals
that show at least some degree of
expression of a mutant genotype. E.g.
if 15% show wild-type appearance, the
gene is said to have a penetrance of
85%.
Expressivity – range of expression of
the mutant genotype (e.g. eyeless)
In the case of the eyeless phenotype,
genetic background and
environmental factors influence its
expression.
Extranuclear inheritance modifies Mendelian patterns
Organelle heredity:
chloroplasts and mitochondria contain DNA
inheritance is typically cytoplasmic/maternal
mutants show deficiency in organelle function
Examples:
poky in Neurospora,
petite in Saccharomyces
Maternal effect:
Phenotype for a particular trait is under the control of maternal gene products
(mRNA, proteins) present in the egg. This is in contrast to biparental inheritance,
where both parents transmit information to the offspring (more later …).
Linkage and chromosome mapping in eukaryotes
Chromosomes in eukaryotes contain many genes whose locations are fixed
along the length of the chromosomes.
Unless separated by crossing over, alleles present on a chromosome segregate
as a unit during gamete formation.
Crossing over between homologs during meiosis creates recombinant gametes
with different combinations of alleles that enhance genetic variation.
Crossing over between homologs serves as the basis for the construction of
chromosome maps.
Genetic maps depict the relative locations of genes on chromosomes in a
species.
Sturtevant and mapping
Summary
 Cells are the fundamental units of life. All present-day cells are believed to have
evolved from an ancestral cell that existed more than 3 billion years ago.
 All cells grow, convert energy from one form to another, sense and respond to their
environment, and reproduce themselves.
 All cells are enclosed by a plasma membrane that separates the inside of the cell
from the environment.
 All cells contain DNA as a store of genetic information and use it to guide the
synthesis of RNA molecules and of proteins.
 The simplest of present-day living organisms are prokaryotes: although they contain
DNA, they lack a nucleus and other organelles and probably resemble most closely
the ancestral cell.
 Different species of prokaryotes are diverse in their chemical capabilities and inhabit
an amazingly wide range of habitats. Two fundamental evolutionary subdivisions are
found: bacteria and archaea.
 Eukaryotic cells possess a nucleus and other organelles not found in prokaryotes.
They probably evolved in a series of stages. An important step was the acquisition
of mitochondria, which are thought to have originated from bacteria engulfed by an
ancestral eukaryotic cell.
Textbooks
Alberts, Bray, Hopkin, Johnson, Lewis, Raff, Roberts, Walter
Essential Cell Biology
3rd edition, 2010, Garland Science
Alberts, Johnson, Lewis, Raff, Roberts, Walter
Molecular Biology of the Cell
5th edition, 2008, Garland Science
Pollard and Earnshaw
Cell Biology - Das Original mit Übersetzungshilfen
2nd edition, 2008, Spektrum Akademischer Verlag