I. CELL:
To understand the structure and function of the genetic information (DNA),
you must
understand cells. This is where DNA is and where it functions.
A. Cell
Biology
1. 1665 - Robert Hooke - cork - 'cells'
2. 1805 - Lorenz Oken stated: "All organic beings originate
from and consist of vesicles or cells."
3. 1809 - Jean Baptiste de Lamarck: "No body can have
life if its constituent parts are not
cellular
tissue or are not formed by cellular tissue."
4. 1838 - Matthais Schleiden (Botanist)
1839 - Theodor Schwann (Zoologist)
Credited with the Cell Theory "all animals and plants are composed of cells."
5. 1858 - Rudolph Virchow: "Omnis cellula e cellula"
All cells come from previously existing
cells - no spontaneous generation.
6. 1862 - Louis Pasteur: no spontaneous generation definitively
supported
B. The
Role of Chromosomes
1. 1868 - Freidrick Miescher: purified an acidic, non-proteinaceous
substance high in
phosphorus, with no sulfur, from cell nuclei "nuclein".
2. 1879 - Walther Flemming: used red dye to stain nuclei
and found granules he called
"chromatin". By staining cells in process of dividing, he saw "chromatin"
coalesce into
thread like bodies "chromosomes". these bodies were separated during
the process
he called mitosis.
3. 1900 - Mendel's laws of segregation and independent
assortment rediscovered by Correns,
Tchermak, and DeVries.
4. 1902 + 1903 - Walter Sutton and Theodore Boveri: described
the movement of chromosomes
during gamete formation (meiosis) and explained how this complemented Mendel's
Laws. Suggested that chromosomes carry genetic information.
The Chromosomal Theory of Inheritance: genetic information must -
- replicate
- store information
- control expression of information (regulate)
- mutate
So, by 1905, we knew what cells were and the role of chromosomes as vectors of genetic information was implicated. How do cells work?
II. Review
of Cell Biology
A.
Structures
1. Plasma (Cell) Membrane (animals) : Plasmalemma (plants):
a. 1972 Sanger & Nicholson "Fluid Mosaic Model"
proteins floating in a phospholipid
bilayer.
b. Control the transport of molecules into and out of
the cell.
c. Cell-Cell recognition:
Cell Coat: glycoproteins and polysaccharides antigenic determinants
(AB & MN)
histocompatibility antigens
d. act as a reactive surface (lots of enzymes)
e. Duchenne Muscular Dystrophy: loss of function
of dystrophin which functions at the cell
membrane in muscles
2. Cell Wall:
a. Plants: cellulose
b. Bacteria: peptidoglycan (protein and sugar)
i. Capsule: mucopolysaccharide
ii. Diplococcus pneumonia: virulence and nonvirulence
depending on presence or
absence of capsule
3. Nucleus: porous bi-membrane sac around the DNA.
In bacteria do not have a nucleus, have a
nucleoid peripheral region where DNA is concentrated.
Chromatin: DNA + Histones (basic proteins) - uncoiled
Chromosomes: Condensed chromatin (mitosis and meiosis)
Nucleolus: synthesis of rRNA, and ribosome construction
NOR: Nucleolus Organizer Region: area of DNA encoding rRNA
4. Cytoplasm:
a. Cytosol: nonparticulate, colloidal material called
b. Cytoskeleton: complex of tubules and filaments which
hold organelles in place.
Microtubules: tubulin
Microfilaments: actin
5. Endoplasmic Reticulum: network of membranous vesicles
that may be continuous with the
nucleus.
Smooth: fatty acid and phospholipid synthesis
Rough: studded with ribosomes, protein synthesis
6. Mitochondria : site of aerobic respiration for energy
production.
Oxidative Phosphorylation
C6H12O6 => 6CO2 + 6H2O
7. Chloroplast: site of Photophosphorylation
6CO2 + 6H2O => C6H12O6
Endosymbiont Theory:
Both mitochondria and chloroplast contain DNA . Can duplicate themselves
and
transcribe and translate their genetic information. Resemble prokaryotic
cells.
Once free living structures that established a symbiotic relationship with
a primitive
eukaryotic cell
8. Golgi apparatus: series of pancake-like membranous
envelopes formed at the end of the ER
tubules. Site of protein modification.
9. Ribosomes: Protein and rRNA structures that read mRNA
and create polypeptides.
10. Centrioles: (animals and some lower plants) contained in a region
called the centrosome.
Associated with spindle fibers in mitosis and meiosis. Derived from
the Basal Body
which has the characteristic 9 X 2 morphology. Contain their own
DNA for replication.
III. Homologous
Chromosomes, Haploidy, and Diploidy
A.
Homologous Chromosomes: Chromosomes that synapse or pair during meiosis.
Chromosomes that
are identical with respect to their genetic loci and cetromere placement.
1. Metaphase chromosomes have distinctive shapes and
lengths
2. Each has a condensed centromere
Centromere: Specialized region of a chromosome to which the spindle
fibers attach during cell
division. Location of the centromere determines the shape of the
crhomosome during the
anaphase portion of cell division. Also known as the primary construction.
a. metacentric: middle
b. submetacentric: between middle and end
c. acrocentric: close to end
d. telocentric: at end
3. Arms of the chromosome
a. p arm: petite, short arm, above centromere
b. q arm: q comes after p, long arm, below centromere
4. Each somatic cell within members of the same species
contain an identical number of
chromosomes: the Diploid Number
a. 2n: each chromosome exists in pairs or homologous
chromosomes
b. Contain identical gene sties: locus (sing.)
loci (pl.)
c. Have identical genetic potential
d. Sexual Reproduction: one set obtained from each
parent - biparental inheritance
e. Thus: each diploid organism contains two copies of
each gene. Alternative forms of the
same gene are called alleles
B.
Sex-determining chromosomes: the exception to the rule
1. May not be homologous is size, centromere placement,
arm ratio, or genetic potential
2. In most mammals and insects: the male has two
different chromosomes (XY) called
heterogametic sex and females have homogametic sex (XX).
3. When the reverse is true (birds): Females are
designated ZW and males ZZ.
C.
Karyotype: display of the chromosomes for an organism
D.
Sister Chromatids: each individual strand of a duplicated chromosome
E.
Haploid Genome: the total set of genes contained on one member of
each homologous pair of
chromosomes
1. Haplodiploid: organisms where ploidy in
the sexes is different
a. bees and wasps: females 2n, males n
2. Supernumerary or B chromosomes: very small chromosomes
which vary in number among
individuals. Devoid of important genes
3. Microchromosomes: birds: number of very small
chromosome. hard to count. Thought to
carry genes and be in a constant number like larger chromosomes
F.
Specialized Chromosomes
1. Polytene Chromosomes: A chromosome that has
undergone several rounds of DNA
replication without separation of the re;oicated chromosomes, forming a
giant, thick
chromosome with alined chromosomes producing a characteisic banding pattern.
a. many replication cycles of paired homologues without
strand separation or cytokinesis
b. Various insect dipterans larval cells (salivary, midgut,
rectal, and malpighian excretory
tubules) and in several species of protozoans and plants.
c. Chromomeres: a coiled, beadlike region of a chromosome
most easily visible during cell
division. The aligned chromomeres of polytene chromosomes are responsible
for their
distinct tive banding pattern.
i. Heterochromatin: the heavily staining. Late replicating
regions of chromosomes that are
premeaturely condensed in interphase.
ii. Euchromatin: chromatin or chromosomal regions
that are lightly staining and are
relatively uncoiled during the interphase protion of the cell cycle.
The region of the
chromosomes thought to contain most of the structural genes.
d. Chromocenter: An aggregation of centromeres
and heterochromatic elements of polytene
chromosomes. An irregular, densely staining mass of heterochromatin
in the
chromosomes, with six arm-like extensions of euchromatin, in the salivary
glands of
Drosophila. In Drosophila the chromocenter in polytene
chromosomes is usually derived
from the fusion of the telomers of the chromosomes.
e. Puffs - visible manifestations of gene activity.
A localized uncoiling and swelling in a
polytene chromosome, usually regarded as a sign of active transcription.
2. Lampbrush: meiotic chromosomes characterized
by extended lateral loops, which reach
maximum extensionduring diplotene. Although most intensively studied
in amphibians, these
structres occur in meiotic cells of organisms rnging from nsects through
humans.
a. Meiotic chromosomes
b. Oocytes in Diplotene stage of prophase I of meiosis:
Active in directing the metabolic
activities of the developing cell
c. Homologues are synapsed pairs held together by chiasmata,
but not condensed
d. Chromomere supports a pair of lateral loops of DNA
IV. Cell
Cycle
A.
Description
1. Interphase (period between mitotic divisions)
a. G1: Gap I : Start
- active metabolic stage (protein synthesis)
- chromosomes diffuse in euchromatic regions. Heterochromatin
still condensed - gives
granular look to nucleus "chromatin"
- Each chromosome has 1 chromatid
- Nucleolus Þ area of high rRNA production
- At a point late in G1, cells follow one of two paths:
G0 or S. Time when dicision is made
is called G1 Checkpoint.
b. S
- DNA replication (synthesis)
- each chromosome has 2 chromatids
- Euchromatin replicated first, then the heterochromatin
is unwound and replicated.
c. G2: Gap II
- Synthesis of structures required for mitosis - in animals
this is centriole.
- Preparatory stage for mitosis
d. G0
- some cells, produced by unspecialized cells that continue
to divide, become arrested in a
productive, non-reproductive stage = Go. Phloem, xylem, epidermis,
nerve, muscle,
RBC.
- R (restriction) point - time when decision to go through
G1 or G0 is decided
- Some cells can remain quiescent in Go, but can be stimulated
to return to G1, thus
reentering the cycle
2. Mitosis
- Organized division of genetic information (Karyokinesis) and cytoplasm
(cytokinesis).
3. Length of time in each stage: cells in culture
have characteristic stage lengths. Usually a
cell will go through
the complete cycle in about 20 hours.
a. S and G2 stages are fairly consistent among different
cell types
i. S depends on cell type and conditions
b. G1: most variation in length of time
c. M: usually the least amount of time
d. In vivo, cycles may be hrs, days, years, usually G1
that varies; S+G2 same.
C.
Regulation
1. In almost all cells, if S is initiated then the cycle
is completed through division. Cue seems to
be cytoplasmic in origin (Put G1 nucleus in S or G2 cell).
2. If it is a protein cue, are there genes that code
for it? Cell Division Cycle (CDC) genes. Lee
Hartwell isolated 150 mutants with >50 different mutations that interrupt
the cell cycle. The
assumption is that the normal gene is then important in correctly regulating
the cycle.
3. Control of the Cell Cycle:
a. Cycle is regulated at three main points called CHECKPOINTS
i. G1/S checkpoint: monitors cell size and status
of DNA (whether it is damaged). Time
when cells become committed to proceeding through the cycle and the subsequent
steps
of mitosis.
ii. G2/M checkpoint: cell physiological conditions are
monitored before entering Mitosis.
DNA repication and repair is monitored.
iii. M checkpoint: Monitoring of spindle fiber system
formation and attachment of spindle
fibers to kinetechoresof chromosomes.
b. Controlled through the interaction of two types of
proteins
i. cdc kinases: enzymes that regulate other proteins
by adding phosphate groups obtained
from ATP. One of the first (and possibly most important kinase found)
comes from the
cdc2 gene which forms the gene product called cdc2 kinase. Cdc2 kinase
may be
necessary for entry into the S phase of DNA synthesis.
ii. Cyclins: Kinases phosphorylate cyclins.
This phosphorylation (or dephosphorylation)
influences cyclin actvity at the cell cycle checkpoints. Act as switches
to turn-on or
turn-off the cell cycle. Cyclins are produced intermittently
and degraded rapidly,
resulting in a pulse of regulatory activity at each point in the cell cycle.
"The successive
appearance of different cyclins and their interactions with different cellular
composnts
seem to propel the cell through the cell cycle in an orderly and precisse
fashion." (Klug
1997)
iii. Cdk protein (cyclin-dependent kinase protein):
the coordination of a cdc kinase with a
cyclin.
c. p53: tumor supressor gene. Active at G1/M
checkpoint. Functions during apoptosis
(programmed cell death). A normal p53 gene product will target cells
for distruction if their
DNA is found to be damaged during the G1/M checkpoint.
d. Oncogenes: Originally discovered in RNA viruses
(retroviruses) oncogenes transform
normal cells into malignant cell (continue dividing). In cells, there
are similar genes called
proto-oncogenes. Apparently, viruses have spliced these out, mutated
them, and
increased the
reproductive rate of host cell.
V. Mitosis
A.
Centrosome: differentiated cytoplasmic area just outside the nucleus
1. In animals and lower plants prophase involves the
migration of two pairs of centrioles to
opposite ends of the cell in the centrosome
2. It is thought that each pair of centrioles consists
of one mature unit and a smaller, newly
formed centriole
3. forms axis along which chromosomal separation occurs.
4. centrioles responsible for organization of cytoplasmic
microtubules into a series of spindle
fibers.
a. although plant, fungi, and certain algae lack centrioles,
spindle fibers are apparent during
mitosis
b. if some other organizer exists, it has not been discovered
B.
Interphase - chromosomes diffuse, but replicated
C.
Prophase (1/3 of mitosis)
1. centrioles migrate to opposite poles involved with
formation of spindle apparatus.
a. as centrioles migrate, the nuclear envelope begins
to break down
b. Nucleolus disintegrates within the nucleus
2. Reorganization of cytoskeletal actin strands attached
to chromosomes and centrioles.
3. Chromatin condenses into distinct chromosomes (supercoil)
a. The two parts of each chromosome are called chromatids
b. sister chromatids:
4. Kinetochore: is actually a granule within the centomere
that attaches to spindle fibers during
mitosis. Two kinetochores form on opposite faces of the centromere,
each attaching one or
the other member of a pair of sister chromatids to the spindle fibers.
D.
Prometaphase and Metaphase - alignment of chromosomes on metaphase plate
by spindle
fibers
1. Prometaphase: refers to the period of chromosome
movement
2. Metaphase: applies strictly to the chromosome
configuration following this movement
3. Migration is made possible by the binding of one or
more spindle fibers to the kinetochore
contained within the centromere of each chromosome. There are two
major classes of
spindle microtubules.
a. Kinetochore microtubules have one end near the centrosome
region and the other anchored
in the kinetochore.
b. Nonkinetochore microtubules grow from the centrioles
of the centrosome, but have their
other ends free. These sometimes interdigitate with one another,
providing a framework to
the spindle and maintaining the separation of the two poles during chromosome
separation.
E.
Anaphase: shortest stage
1. Sister chromatids separate - reeled to opposite poles
by spindle fibers
2. Daughter chromosomes: the two newly formed chromosomes
3. As chromosomes migrate the spindle elongates, extending
the distance between the two
centrosome regions
F.
Telophase
1. Nuclear membrane reforms at poles
2. Cytokinesis
a. Animals - furrowing
b. Plants - vesicles coalesce and form the cell plate
that develops into middle lamella of cell
wall. Primary & Secondary cell walls are deposited on this middle
lamella.
G.
Products
identical to each other and parent cell
H.
Regulation - cytoplasmic initiation of S starts S-G2-M (invariant length)
- division protein accumulates in G1
- state. in DNA poly
- Build to threshold levels inhibition lengthens G1
Sexual Reproduction of Multicellular Organisms demands creation of haploid cells, that, when joined will reconstitute original blueprint.
VI. Meiosis
- 'Reduction-Division'
A.
MEIOSIS 1
1. Overview
a. Homologous chromosomes pair or synapse to form a bivalent
b. Bivalent gives rise to a unit called the tetrad which
consists of four chromatids.
c. Two divisions
i. reductional - Meiosis I (tetrad separates into dyad)
ii. equational - Meiosis II (dyad splits into two monads)
d. Chromosomes may exchange information - crossing over
2. Interphase I: same as mitosis
3. Prophase I
a. Leptonema (n)
Leptotene stage - chromosome begin to condense
Homology Search: initial pairing of homologues
b. Zygonema
Zygotene stage: rough pairing of the homologues
Synaptonemal Complex is formed: the paired structures are called bivalents.
c. Pachynema
Pachytene stage: intimate point by point synapsis of homologous chromosomes
Shortening and coiling of paired chromosomes occurs - bivalents thicken
Can distinguish sister chromatids Bivalent = Tetrad
Crossing over occurs
d. Diplonema
Crossing over occurs at chiasmata
Crossing over involves nonsister chromatids
Genetic variability
Chromatids begin to separate
e. Diakinesis - further shortening of chromosomes and
separation. Linkage to the spindle.
Nuclear membrane and nucleolus disappear.
Terminalization: the chiasmata move toward the ends of the tetrad
END OF
PROPHASE I
4. Metaphase I - chromosomes pair up in metaphase plate.
5. Anaphase I - homologous pairs (tetrads) separate to
form dyads
Disjunction: the separation of tetrads into dyads
Nondisjunction: error where separation does not occur
Random segregation of dyads is the basis for Mendelian principle of independent
assortment.
6. Telophase I - cytokinesis
B.
MEIOSIS II - Equational Division
1. Interkinesis: no DNA synthesis. only a
haploid complement (n)
2. Prophase II: dyad is composed of one pair of sister
chromatids attached by a common
centromere
3. Metaphase II: centromeres directed to the equatorial
plate
4. Anaphase II: sister chromatids of each dyad
are pulled to opposite poles (Haploid)
5. Telophase II: monads and cytokinesis. Each chromosome
is a monad (unreplicated)
VII. Significance
of Meiosis:
A.
Function of Sexual reproduction and Meiosis
1) Maintain chromosome number
2) Increase genetic variability in individuals
Sexual Reproduction: occurs through production of sex cells or gametes.
Gametes fuse to form the zygote: a single cell from which a new individual
develops.
General Scheme:
- Haploid (n) vs. Diploid (2n)
Homologous chromosomes
- Asexual Reproduction:
Single individual produces a new individual identical to itself.
Mitosis
Parthenogenetic: female organism can also produce offspring without fertilization.
B.
Alternation of Generations in fungi and plants.