Chromosomes
Where Are the Genes Found?
What Cellular Structure Holds the
Genetic Information?
• Chromosomes
– Contain the genetic
material: DNA, RNA
• Chromatin
– Is the chromosomal
material in its
decondensed,
threadlike state.
Mitosis
• Form of asexual
reproduction.
• Occurs when organism
grows or replaces
damaged cells.
• Prior to mitosis, cell
undergoes replication.
– Process in which chromatin
is copied.
• Produces diploid cells.
Prophase
• Start of mitosis
• Chromatin condenses
into rod-like
chromosomes
– Each chromosome
consists of sister
chromatids, connected
at the centromere
• Nuclear membrane
disappears
Metaphase
• Chromosomes align
themselves in flat
plane at cell equator.
Anaphase
• Centromeres split.
• Sister chromatidsnow chromosomesare pulled to opposite
poles of the cell.
Telophase
• Chromosomes
unravel, returning the
chromatin to its nondividing threadlike
state.
• Nuclear membrane
assembles.
Cytokinesis
• Division of the
cytoplasm.
• Begins during
anaphase and
telophase.
Cytokinesis
• Differs in animals and
plant cells.
• Plant cells form a cell
plate.
– membranous vesicles
congregate at center
of cell.
– Vesicles contain cell
wall material.
Cytokinesis
• Animal cells form a
cleavage furrow.
– Forms around the
periphery of the
dividing cell.
– Furrow becomes
deeper and deeper
until membrane
pinches off forming
two cells.
Chromosomes Come in Matched
Pairs
• Homologous pairs:
chromosomes that
are closely matched
in size and shape
– Determine the same
traits
• Sex chromosomes:
Those that determine
the gender of the
organism.
Chromosomal Theory of
Inheritance
• The two members of
each pair of a
homologous pair of
chromosomes carry
alleles for the same
genes and, therefore,
affect the same traits.
• Proposed in 1903.
Mapping genes
• Locus: location of a
gene on a
chromosome.
• Currently trying to
map all human genes
to the appropriate
chromosome.
• Example: gene
responsible for sickle
cell anemia is located
on chromosome 11.
Sexual reproduction
• Type of reproduction
in which genetic
information from
female combines with
male.
• Requires fertilization
– Fusion of gametes
(egg and sperm)
– These cells need to
have half the amount
of genetic information.
Meiosis
• Produces haploid cells
(gametes-eggs and
sperm)
– Posses only one member
of each pair of homologous
chromosomes
• Chromosomes replicate
before meiosis.
• Requires two rounds of
division.
Prophase I
• Chromatin condenses
into compact
chromosomes.
• Nuclear envelope
disappears.
• Synapsis occurs.
– Homologous pairs of
chromosomes closely align
allowing exchange of
chromosome segments
Metaphase I
• Aligned pairs of
replicated
chromosomes move
to the equator of the
dividing cell.
Anaphase I
• Members of
homologous pairs of
chromosomes
separate from each
other
• They move to opposite
poles of the cell.
– Chromosomes
experience independent
assortment.
Telophase I
• Chromosomes cluster at
opposite poles of cell and
begin to decondense
• Nuclear envelope may
reform.
• Cytokinesis occurs
• Interkinesis: phase
between meiosis I and
and II.
• Daughter cells now
haploid.
Prophase II
• Partially unraveled
chromosomes
condense again.
Metaphase II
• Chromosomes move
to cell equator.
• No longer
homologous pairs, so
chromosomes line up
singly in middle of
cell.
Anaphase II
• Centromeres divide
and chromosomes
separate.
• Move to opposite
poles of the cell.
Telophase II
• Clustered
chromosomes at cell
pole begin to
decondense.
• Nuclear membrane
develops.
• Cytokinesis occurs.
• Produces four nonidentical haploid cells.
Mitosis vs. Meiosis
Cell Cycle
• Repetitive sequence of events that
characterizes life of cell.
• Consists of two main phases:
– Interphase
• Period that cells are in when not dividing.
• 90% of cell cycle
– M phase
• Includes mitosis and cytokinesis
Cell Cycle
Cell Cycle
• Interphase has three subphases
– G1 (first gap)
• Cell makes copies of organelles and grows larger
– S (synthesis)
• Genetic material is copied
• End of this phase, cells chromosomes are doubled
– Copies are attached; thus total number of chromosomes
remains the same
– G2 (second gap)
• Cell prepares upcoming M phase
Regulation of Cell Cycle
• Cell cycle must be regulated or can result
in cancer
– Uncontrolled cell growth
• Metastasize: cell breaks free from original
cancerous mass and resides in new area in the
body
Regulation of Cell Cycle
• Two checkpoints
– First between G1 and S
– Second between G2 and M
• To pass checkpoints, cell
must possess appropriate
amount of protein in
cytoplasm.
– These proteins activate
other proteins necessary
for production of genetic
material and mitosis
Regulation of Cell Cycle
• To pass checkpoints, cell must possess
appropriate amount of regulating protein in
cytoplasm.
– When regulator concentration is high, cell
cycle progresses.
– When low, cell cycle is suspended at that
stage.
• External and internal regulatory agents
also influence passage through
checkpoints.
Why So Some Genetic Traits Tend
to Travel Together?
• Chromosomes contain genes
– Chromosomes follow law of independent
assortment, not genes
• If two genes are on the same
chromosome,
– the two genes are inherited together or are
said to be linked or in linkage groups
Why So Some Genetic Traits Tend
to Travel Together?
• Example: sex linkage
• Sex chromosomes
– Contain other genes
aside from those to
determine gender.
• Example: eye color
and gender in fruit flies
Chromosomes Can Exchange
Segments During Meiosis
• Crossing over
– Exchange of genetic
material between
chromatids of
homologous
chromosomes.
– Occurs at the
chiasmata.
Chromosomes Can Exchange
Segments During Meiosis
• Crossing over
– Important mechanism
for creating new
combinations of
genes.
– Disrupts linkage
groups.
What Is the Chemical Nature of the
Gene?
• 1860s
– Frederich Meisner studied fundamental
constituents of life
• Discovered unknown substance contains carbon,
nitrogen, oxygen, and phosphorus.
• Found it came from nucleus of cell.
• Named it nucelin
– His students renamed substance nucleic acid
after finding it was acidic.
What Is the Chemical Nature of the
Gene?
• 1881
– Discovered nucleic acids were contained in
chromatin.
• Question: Was the genetic material made
of proteins or nucleic acids?
DNA is the Genetic Material
• 1928
– Fred Griffiths, medical officer for British
Ministry of Health
• Studied the bacteria pneumoncoccus
– Two kinds
» Smooth: Virulent form that appears smooth and
shiny when grown on agar plate
» Rough: harmless form that appears rough when
grown on agar plate.
• Experimented by injecting the two types in mice.
DNA is the Genetic Material
DNA is the Genetic Material
• Griffiths identified the material as the
transforming principle
• Avery, MacLeod and McCartney
– Study transforming principle for 20 years.
– They determined that the transforming agent
was DNA.
DNA is the Genetic Material
• Alfred Hershey and Martha Chase
– Studied viruses that infect bacteria
• Viruses are called bacteriophages
DNA is the Genetic Material
• Viruses are made of
• Protein coat and nucleic acid
• Viruses mix their genes with host genes
– hijack cell machinery and use it to produces
new viruses
– Usually kills host cell
DNA is the Genetic Material
• Hershey and Chase labeled protein and DNA
differently with isotopes
– Variants of elements that share same chemical
properties but differ in number of neutrons
• Label DNA with 32P and protein with 35S
DNA is the Genetic Material