Unit 4
 Chapter 12~
The Cell Cycle
Cell Division: Key Roles
 Genome: cell’s genetic
information
 Somatic (body cells) cells
 Gametes (reproductive
cells): sperm and egg cells
 Chromosomes: DNA
molecules
 Diploid (2n): 2 sets of
chromosomes
 Haploid (1n): 1 set of
chromosomes
Cell Division: Key Roles
 Chromatin: DNA-protein
complex
 Chromatids: replicated
strands of a chromosome
 Centromere: narrowing
“waist” of sister
chromatids
 Mitosis: nuclear division
 Cytokinesis: cytoplasm
division
 Meiosis: gamete cell
division
The Cell Cycle
 Interphase (90% of
cycle)
 Mitotic phase
• Mitosis~ nuclear
division •
 Cytokinesis~ cytoplasm
division
The Cell Cycle
 Interphase (90% of
cycle)
 G1 phase~ growth
 S phase~ synthesis of
DNA
 G2 phase~
preparation for cell
division
The Cell Cycle
 G1 Phase: Growth of cell excluding
nucleus
 S Phase: Synthesis of DNA; Chromosome
number doubles to 92 – There are two
copies of each chromosome (Sister
Chromatids).
 G2 Phase: Growth of cell including all
organelles, cytosol, etc. in preparation for
division
The Cell Cycle
 All cells go through Cell Division, most use
Mitosis.
 Cell differentiation occurs once the
organism has grown sufficiently in size
 Cells stop division with specific signals
from the surroundings.
 Cells unable to stop division are mutagenic;
They cause Cancer
The Cell Cycle
 Mitosis produces two identical daughter
cells.
 Each cell contains equal amounts of the
cytosol, identical organelles and membrance
structure.
 Both daughter cells contain exact copies of
all DNA; DNA is semiconservative which
mean each daughter cell has one original
and one copy in its double-helix DNA
Mitosis
 Prophase
 Prometaphase
 Metaphase
 Anaphase
 Telophase
Prophase
 Chromatin fibers condense tightly
 Chromosomes visible in a microscope
 Nucleoli disappear
 Sister chromatids match up and join together
 Mitotic spindle forms – generated from the
centrosomes and microtubles
 Centrosomes move away from eachother,
heading toward opposite ends of the nucleus
Prometaphase
 Nuclear envelope fragments
 Spindle can now enter the nucleus
 Spindle interacts with chromosomes
 Kinetochore develops which allows
chromosomes to be moved back and forth
 Non-kinetochore microtubles interact with each
other forming additional support structure
Metaphase
 Longest stage of Mitosis, lasting about
20 minutes
 Centrosomes at opposite poles of the cell
 Chromosomes align on the Metaphase plate,
equidistant from the centrosomes
 Kinetochores of sister chromatids attached to
microtubules (spindle)
Anaphase
 Shortest phase of Mitosis, lasting only minutes
 Paired centromeres separate; sister chromatids
liberated becoming chromosomes
 Chromosomes move to opposite poles
 Each pole now has a complete set of
chromosomes
 Cell elongates as microtubules lengthen
Telophase
 Daughter nuclei form
 Nuclear envelopes arise from the fragments of
the original nuclear envelope
 Chromatin becomes less coiled and is no
longer visible through a microscope
 Generation of two complete new nuclei
completes mitosis
Cytokinesis
 Cytoplasmic division: includes all organelles
 Begins before Telophase ends – Both processes
can occur simultaneously
 Animals: cleavage furrow, it pinches the cell,
creating two distinct cells
 Plants: cell plate, similar to the cleavage
furrow, it eventually grows into a cell wall
Cell Cycle regulation
 Growth factors
 Density-dependent
inhibition
 Anchorage
dependence
Cancer
 Transformation
 Tumor: benign or malignant
 Metastasis
Lecture # 2
 Chapter 13~
Meiosis and
Sexual Life Cycles
Heredity
 Heredity: the transmission of traits
from one generation to the next
 Asexual reproduction: clones
 Sexual reproduction: variation
Heredity
 Human life cycle:
• 23 pairs of homologous
chromosomes (46);
• 1 pair of sex and 22 pairs of
autosomes;
• karyotype;
• gametes are haploid (1N)/
all other cells are diploid
(2N);
•fertilization (syngamy)
results in a zygote
 Meiosis: cell division to produce
haploid gametes
Alternative life cycles
 Fungi/some algae
•meiosis produces 1N cells
that divide by mitosis to
produce 1N adults
(gametes by mitosis)
Alternative life cycles
 Plants/some algae
•Alternation of
generations: 2N
sporophyte, by meiosis,
produces 1N spores; spore
divides by mitosis to
generate a 1N
gametophyte; gametes
then made by mitosis
which then fertilize into
2N sporophyte
Meiosis
 Preceded by
chromosome
replication, but is
followed by 2 cell
divisions (Meiosis I &
Meiosis II)
 4 daughter cells; 1/2
chromosome number
(1N); variation
Meiosis I
 Meiosis I is very similar as Mitosis
 Chromosomes are replicated prior to Meiosis I creating
homologous chromosomes
 Prophase I contains tetrads – Homologous chromosomes
linked together
 Prophase I is the only time that crossing over occurs
because of the formation of a tetrad.
Meiosis I
 Metaphase I tetrads line up on the Metaphase
plate
 Anaphase I separates the tetrad (homologous
chromosomes), producing cells with 2n
chromosomes.
 Telophase I and Cytokinesis I results in two
complete cells.
Meiosis II
 Chromosomes are not replicated between
Meiosis I and Meiosis II
 Meiosis II uses Prophase II, Prometaphase II,
Metaphase II, Anaphase II, Telophase II and
Cytokinesis II.
 Meiosis II produces gamete cells, each with 1n
chromosomes
Meiosis vs. mitosis
 Synapsis/tetrad/
chiasmata (prophase I)
 Homologous vs.
individual
chromosomes
(metaphase I)
Meiosis vs. Mitosis
 Sister chromatids
(chromosomes) do not
separate (Anaphase I)
 Meiosis I separates
homologous pairs of
chromosomes, not
sister chromatids of
individual
chromosomes.
Origins of Genetic Variation, I
 Independent assortment:
homologous pair of
chromosomes position and
orient randomly
(metaphase I) and
nonidentical sister
chromatids during meiosis
II
 Combinations possible:
2 n ; with n the haploid
number of the organism
Origins of Genetic Variation, II
 Crossing over (prophase I):
• the reciprocal exchange of
genetic material between
nonsister chromatids during
synapsis of meiosis I
(recombinant chromosomes)
 Random fertilization:
• 1 sperm (1 of 8 million
possible chromosome
combinations) x 1 ovum (1 of 8
million different possibilities) =
64 trillion diploid combinations!
Add Meiotic Problems
 Non-disjunction
– Monosomy & Trisomy
– Down Syndrome
– Edward’s
– Patau
– Kleinfelter’s
– Turner’s
Lecture # 3
 Chapter 14~
Mendel &
The Gene Idea
Mendelian Genetics
 Gregor Mendel was
the father of genetics
 He was a monk who
 He looked at patterns
of inheritance within
pea plants in the
1850’s
Mendelian genetics
 Character





(heritable feature, i.e., fur color)
Trait
(variant for a character, i.e., brown)
True-bred
(all offspring of same variety)
Hybridization
(crossing of 2 different true-breds)
P generation (parents)
F1 generation (first filial generation)
Leading to the Law of Segregation
 Alternative versions of genes
(alleles) account for variations
in inherited characteristics
 For each character, an organism
inherits 2 alleles, one from each
parent
 If the two alleles differ, then
one, the dominant allele, is fully
expressed in the organism’s
appearance; the other, the
recessive allele, has no
noticeable effect on the
organism’s appearance
Leading to the Law of Segregation
 Mendel’s Law of Segregation:
 The alleles for each character
segregate (separate) during
gamete production (meiosis).
 Traits are not linked together,
and each plant can have any
allele combination of traits
Genetic vocabulary…….
 Punnett square: predicts the
results of a genetic cross
between individuals of known
genotype
 Homozygous: pair of identical
alleles for a character
 Heterozygous: two different
alleles for a gene
Genetic vocabulary…….
 Phenotype: an organism’s traits
 Genotype: an organism’s
genetic makeup
 Testcross: breeding of a
recessive homozygote X
dominate phenotype (but
unknown genotype)
The Law of Independent Assortment
 Law of Segregation
involves 1 character.
What about 2 (or
more) characters?
 Monohybrid cross vs.
dihybrid cross
 The two pairs of
alleles segregate
independently of each
other.
The Law of Independent Assortment
 Mendel’s Law of
Independent
Assortment
 Independent
Assortment of two
genes can be shown
using a punnett
square
Non-single gene genetics, I
 Incomplete dominance: appearance
between the phenotypes of the 2
parents. Ex: snapdragons
 Codominance: two alleles affect
the phenotype in separate,
distinguishable ways.
Ex: Tay-Sachs disease
 Multiple alleles: more than 2
possible alleles for a gene.
Ex: human blood types
Non-single gene genetics, II
 Pleiotropy: genes with multiple
phenotypic effect. Ex: sickle-cell
anemia
 Epistasis: a gene at one locus
(chromosomal location) affects the
phenotypic expression of a gene at
a second locus. Ex: mice coat
color
 Polygenic Inheritance: an additive
effect of two or more genes on a
single phenotypic character Ex:
human skin pigmentation and
height
Probability and Genetics
 Probability can be used to solve genetic
problems
 Fertilization and the combination of alleles is a
random event
 Take the total number of that combination over
the total number of all possible combinations
Human disorders
 The family pedigree
 Recessive disorders:
•Cystic fibrosis
•Tay-Sachs
•Sickle-cell
 Dominant disorders:
•Huntington’s
 Testing:
•amniocentesis
•chorionic villus
sampling (CVS)
Lecture #4
 Chapter 15~
The Chromosomal
Basis of Inheritance
The Chromosomal Theory
of Inheritance
 Genes have specific
loci on chromosomes
 Each gene can be
mapped to its own loci
 All chromosomes can
be mapped
Chromosomal Inheritance
 Chromosomes undergo segregation and
independent assortment
 Genes should maintain the same placement
on the chromosome despite crossing over
and independent assortment
 Each chromatid should contain the same
genetic information (the same genes) as its
sister chromatid
Genetic Recombination
 Crossing over
Genes that DO NOT
assort independently of each
other
 Each gene must remain on its
designated chromosome
 Genes that are not assorted
independently are said to be
linked
 They are found VERY near
each other on the chromosome
Genetic Recombination
 Genetic maps
The further apart 2 genes
are, the higher the probability
that a crossover will occur
between them and therefore the
higher the recombination
frequency
 If genes are on a different
chromosome, they will not
exchange genetic information
 Genomic mapping can show
relative distances between
genes based upon
recombination frequency
Genetic Recombination
 Linkage maps
Genetic map
based on recombination
frequencies
 It shows the relative placement
of genes on a particular
chromosome
Genetics at Work
 There are some significant experiments within the
Genetic community
 One of the most significant is the experiment
involving Drosophilia melanogaster
 Experiments involving this species of fly have
shown a great deal about the genetic
recombination and inheritance patters within
multicellular organisms
Chromosomal Linkage
 Thomas Hunt Morgan –
embryologist at Columbia
University
 Drosophilia melanogaster
 Proved a specific gene is
associated with a specific
chromosome
 XX (female) vs. XY (male)
Morgan’s Research
 Sex-linkage: genes located
on a sex chromosome
 Linked genes: genes located
on the same chromosome that
tend to be inherited together
 Wild Type: Normal variation
of a gene
– Ex: Red Eyes
 Mutant phenotype:
Alternatives to the wild type
– Ex: White Eyes
Morgan’s Research
 After breeding fruit flies for about a year Morgan
found 1 White eye male.
 He mated the white eyed male with a wild type
female, and found only red eyed flies suggesting
the wild type is dominant.
 A cross of the F1 generation produced the
expected 3:1 results, except the only white eyed
fly was male.
 What does this indicate?
Morgan’s Research
 This suggests that the white eyed trait is
sex-linked.
 Morgan was not only the first person to
discover linkage, but also sex-linkage of
traits.
 Linked Genes are genes on the same
chromosome that tend to be inherited
together
Morgan’s Research
 Genetic Recombination is produced by
crossing over and independent assortment.
– Offspring that have the same phenotype as the
parent are called parental types
– Offspring that have different phenotypes than
their parents are called recombinants.
Sex-Linked Genes
 SRY gene: gene on Y chromosome that triggers the development of
testes; development is dependent upon a hormonal condition
within the embryo
– In the absence of this gene, the XY individual is male, but does not
produce normal sperm
 Fathers = pass ALL X-linked alleles to ALL daughters only (but
not to sons)
– Fathers give an X chromosome for a daughter and a Y chromosome
for a son
 Mothers = pass X-linked alleles to both sons & daughters
– Mothers give an X chromosome for a son and a daughter
Human sex-linkage
 Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy
(MD); hemophilia

 X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g.,
tortoiseshell gene gene in cats)
Human disorders
 Recessive disorders:
– Cystic fibrosis: Abnormal functioning of the
transport proteins for chloride ions producing
mucus build up in the lungs, liver, digestive tract
– Tay-Sachs: Non-functioning enzyme that breaks
down brain lipids, causing seizure, loss of motor
skills, blindness and death
– Sickle-cell: Red blood cells have a sickle shape that
prevents the binding of oxygen leading to brain and
other organ damage
Human disorders
 Dominant disorders:
– Huntington’s: degenerate disease in the
nervous system; there is no phenotypic effect
until the individual is about 35-45 years old
– Spondyloepimetaphyseal dysplasia: a dominant
allele that causes a form of Dwarfism
Human disorders
 Testing:
– Amniocentesis: Tests
the amnionic fluid for
biochemical changes
• Can be used to detect
Tay Sachs disease
– Chorionic villus
sampling (CVS):
Tests samples of the
fetal tissue from the
placenta.
• Samples are used for
karyotyping
Chromosomal errors, I
 Nondisjunction:
members of a pair of
homologous chromosomes
do not separate properly
during meiosis I or sister
chromatids fail to separate
during meiosis II
 Most nondisjunctions lead
to organisms that are
unable to survive or have
severe problems
Chromosomal errors, I
 Aneuploidy: chromosome
number is abnormal
– Monosomy~ missing
chromosome
– Trisomy ~ extra
chromosome (Down
syndrome)
– Polyploidy~ extra sets
of chromosomes
Chromosomal Errors in Humans
 Down Syndrome is a trisomy of
chromosome 21 characterized by distinct
facial features, heart defects, and mental
retardation
 Klinefelter Syndrome is a trisomy of sex
chromosomes XXY; feminine body
characteristics such as breast enlargement,
smaller testes and sterility
Chromosomal Errors in Humans
 XYY; not characterized by a syndrome
name, but tend to be taller than average
 XXX; cannot be distinguished from normal
genotypes except by karyotype
 Turner Syndrome, an X monosomy,
produces phenotypically females without
mature sex organs, they also have short
stature and are sterile
Chromosomal errors, II
 Alterations of chromosomal structure:
 Deletion: removal of a chromosomal segment
 Duplication: repeats a chromosomal segment
 Inversion: segment reversal in a chromosome
 Translocation: movement of a chromosomal segment to another
Chromosomal errors, II
 Changing human chromosomes in any way,
can cause severe problems
 Even if the chromosome number is normal,
a deletion on a chromosome, even in the
heterozygous state can cause severe
physical and mental problems
Genomic imprinting
 Def: a parental effect on
gene expression
 Identical alleles may have
different effects on
offspring, depending on
whether they arrive in the
zygote via the ovum or via
the sperm.
 Fragile X syndrome:
higher prevalence of
disorder and retardation in
males