Cell Division
Sexual Reproduction = egg & sperm
OR
Asexual Reproduction = single parent, no
egg/sperm
Cell Division: reproduction of cells; “cells
come from cells”
* Basis of all life
2 Main Roles:
1) development of fertilized egg
2) continuation of life (growth, repair)
Prokaryotes = binary fission (split in half)
OR
Eukaryotes = more complex; more genetic
material
chromosome: structure which contains
DNA (deoxyribonucleic acid)
chromatin: long, thin fibers of DNA &
protein clumping together to form
chromosomes
gene: specific region of DNA on
chromosomes
somatic cell: all body cells except egg &
sperm; contain chromosomes
(humans= 46)
Human egg & sperm (gametes) have 23
chromosomes
Prior to Cell Division…
* All chromosomes duplicate…result in
2 identical parts = sister
chromatids (X-shaped)
* joined at centromere
When Cells Divide
* sister chromatids separate..each goes
to separate cell (daughter cell)
* each daughter cell has complete set of
chromosomes
Overview of Cell Division
* eukaryotic cells divide according to
cell cycle
cell cycle: sequence of events including
time a cell divides until its daughter cell
divide
Phases in the Cell Cycle
1) Interphase: most of cycle here
- chromosomes duplicate
- cell grows
2) Mitotic Phase: cell division phase
Includes Mitosis & Cytokinesis
* Mitosis unique to eukaryotes
* Mitosis = continuous process but
separated into defined stages
Stages of Mitosis
1) Prophase
- chromatin fibers coil to form
discrete chromosomes
- sister chromatids
- nuclear membrane breaks near
end
2) Metaphase
- sister chromatids line up along
center of cell
Stages of Mitosis
3) Anaphase
- sister chromatids separate &
migrate to opposite ends of cell
4) Telophase
- nuclear membrane reforms
around chromosomes
Cytokinesis: division of cytoplasm
- usually occurs along with telophase
- daughter cells separate
- Certain drugs can stop cell cycle by
preventing DNA synthesis, or inhibiting
synthesis of necessary proteins
Ex: cancer drugs target rapidly dividing
cells – including hair follicles and
digestive tracts
homologous chromosome: matched pair of
chromosomes; same length, genes for
same traits at same loci
locus (loci = plural): specific location of a
gene on a chromosome
e.g., each chromosome has gene for hair
color at same loci, but the gene may be
for any color of hair … impt pt = gene
results in some color of hair
• homologous chromosomes have
matching loci &
• One chromosome of each pair inherited
from mother & father
Human Example
Somatic cells = 46 chromosomes
23 pairs of homologous chromosomes
22 pairs = autosome chromosomes
(F & M)
1 pair = sex chromosomes; specific to
the sex (M or F)
Sex Chromosomes
Human females
1 pair (2 XX)
Human male
1 pair (1X, 1Y)
• Are human male sex chromosomes
homologous?
diploid cells: cells with 2 homologous sets
of chromosomes in nucleus
total # chromosomes = diploid # = 2n
human diploid # = 46 (2x23=46)
• Humans = diploid animals because most
of our cells = diploid (e.g., somatic cell)
• But, eggs & sperm are not diploid
gametes: egg & sperm cells (sexual
reproduction only)
haploid cells: cells with 1 homologous set
of chromosomes
haploid # = n
human haploid # = 23
• Human gametes are haploid
• Fertilized egg = zygote = ????
Why is there so much variety among
species? (e.g., diversity in humans)
1) Independent orientation of chromosomes
- in Metaphase I --- way that tetrads
line up is due to chance (random)
- Results in different possible
combinations of chromosomes in
gametes
- For humans = 8 million possible
combos.!
2) Random fertilization (1 egg & 1 sperm)
What is probability that 1 of 8 million
possible sperm fertilizes 1 of 8 million
possible eggs????
Humans = (8 M) * (8 M) = 64 trillion
possible combinations of chromosomes
due to random fertilization!
3) Crossing Over
- can result in genetic recombination
genetic recombination: producing gene
combinations different from those
carried by original chromosomes
* During synapsis, tetrad formed –
crossing over possible
1) homologous chromatids break at
similar locations & chromatids join
2) h. chrom. separate at Anaphase I –
crossing over
3) Meiosis II, sister chromatids separate
Mendelian Genetics
genetics = science of heredity
gene: specific region of genetic material
(DNA) that provides provides the cell
with a “map”
Goal: determine patterns of inheritance
Mendelian Genetics
Gregor Mendel – 1860’s monk
significant findings = offspring obtain
discrete heritable factors (genes) from
their parents
Mendelian Genetics
Gregor Mendel – 1860’s monk
-carefully chose organisms to study
(garden pea), controlled pollinations,
chose traits that were easy to observe,
used statistical methods to analyze data
-significant findings = offspring obtain
discrete heritable factors (genes) from
their parents
Terms
self-fertilization: plant’s egg fertilized by
it’s own pollen
cross-fertilization: plant’s egg fertilized by
another plant’s pollen (hybridization)
P generation: parental generation
F1 generation: filial generation; hybrid
offspring of the P generation
F2 generation: offspring produced by F1
generation via self-fertilization
Mendel’s Principles
1) Principle of Segregation – pairs of
genes segregate during gamete
formation; fertilization pairs genes again
monohybrid cross: cross of 2 individuals
that differ in 1 trait
allele: alternate form of a gene found at
same loci of homologous chromosomes
1) Principle of Segregation
Ex: Flower color (P = purple, p = white)
P = 1 Purple (PP) & 1 white (pp)
F1 = all Purple (Pp)
F2 = ¾ Purple (PP & Pp) ¼ white (pp)
homozygous: identical pair of alleles
heterozygous: 2 different alleles for a trait
phenotype: physical trait; appearance of
organism; expressed as phenotypic ratio
genotype: genetic makeup of organism;
expressed as genotypic ratio
• In the flower color example…..
What is the phenotypic ratio?
What is the genotypic ratio?
** For monohybrid cross… phenotypic
ratio is always 3:1 & genotypic ratio is
always 1:2:1
2) Principle of Independent Assortment
• each pair of alleles segregates
independently during gamete formation
dihybrid cross: cross of 2 individuals that
differ in 2 traits
2) Principle of Independent Assortment
Example
P generation: Round (RR) & Yellow (YY) seeds = RRYY
Wrinkled (rr) & Green (yy) seeds = rryy
Gametes = RY and ry
F1 gen:
All RrYy (Round & Yellow seeds)
Gametes = RY, Ry, rY, ry
RY
ry
Male
RrYy
Female
2) Principle of Independent Assortment
Example (continued)
F2 gen:
(Do Punnett Square
RY
RY
Ry
rY
ry
Male
Ry
rY
ry
Female
2) Principle of Independent Assortment
Example (continued)
F2 gen:
(Do Punnett Square
RY
RY
Ry
rY
ry
Male
RRYY
Ry
rY
ry
Female
2) Principle of Independent Assortment
Example (continued)
F2 gen:
(Do Punnett Square
RY
RY
Ry
rY
ry
Male
Ry
rY
RRYY RRYy RrYY
ry
RrYy
Female
2) Principle of Independent Assortment
Example (continued)
F2 gen:
(Do Punnett Square
RY
rY
ry
RY
RRYY RRYy RrYY
RrYy
Ry
RRYy RRyy
RrYy
Rryy
rY
RrYY RrYy
rrYY
rrYy
RrYy
rrYy
rryy
ry
Male
Ry
Rryy
Female
Probabilities
• Probability (chance) of an event occurring ranges
from 0 to 1
Probability = 0 = event will not occur
Probability = 1 = event will occur always
Tossing a Coin
What is the probability of getting a “tails”?
= 0.5 (1/2)
What is the probability of getting a “heads”?
= 0.5 (1/2)
What is the probability of getting a “heads” or a
“tails”?
= P(heads) + P(tails) = 0.5 + 0.5 = 1.0
Tossing 2 Coins
What is the probability of getting a “heads” on
both coins?
= P(heads) x P (heads) = (0.5)*(0.5) = 0.25
Flower Color Example
F1 = Pp = 0.5 P & 0.5 p gametes
F2 = Pp x Pp
1 P (female) x 1 P (male) = 0.5 * 0.5 = 0.25 PP
1 P (female) x 1 p (male) = 0.5 * 0.5 = 0.25 Pp
1 p (female x 1 P (male) = 0.5 * 0.5 = 0.25 Pp
1 p (female) x 1 p (male) = 0.5 * 0.5 = 0.25 pp
• What is the probability of getting a heterozygote?
• What is the probability of getting a homozygote?
Why are some flowers pink?
• Complete dominance = dominant &
recessive alleles
• Incomplete dominance = F1 offspring
have phenotype somewhere
between that of the 2 parents =
both alleles expressed
Ex: Flower color (R = red, r = white)
P = 1 Red (RR) & 1 white (rr)
F1 = all Reddish-White = Pink (Rr)
F2 = ¼ Red (RR), ¼ white (rr), ½ pink (Rr)
Incomplete Dominance
Pleiotropy vs. Polygenic Inheritance
• pleiotropy = 1 gene influence many traits
e.g., sickle-cell anemia = homozygous
recessive disease
sickle-cell gene influences:
- shape of RBC’s
- health of heart, brain, spleen, kidneys
• polygenic inheritance = many genes
influence 1 trait, e.g., skin color
- many genes interact to give diverse skin
color ranging very dark to very light
Chromosomal Basis
Review Problem / “Test-Like Question”
Cross the following (assume complete dominance)
AABbCC X AabbCc
Where: A = no anemia
B = brown eyes
C = climb trees
a = anemia
b = blue eyes
c = don’t climb trees
1) Find genotypes & phenotypes of F1 offspring &
express in ratio format
2) Do a F1 X F1 cross using the AABbCC (one of
the F1 offspring) & find the genotypes and
phenotypes of the F2 offspring; express in ratio
format
3) What is the probability of a F1 offspring having
anemia, brown eyes, and tree-climbing abilities?